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Effects of flooding on the activities of some enzymes of activated oxygen metabolism, the levels of antioxidants, and lipid peroxidation in senescing leaves of tobacco were investigated . As judged by the decrease in chlorophyll and protein levels, flooding accelerated the senescence of tobacco leaves . Total peroxide and the lipid peroxidation product, malondialdehyde, increased in both control and flooding-treated leaves with increasing duration of the experiment . Throughout the duration of the experiment, flooded leaves had higher levels of total peroxide and malondialdehyde than did control leaves . Flooding resulted in an increase in peroxidase and ascorbate peroxidase activities and a reduction of superoxide dismutase activity in the senescing leaves . Glycolate oxidase, catalase, and glutathione reductase activities were not affected by flooding . Flooding increased the levels of total ascorbate and dehydroascorbate . Total glutathione, reduced form glutathione, or oxidized glutathione levels in flooded leaves were lower than in control leaves during the first two days of the experiment, but were higher than in control leaves at the later stage of the experiment . Our work suggests that senescence of tobacco induced by flooding may be a consequence of lipid peroxidation possibly controlled by superoxide dismutase activity . Our results also suggest that increased rates of hydrogen peroxide in leaves of flooded plants could lead to increased capacities of the scavenging system of hydrogen peroxide .Abbreviations : GSH = reduced form glutathione ; GSSG = oxidized form glutathione ; GSSG reductase = glutathione reductase ; MDA = malondialdehyde; SOD = superoxide dismutase . 37

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The effect of flooding on the contents of chlorophyll, protein and starch and the activity of aamylase in tobacco leaves was investigated. Flooded leaves had lower protein and chlorophyll contents than control plants. Chloroplasts in leaves of tobacco whose roots were flooded for 4 days showed a significant decrease in the level of starch and dilation of thylakoids in chloroplasts when examined with the electron microscope. The decrease in the level of starch in flooded leaves was confirmed by chemical analysis of starch level and was found to be associated with an increase in the activity of a-amylase.