Abstract: This experiment explored the role of exogenous melatonin (MT) in enhancing the resistance of tobacco seedlings to drought stress. Moderate drought stress was simulated using 15% polyethylene glycol (PEG-6000), and the drought-tolerant variety ‘Yuyan No.6’ and drought-sensitive variety ‘Yuyan No.10’ were tested for their response following foliar-spraying with MT at concentrations of 0, 0.1, 0.2, and 0.4 mmol·L^-1. Parameters measured included seedling growth, chlorophyll content, photosynthetic characteristics, chlorophyll fluorescence, antioxidant enzyme activities and reactive oxygen metabolism of the tobacco seedlings. It was found that MT application significantly increased total root length, total root surface area, average root diameter and root tip number; enhanced both shoot and root fresh weight of tobacco seedlings under drought stress; increased chlorophyll content, net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r), water use efficiency (WUE), maximal PSⅡ quantum yield (F_v/F_m), PSⅡ actual photochemical efficiency (Φ_PSⅡ) and photochemical quenching coefficient (qP); and decreased the enhancement of intercellular carbon dioxide concentration (C_i) and non-photochemical quenching coefficient (NPQ). Moreover, the stomatal and non-stomatal limitations to photosynthetic ability were effectively reduced by exogenous MT in seedlings under drought stress. The application of MT enhanced the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT); and reduced malondialdehyde (MDA) content and $O^{-·}_{2}$ generation rate. The changes in drought-sensitive ‘Yuyan No.10’ were greater than those in drought-tolerant ‘Yuyan No.6’. Across the range of exogenous MT concentration tested, the seedling growth indexes of the two tobacco varieties showed stimulation at lower rates and inhibition at higher rates. Foliar application with 0.2 mmol·L^-1 MT produced the largest growth promotion effects and drought stress alleviation. In summary, MT application at the appropriate concentration positively stimulated seedlings to capture and convert solar energy, thus promoting seedling photosynthesis and growth. Exogenous MT also enhanced the adaptability of seedlings to drought stress by improving antioxidant capacity and reducing oxidative damage. The positive effect of MT was more obvious for drought-sensitive variety ‘Yuyan No.10’.

Key words: flue-cured tobacco, drought stress, melatonin, photosynthetic traits, chlorophyll fluorescence, antioxidant capacity