Objectives Exogenous chitosan could increase the nodulation of vegetable soybean under salt stress. We studied the root hair growth and the accumulation of endogenous IAA in root tip of vegetable soybean under NaCl stress, to provide new clues to further understand study of the promotion mechanism of chitosan.
Methods Vermiculite cultivation was adopted, the test material was the symbiosis of vegetable soybean variety ‘Green Collar VIII’- slow-growing rhizobium USDA 122. Four treatments were set up in the experiment: nitrogen-free nutrient solution was watered on the roots, distilled water was sprayed on the foliage (CK), and 200 mg/L chitosan aqueous solution was sprayed (T1); N-free nutrient solution containing 50 mmol/L NaCl was watered on the roots, and distilled water was sprayed on the foliage (T2); 200 mg/L chitosan aqueous solution was sprayed (T3). At 2, 3, 4, and 5 days after NaCl stress treatment, the number and morphology of seedling root hairs and the number of deformed root hairs were counted, and the endogenous IAA content, indole acetic acid oxidase (IAAO) and peroxidase (POD) activities of the root tips, as well as the malondialdehyde (MDA) content of the root system were analyzed. The number of nodules, fresh weight of rhizomes, nitrogen content of plants and dry weight of plants were determined at 30 days of NaCl treatment.
Results 1) Exogenous chitosan significantly increased the nodule number, nodule weight, plant N content, and plant dry weight of vegetable soybeans by 84%, 151%, 20%, and 54% under NaCl stress compared to T2, and by 38%, 18%, 12%, and 7% under no stress compared to CK. 2) Exogenous chitosan significantly increased the total number of root hairs, root hair density, root hair length, number of deformed root hairs, and endogenous IAA content in root tips regardless of NaCl stress or not. 3) Exogenous chitosan significantly shortened the initial distance of root hairs, reduced the IAAO and POD activities in root tips, and reduced the MDA content in roots under NaCl stress.
Conclusions Under NaCl stress, exogenous chitosan could reduce the oxidative decomposition of IAA by decreasing the activities of IAAO and POD, maintain the relative stability and integrity of the biofilm by alleviating the lipid peroxidation of the root membrane, promote the polar transport of IAA to the root tip, increase the content of endogenous IAA in the root tip, and induce the formation and growth of root hairs, resulting in the increased number of deformed root hairs, which may be one of the important mechanisms for chitosan to improve the tumor-forming capacity of vegetable soybean.
DownLoad: