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Indoor plant cultivation systems are gaining increasing popularity because of their ability to meet the needs of producing food in unfavourable climatic contexts and in urban environments, allowing high yield, high quality, and great efficiency in the use of resources such as water and nutrients. While light is one of the most important environmental factors affecting plant development and morphology, electricity costs can limit the widespread adoption of indoor plant cultivation systems at a commercial scale. LED lighting technologies for plant cultivation are also rapidly evolving, and lamps for indoor cultivation are often designed to optimize their light emissions in the photosynthetically active spectrum (i.e. red and blue), in order to reduce energetic requirements for satisfactory yield. Under these light regimens, however, little information is available in literature about minimum photosynthetic photon flux density (PPFD) for indoor production of leafy vegetables and herbs, while existing literature often adopts light intensities from 100 to 300 μmol m-2 s-1. This study aims at defining the optimal PPFD for indoor cultivation of basil (Ocimum basilicum L.) and lettuce (Lactuca sativa L.), by linking resource use efficiency to physiological responses and biomass production under different light intensities. Basil and lettuce plants were cultivated at 24 °C and 450 μmol mol-1 CO2 under red and blue light (with red:blue ratio of 3) and a photoperiod of 16 h d-1 of light in growth chambers using five PPFD (100, 150, 200, 250 and 300 μmol m-2 s-1, resulting in daily light integrals, DLI, of 5.8, 8.6, 11.5, 14.4 and 17.3 mol m-2 d-1, respectively). A progressive increase of biomass production for both lettuce and basil up to a PPFD of 250 μmol m-2 s-1 was observed, whereas no further yield increases were associated with higher PPFD (300 μmol m-2 s-1). Despite the highest stomatal conductance associated to a PPFD of 250 μmol m-2 s-1 in lettuce and to a PPFD ≥ 200 μmol m-2 s-1 in basil, water use efficiency was maximized under a PPFD ≥ 200 μmol m-2 s-1 in lettuce and PPFD ≥ 250 μmol m-2 s-1 in basil. Energy and light use efficiencies were increased under a PPFD of 200 and 250 μmol m-2 s-1 in lettuce and under a PPFD of 250 μmol m-2 s-1 in basil. Furthermore, in lettuce grown under 250 μmol m-2 s-1 antioxidant capacity, phenolics and flavonoids were higher as compared with plants supplied with PPFD ≤ 150 μmol m-2 s-1. Accordingly, a PPFD of 250 μmol m-2 s-1 seems suitable for optimizing yield and resource use efficiency in red and blue LED lighting for indoor cultivation of lettuce and basil under the prevailing conditions of the used indoor farming set-up.

Heavy shading is commonly applied during production of pot-plants in order to avoid damage caused by high light intensities; usually the daily light integral (DLI) is limited to 5–8 mol m −2 d −1 photosynthetically active radiation (PAR). However, shading carries a production penalty as light is the driving force for photosynthesis. Diffuse glass has been developed to scatter the incident light in greenhouses. This study aims at investigating the effect of diffuse glass cover and high DLI under diffuse glass cover on the growth of pot-plants; furthermore, to systematically identify and quantify the yield components which are influenced by these treatments. Experiments were carried out with two Anthurium andreanum cultivars (Royal Champion and Pink Champion) in a conventional modern glasshouse compartment covered by clear glass with DLI limited to 7.5 mol m −2 d −1 (average realized DLI was 7.2 mol m −2 d −1 ), and another two glasshouse compartments covered by diffuse glass with DLI limited to 7.5 (average realized DLI was 7.5 mol m −2 d −1 ) and 10 mol m −2 d −1 (average realized DLI was 8.9 mol m −2 d −1 ). Diffuse glass cover resulted in less variation of temporal photosynthetic photon flux density (PPFD) distribution compared with the clear glass cover. Under similar DLI conditions (DLI limited to 7.5 mol m −2 d −1 ), diffuse glass cover stimulated dry mass production per unit intercepted PPFD (RUE) in ‘Royal Champion’ by 8%; whilst this stimulating effect did not occur in ‘Pink Champion’. Under diffuse glass cover, biomass production was proportional to DLI in both cultivars (within the range 7.5–9 mol m −2 d −1 ). Consequently higher DLI led to more flowers, leaves and stems. Furthermore, high DLI resulted in more compact plants without light damage in leaves or flowers in both cultivars. ‘Pink Champion’ produced more biomass than ‘Royal Champion’ in all treatments because of higher RUE which resulted from a more advantageous canopy architecture for light capture and more advantageous leaf photosynthetic properties. We conclude that less shading under diffuse glass cover not only stimulates plant growth but also improves plant ornamental quality (i.e. compactness).

To determine the extent to which transpiration and Ca concentration in the nutrient solution affect the regulation of growth, two independent experiments with young tomato plants were carried out under fully controlled climate conditions and grown hydroponically. The first experiment consisted of the regulation of transpiration by three levels of relative air humidity (RH): 50%, 70% (control) and 95% (corresponding to 1.32, 0.79 and 0.13 kPa, respectively) during 7 days. The second experiment involved four periods of 1, 3, 7 or 14 days of low-calcium (0.5 meq L¿1) compared with the nutrient standard solution (9 meq L¿1). The results show that plant growth was affected more by RH than by the reduction of Ca in the nutrient solution. High humidity reduced the total plant dry matter and total leaf area, increasing the dry matter partitioning into the stems and reducing it into the leaves. However, the low-Ca supply did not affect those parameters. Plant Ca concentration was significantly reduced by low-Ca supply as well as by high RH, but to a much greater extent by the Ca supply than by high RH. Ca concentrations in leaves, stem, and roots were quickly reduced already after 1 day of low-Ca. After 14 days, Ca concentration in all plant organs (leaves, stems and roots) was reduced by approximately 70% compared to control plants. Our data show that calcium supply, and consequently Ca concentration in the tomato plant can be reduced drastically for short-term periods during the vegetative growth stage without any adverse effect on growth whilst higher humidity reduce both growth and Ca concentration in young vegetative tomato plants. Consequently, reduced Ca uptake at high air humidity is not the cause for the reduction in growth. Keywords: Calcium; Transpiration; Dry matter partitioning; Leaf expansion; Lycopersicon esculentum Mill.

In order to analyze the dynamics of growth, water and K uptake, the effects of 1, 3 and 7 days of potassium starvation and the recovery capability during 7 days afterwards were investigated in vegetative tomato plants. After 7 days of K starvation, plant dry matter was reduced by 36% compared to control plants. After 3 days of starvation plants showed a 15% reduction in dry matter and a 25% reduction in growth rate (not statistically significant). K starvation reduced leaf area and specific leaf area (SLA) and it increased leaf dry matter percentage. K starvation enhanced dry matter partitioning into the roots at the expense of the stem. Plant K concentration was reduced by K starvation with the strongest effect in the leaves and roots. When a 3-day K starvation period was followed by 7 days of recovery with full strength nutrient solution, growth and plant K concentration completely recovered, but not after 7 days of K starvation. Xylem sap flow was reduced by K starvation and after 7 days of starvation the K concentration in the sap was reduced by 60%. During the starvation period, the reduction in relative growth rate was linearly related to the plant K concentration. The critical potassium concentration in the plant (the K concentration at which relative growth rate was reduced by 10%) was determined according to the nutrient interruption technique. The critical concentration was 4.3% K which was reached after 2.5 days of K starvation while the potassium concentration of control plants was 6.3%. During recovery the dry matter growth rate seemed to be the most important factor determining K uptake.

A mechanistic model was developed to predict the weekly fresh weight yield of cucumber fruits and the fresh weight and developmental stage of the individual fruits at harvest. The latter two being major criteria of fruit quality. The model consists of modules for greenhouse light transmission, light interception by the crop, leaf and canopy photosynthesis, assimilate partitioning, dry matter production, fruit growth, fruit dry matter content and fruit harvest. A sensitivity analysis showed the total yield to increase with increasing radiation, CO 2 concentration and temperature. The harvest strategy of the grower (frequency of harvesting and threshold weight for harvest) had a great impact on simulated fruit size and fruit developmental stage at harvest. The latter was lowest in summer time, which may have consequences for the shelf life of the fruits. The model was validated by comparing simulation results with production data of 10 commercial growers in 1996 and 14 growers in 1997 (January–May). Input data used for validation were week numbers of planting and removing the crop, weekly data on global radiation outside the glasshouse and glasshouse air temperature and daytime CO 2 concentration. The weekly harvest of total fresh weight averaged over all growers was simulated well by the model. The average error of the weekly prediction of the fresh weight yield was 12.6%, while the error of the annual yield was only 0.3% in 1996. The simulated average fruit size corresponded reasonably well with growers' data, showing an average weekly error of 6.6%. The accuracy of prediction of cucumber yields largely depends on accuracy of the weather prediction.

The shape of cucumber fruits expressed as the length/circumference ratio is an important quality factor. The fruit shape of parthenocarpic cucumber fruits was measured during fruit development and the effects of position within the plant, fruit load (number of fruits per plant) and temperature were analysed. At 25°C the length/circumference ratio increased until 4 days after anthesis and declined thereafter. The length/circumference ratio of the fruits was low at positions around leaf axil 12 and was highest for fruits from the branches. The shape of the fruits was apparently predetermined by their position within the plant. The number of fruits competing for assimilates had no effect on the pattern of development over time of the length/circumference ratio. With decreasing number of fruits per plant the fruits reached the marketable size (weight) at an earlier developmental stage, resulting in marketable fruits with a large length/circumference ratio. An increase in temperature hastened fruit development, but when the length/circumference ratio was plotted against the temperature sum the effects of temperature on the length/circumference ratio were only small.

Cucumber plants were grown at three different irradiances in a glasshouse for a short (4 day) and an extended period (62 day). The biomass allocation to the fruits increased with increasing irradiance when plants were grown over an extended period (62 day) at different irradiances. In the short-term (4 day) the allocation to the fruits did not increase with increasing irradiance. The positive long-term effect of high irradiance on the biomass distribution to the fruits could primarily be ascribed to an increase in the number of fruits growing at the same time on the plant. The growth rate of individual fruits increased with increasing irradiance resulting in a shorter growing period from anthesis until harvest of a fruit. The dry-matter percentage of both the fruits and the vegetative plant parts increased with increasing irradiance.

Cucumber plants were grown at 18°C or 25°C and with four intensities of fruit removal to analyse the effects of temperature and the number and weight of fruits on fruit growth and biomass allocation to the fruits. Increasing the number of fruits per plant increased the total fruit growth at the expense of the vegetative growth. The daily biomass allocation to the fruits showed a saturation type relationship with the total fruit weight on a plant. With the same number or weight of fruits on a plant, the biomass allocation to the fruits was greater at 25°C than at 18°C. When the number of fruits on a plant was not manually restricted, temperature had only a slight effect on the biomass allocation because there were less fruits on a plant at 25°C than at 18°C. With increasing number of fruits per plant or decreasing temperature the growth rate of the individual fruits decreased, resulting in an increase in the growing period from anthesis until harvest of the individual fruits. The dry-matter percentage of the fruits decreased with increasing number of fruits on a plant and with increasing temperature. The fruit fresh weight production increased with increasing temperature, but fruit dry weight production was hardly affected.

An explanatory simulation model was developed to simulate the diurnal course of plant-water relations and their effects on the photosynthesis of greenhouse crops. The simulated effects of global radiation on plant-water relationships and photosynthesis are shown. Measurements of transpiration of a tomato crop (cultivar ‘Turbo’) showed that daily totals as well as the diurnal course of transpiration on a short-time basis were simulated satisfactorily.