Melatonin Alleviates Saline-Alkali Stress in Pakchoi by Protecting Photosynthetic Electron Transport, Ion Homeostasis, and Antioxidant Defense.

Saline-alkali stress severely restricts leafy vegetable growth by impairing photosynthesis, ion homeostasis, and oxidative balance. This study investigated whether exogenous melatonin could alleviate saline-alkali stress in pakchoi (Brassica rapa subsp. chinensis) through coordinated protection of photosynthetic electron transport, ionic balance, and antioxidant defense. Pakchoi plants were subjected to four treatments: normal nutrient solution (CK), 100 μM melatonin (MT), 100 mM mixed saline-alkali stress with NaCl:NaHCO3 at 2:1 (SAS), and saline-alkali stress plus melatonin (SAS+MT). After 7 d, growth traits, gas exchange, chlorophyll fluorescence, OJIP transients, ion contents, osmotic adjustment, oxidative damage, and antioxidant enzyme activities were analyzed. Saline-alkali stress markedly inhibited growth, reduced photosynthetic pigment contents and gas exchange, impaired PSII photochemical performance, disrupted ion balance, and increased ROS accumulation and membrane damage. Compared with SAS, SAS+MT increased net photosynthetic rate (Pn) by 66.2%, effective quantum yield of PSII [Y(II)] by 56.0%, electron transport rate (ETR) by 57.9%, and the performance index on absorption basis (PIABS) from 0.80 to 1.80. Melatonin also reduced Na+ content by 38.7%, increased the K+/Na+ ratio from 1.0 to 2.2, and enhanced superoxide dismutase (SOD),peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) activities by 20.5%, 24.4%, 29.2%, and 33.3%, respectively. These results indicate that melatonin alleviates saline-alkali injury in pakchoi by maintaining PSII electron transport, improving ion homeostasis, and strengthening antioxidant defense.