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Roses in Kenya are mostly grown in soil with an open drip irrigation system which is inefficient as water and nutrients are lost through drainage. A study was carried out from January to December 2013 at a commercial rose farm in Naivasha, Kenya, to evaluate the potential of a cocopeat-based system, which additionally enables re-use of the drain water in a soil-based system. Vegetative growth in both systems was assessed in terms of leaf expansion, number of leaves, stem length, chlorophyll content (represented by the measured SPAD value) and flower head expansion. The water used throughout the year was also measured. Leaf expansion was characterized by an initial slow expansion rate followed by a fast expansion rate before levelling off. Maximum leaf length reached was 63 mm in the cocopeat system, while it was 60 mm in soil system; however, the difference was not significant. The number of leaves did not differ between the soil and cocopeat system. The maximum number of leaves per stem was 20 for both systems. There was no significant difference in stem length of plants in cocopeat system (650 mm) and in soil system (630 mm). Measured SPAD value on plants grown in the cocopeat system were significantly higher than for plants grown in soil (P0.01). Net water use for the cocopeat system was lower than for the soil system, with a difference of 1197 L m-2 or 58%, due to the re-use of water in a soil system. There was a significant substrate effect on the number of stems m-2 and measured SPAD value, which are associated with differences in leaf chlorophyll and nitrogen content. It is likely that the optimized fertigation regime in cocopeat system led to higher growth rates and enabled a higher stem production. Other parameters were not significantly affected by the substrate type under the same greenhouse climatic conditions.

In this study, we present a new light absorption enhancement method for p-i-n thin film silicon solar cells using pyramidal surface structures, larger than the wavelength of visible light. Calculations show a maximum possible current enhancement of 45% compared with cells on a flat substrate. We deposited amorphous silicon (a-Si) thin film solar cells directly onto periodically pyramidal-structured polycarbonate (PC) substrates, which show a significant increase (30%) in short-circuit current over reference cells deposited on flat glass substrates. The current of the cells on our pyramidal structures on PC is only slightly lower than that of cells on Asahi U-type TCO glass (Asahi Glass Co., Tokyo, Japan), but suffer from a somewhat lower open circuit voltage and fill factor. Because the used substrates have a locally flat surface area due to the fabrication process, we believe that the current enhancement in the cells on structured PC can be increased using larger or more closely spaced pyramids, which can have a smaller flat surface area.

Species of the genus Vaccinium are commercially cultivated in Europe for their berries, which are highly valued for dietary and pharmaceutical properties. Cultivation is severely limited due to a range of fungal diseases, especially those caused by species of Diaporthe. A number of Diaporthe isolates have been collected from Vaccinium growing regions in Europe, and initially identified as D. vaccinii based on host association. Using DNA sequence inference of the combined ß-tubulin, calmodulin, translation elongation factor 1-alpha and the internal transcribed spacer region of the nuclear rDNA, along with morphological characteristics, six species were characterised. Diaporthe eres, D. vaccinii and D. viticola are known species and three novel taxa are described here as D. asheicola, D. baccae and D. sterilis. This study is the first confirmed report of D. vaccinii in Latvia and the Netherlands.

This work was conducted to investigate the efficacy of chlorophyll fluorescence (CF) sorting to improve seed germination, seedling emergence, and vigor of seeds produced from different maturity fruits of four different cultivars. Four harvest dates from each cultivar were evaluated by harvesting orange (immature), bright red (half-mature), dark red (mature), and dark red and soft (overmature) fruits. Seeds were either sorted or nonsorted after harvesting and standard laboratory germination, seedling emergence, and controlled deterioration tests were conducted. CF sorting significantly increased laboratory germination, seedling emergence, and seed vigor. Maximum improvements were obtained from seeds harvested from half-mature and mature stages. Mean germination improvement among cultivars between CF-sorted and nonsorted seeds were 14% in the immature seeds, 11% in half-mature seeds, 6% in mature seeds, and 9% in overmature seeds. Improvements in seedling emergence were 21%, 17%, 9%, and 10% and 4%, 11%, 10%, 14% for seed vigor (CD germination) in the all maturity stages of seed lots, respectively. CF has the potential to upgrade seed quality in pepper lots as a non-destructive sorting technology.

Public pressure to use peat alternatives in horticultural rooting media offers room for the re-use of local organic waste materials. The re-use of organic wastes requires value added processing such as composting, co-composting, digestion, fractioning/sieving, pressing, binding, stabilising by torrefaction and charification, washing and nutrient exchange. Three cases are presented. In case 1, the new growing media project showed a successful reduction of peat in potting soil mixes at nurseries of 15 plant species. Peat use decreased from 78 to 27%-v/v. The 51%-v/v extra peat alternatives used were, averaged over all mixes used, 24%-v/v coir products, 13%-v/v wood fibre, 6%-v/v bark products and 2%-v/v for each of compost, rice husks, perlite or rockwool granulate. When compared to the standard control mixes yield decreased for two species, increased for two species and was equal for the remaining species. Important, sometimes limiting, properties were stability, nitrogen fixation, EC level, sodium level, rewetting rate and water holding capacity. In case 2 torrefaction at 250°C was used to turn a fast degrading nitrate fixing reed (Phragmites australis) into a non-toxic potting soil constituent which could be added to potting soil mixes up to 80%-v/v. In case 3 the cation exchange complex of coir was measured and saturated with calcium ions. The amount and concentration of a solution necessary to exchange sodium and potassium was 600 meq kg -1 and was dosed as 10 L of 30 mmol L -1 calcium nitrate. In all 3 cases product quality measurements and adapted processing or cultivation practises were discussed. A set of the most important measurements is presented, including: pH, EC, EC level, sodium level, potassium level, stability, nitrogen fixation, easily available water and rewetting rate. In conclusion additional or adapted processing of organic waste guided by proper measurements can increase the value of such organic waste for use in horticulture.

To fulfill the market demand for year-round high-quality production, the use of assimilation light has increased over the last decades by 10% per year and continues to expand. The electrical consumption involved largely contributes to the high CO2 emission of greenhouse horticulture. Following the Kyoto protocol, the emission of greenhouse gases should be reduced. Light-emitting diodes (LEDs) can contribute to the reduction in carbon footprint by their high efficiency in converting electricity into light. Due to their low heat emission, LEDs can be positioned within the canopy, which allows the design of new, low-carbon production systems. Inter-canopy LED lighting is already commercially used on a small scale. This paper describes the steps taken to further optimize LED-based production systems. Since it is impossible to test all possible strategies of using LED lighting, a 3D functional-structural plant model was used to do scenario calculations to determine the light interception of the canopy and crop photosynthesis at different positions and orientations of the LEDs. Orienting inter-lighting LEDs 30° downwards positively affected the light interception by the crop, provided that there was sufficient leaf area below the LEDs to prevent light loss to the floor. Replacing the conventional high-pressure sodium (HPS) lamps by LED lamps (efficiency 2.3 μmol J-1) reduced the carbon footprint of a tomato crop by approximately 15%, due to a combination of the higher efficiency of LEDs and an increased use of thermal energy to maintain plant temperatures. These calculations were validated in a greenhouse trial, where the production and energy consumption of a HPS+LED hybrid system was compared to those of a LED top-lighting and LED inter-lighting combination. Plant development and production levels were comparable, whereas the electrical consumption in the LED+LED system was 37% lower than under HPS+LED lighting. Approximately half of the reduction in electricity was used for additional heat input to maintain plant development rate, which agreed well with the carbon footprint calculations. Work in the near future will focus on plant architecture and LEDs with altered light emission patterns, aiming to design new LEDbased production systems, which combine a high production level with a low-carbon footprint.

Greenhouse operators strive to be as economic as possible with energy. However, investing in fancy energy-saving equipment is often not cost-effective for smaller operations and in climate zones with mild winters. It is possible, though, for many growers to save energy without buying special equipment. The trick is choosing optimal settings for heating and venting, and checking the graphs.

BACKGROUND: Iodine is an essential trace element for humans. Two billion individuals have insufficient iodine intake. Biofortification of vegetables with iodine offers an excellent opportunity to increase iodine intake by humans. The main aim was to study the effect of iodine form and concentration in the nutrient solution on growth, development and iodine uptake of lettuce, grown in water culture. RESULTS: In both a winter and summer trial, dose rates of 0, 13, 39, 65, and 90 or 129 µg iodine L-1, applied as iodate (IO3-) or iodide (I-), did not affect plant biomass, produce quality or water uptake. Increases in iodine concentration significantly enhanced iodine content in the plant. Iodine contents in plant tissue were up to five times higher with I- than with IO3-. Iodine was mainly distributed to the outer leaves. The highest iodide dose rates in both trials resulted in 653 and 764 µg iodine kg-1 total leaf fresh weight. CONCLUSION: Biofortification of lettuce with iodine is easily applicable in a hydroponic growing system, both with I- and IO3-. I- was more effective than IO3-. Fifty grams of iodine-biofortified lettuce would provide, respectively, 22% and 25% of the recommended daily allowance of iodine for adolescents and adults

This paper briefly describes the foundations and characteristics of biological control in protected cropping and what drivers are behind adoption of this management system within this industry. Examining a brief history of biological control in greenhouses and what makes it a successful management strategy within the industry, the authors describe the rapid growth of biological control in parts of Europe and what this may mean for the industry in other parts of the world. The reaction of the greenhouse industry to several consumer led campaigns aimed at reducing the incidence of pesticides in the marketplace may be replicated in many other parts of the world. The size and robustness of the biological control industry in greenhouses, which is a reflection of the inherent characteristics of this industry that lends itself to biological control, is strong and growing with indications that this trend will be followed in many areas of the world.

Massive amounts of water are required for the production of our food, varying from several cubic metres per kilogram of beef to as low as 4 litres per kilogram for tomatoes grown in high-tech glasshouses. This article presents data on Product Water Use (PWU) of some foods and discusses how the water requirement for fresh tomatoes can be brought down from 300 to 4 litres/kg.

In this manuscript we provide evidence for the seed transmission of Pepino mosaic virus (PepMV) in tomato. Fruit was harvested from a tomato crop artificially infected with both European and CH2 genotypes of PepMV and more than 100,000 seeds were extracted and cleaned using an enzymatic treatment without disinfection. Infection assays using indicator plants confirmed the presence of viable virus on the seeds. Seeds were distributed to ten different laboratories in three separate batches, where they were germinated and the young plants tested by ELISA. In total over 87,000 plants were tested and 23 positives detected, indicating an overall transmission rate of 0.026%. However, the observed seed transmission rates varied from 0.005% to 0.057%, depending on the seed batch used. Results clearly showed that PepMV can be transmitted from seeds contaminated with virus to seedlings, highlighting the risk of using seeds from PepMV-infected plants and the potential for seed transmission to contribute to the further spread of PepMV.

Tomato plants under attack from boyritis fungus give off an aromatic substance that can be measured in greenhouses. This observation is the result of research by Dr. Roel Jansen during his doctoral studies at Wageningen University in the Netherlands. Working with a team of scientists from Wageningen and Germany, Jansen has opened the door to a new way of preventing and managing disease problems in greenhouse horticulture

Pepino mosaic virus (PepMV) has recently emerged as a highly infectious viral pathogen in tomato crops. Greenhouse trials were conducted under conditions similar to commercial tomato production. These trials examined whether tomato plants can be protected against PepMV by a preceding infection with an attenuated isolate of this virus. Two potential attenuated isolates that displayed mild leaf symptoms were selected from field isolates. Two PepMV isolates that displayed severe leaf symptoms were also selected from field isolates to challenge the attenuated isolates. The isolates with aggressive symptoms were found to reduce bulk yields by 8 and 24% in single infections, respectively. Yield losses were reduced to a 0–3% loss in plants that were treated with either one of the attenuated isolates, while no effects were observed on the quality of the fruits. After the challenge infection, virus accumulation levels and symptom severity of the isolates with aggressive symptoms were also reduced by cross-protection. Infection with the attenuated isolates alone did neither affect bulk yield, nor quality of the harvested tomato fruits.

This paper describes a model to calculate the concentrations of (Z)-3-hexenol, a-pinene, a-terpinene, ß-caryophyllene, and methyl salicylate in a greenhouse on the basis of their source and sink behaviour. The model was used to determine whether these volatile organic compounds (VOCs) can be used to indicate Botrytis cinerea infection in a large-scale tomato production greenhouse with a volume of 5 × 104 m3 containing 2.2 × 104 plants. Seven experiments were done to parameterise the model for these VOCs. Based on model predictions, the B. cinerea-induced increase in concentration of methyl salicylate is detectable in a large-scale tomato production greenhouse when: (a) windows are fully opened, and (b) the increase continues for at least 1 h, and (c) 5% of the plants are infected. The B. cinerea-induced increase in concentration of methyl salicylate is also detectable when: (a) windows are closed, and (b) the increase continues for at least 6 h, and (c) 5% of the plants are infected. The B. cinerea-induced increase in concentration of (Z)-3-hexenol is detectable under all conditions studied. However, it is expected that besides infected plants, many additional sources of (Z)-3-hexenol exist including plant debris and nearby field crops especially upon harvest or stress. The B. cinerea-induced increases in concentration of a-pinene, a-terpinene and ß-caryophyllene are probably undetectable in a large-scale tomato production greenhouse. Therefore, it is recommended to focus on the detection of methyl salicylate to indicate B. cinerea infections in large-scale tomato production greenhouses.

Drastic improvements in growing technology in the Netherlands have achieved a large reduction in energy use and a striking increase in production.

Lighting in a greenhouse is surrounded by questions. How much light to supply and when?. What intensity and light sum to aim for? Is it radiation, light growlight, PAR, photons or quanta? How much is joule, watt, lux?. What does wavelength, nanometer, spectrum, UV, IR and NIR mean?

Light Emitting Diodes (LED's) are a promising technology for greenhouse lighting with their efficiency to activate plant photosynthesis potentially higher in red LEDs than in HPS lamps. Due to their particular light colour, LEDs can initiate special effects in plants or steer plant processes and plant balance

Only about half of the energy that enters a greenhouse as sun radiation is in the wavelength range that is useful for photosynthesis (PAR, Photosynthetically Active Radiation). Nearly all the remaining energy fraction is in the Near InfraRed range (NIR) and only warms the greenhouse and crop and does contribute to transpiration, none of which is necessarily always desirable. Materials or additives for greenhouse covers that reflect a fraction of the NIR radiation have recently become commercially available. Besides lowering greenhouse temperature, a NIR-excluding cover has quite a few side-effects that may become quite relevant in the passive or semi-passive greenhouses typical of mild climates. For instance, the ratio of assimilation to transpiration (the water use efficiency) should increase. On the other hand, by lowering the ventilation requirement, such a cover may hinder in-flow of carbon dioxide, thereby limiting the photosynthesis rate. In addition, there are obviously conditions where the warming up caused by NIR may be desirable rather than a nuisance. NIR-reflecting materials are becoming available in forms that are suitable for various types of applications, such as permanent, seasonal or mobile. By means of a simulation study, we discuss in this paper the best form of application in relation to the external climate and climate management options available

The plant's water status is the balance of water uptake and water loss. Transpiration (water loss) in influenced by sun, wind humidity, leaf area, and opening of stomatal pores in the leaves.

Pathogen attack and herbivore infestation have a major impact on plant health. In a model study, these two plant health issues were simulated to study whether plant health can be monitored at greenhouse scale through the analysis of volatile organic compounds (VOCs) in greenhouse atmosphere. To simulate pathogen attack and herbivore infestation, we repeatedly stroked the stems of tomato plants (Lycopersicon esculentum) and repeatedly removed their side shoots. In addition, we studied the effect of fruit picking on the concentration of plant-emitted VOCs in greenhouse atmosphere. Analysis of air samples obtained before these treatments revealed up to 17 VOCs that are known to be released from tomato plants, of which the most dominant one was the monoterpene ß-phellandrene. When plants were 7 weeks old, the concentration of this VOC was approximately 0.06 ppbv before treatment. When plants were 12 weeks old, this concentration was raised to approximately 0.14 ppbv. Stroking of the stems, removing the side shoots and fruit picking resulted in an increase in the concentrations of all mono- and most sesquiterpenes up to 60-fold, which was expected because these VOCs are well-known constituents of trichomes. The treatments did not result in substantially increased concentrations of the stress-related compounds ¿-copaene, methyl salicylate and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. In contrast to stroking and fruit picking, shoot removal resulted in the emission of the lipoxygenase-derived product (Z)-3-hexenol in greenhouse atmosphere expressing cell membrane degradation. The findings presented in this paper focus on the feasibility of monitoring plant health through the analysis of VOCs in greenhouse air, but findings might also be relevant for atmospheric chemistry.

Substrate grown roses appear to be susceptible to chlorosis, which indicates problems with Fe or Mn uptake and hence yield reduction. In common practice this problem is often treated by the addition of extra Fe-chelate, or the use of Fe-EDDHA instead of Fe-DTPA. In previous tests, it was shown that the pH in the root environment is a major factor in the prevention of chlorosis. Moreover, the application of Fe-EDDHA does not always show satisfying improvements in practice. The interaction between Fe-chelate types (EDDHA and DTPA) and pH was studied with roses cv. ‘Kiss’ and ‘Escimo’ on glasswool substrate, reusing drainage water. pH levels compared were about 7, 5.8 and 4.5. The treatments resulted in significant chlorosis and consequently yield reduction at high pH with both cultivars and both chelate types. Highest yields were obtained at low pH, especially with ‘Escimo’. The Fe uptake was clearly affected by the pH with both chelate types. At high pH the Fe-uptake was significantly higher with Fe-EDDHA; however the Mn contents in the plant were significantly lower with these treatments. The uptake of Zn and Cu was also affected by specific combinations of pH and the type of chelate. It was concluded that an optimal pH control was the best method of preventing chlorosis. The choice of the chelate type was less effective and could enhance Mn deficiency.

Detailed measurements of crop photosynthesis at supra-optimal temperatures and high CO2 levels, to validate models for use in model-based greenhouse climate control, are still lacking. We performed CO2 gas exchange measurements to estimate gross crop photosynthesis (Pgc) from measured net crop gas exchange in the daytime, and at night, with temperature and CO2 conditions higher than the normal temperature range, and with two crops of different architecture: tomato, and cut chrysanthemum. From these measurements, Pgc was predicted at photosynthetic photon flux densities (IPPFD) of 300, 600, 900 and 1,200 ¿mol m-2 s-1 for different temperatures (from 20°C to 33°C) and CO2 concentrations (400, 700, and 1,000 ¿mol mol-1). From these predictions, the optimum temperature that maximised Pgc was determined. CO2 concentration had a strong and similar effect on Pgc in both crops, and this effect decreased with increasing CO2 level. For example, at 32°C, there was a 55% or 49% increase in Pgc between 400 and 1,000 ¿mol mol-1 CO2 in chrysanthemum and tomato, respectively. A clear shift to higher optimum temperatures at elevated CO2 levels was observed, and was different for the two crops. Chrysanthemum had a lower temperature optimum than tomato for a maximum Pgc (e.g., at 1,000 ¿mol mol-1 CO2 and 600 ¿mol m-2 s-1 IPPFD, the difference was 3.1°C). Compared to leaf photosynthesis, crop photosynthesis had a lower temperature optimum (the difference could be several °C), and the shift in optimum temperature from a low to a high CO2 level was lower for a canopy compared to a leaf. Therefore, optimising the leaf photosynthetic rate in model-based greenhouse climate control would not result in optimum crop photosynthesis

In 1974 the 1:1½ volume extract was published (Sonneveld et al., 1974) as a water extraction method for the determination of available nutrient elements and of the salinity status of peaty substrates. The analytical data of this extract were related to the data of the “substrate” solution extracted from the substrates with a moisture condition of -3.2 kPa. The method has been widely used and offers a suitable basis for fertilization of peaty substrates. In the years after publication, the composition and application of substrates has undergone much change and the irrigation methods have also been thoroughly adjusted. The latter was responsible for increased water contents in the substrate during crop growth. Therefore, another study was carried out in which the analytical data of the 1:1½ volume extract was compared with the analytical data of the “substrate” solution, where the “substrate” solution was defined at the moisture condition of -1.0 kPa. There was a good agreement between the results of both studies. However, the regression coefficients for the relationships between the data of the extracts differ, as expected, because of the higher moisture contents on which the substrate solution was defined. In addition to the data of major elements published, in the second study, micro nutrients were determined in the extracts, but not published. Therefore in this paper the relationships between the concentrations of micro nutrient as determined in the 1:1½ extract and in the “substrate” solution are given. The relationships presented support the interpretation of analytical data of micro nutrients by means of water extraction.

Levels of abortion of reproductive organs (i.e., buds, flowers, and young fruits) in sweet pepper plants (Capsicum annuum L.) are high, and cyclical fluctuations occur in fruit set. Stages susceptible to abortion are very young buds (

Strategies for improving greenhouse crop production should target both developing advanced technological systems and designing improved plants. Based on greenhouse experiments, crop models and biotechnological tools, this paper will discuss the physiology of plant-greenhouse interactions. It is discussed how these interactions can be applied to control the production process at Northern and Mediterranean climatic conditions. Absorption of light by the leaves is important for maximum crop photosynthesis. For this, it is important to have plants that develop as fast as possible a sufficient leaf area index. The question is: what leaf area index is needed for optimal crop performance? Most of the light is absorbed by the upper part of the canopy. Can we improve the light distribution in the canopy and, moreover, does this increases yield or quality? Virtual plant models may help to address this question. In some cases removal of older leaves can improve yield, while in other cases removal of young leaves may accomplish the same objective. In summer time the light transmission of the greenhouse is often reduced by growers to avoid plant stress. However, in several cases this stress is only an indirect effect of light, because other growth factors (e.g. temperature, humidity) tend to be suboptimal. In Northern countries CO2 supply is commonly used. The introduction of semi-closed greenhouses allows to maintain high CO2 concentrations all year round. In Mediterranean countries, a large yield increase is still feasible by CO2 supply. Optimum growth conditions means that there is a good balance among different climate conditions. The source/sink ratio of a crop (ratio between production and demand of assimilates) often reflects whether these conditions are balanced. Variation in the source/sink balance affects formation and abortion of organs, product quality and production fluctuations. Some examples are shown on temperature control based on the source/sink balance of a crop. Drought and salinity may limit production especially in the Mediterranean. Morphological and metabolic traits, with known genetic bases, can be functionally altered to test current hypotheses on plant-environment interactions and eventually design a greenhouse plant. Reasonably, such a plant should have specific shoot vs. root developmental patterns, efficient water and nutrient uptake systems as well as other specific features that have not been sufficiently explored. Elucidation of the complex plant-greenhouse interactions would establish a physiological basis to improve both product quality and resource use efficiency in greenhouse

The aim of the studies was to correlate results of a fast extract-based toxicity test with results of an established growing test with mostly pre-grown plants. A standard white peat was contaminated with four levels of TCA (trichloroacetic acid), a known toxic substance. A range was composed of five levels of TCA, including zero, 0.0013, 0.013, 0.13, and 1.3 g/L of peat. Part of the material was entered in Phytotoxkit containers and covered with a filter paper. At the other side of the filter paper two dicotyledonous species, garden cress (Lepidium sativum) and mustard (Sinapis alba) and one monocotyledon, Sorghum (Sorghum saccharatum) were allowed to germinate for a three day period on the extract from the substrate in a climate-controlled cabinet. The same range of amended peats was used in the standard test to grow transplanted lettuce (Lactuca sativa), Chinese cabbage (Brassica rapa subsp. pekinensis) and directly sown barley (Hordeum vulgare). For the Phytotoxkit the above ground length and root length were recorded, and for the standard test the fresh weight and dry weight of the above ground parts were recorded. Results showed a growth reduction for all levels of TCA when using the standard method. When using the Phytotoxkit only the two highest concentrations of TCA reduced growth. For screenings of product alternatives or dissolution series the Phytotoxkit is a faster and cheaper alternative

The objective of this paper is to demonstrate the need of a multiple design parameter approach to greenhouse design. To illustrate this need, we determined the combined effects of cover design parameters on tomato production of a passive greenhouse, that is a greenhouse with only natural ventilation and seasonal whitewash for climate management. The design parameters investigated in this research were the transmission of the cover for photosynthetically active radiation (PAR) and near infrared (NIR) radiation, the emission coefficient for long wave radiation of the cover and the ventilation area. First, we developed a model to link the tomato yield to the cover design parameters, through their effects on greenhouse climate. The model was validated by comparing the simulated greenhouse climate and yield with data obtained from field studies conducted in Almería, Spain. Thereafter, the sensitivity of the yield to the cover design parameters was analysed for three greenhouse configurations. This analysis gave insight into the effects of the cover design parameters on crop yield. Results showed that the sensitivity of the yield to a single design parameter depended on the absolute values of the other ones. For example, the yield in a greenhouse with a high ventilation capacity was the most sensitive to PAR transmission (0.45 % more yield for each 1% increase of PAR transmission) while in a greenhouse with a low ventilation capacity the crop yield is most sensitive to the ventilation area (0.63 %) and NIR transmission (-0.56 %). In addition, the yield sensitivity to the design parameters also varied over time because of changing outdoor climate conditions. In conclusion, a significant improvement of greenhouse design can be attained only through a multifactorial approach that accounts for the joint effect of design parameters, local climate and desired production period upon crop yield.

Earlier research has revealed that considerable energy savings can be achieved by maintaining an average temperature in the greenhouse in stead of maintaining rigid pre-defined temperature `blue-prints¿. A model based optimal control approach has proven to be a suitable framework to tackle these kind of control problems and it has been shown that these algorithms can be implemented on-line. But, when on-line optimal temperature control is considered, interesting questions arise, some of which are still unresolved. The issue tackled in this paper concerns the relation between the resolution of the control strategy (sample time) and energy savings of the control strategy. One would expect that an accurate and frequent anticipation to changing outdoor climate conditions might result in reduced energy consumption. It was indicated in the literature that a sample-time of 0.25 h or 1 hour should be sufficient, but these choices were hardly motivated. In this research, the relation between the control resolution and energy savings was quantitatively investigated using a dynamic greenhouse climate model and measurements of Dutch outdoor climate conditions containing high-frequency components. The results indicate that for an open-loop optimal control problem concerning the realization of an average temperature during a fixed period of one day using a minimum amount of energy with full a-priori knowledge of the outdoor weather, a resolution of the heating profile between half an hour and a hour suffices to produce accurate results in terms of energy conservation. These results were not much affected by parameter variations (heat capacity of the air, the solar heating efficiency) or opening and closing of thermal screens.

Light is not evenly distributed in Dutch glass greenhouses, but this can be improved with diffuse light. Modern greenhouse coverings are able to transform most of the light entering the greenhouse into diffuse light. Wageningen UR Greenhouse Horticulture has studied the effect of diffuse light on crops for several years. Modelling and experimental studies showed that crops such as fruit vegetables with a high plant canopy as well as ornamentals with a small plant canopy can utilize diffuse light better than direct light. Diffuse light penetrates the middle layers of a high-grown crop and results in a better horizontal light distribution in the greenhouse. Diffuse light is absorbed to a better degree by the middle leaf layers of cucumber, resulting in a higher photosynthesis. The actual photosynthesis of four pot plant species was found to be increased and crop temperatures were lower during high irradiation. The yield of cucumbers was increased, and the growth rate of several potted plants was increased. These investigations have resulted in a quantitative foundation for the potentials of diffuse light in Dutch horticultural greenhouses and the selection and verification of technological methods to convert direct sunlight into diffuse light.

In order to develop new eco-sustainable technologies to set up biodegradable films for agricultural activities, spray mulching coating have been planned, prepared and tested on experimental fields. The suitable polymers used to this purpose were Arabic Gums and Agarose. Cellulose fibres were added to the polymeric water solution to improve the mulching power and to increase the tensile strength of the composite; glycerol as plasticizer was added in order to improve the mechanical response of mulching coatings. On the composites sprayed and naturally aged on the soil, both in greenhouse and in open field, mechanical, water vapor transmission rate and morphological analyses were carried out; the different environment influenced the outcomes of studied properties, evidencing better results and promising perspectives for composites employed and aged under controlled atmosphere of greenhouse.

Experimental results and validation of a simple greenhouse climate model are analysed according to data sets from six prototype greenhouses with three different plastics (reference N0, and two levels of near-infrared reflecting pigments N1 and N2); two ratios of ventilation openings to greenhouse covering area (0.223 and 0.427); a wide range of tomato leaf area index (0.01–4.97); and for three crop-growing periods that represent year-long tropical lowland climatic conditions in Purwakarta (107°30′E, 6°30′S, altitude 25 m), Indonesia. The model with a calibration factor for indirect absorbed solar radiation (indicating the part of the radiation absorbed by the greenhouse cover, structural elements, and soil surface released into the air) Λ of 0.1 satisfactorily calculated greenhouse air temperature TAir with less than 2% error and greenhouse air water vapour pressure deficits DAir with less than 10% error. The errors were higher at low values for the leaf area index. The model performance slightly improved by including the effect of leaf area index on Λ as an exponential term. Measurements and calculations demonstrated that TAir was affected more by variations of ventilation and leaf area index than by the applied cover properties. The leaf area index had the highest impact on greenhouse air temperature, implying that a large proportion of the cooling is achieved by the crop itself. The results enable the model to be used in the design of optimum greenhouse systems for tropical lowland Indonesia.

Greenhouse production in North-West Europe consumes a lot of energy. The energy is needed for heating the greenhouse and controlling air humidity. Transpiration of a crop increases the energy use. The aim of this study was to explore the possibilities for the application of anti-transpirants to save energy by reducing crop transpiration without reducing crop yield. Literature and model calculations were used to explore the effects of increased leaf resistances on transpiration, energy use and production in tomato, cucumber and sweet pepper. In literature a large number of compounds are described that act as anti-transpirant. A two to five fold increase in stomatal resistance can be expected from treatment with anti-transpirants. Model calculations for tomato showed that increasing the stomatal resistance (from 2 to 5 times) throughout the whole year leads to substantial yield reduction: crop growth was reduced by 6-19%, while transpiration by 15-42% and consequently energy use by 9-16%. However, in the winter period (beginning of October/end of March) the growth reduction was only 0.3-1.3%, as in this period light levels are low and CO2 concentrations in the greenhouse are relatively high. Raising the (maximum) set-point for CO2 concentration from 1000 ppm to 3000 ppm, increased the actual concentration during day-time from 892 to 1567 ppm (flue gases were the only source of CO2). When the application of anti-transpirants was combined with raising the set-point for CO2 concentration, the model showed no growth reduction due to the application of anti-transpirants, while the annual energy use was reduced by 5.5-10.4% in tomato. Similar results were obtained for sweet pepper (5-9% energy saving) and cucumber (2-5% energy saving). These model calculations show that increasing stomatal resistance by anti-transpirants during the winter period may potentially save a substantial amount of energy (2-10%), without affecting yield of vegetables such as tomato, cucumber and sweet pepper. It is concluded that increasing the stomatal resistance by anti-transpirants in wintertime may lead to substantial energy saving due to the reduced transpiration and need for humidity management, without yield reduction. Such model calculations are useful to analyse beforehand the chances of a good combination of energy saving and yield loss of a possible application. Experiments will be needed to verify the results

For routine toxicity testing of composts and growing media, two different types of assays are usually applied. One approach is the use of growing trials in pots with various mixtures of the material. In extract-based tests, seeds are germinated in petri dishes and exposed to an exudate from the material to be tested. In this study, one of these methods, the Phytotoxkit microbiotest, was compared to the standard phytotoxicity test with Lactuca sativa as used by the RHP foundation. The Phytotoxkit test was performed in transparent test containers which allow for direct observation and length measurements of the seedlings by means of image analysis. Two dicotyles (Lepidium sativum and Sinapis alba) and one monocotyle (Sorghum saccharatum) were used. When measuring a presumably phytotoxic bark, in comparison with a reference bark, the Phytotoxkit showed a seedling length inhibition of 60¿80% for the dicotyles and less than 20% for the monocotyle after three days. The RHP growing trial showed 30% weight inhibition of the lettuce after two weeks of plant growth when comparing the same barks in mixtures. This preliminary comparison showed that the Phytotoxkit microbiotest is a quick and practical bioassay with a high resolution which has a potential to become an international standard.

The objective of this paper is to demonstrate that optimal greenhouse design must account for (and be combined to) optimal climate management. We prove this by showing that different strategies and set-points to control the greenhouse ventilators result in different ¿optimal sets¿ of design parameters. We determined these optimal sets for a passive greenhouse in Almería, Spain where tomatoes were grown. The greenhouse design parameters investigated in this research were: 1) the transmission of the cover for photosynthetically active radiation (PAR), 2) the transmission of near infrared (NIR) radiation and 3) the emission coefficient for longwave radiation of the cover. Six optimal sets of design parameters were determined by maximising the marginal revenues (crop yield minus costs of design parameters), under given climate conditions, and for different ventilation control strategies. Each ventilation control strategy had different set-points for the air temperature and carbon dioxide concen¬tration to control the greenhouse ventilators. To solve this optimization problem we used a dynamic crop-greenhouse model and an optimization algorithm. The model described the combined influence of the relevant design parameters, outdoor climate and ventilation control upon economic crop yield, through their effect on indoor climate. The yearly costs of the design parameters were empirically derived from prices, physical properties and lifespan of a number of greenhouse cover materials. Results showed that indeed for different strategies and set-points to control the green¬house ventilators different ¿optimal sets¿ of design parameters and marginal revenues were obtained. For example, the difference between the highest optimal NIR trans¬mission 1.00 and the lowest optimal NIR transmission 0.40 was 60%, while the highest marginal revenues 16.94 ¿m-2 differed 18,7% with the lowest marginal revenues of 13.77 ¿ m-2. Additionally, it was found that the cover design parameters were time dependent. In conclusion, only a combined optimal control and design approach that takes into account the best climate control strategy and the time dependency of the design parameters will ensure optimal design parameters and maximum marginal revenues.

This paper reports osmotic effects from a series of experiments where greenhouse crops were grown in substrates with different EC values of the nutrient solution. Results of yield characteristics were published elsewhere: in the present paper, the nutrient uptake in relation to the EC will be discussed in the light of growth of lily, lettuce and kohlrabi. The crops were grown in substrates within a circulation system and thus, the water absorption of the crops could be precisely determined. The nutrient absorption was determined by tissue analysis. For lettuce whole heads were sampled and analysed; for lily, bulbs and leaves and for kohlrabi, tubers and leaves were sampled and analysed separately. The results showed a strong increase of plant nutrient concentrations in the EC-domain until the optimum growth response of the crops. Higher EC values did not affect the plant nutrient dry matter concentrations seriously, except for K concentrations of the lily crop. Calculations showed that the K concentration in the plant sap played an important role in the osmotic adjustment of the different crops to high external concentrations. The uptake concentrations of the different crops were presented and discussed in relation to the EC value in the external solution. External nutrient concentrations and plant nutrient concentrations in relation to optimum yield are presented. Calculations are made of the nutrient efficiency of a system with drainage to waste compared with a system with reuse of drainage water. INTRODUCTION Results of a study of osmotic effects on greenhouse crops grown in substrate (Sonneveld et al., 2004) outlined the relationships between the yields of a series of crops and osmotic potentials in the external solution. In the experiments, differences in the osmotic potential of the external solution were achieved by addition of nutrients. In the present paper results of nutrient uptake in three crops, in relation to the osmotic potential of the external solution, will be discussed. These include a tuber crop, a leaf crop and a bulb crop, represented by kohlrabi, lettuce and lily, respectively. METHODS AND MATERIALS The crops were grown in sand and as well in granulated rock wool substrates placed in a 0.15 m thick layer in basins with a size of 0.8 m x 1.6 m. Nutrient solutions of different concentrations were prepared and added with the aid of a sprinkler system. The irrigation time, the quantity of water and the concentration of nutrients in the water were varied according to the crop, the growing conditions and the osmotic potential aimed at. The leaching fraction in the growing system was focused on 0.25 and 0.50 and the drainage water was reused in the system. Six treatments were laid out in four parallel-randomized blocks in which the EC in the substrate solution roughly varied between 1 and 8 dS m. The composition of the nutrient solutions used was tuned to the need of the crop according to the recommendations for growers (Sonneveld and Straver, 1994). The different EC values were achieved by addition of various amounts of major elements. NH4 and P, however, were kept constant, since NH4 may influence the pH of the nutrient solution (Sonneveld, 1991) Proc. IS on Growing Media Ed.: J.-C. Michel Acta Hort. 779, ISHS 2008 314 and P can become toxic to some crops (Howell and Bernhard, 1961), when added with too high concentrations. The rough composition of the nutrient solution as used related to an EC value of 1.8 dS m was following: in mmol L NH4 1.0–1.3, K 6.0–7.5, Ca 2.8– 3.3, Mg 0.8–1.5, NO3 12.6–13.5, SO4 0.8–1.5, H2PO4 1.0–1.3 and in μmol L Fe 10–60, Mn 0–20, B 15–30, Cu 0.25–0.75, Mo 0.5–0.75. Sufficient Zn was available in the primary water, either rain water or desalinated water. The average EC of this water was 0.15 dS m and the concentrations of Na and Cl were about 0.6 mmol L. The kohlrabi crop cv. Quickstar was grown between Feb.-Apr., 1991; the lettuce between cv. Cortina Jun.-Jul., 1992; and lily cvs. Connecticut King and Star Gazer between Feb.-Jun., 1995. Both cultivars of lily were grown in the same experimental plots; thus part of the calculations could not be made for the cultivars separately. In such cases average values of both cultivars are given. The planting density was 20.3, 18.8 and 56.2 plants per m for kohlrabi, lettuce and lily respectively, in agreement with horticulture practices in The Netherlands. The concentrations of nutrients and the EC in the root environment were estimated by measurement of these parameters in the solution supplied and in the drainage water. The quantities of water supplied and drained out were recorded and used for calculation of the water absorption of the crop. Tissue samples were gathered at the end of the cropping periods and analysed for major nutrients, as well as Na and Cl. For kohlrabi tubers and tops, and for lilies bulbs and tops were gathered separately. With lettuce, representative tissue samples were taken from whole heads. The samples of the lily bulbs grown in the rock wool could not be cleaned sufficiently from these fibres and thus were not analysed. All other samples were dried at 80°C, ground and extracted by total destruction (acid digestion) to determine the element concentrations. At the beginning of the experiments, small kohlrabi plants in rock wool plugs were set out, and with lettuce small plants in peat cubes were used. The quantities of nutrients brought in with these materials were very small and considered neglected for the purpose of calculations. For lily young bulbs were planted out and tissue analysis showed that 5 to 45% of the nutrients absorbed by the crop were already available in the bulbs used at the start. These quantities were taken into account in the calculations of the nutrient uptake. RESULTS AND DISCUSSION The relationships between the yields of the crops and EC values in the root environment are shown in Fig. 1. A function of an exponential model showed the best relationship between the EC and the yield characteristics (Sonneveld et al., 2004). The maximum yield was obtained at EC values of 2.3, 2.7, 0.5 and 1.1 for kohlrabi, lettuce, lily cv. Connecticut King and lily cv. Star Gazer respectively. The water use in the experiments, covering the absorption by the crop and the evaporation by the substrate surface, was not different for the EC values of the kohlrabi and the lettuce and amounted on average 109 and 100 L m respectively. The water use of the lily decreased from 325 till 229 L m with increasing EC in the root environment from 1.2 to 6.0. This reduction in water use of 30% is in agreement with the previously reported growth reduction in this range, which was also about 30% (Sonneveld et al., 2004). The fact that kohlrabi and lettuce did not show differences in water use with increasing EC can be explained by the type of canopy formed by these crops. Both crops quickly form a closed canopy over the total area, even when there is a moderate growth reduction. In such cases the transpiration is related to the area covered. Lily, however, has an open canopy in which the transpiration will be related to the total leaf area. These results contradict the (still) generally held believe that the principal factor associated with growth reduction of crops by salinity is water availability, and support conclusions of previous workers such as Lagerwerff and Eagle (1961), and Maas and Nieman (1978), who concluded that the plants adjust for the water availability. Tissue analysis between the crops grown in sand and those grown in rock wool

The whole construction parameters of the net, combined with the shape of the structure, the position of the sun and the sky conditions affect the radiometric performance of the permeable covering system. The radiometric properties of the permeable membrane influence the quality of the agricultural production and the aesthetic characteristics of the netting system. Moreover, the colour of the material and the light reflection- especially of the wavelengths visible for the human eye (VIS, 380-760nm)- is an interesting criterion to determine the aesthetic value of the net structure and its environmental impact. In order to investigate the influence of the threads colour on the radiometric properties of the net, a set of field tests were performed by means of a spectroradiometer in combination with an experimental setup 120x120x50cm covered with membranes formed by threads with different colour. A second set of experiments were performed, on the same kind of nets, in laboratory by means of a combination of a large integrating sphere and a small one: the transmissivity from a direct (tauDIR) and diffuse ((tauDIF) source and the reflectivity from diffuse source (?) of 50x50cm samples were measured in the PAR range. The evaluation of the transmissivity values shows that the colour of a net influence spectral distribution of the radiation passing through the net absorbing their complementary colours. The transmissivity of black nets is almost constant in the visible range and the reduction of the incoming radiation is proportional to the solidity of the net. In the PAR range transparent and black nets doesn?t cause an alteration of the spectrum of solar radiation and transmittance is almost constant with a slight growth in nets having lower porosity.

At the moment, there are a large number of agricultural net types on the market characterized by different structural features such as type of material, type and dimensions of threads, texture, mesh size, porosity / solidity and weight; by radiometric properties like color, transmissivity/reflectivity/shading factor; by physical properties like air permeability and several mechanical characteristics such as tensile stress, strength, elongation at break, and durability. Protection from hail, wind, snow, or strong rainfall in fruit-farming and ornamentals, shading nets for greenhouses and nets moderately modifying the microenvironment for a crop are the most common applications. A systematic review of the current state-of-the-art of structural parameters, standard and regulations, most common agricultural net applications, and their supporting structures has been developed by means of a literature study, technical investigations, concerning characteristics and use of nets. As a result, the survey highlighted that in many cases different, not even similar, net types were adopted for the same application and the same cultivations by various growers. Results show that neither growers nor net producers have clear ideas about the relationship between the net typology optimization for a specific application and the construction parameters of the net. The choice often depends on empirical or economic criteria and not on scientific considerations. Moreover, it appears that scientifically justified technical requirements for nets used in specific agricultural applications have not been established yet.

SUMMARY The world needs sustainable, efficient, and renewable energy production. We present the plant microbial fuel cell (plantMFC), a concept that exploits a bioenergy source in situ. In the plant-MFC, plants and bacteria were present to convert solar energy into green electricity. The principal idea is that plants produce rhizodeposits, mostly in the form of carbohydrates, and the bacteria convert these rhizodeposits into electrical energy via the fuel cell. Here, we demonstrated the proof of principle using Reed mannagrass. We achieved a maximal electrical power production of 67 mW m � 2 anode surface. This system was characterized by: (1) nondestructive, in situ harvesting of bioenergy; (2) potential implementation in wetlands and poor soils without competition to food or conventional bioenergy production, which makes it an additional bioenergy supply; (3) an estimated potential electricity production of 21 GJ ha � 1 year � 1 ð5800 kWh ha � 1 year � 1 Þ in Europe; and (4) carbon neutral and combustion emission-free operation. Copyright # 2008 John Wiley & Sons, Ltd.

The paper presents results of research on inverse kinematics algorithms to be used in a functional model of a cucumber harvesting robot consisting of a redundant manipulator with one prismatic and six rotational joints (P6R). Within a first generic approach, the inverse kinematics problem was reformulated as a non-linear programming problem and solved with a generic algorithm. Solutions were easily obtained, but the considerable calculation time needed to solve the problem prevented on line implementation. To circumvent this problem, a second, less generic approach was developed consisting of a mixed numerical-analytic solution of the inverse kinematics problem exploiting the particular structure of the P6R manipulator. This approach facilitated rapid and robust calculation of the inverse kinematics of the cucumber harvester. During the early stages of the cucumber harvesting project, this inverse kinematics algorithm was used to off-line evaluate the ability of the robot to harvest cucumbers using 3D-information of a cucumber crop obtained in a real greenhouse. Thereafter, the algorithm was employed successfully in a functional model of the cucumber harvester to determine if cucumbers were hanging within the reachable workspace of the robot and to determine a collision-free harvest posture to be used for motion control of the manipulator during harvesting.

Deregulation and globalisation has altered the views of public involvement in development and led to strategies focusing on private sector participation. An implicit assumption seems to be that these linkages will enhance the technological capacity of smallholder producers by way of more cost-efficient technologies trickling down through the value chain or by quality requirements inducing best practices. The argument put forward in this paper is that sustainable non traditional agricultural chain development requires more purposeful actions and institutional transitions, both in the public and private spheres, targeting improved upstream innovative capacities. Empirical findings from a Dutch-Vietnamese partnership on sustainable floriculture development are used. Research revealed that the pest and disease control solutions applied by smallholder rose growers were incremental adaptations of experiences obtained in former food crop cultivation practices. Floriculture however may require more drastic changes in cultivation practices to make the sector more environmentally benign. In the case of smallholder Vietnamese flower producers, this implies adaptation of knowledge and skills currently not present. An important hindrance in promoting this knowledge and skills appears to be the weak vertical linkages between flower growers and public and private research and development organizations.

The objective of this study is to dimension a system capable of removing water vapour mainly generated by evaporation of the crop when a thermal screen is applied. The humid greenhouse air is replaced by cold dry outside air using an air distribution system. The dry air is injected above the greenhouse floor thereby forcing humid air to pass through the thermal screen. The excess air in the greenhouse will flow away though leaks in the cover. The common procedure to remove moisture from the greenhouse where a thermal screen is applied is by slightly opening the screen. This results in an air exchange of relatively dry air from above the screen and the humid air below the screen. This procedure is difficult to control and causes horizontal temperature differences in the greenhouse. By mechanically controlling the exchange of the greenhouse air and outside air these problems can be resolved. The airflow through the system needed over the year resulting from the evaporation of the crop and the outside conditions is determined using a greenhouse simulation model KASPRO. Based on this result the dimensions of the system are calculated and a control strategy is suggested. The KASPRO calculations also showed that using outside air for vapour removal is more energy-efficient as using the air above the thermal screen. The greenhouse climate resulting from using the conventional method of vapour removal as well as with the forced ventilation is investigated using CFD. The three dimensional CFD calculations show that the climate using the forced ventilation system is much more homogenous and the control is more efficient compared to the conventional method of vapour removal. The system proves to be economical since investment costs are low (expected to be around 3 EURO/m2 maximum) and it ensures a proper well-controlled climate under the thermal screen resulting in a higher use of the screen through the year. The system is currently being tested in practise at a Dutch commercial tomato grower.

The scope of this investigation is the design and development of a new type of greenhouse with an integrated filter for rejecting near infrared radiation (NIR) and a solar energy delivery system. Cooled greenhouses are an important issue to cope with the combination of high global radiation and high outdoor temperatures. As a first measure, the spectral selective cover material, which prevents the entrance of NIR radiation, is investigated. The special spectral selective properties of these materials have to block up to 50% of the solar energy outside the greenhouse, which will reduce the needed cooling capacity. The second measure is the integration of a solar energy system. When the NIR reflecting coating is designed as a parabolic or circular shaped reflector integrated in the greenhouse, the reflected solar energy of a PhotoVoltaic (PV) cell in the focus point delivers electric energy. With a ray tracing computer program the geometry of the reflector was optimally designed with respect to the maximum power level. The PV cells mounted in the focal point require cooling due to the high heat load of the concentrated radiation (concentration factor of 30-40). The properties of different materials, Ge, GaSb, CIS and Si cells were investigated to find the optimal cell for this application. All parts are integrated in a greenhouse structure with a size of about 100m2.

It has been shown that many insects have Enterobacteriaceae bacteria in their gut system. The western flower thrips, Frankliniella occidentalis Pergande [Thysanoptera: Thripidae], has a symbiotic relation with Erwinia species gut bacteria. To determine if other Thripidae species have similar bacterial symbionts, the onion thrips, Thrips tabaci, was studied because, like F. occidentalis, it is phytophagous. Contrary to F. occidentalis, T. tabaci is endemic in Europe and biotypes have been described. Bacteria were isolated from the majority of populations and biotypes of T. tabaci examined. Bacteria were present in high numbers in most individuals of the populations studied. Like F. occidentalis, T. tabaci contained one type of bacterium that clearly outnumbered all other types present in the gut. This bacterium was identified as an Erwinia species, as was also the case for F. occidentalis. However, its biochemical characteristics and 16S rDNA sequence differed from the bacteria present in F. occidentalis.

Horticultural production is a complex process, the efficiency of which is rarely attributable to a single factor. In addition, optima tend to vary with internal states and outside conditions. To manage their production process, growers generally rely on rules based on experience. However, much of this experience has been acquired under conditions of relatively low fuel prices. In view of increasing fuel costs and worldwide initiatives to reduce CO2 emission, horticultural producers are faced with the need to adapt their practice in order to remain economically viable. The horticultural research community can provide knowledge which may help growers to adapt to the changing conditions. In The Netherlands, internet technology is being used to facilitate efficient communication between growers and scientists. Information is collected automatically from process computers at five commercial nurseries, dynamic models are used to monitor crop and climate conditions, and nearly real-time results are published on a web portal, where it is linked with an information database and web logs by growers, advisors and researchers. The web portal (in Dutch) can be viewed at http://www.kijkindekas.nl.

The responses of water relations, stomatal conductance (gs) and growth parameters of tomato (Lycopersicon esculentum Mill. cv. Royesta) plants to nitrogen fertilisation and drought were studied. The plants were subjected to a long-term, moderate and progressive water stress by adding 80 % of the water evapotranspirated by the plant the preceding day. Well-watered plants received 100 % of the water evapotranspirated. Two weeks before starting the drought period, the plants were fertilised with Hoagland’s solution with 14, 60 and 110 mM NO3 − (N14, N60 and N110, respectively). Plants of the N110 treatment had the highest leaf area. However, gs was higher for N60 plants and lower for N110 plants. At the end of the drought period, N60 plants showed the lowest values of water potential (Ψw) and osmotic potential (Ψs), and the highest values of pressure potential (Ψp). N60 plants showed the highest Ψs at maximum Ψp and the highest bulk modulus of elasticity.