Chitosan mitigates the adverse effects and improves photosynthetic activity in rice (Oryza sativa L.) seedlings under drought condition

The ability of chitosan to promote rice growth, physiological traits, and photosynthetic performance in rice seedlings under drought stress was investigated. Rice seedlings (Oryza sativa L. cv. KDML105) were treated with 100 mg l⁻¹ low and high molecular weight (MW) chitosan via a combination of seed priming and foliar spray. The seedlings were subjected to drought stress by withholding water for 4 days, which resulted in the U-shaped (scale 5) leaf rolling. The results showed that drought significantly decreased shoot and root growth. Chitosan application, particularly with high MW chitosan, improved shoot and root growth under drought stress. Chitosan treatment also alleviated the effects of drought stress by elevating relative water content, as well as reducing electrolyte leakage and malondialdehyde content. Antioxidant enzyme activities, including guaiacol peroxidase (GPX) and ascorbate peroxidase (APX), were increased in response to chitosan. Additionally, treating the plant with chitosan improved photosynthetic efficiency as evidenced by increased CO₂ response, the maximum rate of Rubisco carboxylase activity (Vc_max), and photosynthetic rate. We conclude that the exogenous application of chitosan aids the plant in coping with the severity of drought stress. While both low and high MW chitosans were effective at alleviating the effect of drought on rice seedlings, high MW chitosan might have a slight advantage with respect to increased effective duration.