MITIGATION OF DROUGHT STRESS IN RAPESEED (Brassica campestris L.) BY EXOGENOUS APPLICATION OF OSMOLYTES

Abstract

The experiment was conducted at the Laboratory of Plant Stress Responses, Faculty of Agriculture, Kagawa University, Japan, during June, 2016 to December, 2016 to investigate the relative protective effects of three osmolytes, proline (Pro), glycine betaine (GB) and trehalose (Tre) in mitigating the adverse effects of drought stress on rapeseed seedlings. The experiment was carried out with Brassica campestris L. cv. BARI Shariasha-15 and it consisted of eight treatments viz. control (well watered), Pro (0.5 mM), GB (0.5 mM), Tre (0.5 mM), Drought (20% Polyethylene glycol, D), Pro+D, GB+D and Tre+D. The experiment was laid out in a Completely Randomized Design (CRD) with three replications. In the present study, reduced plant height, fresh weight and dry weight of seedlings were observed under drought stress condition, compared to control. Leaf relative water content (RWC) and chlorophyll (chl) contents were also reduced due to drought stress with increased levels of endogenous Pro, GB and Tre. Enhanced levels of the malondialdelyde (MDA) and hydrogen peroxide (H2O2) were evident with the higher superoxide (O2•−) generation under the same water deficit condition. Seedlings subjected to drought stress had significantly uplifted levels of ascorbate (AsA), reduced glutathione (GSH), glutathione disulphide (GSSG) and methylglyoxal (MG) whereas the glutathione/glutathione disulphide (GSH/GSSG) ratio was decreased. Activities of several antioxidative enzymes were increased viz. ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), catalase (CAT), glutathione peroxidase (GPX), glyoxalase I (Gly I) but glyoxalase II (Gly II) showed lesser activity in response to drought stress. However, exogenous application of Pro, GB and Tre with drought stress increased the GSH content and decreased the GSSG content and finally improved the GSH/GSSG ratio. Pre-treatment with Pro, GB and Tre reduced oxidative stress by decreasing MDA, H2O2 and MG contents and by upregulating the antioxidant enzymes activities. Exogenous protectants further improved the chl contents and water status of rapeseed seedlings which finally improved the growth of the plants. Among the three protectants, proline (Pro) showed the best performance in alleviating drought stress in rapeseed compared to glycine betaine (GB) and trehalose (Tre).