Activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), as well as malondialdehyde (MDA) contents and solute potentials, were studied in seedlings of seven wheat (Triticum) species (nine genotypes representing three ploidy levels: hexaploid, tetraploid, diploid) subjected to water stress for 4, 8, and 12 days by withholding water. Solute potentials of all genotypes were lowered by water stress. In most species, SOD and CAT activities showed an increase or maintenance in the early phase of drought and then a decrease with further increase in magnitude of water stress. On the contrary, POD activities and MDA contents greatly increased in response to water stress. Enzymatic activities partly recovered and MDA contents decreased with rewatering. Under drought, hexaploid wheats had higher POD activities and MDA contents than tetraploid and diploid wheats; solute potentials and activities of SOD and CAT, however, were similar among the three groups. These results suggest that water stress alters the equilibrium between free radical production and enzymatic defense reactions in wheat species and that hexaploid wheats have less efficient antioxidant systems (e.g., the ascorbate-glutathione cycle and the nonenzymatic system) than tetraploid and diploid wheats.
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