Effects of small amounts of NaCl on alleviating damage caused to the photosynthetic activity of alfalfa seedling roots and leaves by KCl stress

Salinized soils in Xinjiang are rich in various types of salt ions, which significantly affect the growth of alfalfa (Medicago sativa) and restrict the development of the alfalfa industry in Xinjiang. In this study, the effects of different ratios of mixed sodium and potassium salts on the root growth and photosynthetic activity of alfalfa seedlings were investigated. The results showed that the electrical conductivity and malondialdehyde and proline levles of alfalfa roots and young leaves gradually increased while root vigor, chlorophyll content, and the maximum photochemical efficiency (F_v/F_m) of the leaves gradually decreased with increasing concentrations of NaCl and KCl alone, and the decrease was greater in the treatment with KCl alone at the same concentration. This suggests that salt stress disrupted the cell membrane integrity of alfalfa seedlings, leading to severe membrane lipid peroxidation, and that the osmoregulation of the cells itself reduced root vigor and photosynthetic efficiency, causing the seedlings to be more damaged by KCl stress. However, after a small amount of NaCl was added to the high concentration KCl treatment, the changes in the above indicators decreased compared with those observed in the group treated with KCl alone, which indicated that a small amount of Na⁺ ions alleviated the damage to the cell membrane caused by K⁺ ion stress, reduced the oxidative damage to the cell, and increased osmoregulation, thus alleviating the effect on root vigor. In addition, a small amount of NaCl alleviated the oxidative damage to young leaves under KCl stress and reduced the degradation of chlorophyll; thus, maintaining the photosynthetic activity of the leaves.