Drought stress is the most pervasive threat to sustainable rice production and mainly disrupts membrane structure and cell-water relations. Exogenously applied brassinosteroids (BRs) may produce profound changes that may improve drought tolerance in rice. In this study, we monitored some physiological basis of the exogenously applied BRs in improving drought tolerance in fine grain aromatic rice (Oryza sativa L.). Two BRs i.e. 28-homobrassinolide (HBL) and 24-epibrassinolide (EBL) were used both as seed priming and foliar spray. To prime, the seeds were soaked in 0.01 μm aerated solution each of HBL and EBL for 48 h and dried back to original weight. Treated and untreated seeds were sown in plastic pots with normal irrigation in a phytotron. At four-leaf stage (3 weeks after sowing), plants were subjected to drought stress at 50 % field capacity by cutting down the water supply. For foliar spray, 0.01 μm of HBL and EBL solutions were sprayed at five-leaf stage. Drought stress severely reduced fresh and dry weights, whilst exogenously applied BRs improved net CO2 assimilation, water use efficiency, leaf water status, membrane properties, production of free proline, anthocyanins, soluble phenolics, but declined the malondialdehyde and H2O2 production. In conclusion, BRs application improved the leaf water economy and CO2 assimilation, and enabled rice to withstand drought. Moreover, foliar spray had better effect under drought than seed treatments and of the two BRs, EBL proved more effective.
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