Cadmium effects in sunflower: Membrane permeability and changes in catalase and peroxidase activity in leaves and calluses

Mechanisms preventing oxidative burst in cells exposed to high metal concentrations are crucial for cell survival. In this report, sunflower ( Helianthus annuus L.) was used as a model species to investigate: (a) how cadmium (Cd) affects plant defense pathways (in particular, those involved in preventing oxidative stress), and (b) whether antioxidative enzymes of plants and of in vitro cell culture (calluses) have similar responses to Cd exposure. For this experiment, plants (grown hydroponically on Long Ashton medium) and calluses (grown on Murashige and Skoog medium) were exposed for three weeks to different Cd concentrations (0, 5, 50, and 500 mu M). Solute and electrolyte leakage increased significantly with the increase in Cd concentration in the external medium and in exposed leaves, and less in exposed calluses. Cadmium exposure also reduced significantly soluble protein contents in both leaves and calluses. The activities of catalase and peroxidase decreased significantly in 50 and 500 mu M-exposed leaves compared with the control, but increased in 5 mu M-exposed calluses, decreasing in the calluses exposed 500 mu M Cd. The decrease in antioxidative enzymes activities is congruent with the decrease in membrane integrity and suggests that calluses develop antioxidant mechanisms that respond better to Cd stress than do leaves. On average, 100% of plants exposed for 21 d to 500 mu M and 95% of plants exposed for 45 d to 50 mu M Cd were dead. After 45 d, only 53% of 50 mu M-exposed calluses had died. These surviving calluses were maintained on 50 mu M for six months. Six-month-old exposed calluses had higher activities of peroxidase and catalase when compared with control calluses, as well as lower membrane degradation. These data show that calluses are more tolerant than plants to Cd exposure, and that antioxidant mechanisms under Cd exposure may differ between the two cell systems and vary with time.