Shading by sunlit leaves causes a low red (R) to far-red (FR) ratio that results in a low phytochrome stationary state (PSS). A low PSS induces an array of shade avoidance responses that influence plant architecture and development. It has often been suggested that this architectural response is advantageous for plant growth due to its positive effect on light interception. In contrast to sunlight, artificial light sources such as LEDs often lack FR, resulting in a PSS value higher than solar light (∼0.70). The aim of this study was to investigate how PSS values higher than solar radiation influence the growth and development of tomato plants. Additionally, we investigated whether a short period of FR at the end of the day (EOD-FR) could counteract any potentially negative effects caused by a lack of FR during the day. Tomato plants were grown at four PSS levels (0.70, 0.73, 0.80, and 0.88), or with a 15-min end-of-day far-red (EOD-FR) application (PSS 0.10). Photosynthetic Active Radiation (PAR; 150 μmol m -2 s -1 ) was supplied using red and blue (95/5%) LEDs. In an additional experiment, the same treatments were applied to plants receiving supplementary low-intensity solar light. Increasing PSS above solar PSS resulted in increased plant height. Leaf area and plant dry mass were lower in the treatments completely lacking FR than treatments with FR. EOD-FR-treated plants responded almost similarly to plants grown without FR, except for plant height, which was increased. Simulations with a 3D-model for light absorption revealed that the increase in dry mass was mainly related to an increase in light absorption due to a higher total leaf area. Increased petiole angle and internode length had a negative influence on total light absorption. Additionally, the treatments without FR and the EOD-FR showed strongly reduced fruit production due to reduced fruit growth associated with reduced source strength and delayed flowering. We conclude that growing tomato plants under artificial light without FR during the light period causes a range of inverse shade avoidance responses, which result in reduced plant source strength and reduced fruit production, which cannot be compensated by a simple EOD-FR treatment.

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Over the last decade, LED lighting has gained considerable interest as an energy-efficient supplemental light source in greenhouse horticulture that can change rapidly in intensity and spectral composition. Spectral composition not only affects crop physiology but may also affect the biology of pathogens, pests, and their natural enemies, both directly and indirectly through an impact on induced plant resistance. In this study, we investigated the effects of light spectrum against a background of sunlight on growth and development of Solanum melongena. These effects were related to the spectral effects on the establishment of populations of the predatory mite Amblyseius swirskii and plant resilience against the biotrophic fungus powdery mildew, the necrotrophic fungus botrytis, and the herbivorous arthropod Western flower thrips. The effects of a reduced red/far-red (R:FR) ratio were studied under two ratios of red to blue light. Far-red light either was supplied additionally to the photosynthetic photon flux density (PPFD) or partially replaced PPFD, while maintaining total photon flux density (PFD). Effects of white light or additional UV-B light on plant resilience was tested, compared to the reference (5% blue, 5% green, and 90% red light). Plant biomass in the vegetative phase increased when additional far-red light was supplied. Stem length increased with far-red, irrespective of PPFD and the percentage of blue light. In the generative phase, total shoot biomass and fruit fresh weights were higher under additional far-red light, followed by the treatments where far-red partly replaced PPFD. Far-red light increased biomass partitioning into the fruits, at the expense of the leaves. There were no differences in population growth of A. swirskii mites between light treatments, nor did light treatment have an effect on the vertical distribution of these predatory mites in the plants. The treatments with additional far-red light reduced the infection rate of powdery mildew, but increased botrytis infection. These differences might be due to the plant defenses acting against these pathogens evolving from two different regulatory pathways. These results show that positive effects of altered spectral compositions on physiological responses were only moderately compensated by increased susceptibility to fungal pathogens, which offers perspective for a sustainable greenhouse horticulture.

In de afgelopen 20 jaar is het elektriciteitsgebruik in de glastuinbouw snel toegenomen door toenemende arealen belichte teelt en een sterke toename van het geïnstalleerd vermogen aan belichting. Deze trend is te keren door de vervanging van de huidige SON-T belichting door LED belichting, die efficiënter zijn in het omzetten van elektriciteit in licht. LED belichting biedt nieuwe perspectieven voor de teelt en sturing van gewassen omdat het lichtspectrum aangepast kan worden aan de behoeftes van het gewas. Dit biedt mogelijkheden om de morfologie, ontwikkeling en fysiologie van planten te sturen. In het project "Denkkader licht" hebben we bestaande kennis, proefresultaten en praktijkervaringen geïnventariseerd en geanalyseerd om bij te dragen aan de kennis die nodig is om te komen tot een beter gebruik van de mogelijkheden van LED belichting in de glastuinbouw. Hierbij is niet alleen gekeken naar de effecten op gewas, maar ook op de plantweerbaarheid en natuurlijke vijanden. Dit project levert daarmee geen standaard lichtrecepten op, maar biedt een denkkader om te begrijpen waarom bepaalde effecten optreden en hoe per gewas de kennis opgebouwd moet worden om wel te kunnen komen tot de meest effectieve belichtingsstrategieën.

Voor telers is het van belang te weten hoeveel reserves hun gewas heeft. Dat kan nu alleen gemeten worden door bladeren of vruchten in te sturen naar een laboratorium en te wachten op de analyse. In dit project is gekeken of het mogelijk is met hyperspectraal camera's direct gehaltes aan suikers en zetmeel, droge stof, chlorofyl en nutriënten in het gewas te meten. De resultaten zijn veelbelovend: met hyperspectraal camera's zijn gehaltes aan suikers in bladeren en vruchten, chlorofyl en droge stof percentage en de bladdikte goed te bepalen. Dit maakt het mogelijk om op basis van deze camerabeelden klimaat te sturen of teeltstrategie aan te passen. AbstractGrowers would like to know the status of their crop to determine climate strategy and crop management practices. Chemical composition of the crop can now only be determined by sampling leaves or fruits, send this to a laboratory and wait for the analysis. In this project, we aimed to use hyperspectral imaging to determine the contents of sugars and starch, dry matter percentage, chlorophyll and nutrient composition in the crop. The results are promising. Hyperspectral cameras are very well capable to estimate the concentrations of sugars in leaves and fruits, chlorophyll content, dry matter percentage and specific leaf area. This allows the growers to adjust their climate settings or crop management based on these hyperspectral images.