Objective Tolerance to short-term salt stress and determinant indicators for the property of varieties of Azolla plants were studied for saline-alkali land restoration and sustainable agriculture.

Methods Nine strains of Azolla of different subgenera and resource types were subjected to 0.8% NaCl stress for 6 d to measure the relative growth rate (RGR), salt tolerance index (STI), and physiological factors related to photosynthesis, nitrogen fixation, osmotic regulation, and oxidative stress of the plants. Key indicators that determined the salt tolerance were identified by correlating them to the growth of the aquatic ferns as well as analyzing the principal components and clustering of the tested factors.

Results The 9 strains exhibited significantly different degrees of tolerance to the salt treatment. A. caroliniana Guilin Guanyang, A. filiculoides Luxi Luxing, and the Interspecific Hybrid 8010 had high salt tolerance index (STI) and were classified as salt-resistant strains, and A. mexicana YAHZ and A. pinnata 7001 with low STI as salt-sensitive strains. The salt-resistant azolla plants under the salt stress would maintain a constant dry weight RGR with stable functions on the synergistic photosynthesis, nitrogen fixation, and osmotic regulation without much oxidative damage. Fresh weight RGR (RCR_RGRFW), chlorophyll content under salt stress (Chl-SS), and nitrogenase activity (RCR_Nit) were the critical indicators for salt tolerance evaluation in Azolla. The cumulative variance contribution of principal components reached 67.70%. And the clustered indicators effectively distinguished the salt tolerance and physiological responses of the various Azolla strains.

Conclusion By using the short-term salt stress test, 9 Azolla strains of different varieties could be evaluated for their resistance to the salt stress based on 9 quantifiable indicators of the plants.