Beneficial Effects of Hemin on Antioxidative Capacity and Anatomical Characters of NaCl-Stressed Rice Plants

Salt stress has become one of the most widespread abiotic stresses globally, negatively affecting crop growth, development, and yield. Rice is an important economic crop affected by salt stress. This study used Huanghuazhan (‘HHZ’) as the test material to investigate the mitigating effect of spraying Hemin (alone) or with Hb and ZnPP (in combination) on the oxidative damage induced by 100 mM NaCl solution. The results showed that Hemin treatment maintained the growth of rice seedlings under NaCl stress and improved shoot (plant height, stem base width, leaf area) and root (root length, root surface area, root volume, average root diameter and number of root tips) morphological parameters. In addition, exogenous Hemin increased root activity, raised the content of various photosynthetic pigments, improved leaf structure, and increased the area of vascular bundles, which sustained photosynthesis and promoted the accumulation of biomass. Furthermore, Hemin reduced the accumulation of malondialdehyde (MDA) and hydrogen peroxide (H2O2), by activating the activities of various antioxidant enzymes and increasing the content of non-enzymatic antioxidants. While ZnPP (a specific inhibitor of heme oxygenase-1 (HO-1)) and Hb (CO scavenger) reversed the positive regulation of Hemin, and the indexes (biomass of rice seedlings, root activity, and the activity of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX)) somewhat decreased. In conclusion, this study demonstrated that Hemin reduced ROS accumulation, maintained leaf structure and photosynthetic pigment content, and mitigated the adverse effects of oxidative damage caused by NaCl stress in rice through improving the antioxidant capacity of leaves and roots. This research provides a theoretical basis for Hemin to regulate salt tolerance in other crops. However, the molecular mechanisms involved in Hemin regulation need to be further explored.