Antioxidative enzymes in cultivars of pepper plants with different sensitivity to cadmium

The effect of growing five different cultivars of pepper plants (Capsicum annuum L.) with CdCl₂ concentrations ranging from 0.125 to 0.5 mM on different physiological parameters, and antioxidative enzyme activities of leaves was studied. On the basis of growth parameters, pepper plants were relatively tolerant to cadmium, although metal concentrations higher than 0.125 mM produced a significant inhibition of growth, net photosynthesis, and water use efficiency. Different sensitivities to Cd⁺⁺ ions were observed among cultivars, Abdera being the most resistant to cadmium stress, while Mondo and Herminio were the most sensitive cultivars. Cadmium concentrations of 0.5 mM produced an increase in the activity of glutathione reductase, and guaiacol peroxidase in most cultivars, while catalase and superoxide dismutase (SOD) were slightly depressed. The analysis of the SOD activity pattern by native-PAGE showed the presence in most cultivars of four SODs which were identified as Mn–SOD, Fe–SOD, CuZn–SOD I and CuZn–SOD II. However, the two CuZn–SODs were absent in the Cd-sensitive cv. Herminio. The growth of pepper plants with 0.5 mM cadmium inhibited the activity of CuZn–SODs in all cultivars, while the activity of Mn- and Fe–SOD was enhanced. The activity of NADPH-dehydrogenases (glucose-6-P-dehydrogenase, 6-phosphogluconate dehydrogenase, NADP–isocitrate dehydrogenase and malic enzyme) showed a Cd-dependent enhancement in most cultivars, the highest increase being observed in the tolerant cv. Abdera. These results suggest that in pepper plants the tolerance to Cd toxicity is more dependent on the availability of NADPH than on its antioxidant capacity.