Abstract
Salt stress is one of the primary abiotic stresses that negatively affects agricultural production. Melatonin, as a useful hormone in plants, has been shown to play positive roles in crop improvement to abiotic stress conditions. However, it remains unclear whether spraying melatonin could reduce the halotolerance gap between tomato genotypes with different salt sensitivities. Here, plant growth, H2O2 content, electrolyte leakage, antioxidant system, gas exchange, pigment content, and chloroplast ultrastructure of salt sensitive genotype (SG) and resistant genotype (RG) at CK (control), M (spraying melatonin), S (salt), and SM (spraying melatonin under salt stress) were investigated. The results showed that the weight, height, and stem diameter of the plant at SM from both genotypes significantly increased compared with S. The plant undergoing SM from both genotypes showed significantly decreased H2O2 but increased activity of SOD, APX, GR, and GSH, as well as net photosynthetic rate and Fv/Fm, as compared with S. The ratio between SM and S (SM/S) of SG was significantly higher than that of RG in terms of plant height and stem diameter, whereas antioxidant parameters, H2O2 content, and electrolyte leakage showed no difference between RG and SG in SM/S. The SM/S of SG in terms of photosynthetic parameters and pigment content were significantly higher than that of RG. Chloroplast ultrastructure showed remarkable changes under salt stress, whereas spraying melatonin reduced the destruction of chloroplasts, especially for SG. We concluded that spraying melatonin reduces the halotolerance gap between SG and RG by photosynthesis regulation instead of the antioxidant mechanism. This indicated that the positive roles of melatonin on tomato plants at salt stress depend on the genotype sensitivity.
Highlights
Melatonin (N-acetyl-5-methoxytryptamine) is a molecular compound with physiological activity and is widely studied in animals for its important roles
Compared with CK, the fresh weight, dry weight, plant height, and stem diameter for M treatment showed no significant difference in both resistant genotype (RG) and sensitive genotype (SG) (Figure S1a,b,d,e), treatments (CK, S, M, and SM) within genotype
Compared with CK, the H2O2 content and electrolyte leakage for M treatment showed no significant difference in both RG and SG (Figure S2a)
Summary
Melatonin (N-acetyl-5-methoxytryptamine) is a molecular compound with physiological activity and is widely studied in animals for its important roles. Many studies have shown that melatonin can effectively relieve the damage caused by stress in plants and improve plant resistance [3,4,5,6,7]. A recent study found that exogenous melatonin improved salt resistance of tomato seedlings mainly by controlling ROS levels and enhancing photosynthesis via non-stomatal factors [27]. There are no reports about the effects of melatonin on tomato genotypes with different salt sensitivity. The response of sensitive and resistant genotypes to salt treatment in the aspects of plant growth, H2O2 content, electrolyte leakage, antioxidant system, gas exchange, pigment content, and chloroplast ultrastructure was investigated. We will help to understand the effects of melatonin on different tomato genotypes in salinity conditions
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