Mitigation of PEG-induced drought stress in rapeseed (Brassica rapa L.) by exogenous application of osmolytes

Considering the adverse effect of drought on plants growth, development and physiology, we studied the protective role of three osmolytes, proline (Pro), glycine betaine (GB) and trehalose (Tre) for the mitigation the adversity of drought on rapeseed (Brassica rapa L. cv. BARI Sharisha-15) seedlings. The experiment consisted of eight treatments viz. control, Pro (0.5 mM), GB (0.5 mM), Tre (0.5 mM), Drought (20% Polyethylene glycol, D), Pro + D, GB + D and Tre + D. Reduced plant height, seedling fresh and dry biomass, relative water content (RWC) of leaf and chlorophyll (chl) contents were observed due to drought stress with increased levels of endogenous Pro, GB, and Tre. Drought stress also induced oxidative stress, manifested by the overgeneration of toxic superoxide (O-2(center dot-)) radicals, increased hydrogen peroxide (H2O2), malondialdehyde (MDA), and methylglyoxal (MG) content, as well as impaired antioxidant defense and redox balance. However, pretreatment with Pro, GB and Tre upregulated the antioxidant defense mechanisms and reduced oxidative damage by reducing MDA, H2O2 and MG contents. Moreover, increased the glutathione (GSH), and decreased oxidized glutathione (GSSG) levels, finally improved the redox balance. In addition, exogenously applied osmolytes improved the chl contents and water balance, which improved the rapeseed seedlings development. Among the three osmolytes studied, Pro performed best in alleviating drought stress in rapeseed at the early seedling stage.