Effect of heavy metals Co2+, Ni2+ and Cd2+ on growth and metabolism of cabbage

Exposure of cabbage plants to excess (500 muM) of Co2+, Ni2+ and Cd2+ in sand culture led to increased accumulation of the metals, inhibition of growth and induction of visible symptoms of metal toxicity. In addition to chlorosis, Co2+, treated plants exhibited reddish purple coloration along leaf tnargins, Ni2+ treated plants exhibited black spots near leaf margins, and Cd2+ treated plants developed purple coloration along leaf margins. At equimolar concentration, inhibition of growth was most severe with excess Cd2+ and induction of visible symptoms was most severe with excess Ni2+. Exposure to excess concentration of the heavy metals decreased the uptake of Fe and its tran slocation to leaves. Exposure to each Co2+, Ni2+ and Cd2+ decreased chlorophyll content (Ni2+ > Cd2+ > Co2+). concomitant with decrease in the activities of the Fe enzymes-catalase and peroxidase, suggesting reduced availability of Fe for chlorophyll-heme biosynthesis. Each Co2+, Ni2+ and Cd2+ decreased water potential and transpiration rate, associated with increase in diffusive resistance showing development of water stress. This was further substantiated by enhanced accumulation of proline in the leaves of plants exposed to Co2+, Ni2+ and Cd2+. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.