Pretreatment with different molecular weight chitosans encourages drought tolerance in rice (Oryza sativa L.) seedling

Drought is a critical environmental constraint limiting plant growth and productivity. Chitosan has been utilized as a potential biostimulant and proven to be effective against drought stress in many plant species. The objective of this study was to determine the effects of pretreatment with different molecular weight (MW) chitosans on some physiological characteristics of rice seedlings under drought stress. Rice seedlings were treated with low (50-190 kDa), medium (190-310 kDa) and high (310-375 kDa) MW chitosans by seed priming and foliar spray. The seedlings were subjected to drought by withholding water for four days. The relative water content (RWC) was reduced from 93% in the control plants to 74% in the droughted plants. The results revealed that treating with chitosan, especially with low MW chitosan, promoted root growth under drought stress. All chitosan treatments led to higher relative water content and photosynthetic pigment under drought condition. Pretreatment with chitosan also induced sugar accumulation, and treating with low MW chitosan significantly increased starch accumulation under drought stress. In addition, chitosan treatments alleviated the effects caused by drought stress as represented by the decreases in electrolyte leakage, malondialdehyde (MDA) as well as hydrogen peroxide (H2O2), corresponding with the increases in activities of antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX) activity.