Physiological and biochemical responses of cotton (Gossypium hirsutum) seedlings to NaCl stress and analysis of salt tolerance thresholds

Soil salinisation is increasing in extent and area, which seriously limits the growth of crops. In this experiment, we investigated the differences in physiological responses and salt (NaCl) tolerance thresholds between salt-tolerant (‘Xinluzao 53’) and salt-sensitive (‘Xinluzao 60’) varieties of cotton (Gossypium hirsutum). Peroxidase activity of ‘Xinluzao 53’ and ‘Xinluzao 60’ increased by 29.37% and 59.35%, compared with the control, respectively. Catalase activity of ‘Xinluzao 53’ and ‘Xinluzao 60’ was 101.00% and 61.59% higher than that of the control, respectively. Overall increase of malondialdehyde (MDA) content in the leaves of ‘Xinluzao 53’ was less than ‘Xinluzao 60’, which was lower in ‘Xinluzao 53’ than ‘Xinluzao 60’ under the salt treatments of 8 g kg−1 (32.59% lower) and 10 g kg−1 (35.27% lower). Net photosynthetic rate (Pn) of ‘Xinluzao 60’ was reduced by 13.31%, 22.83%, and 21.52% compared to ‘Xinluzao 53’ at salt concentrations of 2, 8, and 10 g kg−1, respectively. ‘Xinluzao 53’ protected the cell membrane structure by maintaining higher antioxidant enzyme activities, lower MDA content, and electrolyte leakage under salt stress. Higher SPAD values, chlorophyll fluorescence parameters and photosynthetic rates were further maintained to safeguard normal physiological metabolism and photosynthetic system, higher salt tolerance than ‘Xinluzao 60’. The orrelation analysis and quadratic regression equation established an integrated, comprehensive, and reliable screening method for cotton seedling salt tolerance threshold in combination with the actual growth of seedlings. The salt tolerance threshold of salt-tolerant ‘Xinluzao 53’ seedlings was 10.1 g kg−1, and the salt tolerance threshold of sensitive ‘Xinluzao 60’ seedlings was 8.5 g kg−1.