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The sap flow of plants directly indicates their water requirements and provides farmers with a good understanding of a plant’s water consumption. Water management can be improved based on this information. This study focuses on forecasting tomato sap flow in relation to various climate and irrigation variables. The proposed study utilizes different machine learning (ML) techniques, including linear regression (LR), least absolute shrinkage and selection operator (LASSO), elastic net regression (ENR), support vector regression (SVR), random forest (RF), gradient boosting (GB) and decision tree (DT). The forecasting performance of different ML techniques is evaluated. The results show that RF offers the best performance in predicting sap flow. SVR performs poorly in this study. Given water/m2, room temperature, given water EC, humidity and plant temperature are the best predictors of sap flow. The data are obtained from the Ideal Lab greenhouse, in the Netherlands, in the framework of the European Funds for Regionale Ontwikkeling (EFRO) EVERGREEN Greenport Noord Holland Noord project (2018-2020).

Vegetables have low taste intensities, which might contribute to low acceptance. The aim of this study was to investigate the effect of taste (sweetness, sourness, bitterness, umami, and saltiness) and fattiness enhancement on consumer acceptance of cucumber and green capsicum purees. Three concentrations of sugar, citric acid, caffeine, mono-sodium glutamate, NaCl, and sunflower oil were added to pureed cucumber and green capsicum. Subjects (n = 66, 35.6 ± 17.7 y) rated taste and fattiness intensity. Different subjects (n = 100, 33.2 ± 16.5 years) evaluated acceptance of all pureed vegetables. Taste intensities of vegetable purees were significantly different (P

Vegetables have low taste intensities, which might contribute to low acceptance. The aim of this study was to investigate the effect of taste (sweetness, sourness, bitterness, umami, and saltiness) and fattiness enhancement on consumer acceptance of cucumber and green capsicum purees. Three concentrations of sugar, citric acid, caffeine, mono-sodium glutamate, NaCl, and sunflower oil were added to pureed cucumber and green capsicum. Subjects (n = 66, 35.6 ± 17.7 y) rated taste and fattiness intensity. Different subjects (n = 100, 33.2 ± 16.5 years) evaluated acceptance of all pureed vegetables. Taste intensities of vegetable purees were significantly different (P0.05) between the three tastant concentrations except for umami in both vegetable purees, sourness in green capsicum puree, and fattiness in cucumber puree. Only enhancement of sweetness significantly (P0.05) increased acceptance of both vegetable purees compared to unmodified purees. In cucumber purees, relatively small amounts of added sucrose (2%) increased acceptance already significantly, whereas in green capsicum acceptance increased significantly only with addition of 5% sucrose. Enhancement of other taste modalities did not significantly increase acceptance of both vegetable purees. Enhancing saltiness and bitterness significantly decreased acceptance of both vegetable purees. We conclude that the effect of taste enhancement on acceptance of vegetable purees differs between tastants and depends on tastant concentration and vegetable type. With the exception of sweetness, taste enhancement of taste modalities such as sourness, bitterness, umami, and saltiness was insufficient to increase acceptance of vegetable purees. We suggest that more complex taste, flavor, or texture modifications are required to enhance acceptance of vegetables.Results can be used by cultivators to select and grow vegetable varieties with enhanced taste and flavor. Especially for cucumber, relatively small sweetness enhancement is sufficient to increase acceptance.

In the horticultural fresh food supply chain network in the Netherlands, a crisis is emerging. The market is out of balance and many growers are facing bankruptcy, in the period of 2011–2013, 50% of the growers were not able to pay interest and redemption. Trust between participants in the supply chain network has decreased. This chapter presents the currently not established and identiies design requirements for new systems to address this challenge and provide directions for possible improvement. As a result, this chapter introduces the concept of SamenMarkt®, a participatory system in which multi-agent system technology enables distributed price negotiation, distribution and communication between producers, retailers and consumers. A SWOT analysis of the concept of SamenMarkt® is provided together with a research and development plan in which simulation and emulation create the basis for stakeholder- and participant involvement in the design process of a distributed digital market place. Further research aims to study how SamenMarkt® can provide a solution space for the emerging global food crises. At present, we are using agent-based modelling to simulate the present market and scenarios. The next step will be to build the actual agent-based platform for real-time negotiations and business intelligence.

PurposeMeasurement of the performance of entire supply chains is an important issue because it allows for “tracking and tracing” of efficacy and efficiency failures and leads to more informed decision making with regard to chain design. However, the choice of appropriate supply chain performance indicators is rather complicated due to the presence of multiple inputs and multiple outputs in the system. Therefore, this paper aims to evaluate the usefulness of a novel conceptual model for supply chain performance measurement in an agri‐food supply chain.Design/methodology/approachA conceptual model for integrated supply chain performance measurement is evaluated in a Dutch‐German tomato supply chain by means of a case study approach.FindingsThe proposed conceptual framework is found to be useful for measuring performance of the tomato supply chain. From the case study it is concluded that four main categories of performance measures (i.e. efficiency, flexibility, responsiveness, and food quality) are identified as key performance components of the tomato supply chain performance measurement system.Originality/valueThis research evaluates a novel concept for measuring the performance of agri‐food supply chains. This concept is the first step in developing an integrated performance measurement system that contains financial as well as non‐financial indicators combined with the specific characteristics of agri‐food supply chains. Based on a case study in the tomato supply chain, this concept is found to have potential.

The present work was aimed at understanding and quantifying the effect of sink–source relationships on flower size, using chrysanthemum as a model system. Sink/source ratio was manipulated by flower bud removal (leaving one, two or four flowers, and a control), axillary shoot removal, and varying daily light integral. Furthermore, the influence of flower position within the stem on the flower size was investigated. All means applied to reduce sink/source ratio resulted in a significantly higher individual flower dry mass and area in plants with a fixed number of flowers. Nevertheless, control plants responded to supplementary assimilation light with an increased number of flowers rather than producing larger flowers. Flower position had a negligible effect on flower size in both disbudded and control plants, except that the second-order lateral flowers were significantly smaller than the first-order ones. Singly flowered plants without side shoots represented the greatest potential flower size; they had flowers up to 2.4 times heavier than the control plants. Total aerial plant dry mass was only reduced at very low sink strength treatments, whereas flower mass ratio showed a saturating response to the number of flowers per plant. The results indicate that individual flower size is very sensitive to total plant sink strength, but it does not change with plant source strength when the number of flowers is not manipulated.

In recent years, supply chain performance measures have been in the spotlight of many authors. Many performance measures have been used. In this paper, efficiency was measured as an indicator of performance in the vegetable supply chain. The study consisted of two stages. In the first, data envelopment analysis (DEA) evaluated technical and scale efficiency of the Dutch vegetable sector relative to the type of final distribution (mixed marketing channels versus traditional auctions). In the second stage, a truncated regression model (TRM) was applied to explain grower efficiency from a managerial point of view. The results of the first stage showed that growers who sell their products through auctions are less efficient. The second stage revealed that variables such as firm size, age, ownership structure, and distribution channel strategy significantly contribute to the explanation of technical efficiency of the growers, while the variables firm location, firm size, and firm age affect their scale.

The keeping quality for a cucumber, defined as the time the colour remains acceptable to the consumer, depends on the state of the chlorophyll metabolism. By building a physiological model of the chlorophyll metabolism for cucumbers and using colour data from cucumbers stored at 12, 20 and 28 °C, the parameters of the model were estimated with time and temperature simultaneously as explaining variables. Colour data were expressed as ratios of the separate intensities of the red, green and blue (RGB) values from a 3CCD digital camera, namely blue/red (B/R) and blue/green (B/G). The explained variance obtained was more than 93%. The model structure is generic in nature and describes the postharvest colour behaviour in time and temperature for cucumbers, irrespective of growing conditions and cultivar. For six batches, from three cultvars over two growing seasons, the batch keeping quality, i.e. the time that 95% of all cucumbers in a batch had an acceptable colour, was obtained at 20 °C. A batch was defined as all cucumbers from one grower, one cultivar and one harvest. It is shown that, for batches grown in the spring season, high (colour) keeping quality related to high resistance against bacterial spoilage. Model parameters were estimated using either B/R or B/G colour data. Comparing the model parameters, it was deduced that the average initial colour measurement, expressed as the ratio of the red and green intensities (R/G), would be indicative for the batch keeping quality. It was possible to predict batch keeping quality, on the basis of initial colour measurements only, with a percentage variance accounted for R2adj of 94%. As the basis of the batch keeping quality predictions is a generic model, it might be possible to predict batch keeping quality for many other products which have green colour as the limiting quality attribute.

Consumers expect product availability as well as product quality and safety in retail outlets. When designing or re-designing fruit and vegetables supply chain networks one has to take these demands into consideration next to traditional efficiency and responsiveness requirements. In food science literature, much attention has been paid to the development of Time-Temperature Indicators to monitor individually the temperature conditions of food products throughout distribution as well as quality decay models that are able to predict product quality based upon this information. This chapter discusses opportunities to improve the design and management of fruit and vegetables supply chain networks. If product quality in each step of the supply chain can be predicted in advance, good flows can be controlled in a pro-active manner and better chain designs can be established resulting in higher product availability, higher product quality, and less product losses in retail. This chapter works towards a preliminary diagnostic instrument, which can be used to assess supply chain networks on QCL (Quality Controlled Logistics). Findings of two exploratory case studies, one on the tomato chain and one on the mango chain, are presented to illustrate the value of this concept. Results show the opportunities and bottlenecks for quality controlled logistics depend on product—(e.g. variability in quality), process—(e.g. ability to use containers and sort on quality), network- (e.g. current level of cooperation), and market characteristics (e.g. higher prices for better products).

This study was designed to assess the effect of Diammonium Phosphate (DAP) fertilizer and cattle manure on the accumulation of total phenolics, quercetin and oxalates during growth and storage of vegetable amaranth (var. Amaranthus hypochondriacus L). Field trials were set up at the University of Nairobi Field Station-Upper Kabete Campus, Kenya during the long (March –May) rains of 2007 and 2008. Trials were laid out in a randomized complete block design with four fertilization treatments: 20, 40, and 60kg N.ha -1 supplied by DAP (18:46:0), cattle manure (40 kg Nha -1 ) and an unfertilized control variant. The vegetables were harvested at 6, 7, and 8 weeks after planting and either refrigerated (4+1 o C) or stored at ambient temperature (20+3 o C) for a period of 4 days. The vegetables were then analyzed for total phenolic, quercetin, total and soluble oxalates content before and after storage. The total phenolic content decreased with increasing N, the extent of decrease depended on the level and source of N. DAP at 20kg Nha -1 had the same effect on the phenolic content as that of cattle manure at 40kg Nha -1 . The levels of phenolics and quercetin increased with crop maturity while the converse was true for the oxalate content. During refrigeration, levels of phenolics increased whereas those of oxalates did not change appreciably. There was a decrease in phenolics content during storage at ambient temperatures. The vegetable amaranth accumulates higher levels of total phenolic and quercetin when grown with manure than with DAP for similar levels of N. Total phenolics and quercetin levels increased, while that of oxalates decreased, with maturity of the vegetables. The accumulation of oxalates during the periods considered normal for growth are low enough to maintain their levels within safe limits for human consumption. Storage of the fresh vegetables at 4 o C for limited periods improves their quality by increasing total phenolics and quercetin contents that also fulfill beneficial health functions in the human body.

A constant high relative air humidity (RH) during cultivation can strongly reduce the vase life in some cut rose cultivars. We studied three contrasting cultivars in their tolerance to high RH in order to analyse in detail the water relations during postharvest and better understand this genotypic variation. Plants were grown at moderate (60%) and high (95%) RH, and cut flowers were placed in water immediately after cutting. Flowers of cv. Pink Prophyta grown at high RH did not open throughout vase life, while flower opening of cvs. Frisco and Dream was not affected by preharvest RH. Cultivation at high RH resulted in about 80% shorter vase life in Pink Prophyta, whereas in Dream and Frisco the negative effect was considerably smaller (15 and 9% shorter vase life, respectively). The shorter vase life and reduced flower opening of cut roses grown at high RH was due to a higher rate of transpiration both in the light and dark periods. It was found that the leaves of Pink Prophyta grown at high RH could partly close their stomata upon lowering of the water potential or when flower stalks were fed with abscisic acid, but stomata remained far more open than in leaves grown at moderate RH. The RH during cultivation did not affect stem hydraulic conductivity and its recovery after air emboli induction. Preventing vascular occlusion largely alleviated the high-cultivation-RH effect on vase life and flower opening, showing that the effect of high-cultivation-RH becomes only important if water uptake is limited.

Background The xylem vascular system is composed of fused dead, hollow cells called tracheary elements (TEs) that originate through trans-differentiation of root and shoot cambium cells. TEs undergo autolysis as they differentiate and mature. The final stage of the formation of TEs in plants is the death of the involved cells, a process showing some similarities to programmed cell death (PCD) in animal systems. Plant proteases with functional similarity to proteases involved in mammalian apoptotic cell death (caspases) are suggested as an integral part of the core mechanism of most PCD responses in plants, but participation of plant caspase-like proteases in TE PCD has not yet been documented. Results Confocal microscopic images revealed the consecutive stages of TE formation in Zinnia cells during trans-differentiation. Application of the caspase inhibitors Z-Asp-CH2-DCB, Ac-YVAD-CMK and Ac-DEVD-CHO affected the kinetics of formation and the dimensions of the TEs resulting in a significant delay of TE formation, production of larger TEs and in elimination of the 'two-wave' pattern of TE production. DNA breakdown and appearance of TUNEL-positive nuclei was observed in xylogenic cultures and this was suppressed in the presence of caspase inhibitors. Conclusions To the best of our knowledge this is the first report showing that caspase inhibitors can modulate the process of trans-differentiation in Zinnia xylogenic cell cultures. As caspase inhibitors are closely associated with cell death inhibition in a variety of plant systems, this suggests that the altered TE formation results from suppression of PCD. The findings presented here are a first step towards the use of appropriate PCD signalling modulators or related molecular genetic strategies to improve the hydraulic properties of xylem vessels in favour of the quality and shelf life of plants or plant parts.

Firmness is one of the main quality attributes of sliced tomatoes. Here, a physiology-based mechanism is proposed that describes the development of firmness as measured by limited compression. The mechanism assumes that softening of slices is radically different from that of whole tomatoes and depends on the stage of ripening at slicing. During slicing the pericarp solubility will quickly increase because of exposure to, amongst others, vacuolar content with a low pH. This exposure will have a more serious effect on the firmness behaviour of ripe sliced tomatoes as in that case the pH will already be lower due to normal ripening. This mechanism has been translated into a kinetic model that describes the individual firmness behaviour over time and a batch model that describes the variation in firmness within batches of slices. Firmness data were recorded for different cultivars as a function of time at different temperatures, initial maturity, type of compression and treatment with ethylene and 1-MCP just before slicing. Both models were well able to describe the trends present in the firmness data, with an average percentage variance accounted for of 89%, despite the high variability in the raw data. The firmness decay due to the wounding was very fast, within 12 h. The effect of the initial maturity on the final firmness level was large. On the other hand, the effect of treating whole tomatoes with ethylene and 1-MCP on the firmness development of slices was minimal. This indicates that for consumer acceptance of sliced tomatoes, tomato cultivars need to be selected with a sufficient final (structural) firmness combined with a thorough assessment of the maturity at slicing.

The Zinnia elegans mesophyll cell culture is a useful system for xylogenesis studies. The system is associated with highly synchronous tracheary element (TE) differentiation, making it more suitable for molecular studies requiring larger amounts of molecular isolates, such as mRNA and proteins and for studying cellulose synthesis. There is, however, the problem of non-uniformity and significant variations in the yields of TEs (%TE). One possible cause for this variability in the %TE could be the lack of a standardized experimental protocol in various research laboratories for establishing the Zinnia culture. Mesophyll cells isolated from the first true leaves of Z. elegans var Envy seedlings of approximately 14 days old were cultured in vitro and differentiated into TEs. The xylogenic culture medium was supplied with 1mg/l each of benzylaminopurine (BA) and alpha-naphthalene acetic acid (NAA). Application of this improved culture method resulted in stable and reproducible amounts of TE as high as 76% in the Zinnia culture. The increase was mainly due to conditioning of the mesophyll cell culture and adjustments of the phytohormonal balance in the cultures. Also, certain biochemical and cytological methods have been shown to reliably monitor progress of TE differentiation. We conclude that, with the adoption of current improvement in the xylogenic Z. elegans culture, higher amounts of tracheary elements can be produced. This successful outcome raises the potential of the Zinnia system as a suitable model for cellulose and xylogenesis research.

In photosynthesis light energy is utilized to produce ATP and NADPH. If, for whatever reason, the supply of light does not match the availability of substrates, the plant responds by adapting its photosynthetic apparatus. Recent advances in instrumentation and methodology in photosynthesis research are summarized and an application is given illustrating the effect of airpollutants on Douglas firs.

Growth and liberalization of world trade have increased the risks of introduction of quarantine plant pests into importing countries. Import inspection of incoming commodities is a major tool for prevention of pest introductions related to world trade, but inspection capacities are limited. This article develops a theoretical and an empirical model for the optimal allocation of inspection effort for phytosanitary inspection of imported commodities when the inspecting agency has a limited capacity. It is shown that the optimal allocation of inspection effort equalizes marginal costs of pest introduction across risky commodity pathways. The numerical illustration finds the optimal allocation of inspection effort of chrysanthemum cuttings imported in the Netherlands. The numerical results suggest that ceteris paribus, greater inspection effort should be allocated to pathways whose inspection yields a greater reduction in the expected costs of pest introduction. The numerical results also suggest that import inspection has a high marginal benefit. In particular, we found that each additional euro of the inspection capacity decreases the expected costs of pest introduction from 18 to 49 euros, depending on the initial inspection capacity.

Tomato batches are characterised by large variation in the quality attributes colour and firmness. Acceptability of tomato batches is consequently affected by the colour maturity and firmness maturity. In this paper, a model for the acceptance period (AP) is presented that describes the acceptability of tomato batches as the time that all quality attributes are considered acceptable as a function of the maturity at harvest. This model takes into account that tomatoes can first be unacceptable due to being over-ripe, then be acceptable, and then be unacceptable again, due to being over-ripe. Furthermore, the AP model takes also into account that consumers may not consume all tomatoes in a truss at the same time, some immediately and some in 3–4 days time. This is accomplished by combining the acceptance period for future and immediate consumption and taking the intersection of those two as the overall AP to indicate whether tomato batches are suited for both types of consumption. The AP model combines the effects of biological variation, varying consumer limits and the variation in the time of consumption into a practical method to assess acceptability of tomato batches at harvest. The AP model was based on three experiments. In the first and second experiment several tomato batches, differing in harvest maturity, were harvested and stored at three different temperatures. Colour and firmness was measured non-destructively and repeatedly over time so as to estimate the colour and firmness maturity at harvest for each batch. The third experiment was used to determine the consumer limits for today's consumption and consumption over the weekend. By combining the results from all experiments it was possible to estimate the AP as a function of the maturity at harvest, storage temperature and storage duration. The AP model might be used as a tool for growers to assess the acceptability of tomato batches periodically to analyse and solve quality issues or by optimising the compliance to consumer acceptance by varying the storage temperature and transport time to the retailers. Both applications may help to increase profitability in terms of repeated purchase by consumers.

Postharvest life of tomatoes is limited by colour and firmness when unripe and by firmness when over-ripe. Up till now, little effort has been devoted to develop quality models that deal with both the colour and firmness development. Here, tomato colour and firmness models are presented as a function of the biological age. Data are analysed using non-linear regression analysis. The colour model focuses on the red components synthesis and describes postharvest colour behaviour as a function of the amount of colour precursor present at the time of harvest, the colour at harvest and the storage time. The colour model includes the effect of the colour precursor on the final (red) colour as a function of the (colour) biological age at harvest. The firmness model describes postharvest firmness behaviour for tomatoes as a function of the firmness at harvest, the storage time and the (firmness) biological age at harvest. Applying the firmness and colour model on colour and firmness data of the same tomatoes enabled the comparison of the colour and firmness biological age at harvest. The average firmness and colour biological age at harvest expressed per batch showed a linear relation that varies per grower, but not per season. The remarkable difference in the synchronisation between firmness and colour for the two growers may be caused by differences in greenhouse design. In practice, knowledge of the synchronisation between colour and firmness biological age per grower might help growers to adapt their growing conditions to their greenhouse design so as to produce tomatoes with a predefined colour–firmness relation. Also, colour measurements of tomatoes should suffice to assess the quality once the synchronisation is known.

Preface Introduction: 1. Quantifying the agri-food supply chain: overview and new research directions J.H.M. Wijnands and C.J.M. Ondersteijn.- Measuring performance in agri-food chains: 2. Performance measurement in agri-food supply-chain networks: an overview J. G.A.J. van der Vorst. 3. Accounting standards for supply chains H. Bremmers. 4. Pricing and performance in agri-food supply chains F. Bunte. 5. Performance indicators in agri-food production chains L. Aramyan, C. Ondersteijn, O. van Kooten, A.O. Lansink.- Sharing costs, benefits, and risk in agri-food chains: 6. Retailers' supply chain, product differentiation and quality standards E. Giraud-Heraud and L.-G. Soler. 7. Liability and traceability in agri-food supply chains J.E. Hobb. 8. On the (dis)ability of the firm to quantify chains: a marketing perspective on sharing financial rewards P. Ingenbleek.- Modelling agri-food chains: 9. Profitability of 'ready-to-eat' strategies: towards modelassisted negotiation in a fresh-produce chain H. Schepers and O. van Kooten.- The value of information in agri-food chains: 10. Information management in agri-food chains G. Schiefer. 11. The insurability of product recall in food supply chains M.P.M. Meuwissen, N.I. Valeeva, A.G.J. Velthuis and R.B.M. Huirne. 12. Strategic alliances and networks in supply chains: knowledge management, learning and performance measurement T.L. Sporleder.- Supply chain organization and chain performance: 13. Quantifying strategic choice along the vertical coordination continuum: implications for agri-food chain performance A.F. Wysocki, H.C. Peterson and S.B. Harsh. 14. From corporate social responsibility to chain social responsibility: consequences for chain organization J.J. de Vlieger. 15. Governance structures in the Dutch fresh-produce industry J. Bijman. 16. Effective partnerships for agri-food chains: the impact of supply-chain partnerships on supplier performance in the UKfresh-produce industry R. Duffy and A. Fearne. List of participants.

Postharvest life of strawberries is largely limited by Botrytis cinerea infection. It is assumed that there are two factors influencing the batch keeping quality: the Botrytis pressure and the resistance of the strawberry to infection. The latter factor will be discussed in this article. A colour model is presented that describes the development of red colour and anti-fungal function of individual strawberries over time. The model was fitted to colour data from strawberries grown at two different nutrient densities (EC) and stored per batch at 5, 10 or 16 °C and constant vapour pressure deficit after harvest. A batch was defined as all strawberries from one EC and one harvest. Spoilage per batch was recorded daily during storage. The fitted initial spoilage per batch was found to relate to the fitted average amount of precursor of both the colour and the anti-fungal compounds. Batch keeping quality could be derived from the initial batch colour distributions for the low nutrient batches. Also, batch keeping quality correlated highly with the time between harvest dates for the low EC batches. An explanation for the ability to link colour distributions to keeping quality without having to use a term describing the pH is presented from pigment HPLC measurements of sub-batches. Indications that the colour distributions of high EC batches may be subject to substantial co-pigmentation are presented. For a practical implementation of predicting keeping quality of batches additional research is needed, especially regarding different Botrytis pressures between greenhouses.

In the absence of defects, colour is one of the few practical criteria for assessing cucumber ( Cucumis sativus L.) quality. However, cucumbers with the same colour at harvest can exhibit large differences in quality upon reaching the consumer. Photochemical yield (Φ PSII ), photochemical quenching ( q P ) and a photosystem I evaluation ( k e ), as internal quality measurements, were used in combination with an external quality measurement, represented by the colour, to test if the keeping quality could be predicted on the basis of these measurements. Keeping quality is defined as the time for a cucumber to reach a certain predefined colour limit. To test the hypothesis, colour changes for 2000 cucumbers were measured during storage at 20°C and 100% RH. The colour data were fitted to a model where a correction for biological age ( C ba ) was applied to each individual cucumber. The correction for biological age could be linked to the measured Φ PSII , q P and k e values. Statistical analyses resulted in values for the maximum (green) and minimum colour (yellow) for each cucumber. The minimum colour could be correlated with different cultivars and the maximum colour with plant growing conditions. After a suitable transformation by a neural network, C ba could be estimated based on the values of Φ PSII , q P and k e and the initial colour. Predictions on the keeping quality by the model resulted in an explained variance of 74% ( R 2 adj =0.74).

Vegetable amaranth is a leafy vegetable traditionally grown in sub-Saharan Africa and Asia where it is the most consumed traditional vegetable. It is considered to have high nutritional quality, containing relatively large amounts of vitamins A and C. We have assessed the influence of the maturity of the vegetable and soil nutrition on the visual and nutritional quality of amaranth. We found that leaf ascorbic acid content is strongly influenced by both maturity and soil nutrition, with leaves of seven week old matured plants having the highest content. β-carotene increased with increasing amount of soil nitrogen and with increasing plant age. The loss of both visual and nutritional quality during storage was influenced more by maturity at harvest and the temperature of storage than the soil nutrition.

Recently a model of photosynthetic free energy transduction based on the chemiosmotic theory has been presented (1,2). The forces and fluxes involved in the model are shown in fig.1. The two photosystems act as proton pumps interconnected by a pool of mobile hydrogen carriers (PQ pool). The rate of oxidation of the PQ pool by PS 1 is attenuated by the proton motive force.

The electric response of a thylakoid membrane to a single turnover saturating light flash differed if measured by the electrochromic absorbance change (P515) from the response as measured with the micro-electrode. The slower kinetic components in the P515 response being absent in the micro-electrode measurements (Vredenberg 1981). These slower kinetic components, called reaction II (Schapendonk et al. 1979), in the P515 response were shown not to be a response to a secondary charge separation, known as the Q-cycle (Van Kooten et al. 1983). Because of the difference between micro-electrode and P515 measurements, it was postulated that reaction II was an intramembraneous electric event (Westerhoff et al. 1983).

Young cuts of poplar (P. euramericana L.) were exposed to concentrations of SO2 and NH3 which are considered normal for certain areas in the Netherlands. The effect of 7 weeks of this treatment was monitored with the modulated fluorescence technique and with gas exchange measurements. Photosynthetic performance during the development of the leaves was also followed with the fluorescence technique. The effect of the fumigation on these stress-’free’ plants was relatively small in comparison to the developmental changes during the 7 weeks of observation and treatment. The photosynthesis measurements revealed a slight increase in light efficiency for the higher concentrations of NH3 and SO2. However this increase was absent when the two gasses were combined. NH3 also revealed an increased Pmax. Fluorescence measurements only revealed an enhanced non-photochemical quenching component under steady state conditions for the NH3 treatment. The SO2 treatment revealed enhanced activation kinetics of the CO2 fixation process (Calvin cycle). When the 2 measurements are combined it becomes evident that the higher SO2 concentration (37.2 ppb) and the NH3 treatment (129 ppb) enhance the quantum yield of photosystem 2 for photochemistry under the condition of an ‘energized’ thylakoid membrane. It is therefore concluded that low concentrations of these air pollutants are able to induce a more pronounced state of sun adaptation (Bjorkman 1981) than their counter parts, which had received either filtered air, a low SO2 concentration (18.8 ppb) or a combination of SO2 and NH3.

The P515 electrochromic bandshift following a single saturating light flash in dark-adapted and well preserved chloroplasts, shows multiphasic rise and decay kinetics (A.H.C.M. Schapendonk et al. 1979). These kinetics cannot be explained in terms of generation and decay of a transmembrane electric field. Such a field would be expected to follow a single exponential dark decay with a rate constant determined by the membrane capacitance and the membrane conductance, provided these are field-independent (A.A. Bulychev, W.J. Vredenberg 1976). According to Schapendonk and Vredenberg (A.H.C.M. Schapendonk et al. 1979), the P515 bandshift has components contributed by two different reactions called reaction 1 and reaction 2. In their interpretation, reaction 1, characterised by a fast rise (μs) and a subsequent single exponential dark decay, is the reflection of the flash-induced formation of a transmembrane electric field followed by a field dissipation in the dark by a passive flux of ions. Reaction 2, however, might be related to intramembranal electric phenomena. In this communication we present results that indicate a different sensitivity of the reaction 1 and the reaction 2 components of the overall flash-induced P515 bandshift towards a) the extend of energization of the thylakoid membrane, and b) the ion concentration in the outer aqeous phase.

Since its discovery (Duysens, 1954) the light-induced absorbance change in the 480–550 nm wavelength region, with its characteristic difference spectrum, has been studied in much detail. Ample evidence has been presented (e.g. Junge, 1977) that this so-called P515 absorbance change is due to an electrochromic band shift of a pigment moiety with a maximum around 515 nm. This has made it a useful intrinsic probe for studying the magnitude and kinetics of formation and decay of the transmembrane electric potential of the inner chloroplast (thylakoids) membranes. There has been found reasonable good agreement on the value of the initial potential generated by saturating single turnover light flashes, measured directly by micro-electrodes and calculated by calibrating the absorbance change caused by salt-induced diffusion potentials (e.g. Vredenberg, 1981). The kinetics of P515 responses in continuous light as well as in single turnover flashes are however at variance with the responses measured with microelectrodes. There is now general agreement, based on observations in intact cells as well as in intact and broken chloroplasts that the P515 absorbance change in single turnover flashes is a multiphasic phenomenon, reflected by a biphasic rise and a decay which consists of at least three different single exponentials.

In the previous two contributions (Vredenberg et al. and Snel et al., these proceedings) a model has been described enabling us to conceptualize the mechanism of free energy transduction in chloroplasts. in this paper we will concentrate on the effect of the Donnan potential and of certain membrane modifying reagents on the rate of ATP synthesis or hydrolysis.

Key words: Cucumis sativus, intracanopy lighting, light-emitting diodes (LEDs), light distribution, light interception, light quality, photosynthesis, photosynthetic acclimation Assimilation lighting is a production factor of increasing importance in Dutch greenhouse horticulture. Assimilation lighting increases production levels, improves product quality and opens possibilities for year round production. As a drawback, this use of assimilation lighting increases energy inputs and CO2-emission. Intracanopy lighting (with LEDs) is a technique to enhance the light use efficiency by changing the position of (a part of ) the lamps from above to within the canopy of greenhouse grown crops. Intracanopy lighting (IL) firstly reduces reflection and transmission losses of the supplemental lighting on crop level. These losses are high in traditional top-lighting systems, hence IL yields a higher light absorption on crop level. Secondly, IL creates a more homogenous vertical light distribution which can result in higher light use efficiencies. The aim of present study was to obtain insights in photosynthetic acclimation in response to irradiance level and spectrum in the framework of the applicability of LEDs as light source for intracanopy lighting in indeterminate growing vegetable crops. Intracanopy lighting may vary in (1) position within the crop, in (2) irradiance level and in (3) spectrum. Leaves deeper in the canopy are older. If leaf age negatively affects the photosynthetic capacity (Amax), then potential positions of IL-lamps reduce. By growing tomato plants horizontally so that irradiance was similar for all leaves from 0-70 days old, it is concluded that during the normal life-span of tomato leaves in cultivation, irradiance and not ageing is the most important factor affecting Amax. In winter, natural irradiances are low so that new developing leaves acclimate to low irradiances. Later on in their life time these leaves could be exposed to higher irradiances owing to IL. The question arose if cucumber leaves which develop under low irradiance can acclimate to a moderate irradiance. Acclimation of photosynthesis occurred within 7 days but photosynthesis at moderate irradiance and Amax did not reach to that of leaves developed under moderate irradiance. This reveals the importance of photosynthetic acclimation during the leaf developmental phase for crop productivity in scenarios with realistic, moderate fluctuations in irradiance that leaves can be exposed to. By growing plants under seven different combinations of red and blue light, blue light is shown to have both a qualitative and a quantitative effect on leaf development. Only leaves developed under red light (0% blue) displayed a dysfunctional photosynthetic operation (“red light syndrome”), which was largely alleviated by only 7% blue. Quantitatively, leaf responses to an increasing blue light percentage resembled responses associated with an increase in irradiance. Leaves developed under red light exposed to a mixture of red and blue (RB) completely recovered within 4 days after exposure to RB-light but remained limited in other leaf parameters, showing limitations in plasticity due to constraints arising from the prior leaf development. Leaves developed under RB also revealed the “red light syndrome” within 7 days of red illumination. Lastly, the effects of intracanopy lighting with LEDs on the production and development of a cucumber crop was investigated in winter. In the IL-treatment, LEDs supplied 38% of the supplemental irradiance within the canopy; the remaining 62% was supplied as top lighting by High-Pressure Sodium (HPS) lamps. The control was 100% top lighting (HPS lamps). Intracanopy lighting resulted in a greater Amax for leaves at deeper canopy layers but did not increase total biomass or fruit production. This was partly due to a reduced light interception caused by extreme leaf curling, which counteracted the expected higher light absorption by the crop, and partly to a lower dry matter partitioning to the fruits, and thus a greater dry matter partitioning to the leaves compared to the control. The effect of these factors on fruit yield was quantified using a explanatory crop model. Model calculations revealed a large negative effect on the fruit yield due to the greater partitioning to the leaves, whereas the negative effect of leaf curling was small. The effect of a greater Amax at deeper canopy layers was slightly positive. The last however might have indirectly caused the greater partitioning to the leaves as the greater Amax was associated with a preserved leaf mass per area.

Key words: Parthenocarpy, Capsicum, fruit set, hormones, cell division, cell expansion, auxin, gibberellin, temperature, carpel-like structures, genotype Parthenocarpy (fruit set without fertilization) is a much desired trait in sweet pepper (Capsicum annuum) production as it minimizes yield irregularity, enhances total yieldandmakes theproduction possible under suboptimal environmental conditions. Beside this, parthenocarpyimproves the commercial value of the fruitsince parthenocarpic fruits are convenient for consumption, much wanted for minimal-processed food, and possess long shelf-life.Parthenocarpy has been widely studied for tomato and Arabidopsis but not for C. annuum.Physiological and morphological characterization of parthenocarpy in C. annuum is the main focus of this thesis with emphasis on finding evidence thattomato and Arabidopsis can be used as model plants to study fruit development in C. annuum.The series of physiological and morphological changes (i.e. pollen tube growth, vascular connection between ovule and carpel, cell division and cell expansion in carpel) that occurs in a post-fertilized ovary ofC. annuumwas similar to that reported in tomato and Arabidopsis. Similar to these two species, C. annuum showed a hierarchy between auxin and gibberellinwhere auxin acts upstream of gibberellin in fruit set, most likely by inducing gibberellin biosynthesis.These findings indicate that fruit set mechanisms in C. annuum are similar to that reported in tomato and Arabidopsis.Parthenocarpy was evident in most of the studied genotypes of C. annuum(n=24) suggesting that somedegree of intrinsic parthenocarpy is already present in C. annuum. External application of auxin and gibberellin on the stigma of emasculated flowers enhanced parthenocarpic fruit production.GA3 did not significantly contribute to the final fruit size. GA3 seems to play an important role in preventing flower and fruit abscission while auxin seems to be important for both fruit set and fruit development. Almost all seedless fruits obtained either by only emasculation or emasculation followed by hormone application showed stronger growth of carpel like structures (CLS) compared to seeded fruits. Structural analogy of CLS with bel1 mutant of Arabidopsis suggests that CLS are transformed from abnormal ovules.Capsicum genotypes with high parthenocarpic potential showed a stronger CLS development suggesting a relation between female sterility and parthenocarpy. The parthenocarpic potential appeared to be controlled by a single recessive gene. The CLS phenotype and parthenocarpy could not be linked to a single locus, suggesting that absence of fertilization induces parthenocarpic fruit development and allows CLS growth, which substitutes developing seeds in promoting fruit development.This thesis provides insight in the physiology and morphology and genetic basis of parthenocarpy in C. annuum.

A greenhouse crop can be approached as an open system that can be affected by a number of parameters such as light, climate or nutrient supply. In the last decades efforts have been made to understand the functioning of this system and the interaction between the different parameters. The intensive nature of greenhouse cultivation combined with the economic necessity to enlarge the farm size makes the development of decision support systems (DSS) imperative to help the growers in managing their farms efficiently. The foundation of DSS are plant models and in order to work more efficiently they should be able to receive information in real time from sensors that measure different plant parameters such as light interception, leaf area index and photosynthetic stress in a non-destructive way. In order to develop functional DSS it is imperative to develop accurate models and monitoring techniques applied in the specific greenhouse environment. The aim of this thesis was to explore different techniques to simulate and monitor light interception and photosynthesis by a greenhouse grown tomato canopy. Since photosynthesis is directly linked to light absorption we opted to develop a three dimensional model that takes into account the explicit plant architecture. Different methodologies to monitor these physiological properties online by means of remote sensing were also explored. A number of physiological tomato models have been proposed the last decades, their main challenge being the correct simulation of fruit yield. For this, an accurate simulation of light interception, and thus photosynthesis, is of primary importance. At present most process-based models and the majority of three dimensional models, include simplifications of plant architecture that can compromise the accuracy of light interception simulations and, accordingly, canopy photosynthesis. In Chapter 2.1 the first steps towards the development of the model are presented. Light interception is highly dependent on the canopy structure, which is affected, among others, by the distance between plant rows, the distance of plants within the row, leaf pruning and crop variety. The model was used to test different crop planting scenarios on their effect on light interception. Light interception from the planting scenarios was then compared with results of a totally homogeneous canopy. Also analysis of differences between manual measurements of leaf length, width, elevation angle and leaf orientation was conducted. Changes of leaf elevation angles at two different times of the day were also measured. In tomato differences in leaf length, width and elevation angle of the leaves were mainly observed in the upper 90cm of the plant, in the still developing zone. Changes of the architectural characteristics of structural plant characteristics affected directly light interception by the crop canopy. Nevertheless even if plant structure stayed the same, light penetration could easily be manipulated by changing the row spacing in the crop, thus affecting light interception and potentially plant production. In Chapter 2.2 the development and calibration of a functional-structural tomato model is fully described. The model was used to investigate the canopy heterogeneity of an explicitly described tomato canopy in relation to temporal dynamics of horizontal and vertical light distribution and photosynthesis under direct and diffuse light conditions. The model consists of an architectural static virtual plant coupled with a nested radiosity model for light absorption and a leaf photosynthesis module. Different scenarios for horizontal and vertical distributions of light interception, incident light and photosynthesis were investigated under diffuse and direct light conditions. Simulated light interception showed a good correspondence to the measured values. Explicitly described leaf elevation angles resulted in higher light interception in the middle of the plant canopy compared to fixed and ellipsoidal leaf elevation angle distribution models, although the total light interception remained the same. The fraction of light intercepted at a north-south orientation of rows differed from an east-west orientation by 10% in winter and 23% on summer days. The horizontal distribution of photosynthesis differed significantly between the top, middle and lower canopy layer. Taking into account the vertical variation of leaf photosynthetic parameters in the canopy, led to ca. 8% increase on simulated canopy photosynthesis. Manipulation of plant structure can strongly affect light distribution in the canopy and photosynthesis. In Chapter 2.3 the idea of identifying different plant ideotypes for optimization of light absorption and photosynthesis was explored. Using the functional-structural tomato model presented in the previous chapters, a range of different plant architectural characteristics were tested for two different seasons in order to find the optimal architecture with respect to light absorption and photosynthesis. Sensitivity analyses were carried out for leaf elevation angle, leaf phyllotaxis, leaflet angle, leaf shape, leaflet arrangement and internode length. From the results of this analysis two possible ideotypes were proposed. Increasing light absorption in the top part of the canopy by 25 %, without changing light absorption of the canopy as a whole, augmented photosynthesis by 6 % in winter and decreased it by 7 % in summer. The measured plant structure was already optimal with respect to leaf elevation angle, leaflet angle and leaflet arrangement for both light absorption and photosynthesis while phyllotaxis had no effect. Increasing the length-to-width ratio of leaves by 1.5 or increasing internode length from 7 to 12 cm led to an increase of 7 – 10 % for light absorption and photosynthesis. The most important architectural traits found were the internode length and the leaf shape as they affect vertical light distribution in the canopy distinctly. A new plant ideotype with more spacious canopy architecture due to long internodes and long and narrow leaves led to an increase in photosynthesis of up to 10 %. In Chapter 3.1 ways to monitor on-line LAI and PAR interception of the canopy, under greenhouse conditions, through reflectance measurements, were explored. LAI and PAR interception were measured at the same moments as reflectance at six wavelengths in different developmental stages of tomato and sweet pepper plants. Normalized Difference Vegetation Index (NDVI) was calculated. Relationships between the measured parameters were established in experimental greenhouses and subsequently these were tested in commercial greenhouses. The best estimation for LAI and PAR interception was obtained from reflectance at 460nm for both tomato and sweet pepper. The goodness of the fit was validated with data from the commercial greenhouses and was also tested in this study. The divergence of the results from the ones reported from field experiments can be traced back to the special greenhouse environment, where more sources of reflectance are added due to construction parts and a white plastic covered background. Reflectance measurements offer a non- destructive way to estimate PAR interception and LAI (up to the value of 3) in greenhouse production systems. The relationship established between reflectance at 460 nm, PAR interception and LAI for both tomato and sweet pepper, can become a good tool for crop online monitoring in greenhouse conditions. Furthermore if information from reflectance sensors is used as input directly into the crop models, new opportunities for decision support systems in greenhouse production could be opened up. Photosynthetic stress induced by water deprivation in plants affects a number of physiological processes such as photosynthetic rate, stomatal conductance as well as the operating efficiency of PSII and non- photochemical quenching. Photochemical Reflectance Index (PRI) is reported to be sensitive to changes of xanthophyll cycle that occur during stress and could possibly be used to monitor changes in the physiological parameters mentioned before. In Chapter 3.2 the use of PRI as an early photosynthetic stress indicator was evaluated. A water stress treatment was imposed on a greenhouse tomato crop. CO2 assimilation, stomatal conductance, light and dark adapted fluorescence as well as PRI and relative water content of the rooting medium RWCs% where repeatedly measured. The same measurements were also performed on well-irrigated plants that acted as a reference. The experiment was repeated in four consecutive weeks. Results showed that PRI can be used as an early stress indicator only when light intensity at crop level was above 700μmol m-2 s-1. At lower values of light intensity the relationship of PRI to RWCs% was poor in comparison to photosynthesis or fluorescence parameters that showed a high correlation to RWCs%. For that reason we can conclude that PRI as water stress indicator cannot be independent of the ambient light conditions and its use can make sense only under conditions of high light. Finally in Chapter 4 the main achievements and limitations of this study are discussed and directions for future research are proposed.

Restriction of leaf water loss, by stomatal closure, is decisive for plant survival, especially under conditions of water deficit. This sensitivity of stomata to low water potential is attenuated by high relative air humidity (RH ≥ 85%) during growth, which impedes the plant’s ability to survive when subsequently exposed to lower humidities due to a negative water balance. This thesis focuses on the extent of the existing variation and the reasons underlying cultivar differences in their tolerance to high RH, as well as the rate and reversibility of stomatal adaptation to elevated RH in the course of leaf ontogeny. Cut rose was used as a model plant. An experiment on the postharvest water relations of three contrasting cultivars in their sensitivity to high RH showed that the sensitive cultivar (i.e. steepest decrease in the cut flower longevity) underwent a higher increase in the water loss compared to the tolerant cultivars. Preventing vascular occlusion considerably extended the time to wilting in the sensitive cultivar grown at high RH, showing that the high rate of water loss, as a result of plant growth at high RH, can only be detrimental for keeping quality under limiting water uptake conditions. Further investigation showed a large genotypic variation in the regulation of water loss, as a result of leaf development at high RH, and stomatal closing capacity was the key element in this process. The degree to which the stomatal anatomical features were affected and the extent that their functionality was impaired were not correlated. However, higher stomatal density, longer pore length and depth contributed to the higher water loss of high RH-grown leaves (16–30% of the effect depending on the cultivar). Reciprocal change in RH showed that stomatal functioning was no longer affected by the RH level after full leaf expansion. However, expanding leaves were always able to partly adapt to the new RH level. For leaves that started expanding at high RH but completed their expansion after transfer to moderate RH, the earlier this switch took place the better the regulation of leaf water loss. This behaviour of expanding leaves experiencing a shift from high to moderate RH was related with the increasing population of stomata exceeding a critical stomatal length. Contrary to this, leaves initially expanding at moderate RH and transferred to high RH exhibited poor stomatal functioning, even when this transfer occurred very late during leaf expansion. This suggests that stomata at various developmental stages were similarly prone to loss of closing ability, when these had been exposed to high RH prior to full leaf expansion. Key words: abscisic acid, cuticular permeability, heterogeneity, hydraulic conductivity, pore aperture, relative air humidity, Rosa hybrida, stomatal anatomy, stomatal conductance, stomatal growth, stomatal initiation, stomatal malfunctioning, stomatal population, stomatal proximity, vase life.

Key words: action spectrum, artificial solar spectrum, blue light, Cucumis sativus, gas-exchange, light-emitting diodes (LEDs), light interception, light quality, non-photosynthetic pigments, photo-synthetic capacity, photomorphogenesis, photosystem excitation balance, quantum yield, red light. A wide range of plant properties respond to the spectral composition of irradiance, such as photosynthesis, photomorphogenesis, phototropism and photonastic movements. These responses affect plant productivity, mainly via changes in the photosynthetic rate per unit leaf area, light interception, and irradiance distribution through the canopy. The spectral environment of plants is dependent on location (e.g. latitude), changes over time (e.g. Sun-angle), shading by other leaves, and, in the case of protected cultivation, the use of growth lamps. Therefore, not only the acclimation of developing leaves to light spectrum is important for plant productivity and survival, but also the capability of mature leaves to respond to changes in spectrum. This thesis focuses on the acclimation of photosynthesis per unit leaf area to the growth-light spectrum, the consequences of spectral acclimation for the wavelength dependence of photosynthetic quantum yield, and photomorphogenetic versus leaf photosynthetic acclimation in relation to biomass production. Cucumis sativus is used as a model plant. Additionally, the consequences of the choice and quality of the actinic light used during photosynthesis measurements are explored. By growing plants under seven different combinations of red and blue light, blue light is shown to have both a qualitative and a quantitative effect on leaf development. Only leaves developed under red alone (0% blue) displayed a dysfunctional photosynthetic operation, which was largely alleviated by only 7% blue. Quantitatively, leaf responses to an increasing blue light percentage resembled responses associated with an increase in irradiance. Next, the wavelength dependence of the quantum yield for CO2 fixation (α) is analysed in detail. Leaves grown under artificial shadelight, which overexcites photosystem I (PSI), had a higher α at wavelengths overexciting PSI (≥690 nm) and a lower PSI:PSII ratio compared with artificial sunlight and blue light grown leaves. At wavelengths overexciting PSII, α of the sun and blue grown leaves was higher. The photosystem excitation balance is quantitatively shown to determine α at those wavelengths where absorption by carotenoids and non-photosynthetic pigments is insignificant (≥580 nm). The wavelength dependences of the photosystem excitation balance calculated via an in vivo and an in vitro approach were substantially in agreement with each other, and where not, carotenoid absorption and state transitions are likely to play a role. Not only is the photosynthetic rate per unit leaf area is important for plant productivity, but also photomorphogenesis. We have engineered an artificial solar (AS) spectrum under which plants produced a dry weight that was, respectively, 2.3 and 1.6 times greater than that of plants grown under fluorescent tubes and high pressure sodium light. This striking difference was due to a morphology of the AS-plants that was more efficient in light interception, and not related to photosynthesis per unit leaf area. These results highlight the importance of a spectrum that is more natural than that of usual growth-lamps for research and possibly also for horticultural production. A technically orientated part of this thesis presents a simple method to quantify the light distribution in leaf chambers, which is shown to be important for the accuracy of photosynthesis measurements by gas-exchange. The match between growth-light and measuring-light spectrum is likewise shown to be important. A mismatch can have significant consequences for the estimate of α in situ, but only minor consequences for the estimate of the light-saturated photosynthetic rate. The relationship between the electron transport rate calculated using chlorophyll fluorescence measurements and the CO2 fixation rate also changed considerably with changes in measuring-light spectrum. The use of erroneous estimates of α as input for crop growth models is shown to have disproportionately large consequences for predictions of plant growth.

KEYWORDS: Traditional leafy vegetables, Amaranth, diammonium phosphate, manure, yields, nutrients, antinutrients, phenolics, oxalates, small-scale farmers, KenyaAgriculture in developing countries faces a number of pressing challenges including population growth, widespread poverty and food insecurity. Widespread poverty persists, contributing to severe malnutrition. This intolerable situation can be remedied in great measure by increasing production and consumption of traditional leafy vegetables (TLVs). The most commonly consumed TLVs in Kenya and most developing countries include the Amaranthus spp. (Pig weed), Vigna unguiculata (Cowpea leaves), Solanum nigrum (Black nightshade), Cleome gynandra (Cat’s whiskers), Cucurbita spp. (Pumpkin leaves) and Corchorus spp. (Jute). A market survey carried out in 31 major supermarkets and green grocers’ stores in the city of Nairobi found that the demand for TLVs was high. Of the TLVs, vegetable amaranth was the most preferred but had problems of a short shelf life and a negative image due to poor production practices. The distance from production sites to the markets for the vegetables in Nairobi ranged between 20 and 40 km. Amaranthus was found to contain reasonable amounts of proteins, ascorbic acid, zinc and iron. The levels of nitrates, oxalates and lead found in these vegetables were within safe limits allowed for human consumption, hence are of no public health concern. In field experiments, Amaranthus hypochondriacus yields were significantly increased when N was supplied via chemical fertilizer (diammonium phosphate (DAP)) but not when the N-source was cattle manure. The yield increased with increasing levels of N from 0 – 40 kg N ha-1 but decreased at 60 kg N ha-1., Leaf nitrate content, however, increased between 0 – 60 kg N ha-1 with the highest amounts being recorded at the highest level of N in DAP. In addition, DAP at 60 kg N ha-1 led to the highest levels of β-carotene, but with reduced ascorbic acid and total phenolic contents, when compared to the other levels of N. DAP at 20 kg N ha-1 and the use of cattle manure led to high amounts of ascorbic acid and total phenolic contents. During storage, the quality of the vegetables in terms of ascorbic acid, β-carotene and total phenolics is maintained when the vegetable was stored at 4oC for a period of up to 4 days. Gross margins calculated using the yield obtained at 8 weeks after planting from both manure (40 kg N ha-1) and DAP treated plots that received 40 kg N ha-1 demonstrated the feasibility of production of A. hypochondriacus as a commercial enterprise for small scale farmers. These gross margins were based on the small-scale farmers’ economic model of what they consider as ‘free’ resources. Overall the study shows that traditional leafy vegetables are sold in major supermarkets and green grocers in the city of Nairobi and are free of lead contamination hence safe for human consumption. It also demonstrates that A. hypochondriacus yields can be increased by using DAP fertilizer at 40 kg N ha-1. The vegetable is a valuable source of nutrients therefore can be used to reduce the malnutrition problems facing developing countries. As a commercial enterprise, A. hypochondriacus can be used to generate income that can be used to reduce poverty and food insecurity problems facing developing countries.

The water status in plants is dependent on the xylem hydraulic conductance. In cut flowers, for example, the preservation of continuous hydraulic conductance is important for maintaining longer vase life, an important index for cut flower quality. Many factors, such as stomata performance, root water uptake efficiency, temperature, light quality and humidity, influence the efficiency of water transport in the soil-plant-atmosphere continuum.It has also been shown that the dimensions of the xylem conduits in the plant, such as the conduit length, diameter and density, are important for plant water transport. Therefore, the regulation of the xylem conduit size during xylogenesis is necessary for providing a particular hydraulic efficiency in a given environment. To understand the regulation of xylogenesis and the influence of the processes to produce xylem conduits of particular dimensions with specific hydraulic capacity, this thesis focuses on the Zinnia elegans tracheary element (TE) differentiation at the cellular level. It further considers the regulation of xylem conduit dimensions in whole plants by using drought stress and temperature as variable environmental factors.In Chapter 1, the mechanisms of plant water transport, as regulated by xylem conduit dimensions with consequent effect on plant quality, are reviewed. Also, the chapter provides general literature on xylem formation, Zinnia elegans as model for xylogenic research, and finally gives the outline of the thesis.In Chapter 2 , two cultivars of the Zinnia elegans cut flowers, 'Envy' and 'Purple Prince', were grown in soils with different water contents with increasing water stress levels: 70, 50 and 20 %(v/v) soil water content (SWC). Plant length increased with SWC. The hydraulic efficiency in the stems of cut flowers decreased with increasing water stress. The xylem conduit length was not different between Purple Prince cut flowers from the three different water stress levels. In the Envy cultivar, a significant difference in conduit length was found between plants from 20 and 50% (v/v) SWCs. The proportion of large-diameter conduits increased with decreasing drought stress, i.e., in cut flowers from both cultivars the proportion of large-diameter vessels was highest in the 70% (v/v) SWC and least in the 20% (v/v) SWC. This means that the decrease in hydraulic conductivity with water stress was largely due to the decrease in the diameter of the conduits making the conduit length less important. The vase life of the cut flowers correlated negatively with the diameter of the xylem conduits, confirming that small-diameter conduits are more efficient in overcoming embolism and therefore maintaining efficient water transport in the stems.In Chapter 3 , the role of average day temperature (ADT) and difference in day and night temperature (DIF) on the length of xylem elements and conduits, and fusion of the individual elements into conduits in planta were investigated. Plant length increased significantly with ADT between 17 and 25 o C. Opposite DIF treatments resulted in different plant length at 4 and 8 o C absolute DIF. The responses to temperature differences were similar in both Zinnia cultivars, although Purple Prince cultivar plants were always bigger. The maximal vessel length (L max ) and half vessel lengths (τ) both increased with increasing ADT, about 110-130% increase from 17 to 25 o C ADT. However, DIF had no effect on these conduit parameters. Measurements of average lengths of individual vessel elements from plants grown at average temperatures 17 and 25 o C showed about 10% and 20% increase in Envy and Purple Prince respectively. This change in vessel element length constitutes a small part of the overall change in whole conduit length, and therefore, it can be deduced that the amount of vessel element fusion is more important during formation of longer xylem conduits in the plant.In Chapter 4 , a method for establishing an in vitroZinnia elegans cell culture for differentiation into tracheary elements (TEs) is described. The xylogenic Zinnia system is rather unique for studying xylogenesis at the cellular level. It offers possibilities to study in detail the effects of certain factors important for regulating the xylogenesis process. The in vitro culture has been well standardized to eliminate the inconsistencies in TE differentiation that have been observed in the past. Additional methods for monitoring and measuring the important hallmarks of TE differentiation, including DNA laddering, vacuolar rupture, cellulose bands and lignin synthesis, have been treated in this section.In Chapter 5 , the effect of leaf osmolarity (LO) induced by different light intensities and electrical conductivity (EC) of soil-less plant culture, on the differentiation of TEs in cell cultures from the two cultivars, Envy and Purple Prince, was investigated. Both light intensity and EC correlated with LO with a similar response, although it was always higher in the Envy cultivar. By increasing the LO, the percentage of TEs (%TE) formed also increased, with a similar trend in cultures from both cultivars. About 60% increase in %TE was realized in the Purple Prince cultivar as compared to earlier reports by different authors. The LO of the starting material also correlated positively with the final size of the TE. This implies that the efficiency of the xylogenic Zinnia system can be improved by adaptations in the growth conditions for the starting material.In Chapter 6 , we investigated the effect of osmotic stress and timed application of NAA and BA (xylem differentiating phytohormones) on the TE differentiation in cultures from the two Zinnia cultivars. The %TE increased with increasing extracellular osmotic stress until 300 mOsm, after which any additional increase resulted in a diminishing %TE. The size of the TEs increased with decreasing extracellular osmolarity within the working range of 100 to 700 mOsm. These results were similar in the cultures from both cultivars. Application of NAA/BA induced a significant increase in the rate of cell division corresponding to a similar increase in TE differentiation. It is proposed that the increase in cell division may be beneficial in providing certain bioactive materials for the development of the TEs. Delayed application of NAA/BA reduced the %TE, but with a significant increase in TE size.In Chapter 7 , the role of programmed cell death (PCD) in determining the final size of the TEs and the %TE was investigated. Application of two protease inhibitors, a general caspase inhibitor (Zasp) and a cysteine protease inhibitor (E64), to xylogenic cultures from the two cultivars, caused a significant increase in the size of TEs but 65-300% decrease in %TE. There proved to be a significant delay or complete inhibition of the hallmarks of PCD, such as DNA laddering and vacuole collapse, in the cultures treated with the inhibitors. It was also found that the second stream of TE differentiation, common in non-inhibited xylogenic Zinnia cultures, was absent in cultures treated with 10nM or higher of each of the two inhibitors.The research experiments and their outcomes in this thesis range from fundamental xylogenesis at the cellular level to whole plant water transport performance. The overall discussion of the experimental approaches, results, and their practical implications is presented in Chapter 8 . This chapter also offers suggestions for future research.

Greenhouse horticulture in The Netherlands is a highly sophisticated form of crop production, resulting in high production levels and good product quality. However, it also requires high energy inputs, representing 15 to 20% of the production costs in most crops. It is important that energy efficiency (amount of product produced per unit energy input) is improved. Breeding for cultivars of greenhouse crops that are better adapted to temperatures below the current economical optimum temperature would contribute significantly to an improvement in energy efficiency. This thesis aims at quantifying and understanding the possibilities for improving energy efficiency of greenhouse crops by plant breeding. Tomato and chrysanthemum are used as model crops. The main emphasis is on exploring possible differences in growth, development and ultimately yield within tomato or chrysanthemum cultivars in response to sub-optimal temperature, but attention is also given to improving yield at optimal temperature. In the present study it was found that there is much more variation for temperature response between chrysanthemum than between tomato cultivars. Therefore it is important that for tomato alternative sources for variation are explored, like wild related species.When breeding for lower temperature demanding cultivars it is important to take into account the reduction in growth at sub-optimal temperature due to a decrease in specific leaf area (SLA). Furthermore, attention should be given to the delay in development at sub-optimal temperatures. Energy efficiency can also be increased by creating higher yielding cultivars. In chrysanthemum the whole above ground plant is harvested and therefore a shift in biomass partitioning might not be profitable unless plant quality is improved, resulting in a higher economical value. In tomato changing partitioning from leaves in favour of the fruits might be possible, if cultivation is adapted such that leaf area index stays high enough. For both crops it is important that light use efficiency (LUE) is further increased.Furthermore, a better light interception early in the cultivation, for example by increasing SLA, can be beneficial for growth of both chrysanthemum and tomato although for tomato, for a large part of the cultivation period light interception can be kept already at a near optimum level with modern cultivation practices.

Keywords: quarantine pest, plant health policy, optimization, import phytosanitary inspection, ‘reduced checks’, optimal allocation of resources, multinomial logistic regression, the Netherlands World trade is a major vector of spread of quarantine plant pests. Border phytosanitary inspection is a major barrier against introductions of quarantine pests through imported commodities, although the inspection resources are limited. This thesis provides conceptual and empirical insights that may help optimise import inspection under limited resources. The developed conceptual models analysed inspection policies under capacity constraints and in the absence of capacity constraints. The empirical models focused on finding the optimal inspection policies of propagating materials imported in the Netherlands. The results indicate that inspection effort should focus on commodities, whose inspection yields ceteris paribus greater marginal reduction in the expected costs of pest introduction. The results show that under binding capacity constraints, inspection of chrysanthemum cuttings in the Netherlands has a high marginal benefit, ranging from 8 to 49 euros for every marginal euro of inspection cost. The results further indicate that in the presence of fixed inspection costs, attaining the unconstrained allocation of inspection effort from the current, capacity constrained levels, is relatively inexpensive and greatly reduces costs to society. To expand current inspection capacities, the costs and likelihoods of pest introduction should be carefully estimated. Using the developed models for optimal allocation of inspection resources, the efficacy of the ‘reduced checks’ import inspection system in the EU was analysed. The results indicate that the expected costs of pest introduction in the EU under reduced checks could further be reduced if the economic impacts of pest introduction through various commodities are accounted for when calculating the frequencies of reduced checks. Finally, a multinomial logistic model was developed to analyse factors that determine the likelihoods of rejecting imported commodities due to phytosanitary and non-phytosanitary reasons. The results suggest that the geographical position of the exporting country, the characteristics of the importing company, the size of imported shipments, and the intended use of the commodity, among others, are significant factors based on which shipments of plant commodities can be targeted for inspection. Inspecting agencies can considerably facilitate the design of optimal inspection frameworks for the management of import phytosanitary risks by sound data-recording procedures that enable scientific analysis and provide a solid basis for reliable and applicable results.

As a result of contemporary horticultural practices relative air humidities (RH) in greenhouses are often very high. In particular in cut flowers, this results in quality problems after harvest when flowers are transferred to low RH conditions at the consumers. The quality problems are related to excessive water loss caused by a disturbance in normal functioning of stomata. The general aim of this study was to investigate the effects of high RH during growth on the stomatal response characteristics of Tradescantia virginiana L. To reach this aim, the quantitative effects of moderate (55%) and high (90%) RH during growth on the stomatal anatomy and responses of well-watered plants in response to desiccation, abscisic acid (ABA) application and light/dark transition were studied. Higher variability of stomatal closure and the presence of some non-closing stomata in high RH grown plants were striking. The dynamics of spatial heterogeneity of stomatal closure within a leaf altered by growth at high RH were studied using a chlorophyll fluorescence imaging system under non-photorespiratory conditions, and the estimates of stomatal closure obtained by means of Φ PSII measurements was correlated with direct measurements of stomatal closure and water relation parameters in plants subjected to water stress. It was shown that following desiccation, leaves grown at high RH had both a greater heterogeneity and a higher PSII efficiency compared to leaves grown at moderate RH. The stomata of high RH grown leaves were less sensitive to decreases in leaf relative water content and water potential. Moreover, the role ofABAin the disturbed stomatal responses of plants during growth at high RH was studied. The concentration ofABAin leaves grown at moderate and high RH was compared and stomatal responses to short-termABAapplication, and stomatal responses to long-termABAapplication was analysed.The results reinforce the proposal that a lowABAconcentration in well-watered plants during growth at high RH could be a reason for the structural or physiological changes of stomata. In addition, the consequences for stomatal responses of maintaining a high or low RH around only one leaf are described. The rate and the reversibility of adaptation of stomatal behaviour to moderate or high RH was also investigated and correlated with leaf ABA concentration and leaf hydraulic properties of the adapted plants. The main achievements and practical implications of this study are discussed, and suggestions for further research are presented.

Keywords : PMS,agri-food, supply chain, efficiency, flexibility, responsiveness, food qualityThe main objective of this research is to contribute to the development of a Performance Measurement System (PMS) foragri-food supply chains that involves the entire chain (i.e. all stages starting from raw materials to retailers) and includes a comprehensive set of performance indicators. For this purpose, Dutch vegetable supply chains have been chosen as case study. The research started by looking at the factors influencing the performance of Dutch vegetables growers. The results show that the choice of marketing channel has an impact on the performance of growers (i.e. growers who used mixed marketing channels, on average, were relatively more efficient than those who sold their total produce through auctions).The next step of the research was a literature review on existing performance indicators and models in the supply chain literature. Based on this literature review a conceptual framework for measuring performance ofagri-food supply chains is developed which includes financial, as well as non-financial indicators combined with specific characteristics ofagri-food supply chains. The conceptual framework is evaluated in a Dutch-German tomato supply chain and further developed into a condensed model with only the key performance indicators. The results show that efficiency, flexibility, responsiveness and food quality are four key performance components and form the basis for a PMS foragri-food supply chains.The application of the conceptual framework is carried out by looking at the perceived impact of different requirements of Quality Assurance Systems (QAS) on the performance of a Dutch tomato supply chain. Results indicate that some QAS requirements are perceived to have a positive impact on some supply chain members' performance, while they are perceived to have a negative impact on other chain members' performance. Overall, results revealed that all selected QAS requirements are perceived to have a positive impact on the performance of the supply chain, although the total impact is relatively small. The PMS framework applied in this study provides an insight in the impact of QAS requirements on performance. In addition, the PMS framework allows to make tradeoffs between different performance dimensions within the own firm, as well as throughout the chain.

At present, the value and production quantity of mango fruits are increasing worldwide. Many studies emphasize how chilling injury phenomena affect the quality of tropical fruits, such as mango, during postharvest handling, transport, and storage. Since mango is one of the most favored and popular fruits in the world market, and is considered to be a climacteric fruit, ripening rapidly after harvest, it is essential to study how storage affects the external and internal fruit quality in order to extend the shelf life. Chilling injury occurs when mango thereafter, are exposed to temperatures below 15 o C, but above the freezing point of the tissue. After a certain period of storage at these temperatures, and especially after a subsequent rise in temperatures, certain injuries in the plant tissue can be seen, such as water soaking, pitting, or browning. The possible causes of this type of injury will be reviewed and the consequences for cell components summarized. A distinction is made between the primary processes of chilling injury and subsequent secondary injuries. The membrane phase change and oxidative stress models of primary chilling injury are introduced. How cell components are affected by chilling injury is then discussed, as is the relationship between pre-harvest conditions, post-harvest handling, and injury. Lastly, the roles of different anti-oxidants and anti-oxidant enzymes are discussed and some techniques used for the measurement of chilling injury are introduced.

Fresh-cut or minimally processed vegetables are those which have been trimmed and/or peeled and/or cut into 100% usable product and still maintain freshness. Contrary to other processing methods (freezing, canning and drying for example) the minimal processing operations reduce the shelf life in relation to the raw material (intact product).In this thesis the changes in quality of tomato fruits after cutting into transversal slices were analysed and quantified in relation to the stage of maturity of the fruit at harvest and to the storage temperature. Quality was analysed in relation to some attributes related with organoleptical (firmness) visual (colour and translucency) and nutritional (lycopene content and total antioxidant activity) quality.The firmness of the tomato pericarp, evaluated through puncture tests,decreased exponentially during storage. Within a maturity stage, the firmness decreased only slightly during storage and major difference was found in the initial firmness at the different stages of maturity.The optical properties were evaluated through video image analysis and expressed as RGB (Red,Green,Blue) colour aspects. A mathematical model was used to describe the changes in the RGB colour aspects resulting from two different processes namely changes in colour due to the production and/or degradation of pigments (maturation) and development of translucency due to watersoaking of the pericarp. It incorporated in a single equation the effects of time, temperature, stage of maturity and background over which the tomato slices were imaged. Both translucency and maturation resulted in a decrease of the RGB values. The development of translucency was highly determined by the stage of maturity of the fruit at harvest and rather independent of temperature. Changes in colour due to maturation were much less pronounced. Using a spectrophotometric technique (Kubelka-Munk analysis) to evaluate the development of translucency these findings were confirmed.Changes in lycopene content were evaluated by HPLC. Cutting did not change the lycopene accumulation when the slices were kept at low temperature, while lycopene production predominated over degradation at temperature higher than 8 o C. The antioxidant activity of fresh-cut and intact tomato fruits was assessed by a radical (ABTS) scavenging method. Cutting induced a reduction of the hydrophilic antioxidant activity, while the lipophilic antioxidant activity was not affected. The antioxidant activity of both fractions increased with maturation.In short, the post cutting life based on appearance was shorter than that based on textural and nutritional value. The greatest range of changes happened in the first 2 days of storage for all quality attributes. The stage of maturity determined the initial values as much as the rate of decrease (or increase) along storage of each attribute. The effect of temperature although present was far lower than the effect of stage of maturity.For a short period (less than 1 week) the low temperature was efficient to prevent most of the deleterious changes in quality (softening, over-ripening, loss of antioxidants) but not to prevent water-soaking of the pericarp and translucency development.