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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).

The stimulating effect of diffuse light on radiation use efficiency (RUE) of crops is often explained by the more homogeneous spatial light distribution, while rarely considering differences in temporal light distribution at leaf level. This study investigated whether diffuse light effects on crop RUE can be explained by dynamic responses of leaf photosynthesis to temporal changes of photosynthetic photon flux density (PPFD). Two Anthurium andreanum cultivars ('Pink Champion' and 'Royal Champion') were grown in two glasshouses covered by clear (control) and diffuse glass, with similar light transmission. On clear days, diffusing the light resulted in less temporal fluctuations of PPFD. Stomatal conductance (g s) varied strongly in response to transient PPFD in 'Royal Champion,' whereas it remained relatively constant in 'Pink Champion.' Instantaneous net leaf photosynthesis (P n) in both cultivars approached steady state P n in diffuse light treatment. In control treatment this only occurred in 'Pink Champion.' These cultivar differences were reflected by a higher RUE (8%) in 'Royal Champion' in diffuse light treatment compared with control, whereas no effect on RUE was observed in 'Pink Champion.' We conclude that the stimulating effect of diffuse light on RUE depends on the stomatal response to temporal PPFD fluctuations, which response is cultivar dependent.

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.

Phalaenopsis growers are continuously trying to grow high quality crops with a shorter crop cycle. To this end growers are using a much higher daily light integral (DLI). It was hypothesized that in order to increase the DLI by a factor 1.5-2 and maintain a balance of resources for the plant, it is necessary to increase the mineral concentration in the nutrient solution. To this end a crop of six Phalaenopsis cultivars were grown at a DLI of 8-10 mol/m·d and 3 nitrogen concentrations (14, 20 and 26 mM), all at 2 EC levels (1.0 and 1.7 mS/cm), 6 treatments in total. The above ground biomass increased with increasing EC and nitrogen. The proportion of plants with 2 or more spikes decreased with increasing nitrogen at the cost of plants with 1 spike. Approximately 15% of the plants produced more than 2 spikes, irrelevant of the EC. Likely due to some nutritional imbalance, a number of the developing spikes became diseased with Fusarium solani, especially at 26 mM nitrogen in the nutrient solution. High levels of nitrogen initially had a positive effect on the growth of Phalaenopsis, but a more dynamic fertilization schedule, lowering the nitrogen concentration in the later developmental stages of Phalaenopsis, might be advisable. Based on the results of this trial with these cultivars, a nutrient solution with an EC 1.0 and 14 or 20 mM nitrogen appears to be the best choice. In this treatment, the highest number of spikes, virtually no problems with spike disease, and a good number of newly formed leaves with potential for flowering spikes was observed.

During wintertime Viburnum opulus var. roseum (snowball) shrubs are forced in warm greenhouses to harvest early cut flowers. Early forcing is occasionally unsuccessful. This is probably due to a lack of hours with low temperatures that is needed to break bud dormancy. To gain more insight about the effect of temperature on breaking dormancy and early forcing results, shrubs were transferred into cold storage in week 41, 43 and 45 (2009) for 4, 6 or 8 weeks at a temperature of 2, 5 or 8°C. After storage, the shrubs were forced at a day-/night temperature of 28/23°C. Shrubs stored for at least 8 weeks from week 41, 6 weeks from week 43 and 4 weeks from week 45 gave good forcing results, regardless of storage temperature. Since growing conditions outside vary every year and differ between nurseries, it is difficult to determine when forcing can be started. To prevent poor forcing results, we aimed to develop a molecular diagnostic assay to determine the moment at which winter dormancy of snowball flower buds is sufficiently broken. In order to identify genes involved in bud breaking dormancy, an experiment was performed on a commercial nursery from October 2010 to January 2011. Flower buds of snowball shrubs outside were sampled weekly and in December and January batches of shrubs were placed every week in a greenhouse to observe forcing results. Genes involved in bud breaking dormancy were identified by performing next generation Illumina RNA sequencing on bud samples with poor and good forcing results. A set of candidate genes was validated during a second trial at two different commercial nurseries in 2011-2012.

Commercial protected cultivation of shade-tolerant pot-plants in The Netherlands has expanded enormously in the last decade. Typically, very low daily light integrals are applied (3-5 mol PAR m-2 day-1), which are achieved by use of heavy screening and application of a layer of chalk (CaCO3) on the outside of the greenhouse to increase reflection of incoming irradiance. Although these low daily light integrals are meant to avoid damage by high light intensities and/or high temperatures, it is clear that they carry a production penalty, since potential crop growth is directly related to the amount of light that can be captured and efficiently used. Additionally, it remains unclear whether current daily light integrals are too conservative, which would create room for optimisation. Recently, a number of experiments have been carried out to examine the possibilities and limitations for cultivation of several shade-tolerant pot-plants at higher daily light integrals. For most species, plants could be grown faster when more light was allowed. Also, a significant reduction in energy use for heating could be achieved if more natural irradiance was allowed to enter the greenhouse. However, use of more light required higher levels of relative humidity to avoid light damage. In this paper, we present a synthesis of experiments, as well as an outlook to further improvements.