Exogenous Foliar Melatonin Improves Cotton (Gossypium Hirsutum L.) Physio-Biochemical Characteristics Under the Synergetic Effects of Low Temperature and Salinity Stress

Low temperature and soil salinization during cotton sowing and seedling have adverse effects on cotton productivity. Finding an alternative for reducing the low temperature and salt induced damages during the seedling stage of cotton is a challenge for agricultural researchers nowadays. The physiological mechanism of exogenously applied melatonin (MT) on cotton seedlings under low temperature and salt stress is still unclear. The experiment in a phytotron was comprised with two temperature levels of 15°C and 25°C, and 5 MT treatments of 0, 50, 100, 150, 200 µM, and two salinity levels of 0 and 150 mM NaCl stress. Compared with the control treatments (non-salinity stress under 15°C and 25°C), the coupled stress of salt and low temperature reduced cotton seedlings’ biomass and net photosynthetic rate (Pn), aggravated the membrane damage, reduced the potassium (K+) content and increased the sodium (Na+) accumulation in the leaves and roots. Compared with the NaCl-stressed treatment alone, the exogenous foliar applications of 50-150µM MT significantly increased the biomass and gas exchange parameters of cotton seedlings under the coupled salt and low temperature stress conditions. The exogenously applied MT at 50-150µM under the coupled effect of salt and low temperature stress conditions decreased the degree of membrane damage and regulated the activities of the protective enzymes, ion homeostasis, ion transport and absorption of cotton seedlings. The pairwise correlation analysis of each parameter by MT shows that the parameters with higher correlation with MT at 15°C are mainly malondialdehyde (MDA), peroxidase (POD), and catalase (CAT). The most relevant parameters at 25℃ are K+ concentration in leaves (K+-L), K+ concentration in root (K+-R), Na+ concentration in leaves (Na+-L), Na+ concentration in root (Na+-R), Na+ uptake in-root surface (Na+-uptake), K+ ion translocation (K+-translocation). Stepwise linear regression of the above parameters found that MT is more related to MDA at 15°C, and MT is more related to Na+-L at 25°C.