Effects of oxidative stress induced by paraquat (PQ) on two wild wheats (Aegilops biuncialis and Aegilops cylindrica) and four wheat cultivars (Triticum aestivum L. cv. Bezostaya-1, Hawk, Triticum turgidum L. ev. Kiziltan-91, and Harran-95) were determined. Eight-day-old seedlings of primary leaves were exposed on surface application to different concentrations (0, 15, 30, and 60 mu M) of PQ. The results of chlorophyll a fluorescence indicated that increasing the PQ concentrations caused higher photooxidation in wild wheats than cultivated wheats. The quantum efficiency of PSII open centres in dark-adapted and in light-saturated state, excitation trapping efficiency of PSII, and electron transport rate decreased significantly with increasing the PQ concentrations in both the wheat types but to a higher degree in wild wheats and Hawk. In addition, high concentration of PQ brought about chlorophyll (a + b) and carotenoid loss, and membrane leakage in all genotypes. The higher increase in MDA content of wild wheats and Hawk compared to cultivated wheats was due to the increase in peroxidation of membrane lipids. On the other hand, relative water content of leaves for all genotypes decreased significantly by PQ treatments, but this effect was not found to be significant in Harran-95 except 60 mu M. Analysis and evaluation of these parameters against oxidative stress induced by PQ allowed classification of the genotypes to three tolerance levels: tolerant (cultivated wheat, Harran-95), moderately tolerant (Kiziltan-91 and Bezostaya-1), and sensitive (wild wheats, Ae. cylindrica and Ae. biuncialis, and cultivated wheat Hawk). The antioxidant enzyme response to PQ in tolerant wheat Harran-95 and sensitive Ae. cylindrica was analysed to investigate the possible occurrence of PQ tolerance mechanism. PQ treatment caused a significant activation of all antioxidant enzymes in both genotypes. However, total SOD. APX, and POD activities of Harran-95 were significantly higher than those of Ae. cylindrica in all PQ treatments. PQ stress did not cause oxidative damage to Harran-95 as verified by the electrolyte leakage and MDA content due to endogenous protective mechanisms. In the case of Ae. cylindrica, PQ induced considerable oxidative stress and possibly caused some oxidative damage, despite the upregulation of protection mechanisms. (c) 2005 Elsevier Inc. All rights reserved.