Abstract: 【Objective】Melatonin is a small molecules widely exist in higher plants which is regarded as a new plant growth regulator and biological stimulant. Melatonin plays an important role in improving plant resistance, but research about melatonin involved in regulation of plant photosynthesis under adversity stress is rarely reported. The objective of this study is to explore the influence of exogenous melatonin on photosynthesis of tomato leaves under drought stress. 【Method】Tomato cultivar ‘LiaoYuanDuoLi’ was used as the experimental materials, concentration screening tests were firstly carried out: CK: Leaf spray with water and root application with 50 mL water. R5, R50, R100, R150, R250: leaf spray with water and root application with 50 mL 5, 50, 100, 150, 250 µmol?L^-1 melatonin. L5, L50, L100, L150, L250: root application with 50 mL water and leaf spray with 5, 50, 100, 150, 250 µmol?L^-1 melatonin. This process was repeated in the morning and afternoon for three consecutive days and the drought stress treatment was conducted for subsequent three days (CK0: normal water after leaf spray with water and root application with 50 mL water, CK1: drought treatment after leaf spray with water and root application with 50 mL water). The optimal melatonin concentration was screened by comparing the maximum photochemical quantum yield of PSII (Fv/Fm) and a parameter representing the quantity of efficient PSI complex (Pm). Then the influence of root application and leaf spray with exogenous melatonin on gas exchange parameters, light energy distribution and electron transfer rate of PSI and PSII and the integrity of the thylakoid membrane and ATP enzyme activity of tomato leaves under drought stress were analyzed by using photosynthetic fluorescence synchronous measurement technology. 【Result】Root application and leaf spray with different concentrations of melatonin both increased the Fv/Fm and Pm of tomato leaves under drought stress, and all showed a trend of increase at first and then decreased with the increase of concentration, the values of Fv/Fm and Pm were the highest under L100 and R100, both significantly higher than that of control, thus it was determined that L100 and R100 were the optimal concentration treatments for leaf spray and root application, respectively. L100 and R100 significantly alleviated the inhibition of drought stress on the gas exchange parameters, and the leaf net photosynthetic rate (Pn) was 2.04 µmol?m^-2?s^-1 and 1.71 µmol?m^-2?s^-1, respectively, which both significantly higher than the control (CK1) (0.52 µmol?m^-2?s^-1); transpiration rate (E) was 0.66 mmol?m^-2?s^-1 and 0.54 mmol?m^-2?s^-1, respectively, and both significantly higher than that of CK1 (0.25 mmol?m^-2?s^-1). L100 and R100 treatments significantly increased the stomatal conductance (GH₂O) and the maximum water use efficiency (WUE) and significantly reduced stomatal limitation (Ls) of tomato leaves under drought stress, it was also found that L100 treatment was superior to R100. The results of rapid light response curve showed that L100 and R100 enhanced Fv'/Fm' and qP of tomato leaves, indicated melatonin is beneficial to improve photochemical reaction efficiency of PSII of tomatoes under drought stress. Cyclic electron flow of tomato seedlings under drought stress was significantly enhanced, while melatonin treatment reduced on cyclic electron flow, but strengthened ETRI and ETRII, and they were both higher under L100 compared with R100. Y (I) and Y (II) both improved under L100 and R100 compared with CK1, indicate melatonin treatment strengthened light energy distribution to the direction of photochemical reaction of PSI and PSII under drought stress. P515 induction curves of L100 and R100 were higher than that of CK1 after dark adaptation, and after illumination, P515 signal of CK0 fell fast, followed by L100 and R100, CK1 treatment decreased slowest, showed that exogenous melatonin protected the thylakoid membrane from damage caused by drought stress and strengthened the ATP-synthase activity. 【Conclusion】Root application and leaf spraying with exogenous melatonin can relieve the inhibition of drought stress on photosynthetic performance of tomato leaves, strengthen the photosynthetic efficiency, leaf spraying is a more simple and efficient measure compared with root application. Melatonin can enhance crop photosynthesis adaptability to the environment stresses and has an regulatory role in crop growth and development.

Key words: melatonin, drought stress, tomato, photosynthesis, chlorophyll fluorescence