Plant Photobiology under Artificial Lighting: A Critical Appraisal of the Translation of Light Signalling into Precision Horticultural Production

Abstract

Solid-state lighting has given horticultural producers unprecedented control over the spectral composition, photon flux density, photoperiod and temporal structure of the radiation delivered to crops. This control has been accompanied by an expectation that the detailed molecular understanding of plant photoreceptors accumulated over three decades can be converted directly into prescriptive lighting recipes for commercial production. The present review critically examines that expectation. It evaluates how far mechanistic knowledge of phytochrome, cryptochrome, phototropin and UV RESISTANCE LOCUS 8 signalling, and of the downstream integrators that connect these receptors to growth, actually predicts the yield, morphology, composition and economic performance of crops grown under artificial lighting in greenhouses and plant factories. Literature was identified through structured searching of open scholarly databases and indexes, supplemented by backward and forward citation tracking, with critical appraisal focused on experimental design, environmental control, replication and the adequacy of reported photometric information. The synthesis indicates that photoreceptor-level knowledge predicts direction of response reliably but magnitude poorly, and that the discrepancy arises principally from three sources: strong interaction between spectral treatments and background photon flux, temperature and carbon dioxide concentration; pronounced cultivar-level variation in response amplitude; and the dominance of whole-canopy light interception over leaf-level photochemical efficiency in determining biomass accumulation. Several widely applied heuristics, notably fixed red to blue ratios and the conventional definition of photosynthetically active radiation, are shown to rest on evidence that is weaker or more context-dependent than their routine application implies. Quality traits and physiological disorders respond to lighting in ways that frequently oppose yield objectives, and the economic and environmental evidence base remains thinner than the physiological literature. Confidence is highest for photon-flux and photoperiod effects on biomass, intermediate for spectral effects on morphology and pigmentation, and lowest for transferable spectral prescriptions across genotypes and production systems.