Study on the Relationship of Ions (Na, K, Ca) Absorption and Distribution to Photosynthetic Response of Salix matsudana Koidz Under Salt Stress

To identify the key indicators for salt tolerance evaluation of Salix matsudana Koidz, we explored the relationship of ion absorption and distribution with chlorophyll, fluorescence parameters (leaf performance index, maximum photochemical efficiency), and photosynthetic gas parameters (net photosynthetic rate, transpiration, stomatal conductance, intercellular carbon dioxide concentration) under salt stress. We established 4 treatment groups and one control group based on salinity levels of NaCl hydroponic solutions (171, 342, 513, and 684 mm). The Na+/K+, Na+/Ca2+, chlorophyll fluorescence, and photosynthetic parameters of leaves were measured on the 1st, 3rd, 5th, 8th, 11th, and 15th days to analyze the correlations of chlorophyll, chlorophyll fluorescence and photosynthetic parameters to the ion distribution ratio. The results showed that (1) The ratio of the dry weight of roots to leaves gradually increased with increasing salt concentration, whereas the water content of leaves and roots first increased and then decreased with increasing time. (2) The content of Na+, Na+/K+, and Na+/Ca2+ in roots and leaves increased with increasing salt stress concentration and treatment time, and the difference gradually narrowed. (3) Ca2+ was lost more than K+ under salt stress, and Na+/Ca2+ was more sensitive to the salt stress environment than Na+/K+. (4) Because the root system had a retention effect, both Na+/K+ and Na+/Ca2+ in roots under different NaCl concentrations and different treatment times were higher than those in leaves, and Na+/Ca2+ was much higher than Na+/K+ in roots. (5) Na+/K+ had a higher correlation with fluorescence parameters than Na+/Ca2+. Among them, Na+/K+ had a significantly negative correlation with the maximum photochemical efficiency, and the correlation coefficient R-2 was 0.8576. (6) Photosynthetic gas parameters had a higher correlation with Na+/Ca2+ than with Na+/K+. Among them, significantly negative correlations were noted between Na+/Ca2+ and Gs as well as between Na+/Ca2+ and E under salt stress. The correlation between Na+/Ca2+ and Gs was the highest with a correlation coefficient of 0.9368. (7) Na+/K+ and Na+/Ca2+ had no significant correlation with chlorophylls. Na+/Ca2+ was selected as a key index to evaluate the salt tolerance of S. matsudana Koidz, and the results provided a reference for analyzing the relationship between ion transport and distribution for photosynthesis.