PACLOBUTRAZOL-INDUCED TOLERANCE OF WHEAT LEAVES TO PARAQUAT MAY INVOLVE INCREASED ANTIOXIDANT ENZYME-ACTIVITY

The damage caused by active oxygen generated by paraquat was investigated in two wheat cultivars, Triticum aestivum L. Frederick and Glenlea, which differed in their sensitivity to heat and chilling stress. Application of paraquat induced photoinhibition and loss of fresh mass, protein, and membrane integrity in the leaves. The photosynthetic pigments were lost after paraquat treatment, with Chlb declining the least and carotenoids declining the most. Paclobutrazol, a triazole plant growth regulator, reduced these symptoms and induced a 2-fold increase in paraquat tolerance in both cultivars. This study tested the hypothesis that paclobutrazol protects wheat from oxidative stress by enhancing antioxidant enzyme activity. The enzymes investigated were superoxide dismutase, ascorbate peroxidase, glutathione reductase, as well as cytosolic catalase and guaiacol peroxidase. Paclobutrazol stimulated an overall increase in antioxidant enzyme activities on both a fresh-mass and protein basis in the winter wheat cultivar Frederick, but only on a fresh-mass basis in Glenlea. The spring wheat, Glenlea, which is more sensitive to temperature stress, had lower antioxidant enzyme activity than the tolerant winter cultivar Frederick. Irrespective of the natural difference in the two cultivars to stress, paclobutrazol elevated the plants to a higher level of oxidative stress tolerance.