Scientific publications

The phenomenon of leaf photodamage, characterized by potentially lethal interveinal chlorosis and necrosis, observed in some plants (tomato, eggplant, cucumber, etc.) under long photoperiods, including continuous 24h lighting (CL), was first described over 100 years ago. However, its causes remain unclear. Interest in this phenomenon has been rising significantly in recent years, as plant cultivation under CL with relatively low photon flux is considered as a potentially effective way to conserve resources and augment plant productivity in greenhouses and plant factories with artificial lighting. This article summarizes and analyzes the literature and own data supporting or contradicting currently existing hypotheses explaining the causes of leaf photodamage under long photoperiods. These include photooxidation, hyperaccumulation of starch and soluble sugars, induction of senescence, discrepancy between the frequency of internal (circadian) biorhythms and external light-dark cycles, etc. Despite nearly a century of research, a definitive answer to the question of why CL has a positive effect on some species and a negative effect on others and what causes leaf photodamage has yet to be found. None of the existing hypotheses has yet gained reliable experimental confirmation. Most likely, CL-induced leaf photodamage in sensitive species occurs for a number of reasons, of which it is difficult to single out one primary cause, but perhaps the first in the chain of events terminating in leaf chlorosis and necrosis is circadian asynchrony, that is, a disruption of the endogenous circadian rhythm of many processes that occurs in plants as a result of exposure to an abnormal light-dark cycle.