Involvement of Antioxidant Defense Capacity of Rice and Sunflower in Their Response to Cu and Mn Toxicity under Different Light Intensities

Effects of toxic concentrations (up to 100 µM) of copper (Cu) and manganese (Mn) on growth and oxidative defense system of rice (Oryza sativa L.) and sunflower (Helianthus annuus L.) were studied under three light intensities in hydroponic medium. Rice expressed higher susceptibility to both Cu and Mn toxicity than sunflower. Higher light intensity strengthened the effect of toxicity of Cu while ameliorated that of Mn. Activity of ascorbate peroxidase and glutathione reductase responded to Cu and Mn differently in rice. Cu toxicity increased their activity but Mn did not change or rather decreased it. Activity of catalase was induced in response to both Cu and Mn particularly under higher light intensities. Activity of peroxidase, in contrast to other tested enzymes, was correlated with growth response of plants. Copper was not effective in induction of malondialdehyde accumulation. Mn, in concentrations causing no growth inhibition (100 µM), resulted in a significant increase of malondialdehyde. Changes in the tissue content of proline, similar with peroxidase activity, was mainly associated with stress conditions as judged by growth data. On the other word, activity of peroxidase and accumulation of proline monitored the stress conditions without any protecting role and any effect on adaptation of plants. Our results suggested that the antioxidant capacity of plants is not independently correlated with the growth response of plants without regarding the other physiological responses to metal exposure.