Growth, photosynthetic electron transport, and antioxidant responses of young soybean seedlings to simultaneous exposure of nickel and UV-B stress

The effects of enhanced ultraviolet-B (UV-B, 0.4 W m(-2)) irradiance and nickel (Ni, 0.01, 0.10 and 1.00 mM; Ni-0.01, Ni-0.10, Ni-1.00, respectively) treatment, singly and in combination, on growth, photosynthetic electron transport activity, the contents of reactive oxygen species (ROS), antioxidants, lipid peroxidation, and membrane leakage in soybean seedlings were evaluated. Ni-0.10 and Ni-1.00 and UV-B declined the growth and chlorophyll content, which were further reduced following combined exposure. Contrary to this, Ni-0.01 stimulated growth, however, the effect together with UV-B was inhibitory. Carotenoids showed varied response to both the stresses. Simultaneous exposure of UV-B and Ni as well as UV-B alone reduced the activities of photosystems 1 and 2 (PS1 and PS2) and whole chain activity significantly, while Ni individually, besides strongly inhibiting PS2 and whole chain activity, stimulated the PSI activity. Both the stresses, alone and together, enhanced the contents of superoxide radical (O-2(-)), hydrogen peroxide (H2O2), malondialdehyde (MDA), electrolyte leakage, and proline content, while ascorbate content declined over control. Individual treatments increased the activities of catalase (CAT), peroxidase, and superoxide dismutase (SOD), but Ni-1.00 declined SOD activity significantly. Combined exposure exhibited similar response, however, CAT activity declined even more than in control. Compared to individual effects of UV-B and Ni, the simultaneous exposure resulted in strong inhibition of photosynthetic electron transport and excessive accumulation of ROS, thereby causing severe damage to soybean seedlings.