Possible Physiological Mechanisms of Leaf Photodamage in Plants Grown under Continuous Lighting

Unlike the natural photoperiod that includes the alternation of day and night in the diurnal cycle, continuous (24 h a day) lighting provides uninterrupted supply of light energy required for photosynthesis, permanently promotes photooxidative processes, implies continuous signaling to the photoreceptors, and desynchronizes the internal circadian biorhythms from the external light/dark cycle (circadian asynchrony). The leaves of many plant species grown under constinuous lighting are prone to characteristic and potentially lethal interveinal chlorosis and necrosis. The photodamage of plant leaves exposed to long photoperiods, including daily 24-h illumination was described more than 90 years ago, but the causes of this phenomenon are still not entirely clear. Biological bases underlying this phenomenon are theoretically and practically important, because growing plants under a 24-h photoperiod at a relatively low photon flux density is seemingly an effective way to save resources and increase plant productivity in greenhouses and plant factories with artificial lighting. This review of available literature compiles and evaluates the arguments both supporting and confronting the hypothesis that carbohydrate accumulation, specifically the hyperaccumulation of starch in leaves, is the main cause of photodamage to plants grown under continuous lighting or long photoperiods. The analysis of a large number of studies indicates that the accumulation of carbohydrates is neither the main nor the only cause of leaf injuries in plants grown under a 24-h photoperiod, although the role of this factor in photodamage cannot be ruled out. The appearance and development of photodamage under a 24-h photoperiod is presumably due to several simultaneously acting factors, such as photooxidation, stress-induced senescence, and circadian asynchrony. The contribution of individual factors to photodamage may vary substantially depending on environmental conditions and biological properties of the object (plant species and variety, plant age, and the stage of development).