Effects of chitosan on nodulation and nitrogen fixation of vegetable soybean under NaCl stress

Objectives The effect of stimulating growth and tolerance to salt stress of crop through foliar spray of chitosan was confirmed in this paper.

Methods Vermiculite culture method was conducted inside plant growth chamber, vegetable soybean Glycinemax (L.) Merr. ‘Tezaowang’ and the rhizobium N18 were used as the test materials in the symbiotic system of vegetable soybean-rhizobium. Salt stress was prepared by adding 50 mol/L of NaCl in nitrogen-free nutrient solution and chitosan (CTS) spraying concentration was 200 mg/L in the treatments. Four treatments were setup including spraying water and no salt stress control (CK), spraying CTS under no salt stress (CTS); spraying water under salt stress (Cl) and spraying CTS under NaCl stress (CTS +Cl). After 30 days treatment, the plants were harvested, and the activities of nitrogenase, the number and fresh weight of nodules were measured immediately. Then the root activity and leghemoglobin content were determined, and the plant dry biomass and total N content were tested at last.

Results Without NaCl stress, spraying CTS did not increase the dry plant biomass significantly; Under NaCl stress, the fresh biomass was significantly decreased, and spraying CTS narrowed the decrease significantly but still lower than that in CK (P < 0.05). Without NaCl stress, the nodule number, nodule fresh weight, total N content, root activity, leghemoglobin content and nitrogenase activity of vegetable soybean were significantly increased by spraying CTS. Under NaCl stress, the nodulation and nitrogen fixation of vegetable soybean were significantly inhibited, while spraying CTS significantly increased the nodule umber, nodule fresh weight, total nitrogen content, root activity, leghemoglobin content and nitrogenase activity of vegetable soybean under salt stress, with the increase of 29%, 20%, 17%, 48%, 19%, 21% of the CK respectively, although they were still significantly lower than CK.

Conclusions Under normal condition, spraying chitosan shows significant effect in promoting the nodulation, enhancing the leghemoglobin content and nitrogenase activity, and increase plant growth as a result. Under NaCl stress, spraying chitosan could significantly increase the tolerance of crop to salt stress, although could not offset the diverse effect of salt stress completely. Therefore, foliar application of exogenous chitosan is recommended in the production of legumes.