CONTROL OF CONTINUOUS IRRADIATION INJURY ON TOMATO PLANTS WITH A TEMPERATURE DROP

Using continuous light (CL) is the focus of current research programs. Some CL-grown plants show increased productivity. However, in several plant species including tomato continuous light induces severe injury, which is only poorly understood so far. It is known that diurnal temperature fluctuations (thermoperiods) prevent CL-induced injury in tomato. The present study was undertaken to establish if a daily short-term temperature drop can prevent CL injury in tomato. Our research has revealed that after 3 weeks under CL (150 µmol∙m^-2∙s^-1) control tomato plants grown at 26°C developed light injury symptoms but a temperature drop from 26 to 10°C for 2 h produced healthy plants. Drop-treated plants were more compact and had higher dry weight (DW), larger leaf area (LA), doubled net photosynthesis, considerably higher values of maximum photochemical efficiency of PSII (F_v/F_m), higher chlorophyll a, b content and lower electrolyte leakage compared to the control. The developmental rate did not differ significantly in control and drop-treated plants. A daily short-term temperature drop also showed a benefit compared with thermoperiod (26°C for 12 h and 10°C for 12 h). Thus, drop-treated plants had higher plant DW, larger LA and greater number of leaves compared to plants treated by thermoperiod, although the latest ones did not develop the light injury symptoms either. This study demonstrated that a temperature drop treatment could prevent physiological disorders of tomato plants grown under continuous light, while enhancing growth without delaying plant development. The information obtained supports the hypothesis that a circadian clock entrained by temperature fluctuations can prevent CL-induced injury.