Abstract: Saline-alkali habitats inhibit the normal growth and development of many plant species through ionic toxicity and osmotic stress. Regulation of ion distribution between different tissues is one of the important survival strategies of plants in saline-alkali stress environments. Under this strategy salt is concentrated into particular plant organs so as to reduce the adverse effect of high Na⁺ ion concentration on more important organs. In order to explore the adaptation mechanism of ryegrass under saline-alkali stress, ryegrass plants in hydroponic culture were treated with different saline-alkali Na⁺ concentrations (0, 50, 100, and 200 mmol·L^-1). The growth, ion absorption, transport selectivity and ion compartmentalization were studied. Results showed that: 1) With increasing saline-alkali stress, the relative water content and chlorophyll content of the ryegrass plants showed a decreasing trend, and the electrolyte leakage rate increased 28.6%. The root length, plant height and biomass were significantly decreased at 200 mmol·L^-1, suggesting that the salt tolerance threshold of ryegrass was between 100 and 200 mmol·L^-1; 2) When the saline-alkali Na⁺ concentrations was 50 mmol·L^-1, the K⁺/Na⁺ in the roots increased by 48.3%, and the Ca²⁺/Na⁺ increased by 54.1% without major change in Na⁺ concentration, indicating that the ryegrass roots alleviated the Na⁺ toxicity by increasing the absorption of K⁺ and Ca²⁺ ; 3) Under 50-200 mmol·L^-1 concentrations, the relative contents of Na⁺ in the soluble components and cell wall in ryegrass leaves were increased compared to the control, while the relative content of Na⁺ in cell organelles was decreased, indicating that Na⁺ was compartmentalized in leaf cell vacuoles and cell walls to reduce the toxic effects of Na⁺ on organelles; 4) Scanning electron micrographs of the leaf cross-section showed that the leaf tissue structure changed significantly under saline-alkali stress. Notable changes were thickening of the leaf epidermis and reduction in the number of vessels and in the diameter of the vessels. In summary, the roots in ryegrass plants increased absorption of K⁺ and Ca²⁺ to alleviate the toxicity caused by excessive Na⁺, while changes in microstructure and Na⁺ compartmentalization in leaves of ryegrass plants protected intracellular organelles from ion toxicity. This study provides scientific details of the adaptation mechanisms of ryegrass to environmental saline-alkali stress.

Key words: saline-alkali stress, ryegrass, micro-distribution of ions