Studies on the Physiological Mechanism of Brassinolide to Improve the Resistance of Rice Seedlings to NaCl Stress

In order to explore the effects of NaCl stress on the growth and physiological characteristics of rice seedlings and the mitigation effects of brassinolide (BR), a pot experiment was conducted with rice cultivar "IR29" under 3 g.L-1 NaCl stress. The effects of soaking with BR on the growth and physiological characteristics of rice seedlings at the 2.5 leaves stage and 4.5 leaves stage under NaCl stress were studied. The results showed that NaCl stress inhibited the growth and development of rice, and had adverse effects on the photosynthesis and resistance physiology of rice seedlings. The inhibition of NaCl stress on the seedling growth of rice could be alleviated in different degrees by soaking with BR. Compared with a single NaCl treatment, the stem base width, aboveground dry weight, and underground dry weight of rice seedlings at different stages were increased after soaking with BR at 0.1 mg.L-1. Soaking with BR increased the activities of super-oxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), and phenylalanine ammonia-lyase (PAL) as well as the levels of ascorbic acid (AsA) in the leaves of rice seedlings under NaCl stress were increased. It inhibited the accumulation of reactive oxygen species (ROS), reduced the content of malondialdehyde (MDA), and increased the soluble protein content. The MDA and H2O2 contents in rice leaves decreased by 23.6%, 10.8% and 19.9%, 1.4%, respectively. Treatment of soaking with BR increased the chlorophyll content and Pn of rice seedlings at the 2.5 leaves stage and 4.5 leaves stage under NaCl stress, the Pn increased by 27% and 10.06%, respectively. In summary, exogenous BR effectively improved the salt tolerance of rice seedlings, mainly because BR can improve the osmotic adjustment and antioxidant capacity, removed excess ROS, and alleviated oxidative damage and osmotic stress, thereby improving the photosynthesis of rice seedlings and alleviating the damage of NaCl stress to rice seedlings.