Effects of chitosan oligosaccharides on watermelon seedlings with different salt tolerance under NaCl stress

Abstract: 【Objective】Chitosan oligosaccharides (COS) can induce plant resistance to biotic stress and abiotic stress, and the effects of COS on physiological characteristics of watermelon seedlings with dif- ferent salt tolerance under NaCl stress were investigated, in order to provide a theoretical basis for wa- termelon resistance breeding and watermelon cultivation in saline-alkali land【. Methods】Two watermelon materials，‘HQ-2’and‘HQ-3’，were used to study the effects of COS on the dry and fresh mass, photosynthetic pigment content, SOD, CAT, POD activity and MDA content of watermelon seedlings in different treatments‘. HQ-2’is self line material, which is one of the parent materials of the main variet- ies.‘HQ-3’is a double haploid material. The selected materials are homozygous and the characteristics are stable, so as to facilitate the follow-up study of molecular biology. Four treatments were set up, in- cluding CK (water), T1 (180 mmol·L^-1 NaCl), T2 (only COS), T3 (180 mmol·L^-1 NaCl+COS). 30 mL of solution was added for two days after solution treatment, and fresh water was added for control, for a total of 10 days. After 10 days, relevant indexes were determined. Direct measurement of biomass and the contents of photosynthetic pigment, antioxidant enzymes and MDA were completed according to the guidance of plant physiological experiment.【Results】The results showed that the addition of 1 g·L^-1 chitosan oligosaccharide could improve the biomass and the content of chlorophyll a, chlorophyll b and carotenoid of‘HQ-2’. For salt resistant material‘HQ-3’, the fresh and dry weight of root between the three treatments and the control group, has no significant difference. But the fresh weight of root in the control group was the maximum. There was no significant difference in the fresh weight of the above ground among the treatments, and the fresh weight of the above ground of the control was the maxi- mum. The photosynthetic pigment content of‘HQ-2’was significantly lower than that of‘HQ-3’, but there was no significant difference between the treatments. The SOD activity of T1 treatment was signif- icantly higher than that of the other three treatments of‘HQ-2’. Except CK, the activity of SOD activi- ty in other treatments of‘HQ-3’was higher than‘HQ-2’. For the salt resistant material‘HQ-2’, the POD activity of T1 treatment reached the highest of the four treatments, while for the salt resistant mate- rial‘HQ-3’, the POD activity of T2 treatment was higher than the other three treatments. CK had the lowest POD activity. The POD activity of T3 was significantly lower than that of T1. Under T2 treat- ment, POD activity of‘HQ-3’was significantly higher than that of‘HQ-2’. The CAT activity of T1 treatment of‘HQ-2’was higher than CK. Salt stress had an effect on salt intolerant materials, which in- creased the content of H2O2, and then promoted the CAT activity. However, the addition of chitosan oli- gosaccharide under salt stress could inhibit the damage of salt stress. The CAT activity of T2 treatment was significantly lower than that of the other three treatments of‘HQ-3’. But CAT activity of T1 treat- ment was significantly higher than CK. Under T1 treatment, CAT activity of salt resistant material‘HQ- 3’was higher than that of salt resistant material‘HQ-2’. Except T2 treatment, CAT activity of salt re- sistant material‘HQ-2’was higher than‘HQ-3’. The content of MDA in T1 treatment was significant- ly higher than that in other treatments of‘HQ-2’, which was 1.38 times of CK. The content of MDA in T2 treatment was significantly lower than that in T3, and there was no significant difference between CK and T2 treatment. For salt resistant material of‘HQ-3’, the MDA content of T1 and T3 treatments was significantly higher than that of the other two treatments, about 22.1 and 21.6 percentage points higher than CK. The MDA content of T2 was significantly lower than T3. Under T1 and T3 treatment, the MDA content of salt resistant materials was significantly higher than that of salt resistant materials.【Conclusion】The results showed that under 180 mmol·L-1 NaCl stress, 1 g·L-1 chitosan oligosaccha- ride could improve the salt tolerance of watermelon seedlings and alleviate the harm of salt stress to a certain extent. In addition, comparing the watermelon materials with different salt tolerance, it was found that the effect of chitosan oligosaccharide was more obvious to the non salt tolerance materials, and the salt tolerance materials themselves had a certain tolerance to salt stress, but the effect was not significant. The effects of the optimum time and the mechanism of chitosan oligosaccharides in different salt resistant materials need further study.