Proline Protects Atropa belladonna Plants against Nickel Salt Toxicity

Atropa belladonna L. plants were grown in water culture for 8 weeks before the nutrient medium was supplemented with NiCl2 to final concentrations of 0 (control treatment), 50, 100, 150, 200, 250, and 300 mu M. After 4 days of plant growing in the presence of nickel chloride, the content of water, proline, Ni, Fe, free polyamines, as well as lipid peroxidation rates were measured. The addition of 100-150 mu M Ni to the medium significantly reduced the fresh weight increments and water content in comparison with these parameters for untreated plants; 200 mu M Ni caused serious, although nonlethal damage to the plants, whereas 250 and 300 mu M Ni proved to be lethal. In the aboveground organs, the major part of Ni was accumulated in the apical leaves. When the plants were treated with 200 mu M Ni, the Ni content in apical leaves was 220 mu g/g dry wt, while Ni content in roots reached 1500 mu g/g dry wt. The treatment of plants with proline in the presence of 200 mu M Ni inhibited Ni accumulation in tissues. The proline-treated plants exhibited elevated iron content in leaves and especially in roots and were characterized by comparatively low rates of lipid peroxidation and by sustained leaf water status. When 200 mu M Ni was applied, the content of free putrescine decreased, while the contents of spermine and spermidine in leaves increased appreciably with respect to the control values. The toxic effect of nickel was accompanied not only by an enhanced accumulation of high-molecular-weight polyamines but also by their oxidative degradation, which was evident from the 14-fold increase in the content of 1,3-diaminopropane. The protective effect of exogenous proline in the presence of high nickel concentrations was manifested in lowered lipid peroxidation rates, alleviation of iron deficiency, and in retarded oxidative degradation of polyamines.