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

The damage caused by active oxygen generated by paraquat was investiga ted in two wheat cultivars, Triticum aestivum L. Frederick and Glenlea , which differed in their sensitivity to heat and chilling stress. App lication of paraquat induced photoinhibition and loss of fresh mass, p rotein, and membrane integrity in the leaves. The photosynthetic pigme nts were lost after paraquat treatment, with Chlb declining the least and carotenoids declining the most. Paclobutrazol, a triazole plant gr owth regulator, reduced these symptoms and induced a 2-fold increase i n paraquat tolerance in both cultivars. This study tested the hypothes is that paclobutrazol protects wheat from oxidative stress by enhancin g antioxidant enzyme activity. The enzymes investigated were superoxid e dismutase, ascorbate peroxidase, glutathione reductase, as well as c ytosolic catalase and guaiacol peroxidase. Paclobutrazol stimulated an overall increase in antioxidant enzyme activities on both a fresh-mas s and protein basis in the winter wheat cultivar Frederick, but only o n a fresh-mass basis in Glenlea. The spring wheat, Glenlea, which is m ore sensitive to temperature stress, had lower antioxidant enzyme acti vity than the tolerant winter cultivar Frederick. Irrespective of the natural difference in the two cultivars to stress, paclobutrazol eleva ted the plants to a higher level of oxidative stress tolerance.