Lipoic Acid Positively Regulates Tomato Growth and Yield by Improving Organic Osmolytes and Antioxidant Defense System Under Saline Conditions

Lipoic acid (LA) is a naturally synthesized antioxidant in plants, and its protective role against abiotic stresses is well known. However, its protective role on tomato growth and yield under salt stress is not clear. In this study, we explored the impact of different doses of LA (0, 2, 4, and 6 mM) on two tomato cultivars (Nadir and Naqeeb) under saline conditions (0, 120 mM). Tomato seedlings were shifted to plastic pots containing sterile soil. LA was sprayed on tomato foliage after salt stress application. Foliar application of LA significantly enhanced plant growth by improving 34% shoot length (SL), 28% shoot fresh weight (SFW), and 29% leaf area (LA) in Nadir under salt stress. Leaf photosynthetic pigments were improved viz. chlorophyll a (Chl a) by 29%, chlorophyll b (Chl b) by 35%, total chlorophyll (total chl) by 42%, and carotenoids (carot) by 25% in Nadir under salt stress. The improvement in plant growth was related with a decrease in salt-induced oxidative damage like reduction in leaf malondialdehyde (MDA) by 23% in Nadir. Generally, LA application showed higher antioxidant enzyme activities under control and stress conditions. Peroxidase (POD) activity was increased by 11% and superoxide dismutase (SOD) activity by 21% in Nadir under saline condition. Interestingly, LA application helped to reduce sodium (Na+) concentration in leaf (14%) and roots (25%) in Nadir under salt stress. In contrast, potassium (K+) concentration was increased in both roots (48%) and leaves (77%) in Nadir under stress. Moreover, tomato yield was also increased by LA application under stress. These results exhibited that LA application may reduce the negative effect of salt stress on tomato growth and yield by modulating its enzymatic antioxidants and biochemical traits and hence could be used effectively for the induction of salinity tolerance in plants.