Effect of silicon addition on the antioxidant system and ultrastructure of alfalfa under high-temperature stress

Current research reports that silicon addition plays an important role in alleviating high temperature stress in plants, and alfalfa is a high-quality legume forage whose growth and yield are severely reduced under high temperature stress. Studying the effect of silicon addition on alfalfa growth under high-temperature stress and its related mechanism is of great significance for improving yield. The treatments consisted of planting alfalfa in pots under two silicon concentrations (0 mmol· L⁻¹, labeled Si₀; 2 mmol· L⁻¹, labeled Si₂) and two temperatures (optimal temperature, 20 ℃/25 ℃; high temperature, 35 ℃/40 ℃). The adaptation mechanism for silicon to alleviate alfalfa under high temperature stress was studied from two aspects: antioxidant system and leaf ultrastructure. The results demonstrate that the biomass and plant height; chlorophyll characteristics, such as chlorophyll a, chlorophyll b, and total chlorophyll content; actual photosynthetic efficiency of photosystem Ⅱ; and maximum photosynthetic efficiency of leaf photosystem Ⅱ were significantly reduced under high-temperature stress (P < 0.05). These parameters were significantly improved by the addition of silicon. In contrast, the relative conductivity, and superoxide dismutase (SOD) and catalase (CAT) activities decreased significantly under high-temperature stress. SOD and CAT activities significantly increased by the addition of silicon, but the relative conductivity was significantly reduced (P < 0.05). In addition, the high-temperature stress forced the cell membrane and chloroplast thylakoids of the leaves to different degrees of damage. The number of plastid globules increased, while the leaf cells with silicon adding were complete; the chloroplast thylakoids were in good shape, with only a small number of plastid globules. The addition of silicon can promote alfalfa growth, increase the activity of antioxidant enzymes, maintain the stability of the micromorphological structure of leaves, and improve the photosynthetic ability under high-temperature stress, thereby improving alfalfa heat tolerance.