Characterization of Leaf Gas Exchange and Anti-oxidant Defense of Rice (Oryza sativa L.) Cultivars Differing in Submergence Tolerance Owing to Complete Submergence and Consequent Re-aeration

The impact of submergence on the photosynthesis and antioxidant capacities in three Indica rice cultivars namely FR 13A, IR 42, and Sabita having differential response to flooding were examined under 8 days (d) of complete submergence and subsequent re-aeration for 24 h. All upon three genotypes showed inhibition of CO2 photosynthetic rate due to complete submergence; this was accompanied with the decrease in the Rubisco activity, stomatal conductance (gs), and leaf chlorophyll (Chl) content. These parameters decreased further after 24 h of re-aeration. The decrease was more pronounced in sensitive cultivar, IR 42, and elongating type cultivar, Sabita. Sensitivity of elongating type cultivar under complete submergence was alike to submergence sensitive type cultivar. The activities of antioxidant enzymes such as superoxide dismutase, catalase, guaiacol peroxidase, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase and levels of ascorbate were found to be significantly high in tolerant cv. FR 13A during submergence and subsequent re-aeration period. This study indicates that the suppression of CO2 photo-assimilation under submergence was not only due to decrease of gs and leaf chlorophyll content but mainly due to the inhibition of rubisco activity. A greater protection of the photosynthetic system was observed through antioxidant system, since the degree of protection was more, when the levels of ascorbate and the activities of antioxidant enzymes were high.