Effects of the degree of grassland desertification on water distribution strategy and resistance physiology of Astragalus bhotanensis

Four varying degrees of desertification, i.e., non-sandy (ND), light (LD), moderate (MD), and heavy (HD) desertification are observed in a grasslands of Hongyuan County at an altitude of 3497 m. Astragalus bhotanensis, which in its flowering and fruiting stage exhibits supplementary sowing and natural succession for 3 years, was used for research. The water content and ratio of components, relative water content, and water saturation deficit, as well as the chlorophyll content and the content of resistant substances, such as superoxide dismutase (SOD), malondialdehyde (MDA), and total antioxidant capacity (T-AOC), was determined to analyze the different effects of the degree of desertification on water absorption, and to study the water use strategy and physiological response mechanism of A. bhotanensis to the varying degrees of grassland desertification. The results showed that 1) soil desertification affected the component water content in the following order: root > leaf > stem > fruit > flower, and the effect on distribution ratio was as follows: leaf > root > stem > fruit > flower. LD reduced stem water content, and MD reduced leaf water content, but water was distributed preferentially in roots, flowers, and fruits, so that the ratio of root cap and reproductive water increased with the increase of desertification degree (P < 0.05). 2) The soil desertification leads to component water saturation deficit in the following order: fruit > stem > leaf > flower > root. Fruit water saturation deficit is the most sensitive to desertification, and root water saturation deficit is not generally affected by desertification. 3) The effects of soil desertification on chlorophyll were Chl b > CHL (a + b) > Chl a/b > Chl a. LD only decreased Chl b, MD simultaneously decreased Chl a and Chl b, HD rapidly decreased Chl b, and desertification decreased CHL (a + b) and increased Chl a/b. 4) The antagonistic physiological effects of soil desertification were SOD > MDA > T-AOC > H₂O₂. The greater the degree of desertification, the higher the SOD activity and MDA content, which stabilized the H₂O₂ content, but reduced the T-AOC activity. In conclusion, A. bhotanensis exhibits an efficient water use strategy and physiological response corresponding to the degree of desertification, which can be popularized and applied as a sand-resistant and sand-fixing grass species in alpine grassland regions.