Abstract: 【Objective】The pomegranate, a significant economic forest fruit tree predominantly cultivated in Northwest China, is favored by consumers due to its extensive nutritional and medicinal benefits. However, the escalating issue of land salinization has resulted in reduced pomegranate yields and diminished fruit quality, thereby significantly hindering the advancement of the pomegranate industry. While the selection and breeding of salt-tolerant plants, along with the application of transgenic plants, represent crucial strategies for enhancing and utilizing saline soils, these methods are constrained by their prolonged duration and inherent challenges. In recent years, melatonin (MT) has emerged as a significant phytohormone capable of mitigating the detrimental effects of salt stress on plants. This study aims to examine the impact of exogenous MT on the photosynthetic characteristics and antioxidant capacity of pomegranate (Punica granatum L.) seedlings subjected to salt stress, with a focus on identifying the optimal concentration for alleviating such stress.【Methods】Tunisian soft-seeded (salt-sensitive) and Yecheng acid (salt-tolerant) were used as test materials, and the method of substrate potting was adopted (vermiculite: perlite: grass charcoal = 1:1:1, V/V/V). Six treatments were set up, namely, non-salt stress (CK), 200 mM NaCl (T1), 200 mmol·L-1 NaCl +50 μmol·L-1M MT (T2), 200 m mol·L-1 NaCl +100 μmol·L-1 MT (T3), 200 m mol·L-1 NaCl +150 μmol·L-1 MT (T4), and 200 m mol·L-1 NaCl +200 μmol·L-1 MT (T5) to investigate the effects of different concentrations of MT treatments. This study examined the impact of varying concentrations of melatonin (MT) on chlorophyll content, photosynthetic gas exchange parameters, chlorophyll fluorescence, osmoregulatory substances, and antioxidant capacity in the leaves of two pomegranate seedling cultivars over treatment durations of 5, 10, and 15 days. Additionally, to identify key physiological indicators of exogenous MT effects on pomegranate seedlings under salt stress, a correlation analysis and principal component analysis were conducted on 16 physiological indices across the two pomegranate cultivars.【Results】Exposure to 200 mmol·L-1 salt stress resulted in a significant reduction in leaf chlorophyll content (Chl a, Chl b, Chl a+b) and peroxidase activities (SOD, POD, CAT) in both cultivars. The application of melatonin (MT) significantly enhanced chlorophyll content and peroxidase activities. Nevertheless, the Yecheng acid cultivar exhibited a degree of salt tolerance compared to the Tunisian soft-seeded cultivar, despite the occurrence of electrolyte leakage. At 15 d of salt stress, application of exogenous MT at suitable concentrations (50, 100, 200 μmol·L-1) effectively enhanced net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), photochemical quenching coefficient (qP), actual photochemical efficiency (YII), and maximal photochemical efficiency (Fv/Fm) of pomegranate leaves, and the Tunisian soft-seeded pomegranate increased by 32.89 % - 45.06 %, 16.40 % - 37.49 %, 26.09 % - 53.10 %, 0.41 % - 19.90 %, 28.44 % - 48.04 %, and 11.89 % - 20.39 %, respectively, and in Yecheng sour pomegranate by 28.81 % - 42.47 %, 17.13 % - 34.06 %, and 12.50 % - 37.78 %, 2.94 % - 20.59 %, 46.59 % - 48.58 %, and 24.93 % - 32.00 %, respectively. Under salt stress conditions, pomegranate leaves exhibited a significant increase in electrolyte extravasation (REC) and malondialdehyde (MDA) content compared to the control (CK). The application of melatonin (MT) resulted in a maximum reduction of REC and MDA by 12.22% and 28.91%, respectively. Nonetheless, the application of a high concentration (400 μmol·L-1) of exogenous MT exacerbated oxidative and osmotic stress, ultimately impairing the photosynthetic capacity of the seedlings. This may be related to the disruption of intracellular ionic homeostasis by high concentrations of MT. Principal component analysis showed that Pn, Tr, Gs, Ci, Chl a, Chl b, Chl a+b, qP, NPQ, Fv/Fm, YII, REC, and MDA were representative physiological indices of exogenous MT on pomegranate seedlings under salt stress. 【Conclusion】 Salt stress induced several detrimental effects on pomegranate seedlings, including the restriction of photosynthesis and fluorescence mediation, a decrease in antioxidant enzyme activities, and oxidative damage to cell membranes. Following the exogenous application of melatonin (MT) treatment, there was a significant enhancement in the photosynthetic parameters and chlorophyll content of the pomegranate seedlings. Additionally, the intercellular CO2 concentration was reduced, and stomatal conductance was increased, thereby improving the photosynthetic rate and the photochemical efficiency of photosystem II (PS II). The antioxidant capacity of pomegranate seedlings was augmented through the upregulation of antioxidant enzyme activities, resulting in a significant reduction in relative electrolyte conductivity (REC) and malondialdehyde (MDA) levels. This reduction provided protection to the seedlings against membrane peroxidation damage induced by salt stress. A comprehensive evaluation using the rank sum ratio method indicated that exogenous melatonin (MT) concentrations of 200 μmol·L-1 and 100 μmol·L-1 were most effective in mitigating salt stress injury in Tunisian soft-seeded pomegranate and Yecheng acid pomegranate, respectively.
PDF （）