Responses of Tomato and Eggplant to Abnormal Light/Dark Cycles and Continuous Lighting

Tomato and eggplant are the most sensitive to continuous lighting (CL) crops. The responses of tomato and eggplant to CL and abnormal light/dark (L/D) cycles were studied in order to elucidate the reason for CL-induced leaf injuries. Four light treatments were set: 16/8 h (control), 24/0 h (CL), 6/6 h, and 24/24 h (abnormal L/D cycles). These light treatments provided average daily light integrals (DLI) of 17.3, 25.9, 13.0 and 13.0 mol/(m² day), respectively. The obtained results have shown that in both tomato and eggplant abnormal L/D cycles caused photoinhibition and leaf injuries similar to those in CL-grown plants. The induced defense mechanisms were not strong enough to contend against oxidative stress caused by abnormal L/D cycles, despite the fact that they provided plants with an even lower DLI than the 16/8 h photoperiod. Abnormal L/D cycles were injurious, while dark periods made up half the time. It is concluded that photooxidative stress induced by CL is not solely due to excessive DLI or to the continuity of light itself (the absence of dark periods). Therefore, we hypothesize that circadian asynchrony is possibly the main factor triggering CL-induced leaf injuries.