Growth and leaf injury in tomato plants under continuous light at different settings of constant and diurnally varied photosynthetic photon flux densities

In tomato, continuous light inflicts distinctive injuries, including leaf chlorosis and epinasty, a phenomenon that is not fully understood. To gain insights regarding the mechanism underlying this phenomenon, we cultivated tomato seedlings (Solanum lycopersicum cv. Momotaro) for 14 days under continuous light with one of three constant photosynthetic photon flux densities (PPFD) (300, 175 or 50 mu mol.m(-2).s(-1)) or two diurnal variations of PPFD (300 mu mol.m(-2).s(-1) for 14 h alternating with 50 or 5 mu mol.m(-2).s(-1) for 10 h). We quantitatively evaluated and compared the levels of leaf chlorosis and epinasty of these plants with those grown under a 14/10.h photoperiod. The results showed that under continuous light with constant PPFD, the level of leaf chlorosis did not increase with higher light intensity, whereas the level of leaf epinasty increased with increasing light intensity, suggesting that these injuries are inflicted through different mechanisms. Relative to continuous light with constant PPFD, diurnal variations in PPFD significantly mitigated leaf chlorosis, slightly reduced leaf epinasty and improved plant growth. The leaf carbohydrate concentrations, including glucose, fructose, sucrose and starch concentrations, did not follow the trend in the level of leaf chlorosis. However, a daily pattern of increase and decrease of the sucrose content resulted from diurnal variation in PPFD, which seemed to be associated with the mitigation of continuous light injury. These results suggest that the circadian rhythm rather than the overaccumulation of carbohydrates is involved in the induction of continuous-light-induced injuries in tomato.