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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.

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.

Semi-closed greenhouse management may increase greenhouse productivity. However, it relies on the application of mechanical cooling. Cooling can be applied from above or below the canopy. The positioning of the cooling affects the vertical climate profile in the canopy. In order to determine how this affects the vertical profile of transpiration and crop temperature, in this work we used the energy balance of different crop layers to develop a modified “big leaf” model for its temperature and transpiration. The model was validated with data from a large greenhouse experiment (tomato) where the position of cooling elements and climate set-points ensured different vertical profiles of air properties. In particular, we had the combination of two positions of cooling elements (above and below the crop) and two temperature set-points; and a control (not cooled and naturally ventilated) compartment. For the validation we used measurements of crop temperature at different heights in the canopy and of transpiration of the whole crop. Finally we used the model to determine and discuss the effect of the various types of air conditioning on the vertical profile of transpiration within the crop. The compartment with natural ventilation had the highest simulated transpiration (which agreed with the measurements) and the largest uniformity of distribution of transpiration among layers. The least homogeneous distribution was with the cooling elements below, which had also the smallest total transpiration, which was 95% of the transpiration of the crop cooled from above.

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.

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.

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?

For the pilot crop tomato, business concepts of low emission greenhouses for the midterm (10 year) have been designed. The study is carried out in cooperation with innovators in the horticultural sector, suppliers and extension services. The business concepts are evaluated for different indicators in the field of planet and compared to the current situation. The focus is on the reduction of energy consumption and CO2 emission for the cultivation with and without supplementary lighting. The energy concepts differ in the way the heat and power are produced or supplied on business level. The results show that the energy concepts without supplementary lighting have a lower environmental impact and have a better energy efficiency than the energy concepts with lighting. The energy concepts without supplementary lighting show that simultaneous production of heat and power on business level and the delivery of electricity to the public grid is most favorable looking at the energy use and CO2 emission on a national level. Second best is the conditioned greenhouse. In this energy concept solar energy is collected in summertime, stored and re-used in wintertime. Conditioned greenhouses also have good perspectives in reducing the use and emission of pesticides.

Crop models are powerful tools to test hypotheses, synthesize and convey knowledge, describe and understand complex systems and compare different scenarios. Models may be used for prediction and planning of production, in decision support systems and control of the greenhouse climate, water supply and nutrient supply. The mechanistic simulation of tomato crop growth and development is described in this paper. The main processes determining yield, growth, development and water and nutrient uptake of a tomato crop are discussed in relation to growth conditions and crop management. Organ initiation is simulated as a function of temperature. Simulation of leaf area expansion is also based on temperature, unless a maximum specific leaf area is reached. Leaf area is an important determinant for the light interception of the canopy. Radiation shows exponential extinction with depth in the canopy. For leaf photosynthesis several models are available. Transpiration is calculated according to the Penman-Monteith approach. Net assimilate production is calculated as the difference between canopy gross photosynthesis and maintenance respiration. The net assimilate production is used for growth of the different plant organs and growth respiration. Partitioning of assimilates among plant organs is simulated based on the relative sink strengths of the organs. The simulation of plant-nutrient relationships starts with the calculation of the demanded concentrations of different macronutrients for each plant organ with the demand depending on the ontogenetic stage of the organ. Subsequently, the demanded nutrient uptake is calculated from these demanded concentrations and dry weight of the organs. When there is no limitation in the availability at the root surface, the actual uptake will equal the demanded uptake. When the root system cannot fulfil the demand, uptake is less, plant nutrient concentration drops and crop production might be reduced. It is concluded that mechanistic crop models accurately simulate yield, growth, development and water and nutrient relations of greenhouse grown tomato in different climate zones

The properties of vermicompost, green compost, and their mixes as substitutes for peat were evaluated regarding their recommendation for potting media. The mixes with a maximum of 50% of vermicompost or green compost had acceptable air filled porosity (AFP) and easily available water (EAW). In the vermicompost the level of organic matter (OM), dry bulk density (DBD) and shrinkage were acceptable; however, the AFP and EAW together were not at the recommended level in the different batches; as a consequence, vermicompost should not be used alone for potting media. In the green compost, the level of OM was low which increased the DBD and consequently the AFP was diminished. The particle size distribution (PSD) was different among peat, vermicompost and green compost. The coarse peat had the lower proportion of particles from 0.25 to 2.00 mm (41%) whereas the green compost had the lower coarseness index (CI: percentage by weight of particles larger than 0.5 mm in diameter), 48.4%. The direct effect of the PSD, OM and DBD in the water and air availability was confirmed. Moreover, there were high correlations between the OM, DBD, shrinkage, pore volume and PSD with the water release curve. Those properties should be considered in order to increase the level of substitution of vermicompost in peat-based potting media.

Soil-dwelling predatory mites can be very effective as biological control agents against larvae of the lepidopteral pest Duponchelia fovealis. Some growing media were reported to have natural high level and stable populations of predatory mite. The objective of this experiment was to define conditions to establish stable predatory mite populations in the rooting medium and to assess the direct effect of the rooting media on pest development. Eight rooting media were prepared, including a range of degradabilities as measured with the Oxygen Uptake Rate method (OUR). The OUR range was created by mixing peat products, coir dust, bark, perlite, compost and wood fiber. Each treatment was split: half with and half without a commercially used mulch to create a drier top layer. Kalanchoës were grown on these rooting media. After one week soil-dwelling predatory mites (Hypoaspis miles) were added. Adults of the pest Duponchelia fovealis were released during a number of weeks. Both populations were counted. Results show that the OUR range was successfully achieved. The commercial mulch, a cork based fine granulate, reduced the numbers of Duponchelia by 32%. The number of predatory mites was related to the oxygen uptake of the rooting media (R2=0.87). The predatory mite reduced the numbers of Duponchelia larvae on average by 58%. Thus, biological control by soil-dwelling predatory mites can be improved by offering rooting media with an increased degradability as measured by the oxygen uptake rate. The combined effects of using predatory mite and mulch layers are discussed.

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 system, developed under the FLOW-AID (an FP6 project), is a farm level water management system of special value in situations where the water availability and quality is limited. This market-ready precision irrigation management system features new models, hardware and software. The hardware platform delivers a maintenance-free low cost dielectric tensiometer and several low-end irrigation or fertigation controllers for serving different situations. The software includes a complete, web based, Decision Support System (DSS) that consists of an expert planner for farm zoning (MOPECO) and a universal irrigation scheduler, based on crop-water stress models (UNIPI) and water and nutrient uptake calculations. The system, designed also to service greenhouse fertigation and hydroponics, is scalable from one to many zones. It consists of 1) a data gathering tool which uploads agronomic data, from monitored crops around the world, to a central web Data Base (DB), and 2) a web based Decision Support System (DSS). The DSS processes intelligently the data of the crop using Crop Response Models, Nutrient Uptake Models and Water Uptake Models. The central system returns over Internet to the low-end controller a command file containing water scheduling and nutrient supply guidelines

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.

n a greenhouse without carbon fertilization, the CO2 absorbed in the process of photosynthesis must ultimately come from the external ambient through the ventilation openings. The ventilation of the greenhouse implies a trade-off between ensuring inflow of carbon dioxide and maintaining an adequate temperature within the house, particularly during sunny but chilly days. Crop production is known to increase both with carbon dioxide concentration and with [average] temperature. Therefore, the management of ventilation in such conditions is looking for ¿the lesser of two evils¿. After recalling the conclusion of a previous paper that carbon fertilization up to at least external concentration is the surest and cheapest way to increase productivity in such conditions, we deal with the question of optimal fertilisation in presence of natural ventilation. Allowing for a higher than external concentration obviously reduces the efficiency of the supply, but it does not necessarily reduce profit. By applying some economics to a simple assimilation model, we show that in many conditions¿particularly with relatively high radiation¿ maintaining higher than external concentrations does make economic sense, certainly up to ventilation rates of 10 per hour. We conclude that the optimal management of carbon fertilisation should aim at concentrations well above 1000 vpm in the absence of ventilation, and gradually decrease to maintaining the external value at ventilation rates well in excess of 10 per hour. Market conditions (value of produce v price of CO2) should determine the trend between these two extremes, that is, how fast or gradually should a grower limit fertilisation to only maintaining the external concentration.

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.

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.

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

In the present study a commercial CFD code was used in order to investigate the dispersion of a pesticide inside an arch type tunnel greenhouse with continuous side vents. The greenhouse was cultivated with a tomato crop planted in double rows, which, during the period of measurements had a height of 1.5 m. In parallel, measurements were carried out in order to experimentally determine the decay rate of pesticide concentration. Air samples were continuously taken at seven points inside the greenhouse (six air pumps and one gas analyser). In the 3D numerical model calculations were done for several wind directions and wind speeds, using the experimental values as boundary conditions. The final solution for every case of wind direction and wind velocity was obtained, firstly by a converge solution under steady – state condition; and secondly by an unsteady one, where at the time which equals to zero the air volume in the experimental greenhouse was considered to contain a mixture consisting of air and the used pesticide. The simulation results that were in the same order of magnitude with the experimental values only during the first 5-6 minutes of unsteady solution and only qualitatively good agreement for the rest time, while the decay process was simulated. Even if different wind characteristics, as boundary conditions, were introduced in the CFD model every minute, the agreement remained in a qualitative level, showing that the concentration of the pesticide inside the greenhouse is a function not only of the greenhouse ventilation rate but also of the pesticide evaporation-volatilisation rate from the greenhouse crop, soil and cover surfaces. As the simulation model almost always underestimates the concentration levels inside the greenhouse, more investigation has to be carried out in order to obtain better agreement between measured and estimated values of pesticide concentration inside the greenhouse

Concerning the greenhouse environment, the ultimate goal of an investigation would be to determine the climatic parameters for all locations in the study area. Objective of the present study is to analyse the distribution of air temperature and air velocity of an experimental greenhouse with tomato crop, equipped with fan and pad evaporative cooling system, using geostatistical methods. The main aspects of geostatistics in terms of theoretical background for understanding spatial correlation models and kriging applications are presented. The most common variogram models were fitted to the experimental data sets obtained during summer period from an experimental greenhouse equipped with fan and pad evaporative cooling system. The Kriging approach was applied using the semivariograms corresponded to these data sets. Finally, the prediction maps of air temperature and air velocity were produced in different height levels inside the tomato crop canopy showing a great variability. Geostatistic analysis may be applied to determine not just optimal spatial predictions but also probabilities associated with risk-based analysis in order to improve the suitability and efficiency of climatic controls systems in greenhouses.

Natural ventilation to discharge excess heat and vapour from the greenhouse environment has serious drawbacks. Pests and diseases find their way through the openings; carbon dioxide fertilisation becomes inefficient and the inescapable coupling of heat and vapour release results often in sub-optimal conditions for either temperature or humidity. The present trend, therefore, is to reduce ventilation as much as possible, also in Mediterranean conditions. This relies obviously on improved means for diminishing the heat load and proper use of cooling equipment. Especially the latter can combine the benefits of cooling the greenhouse air with serious energy conservation. However, opposite to the clear benefits there are also serious investments associated with active cooling of greenhouse. Therefore, there is a growing demand for some computational tool that enables quantitive comparisons between the vast number of alternatives with respect to the different components of (semi) closed greenhouse systems. The benefits in terms of improved production (quality, ornamental value and quantity) are quite difficult to quantify, due to the complexity of the biological processes involved. On the energy side of the balance, however, since the physics of greenhouses, climate controllers and horticultural hardware can be described very well, it is quite possible to develop such a tool for predicting the energy consumption of a (semi) closed greenhouse for a wide range of horticultural and outside climate conditions. This paper gives an outline of such a tool and discusses some results. Just as an illustration, a number of quantitative effects are shown of changing the fraction of closed green¬house surface in a 1 hectare enterprise that consists of closed and non-closed compartments. This analysis is made for both a Dutch climate situation and a Mediterranean weather data set.

The so-called closed greenhouse (closed ventilation windows) is a recent innovation in Dutch greenhouse industry. The technical concept consists of a heat pump, underground (aquifer) seasonal energy storage as well as daytime storage, air treatment units with heat exchangers, and air distribution ducts. Savings of up to 30% in fossil fuel and production increases by up to 20%, mainly because of the continuously high CO2 concentration, have been reported. Economic feasibility of this innovative greenhouse highly depends on the yield increase that can be obtained. In this simulation study the effects of greenhouse climate on tomato yield in a closed greenhouse are presented. The explanatory model INTKAM was used, which has several submodels e.g. for light interception, leaf photosynthesis and biomass partitioning. The closed greenhouse offers possibilities for combinations of light, temperature, air humidity and CO2 concentration that are impossible in a conventional greenhouse. At high CO2 concentration and high light intensity, leaf photosynthesis shows a more narrow optimum for temperature than at high CO2 and moderate light intensity. However, the response of crop photosynthesis to temperature has a much broader optimum than that of leaf photosynthesis. Besides photosynthesis, temperature also influences aspects like partitioning, leaf area development and fruit development. Yield potential reduces at temperatures above 26°C, with fruit set being one of the first processes that is negatively influenced by supra-optimal temperatures. Based on actual climatic conditions in a conventional and a closed greenhouse (same crop management) measured during two years, INTKAM predicts an increase in yield by about 17%. Hence, in a closed greenhouse a higher stem density can be maintained for obtaining the same average fruit weight (size) as in a conventional greenhouse. In 2005 actual yield increase was similar to the simulated one (16%), but in 2004 only a 9% higher yield was realized, at least partly because of botrytis infection in the closed greenhouse.

In the last 15 years a significant increase in greenhouse area has occurred in Mexico, from a modest 50 hectares in 1990 to over 2,000 hectares in 2004. The rapid increase in greenhouse area is a result of an attractive export market, USA. Mexican summer midday temperatures are well above crop optimum and cooling is needed if heat stress induced crop growth reduction is to be prevented. The objective of this study was to determine the effectiveness and feasibility of greenhouse cooling systems for tomato culture under desert, humid tropic and temperate Mexican weather conditions. These climate regions are represented by Mexicali, Merida and Huejutla respectively. The cooling systems included a variety of passive and active systems, which through an engineering design methodology were combined to suit the climate conditions of the 3 regions. The evaluation was conducted via simulation, taking into account the most important temperature effects on crop growth and yield. The results showed that the cooling systems were effective in decreasing heat stress to plants. Investment costs of greenhouse with cooling equipment were under USD 50 m-2 and operational costs were under USD 10 m-2 for all equipment combinations and treatments except for the humid tropic climate of Merida. Solutions for Merida were both economically and physically not feasible due to too high humidity levels. This model study clearly indicates that cooling is feasible in desert and moderate climate regions of Mexico but in humid tropic climate regions feasibility is a problem. Application of design methodology and design evaluation with help of simulation greatly contributed to pointing out effective and non-effective solutions to reduce heat stress in hot climates.

Fogging systems are increasingly used to cool greenhouses and prevent water stress. More recently, fogging systems are applied also in relatively low radiation environments (such as The Netherlands), for a better control of product quality than whitewashing and to reduce need for natural ventilation ¿ thus allowing for higher CO2 concentrations to be maintained in the greenhouse. Most commonly the steering of such systems is done by setting an upper limit to the deficit of specific humidity that, whenever exceeded, triggers the fogging system. In both cases, however, one may wonder whether static and pre-fixed set points are the most effective choice. In the experiment presented in this paper, fogging and venting were controlled with the purpose of steering crop transpiration. The desired transpiration rate was the input of an algorithm that calculated on-line the required humidity and air temperature set points in view of the current weather factors. The set points were then the input of a standard P-controller that calculated vent opening and time of operation of the fogging system. In this paper, the resulting climate and actuator control operations are discussed and compared with a similar greenhouse controlled in a traditional fashion. The study concluded that a desired crop transpiration rate (an all-round indicator of crop well-being) could be used to select dynamic set points for the climate control in a greenhouse equipped with a fogging system.

An experimental greenhouse equipped with fan and pad evaporative cooling is analysed using two different models. The first one consists of a numerical simulation approach applying a commercial CFD code. The main aspects of evaporative cooling systems, in terms of heat and mass transfer and both the external and internal climatic conditions were integrated to set up the numerical model. The crop (tomato) was simulated using the equivalent porous medium approach by the addition of a momentum and energy source term. The temperature and humidity of incoming air, the operational characteristics of exhaust fans and the pressure drop occuring in the pad, were specified to set up the CFD model. The second model considers the greenhouse as a heat exchanger. Based on greenhouse structural characteristics, external climatic conditions, pad efficiency and ventilation rate, the air temperature distribution is predicted. The results, concerning the air temperature, provided both by numerical and analytical model, were validated by experimental measurements obtained at a height level of 1.2 m above the ground in the middle of the crop canopy. The correlation coefficient (R2) between computational results and experimental data was at the order of 0.96 for the numerical model and 0.77 for the analytical one, with average percentage error of 3.5% and 7.6%, respectively. The analytical model proved to be a useful simple evaluation tool, but the numerical one provides a more accurate overview of the air flow in the greenhouse showing that fan and pad evaporative cooling system could be effectively parameterized in numerical terms, in order to improve system’s efficiency

The need for a closed and sustainable water system in greenhouse areas is stimulated by the implementation in the Netherlands of the European Framework Directive. The Dutch national project Kasza: Design of a Closed Water System for the Greenhouse Horticulture will provide information how the water system in a greenhouse horticulture area can be closed. In this paper the conceptual design of two systems to close the water cycle in a greenhouse area is described. The first system with reverse osmosis system can be used in areas where desalination is required in order to be able to use the recycle water for irrigation of all crops. The second system with advanced oxidation using UV and peroxide can be applied in areas with more salt tolerant crops and good (low sodium) water sources for irrigation. Both systems are financially feasible in new greenhouse areas with substantial available recycle water.

Wageningen UR Glastuinbouw onderzoekt hoe het teeltproces in semi-gesloten kassen beter is te beheersen. Achterblijvende producties werken nu vertragend op een snelle introductie van de veelbelovende techniek. Mede door uitgebreide metingen hopen onderzoekers knelpunten op te lossen

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.

Koelen, ontvochtigen, bevochtigen, lucht in beweging brengen, zijn teeltmaatregelen die met de opkomst van (semi-) gesloten kassen volop in de belangstelling staan. Onderzoeker Peter van Weel van Wageningen UR Glastuinbouw kijkt in dit artikel vanuit de plantfysiologie naar effecten van klimaatsturing op het gewas.

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.

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.

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.

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 closed greenhouse is a new development in protected cultivation. As air is not ventilated to the outside environment, CO2-concentrations are higher than in conventional greenhouses, causing 15¿20% increased production. The question remained as to whether increased air flow rates in a closed greenhouse cause photosynthetic adaptation. This was evaluated in a tomato crop that was planted in the summer of 2005 in The Netherlands in regular greenhouse compartments, in which air tubes were placed low and high in the canopy. Photosynthesis light-response curves were established at these two heights at 400, 700 and 1000 ppm of air CO2. Increased CO2 concentration and a higher position in the canopy caused an increase of the maximum photosynthetic rate, confirming earlier knowledge. However, the pattern of air circulation did not change the photosynthesis light-response curve. This corresponded with the absence of differences in total dry matter production and cumulative fruit growth. The INTKAM tomato model adequately simulated growth and development. It is therefore concluded that the pattern of air circulation did not cause adaptation of the photosynthetic apparatus, and that yield increases are attributable to the instantaneous effects of elevated CO2-concentrations.

Energy conservation in horticulture can be achieved by allowing temperatures to fluctuate within predefined bandwidths instead of using rigid set points for heating and ventilation. In temperature integration, plants are supposed to compensate effects of temporarily deviations of the average temperature some time later by deviations in the opposite direction. However, little is still known on the effects of integration periods exceeding 1 day. In this study, effects of temperature integration on growth and development of single-stemmed cut rose plants were determined. Pruned rose shoots were placed in climate chambers in which light levels switched daily (2 days integration period) or weekly (14 days integration period) from high light intensity (300 μmol m −2 s −1 ) to low light (150 μmol m −2 s −1 ). Temperatures were kept continuously at 20 °C (control) or changed with the light intensity (phase, high temperature at high light intensity, low temperature at low light intensity) or changed opposite to the light intensity (counter phase). Bandwidths of temperature integration were 0, 6 or 10 °C. Under these conditions, buds grew out to harvestable shoots in approximately 45 days. At both integration periods, shoot length was significantly reduced with increasing bandwidths of temperature integration. Shoot dry weights were reduced when a bandwidth of 10 °C was applied. At both integration periods, rates of photosynthesis were primarily determined by light intensity. However, in the counter phase treatments, photosynthesis rate at high light and low temperature was reduced compared to the high light condition of the control. Under these conditions, starch content increased to approximately 10%, suggesting a feedback inhibition of the rate of photosynthesis. However, this did not (yet) affect plant growth or development.

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.

The combined effects of electrical conductivity (an EC of 2.5 dS m-1 or 8 dS m-1 in the root zone) and fruit pruning (three or six fruit per truss) on tomato fruit quality were studied in a greenhouse experiment, planted in January 2005. Taste-related attributes [dry matter content (DM), total soluble solids content (SSC), titratable acidity (TA), glucose, fructose and citric acid content] and health-promoting attributes (lycopene, ß-carotene, vitamin C, and total antioxidant activity) of tomato fruits harvested on the vine from the fifth or tenth truss positions were determined. The quality of tomato fruits was improved by high EC. A high EC in the root zone increased the DM content, total SSC, TA, as well as glucose, fructose and citric acid contents. A significantly higher lycopene and ß-carotene content was also observed [on a fresh weight (FW) and dry weight (DW) basis] with a high EC in the root zone. The accumulation of different compounds that determine tomato fruit quality differed between the fifth and tenth truss. In particular, the lycopene content was reduced, whereas the ß-carotene content was increased in the tenth truss with respect to the fifth truss, most likely because of higher temperatures during ripening of the tenth truss. Fruit pruning increased fruit FW by 42% and positively influenced the DM content and total anti-oxidant activity, while a negative effect was observed on lycopene and citric acid contents (on a FW and DW basis). EC and fruit pruning both had a strong effect on fruit size; however, EC had a much stronger impact on taste and health-related fruit quality attributes. A small interaction between EC and fruit pruning was found for marketable yield, fructose and glucose content, fruit firmness, and P and Ca concentrations in fruits.

A leaf picking device for cucumbers was designed and evaluated. The picking device is manually operated but can be used as a picking tool for a robot. The device consisted of a picking rotor composed from knives and brushes, a motor and a vacuum cleaner. The performance of removal, cutting, torque and shredding was investigated in the laboratory experiments. In the greenhouse experiments, the performance of picking and cutting was investigated. The results were as follows: (1) the highest removal success rate was achieved at a rotation speed of 1000 min¿1, a rotor configuration of `two knives and two brushes¿ and an insertion speed of 50 mm s¿1; (2) in this mechanical setting, the percentage of the summation of the smooth cut surface of the leaf stalk and the smooth cut surface with small skin was 90%; (3) the required torque was 0.09¿0.96 N m and the average particle size of the shredded leaves was 7.3¿21.2 cm2; (4) the percentages of area of the dropped particles to the average area of leaves was 10¿16%; (5) the average execution time per leaf was 1.1¿2.3 s.