Abstract
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.
Highlights
Light is considered to be the most important environmental factor affecting plant development, growth, and production in greenhouse horticulture
At the end of the vegetative phase, plants that received farred light were taller than the treatments without far-red light (Table 2)
Specific leaf area (SLA) was lower for the treatments with additional far-red light, implying that they are capable of intercepting more light with a comparable leaf biomass
Summary
Light is considered to be the most important environmental factor affecting plant development, growth, and production in greenhouse horticulture. The potential of supplemental lighting to foster off-seasonal production in the Mediterranean region is under investigation (Palmitessa et al, 2020; Paucek et al, 2020). Under low-light conditions, natural light can be supplemented by artificial lighting, with high pressure sodium (HPS) fixtures currently being the predominant greenhouse lighting source. Before the full potential of LEDs as light source for plant production in greenhouses can be used, plant responses to spectral composition of the light as well as the effects on the biology of pathogens, pests, and their natural enemies have to be quantified
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