Effects of exogenous chitosan on antioxidant system in chloroplast of vegetable soybean under NaCl stress

Objectives Chloroplasts are plant organelles that produce reactive oxygen species (ROS). Chloroplasts are extremely sensitive to salt stress. In order to investigate the mechanism of chitosan on regulating photosynthesis under NaCl stress, the improvement of antioxidant system in the chloroplasts by applying exogenous chitosan on soybean Glycine max (L.) Merr. was studied.

Methods A pot experiment was conducted inside greenhouse of the Experimental Base of Inner Mongolia University from April to June 2014. Vermiculite was used as culture substrate, salt sensitive soybean cultivar of ‘Lixiang gaochan 95-1’ (LX) and not tolerant one of ‘Lvling’ (LL) were used as test materials. Four treatments were designed: 1) Soybean leaves were sprayed with water, and roots fed with the nutrient solution (CK); 2) Only the leaves were sprayed with chitosan solution, and the roots fed with nutrient solution; 3) Leaves were sprayed with water, and the roots were fed with nutrient solution containing NaCl; 4) The leaves were sprayed with chitosan solution, and the roots were fed with nutrient solution with NaCl.

Results Under NaCl stress and exogenous chitosan, the chloroplast H₂O₂ content of two varieties was significantly decreased, while the activities of peroxidase (POD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydro ascorbic acid reductase (DHAR), glutathione reductase (GR) and glutathione peroxidase (GPX) were increased markedly, meanwhile the dehydroascorbic acid (AsA) and dehydroascorbic glutathione (GSH) contents during the mid-term of NaCl stress were also increased. Salt-resistant cultivar LL maintained lower H₂O₂ content, and higher DHAR activity and AsA content in the mid and late period (6-15 d) of NaCl stress, maintained higher APX, GR and GPX activities in the whole period, and maintained higher GSH content in the late period (12 d, 15 d) of the stress, as compared with those of salt-sensitive cultivar LX.

Conclusions Exogenous chitosan supply significantly induces POD activity and AsA-GSH cycle in the chloroplast under NaCl stress. The AsA-GSH cycle in salt tolerant cultivar LL maintain relatively stronger activity than that in salt sensitive cultivar in reactive oxygen scavenging, which might explain a reason for salt tolerant cultivar to maintain strong photosynthetic capacity, and be capable of maintaining active growth of soybean.