Changes in physiological and antioxidant activity of indica rice seedlings in response to mannitol induced osmotic stress

Morphological, physiological, and biochemical plant responses were studied in unstressed and stressed seedlings of indiai rice (Oryza saliva L.) genotypes. The effect of NB medium supplemented with a 100 mM mannitol treatment, which induced drought stress conditions, was measured for relative growth rate, cell membrane stability, antioxidant enzyme activity (superoxide dismutase [SOD], catalase [CAT], and peroxidase [POD1), and total antioxidant capacity by DPPH (2,2-d i phenyl-1.-pi crylhydrazyl) and ABTS (2,2'-azinobis[3-ethylbenzothiazoline-6-sulfonic acid] diammonium salt) assays. Results of morphological and physiological factors showed two contrasting rice groups, drought-sensitive and drought-tolerant genotypes. After drought stress, the increased rate of SOD activity was lower in drought-tolerant than in drought-sensitive genotypes, but the increased rate of CAT and POD activity and total antioxidant capacity was higher in drought-tolerant than in drought-sensitive genotypes. Increasing changes in activity levels of antioxidant enzymes and total antioxidant capacity indicated more rapid free radical scavenging compared with relative growth and cell membrane stability in drought-tolerant genotypes under shortterm treatment. The trend toward change in CAT and POD antioxidant enzyme activity and total antioxidant capacity of stressed seedlings, as well as the correlation between these changes, and the morphological and physiological responses (096 correlation coefficient [R-2] between relative growth rate and CAT; 0.93 R-2 between relative growth rate and POD enzyme activity; 0.96 R-2 between relative growth rate and Trolox equivalent antioxidant capacity values) coincided with the degree of drought tolerance,