A pot experiment was conducted to study the oxidative response of drought-stressed black rice to foliar application of pyridoxine (0.03mM) and nicotinic acid (1.5mM). Drought stress was imposed by withholding water for 10 days, until leaf rolling symptoms was observed. After ten days from imposition of drought stress, water deficit resulted in a significant increase in H2O2 content thereby increased lipid peroxidation product; malonaldhyde (MDH) accompanied by high rate of ion leakage (IE) and consequently reduced the relative water content (RWC) compared with their corresponding controls. Drought stress resulted in a considerable increase in the activities of phenol peroxidase (POX), ascorbate oxidase (ASO), and catalase (CAT) enzymes of both shoots and roots of black rice seedlings, while the activities of glutathione reductase (GR), polyphenol oxidase (PPO) and superoxide dismutase (SOD) significantly decreased. Foliar application of pyridoxine or nicotinic acid mitigated the injurious effects of drought stress through the significant reduction in ion leakage, reducing membrane peroxidation, high ratio of chlorophylls/carotenoids thus restoring the membrane integrity and enhancing the defense system in terms of ascorbic acid, glutathione and total phenol contents as well as stimulating the activities of some antioxidant enzymes (GR, SOD, and POX). It may be suggested that the enhancement of the antioxidants in black rice seedlings is one of the mechanisms of drought tolerance under stress condition.
Key words: Black rice, Drought, Antioxidants, Pyridoxine, Nicotinic acid
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