Effects of cadmium on growth of Helianthus annuus seedlings: physiologicalaspects

Sunflower seedlings (Helianthus annuus hybrid Select) were grown in a compl ete nutrient solution in the absence or presence of Cd2+ (10 and 20 mu M). Analyses were performed to establish whether there was a differential effec t of Cd2+ on mature and young leaves. After 7 d the growth parameters as we ll as the leaf area had decreased in both mature and young leaves. Accumula tion of Cd2+ in the roots exceeded that in the shoots. Seedlings treated wi th Cd2+ exhibited reduced contents of chlorophyll and CO, assimilation rate , with a greater decrease in young leaves. The photochemical efficiency of photosystem II (PSII) was not altered by Cd2+ treatment in either mature or young leaves, although during steady-state photosynthesis in young leaves there was a significant alteration in the following parameters: quantum yie ld of electron transport by PSII (Phi(PSII)), photochemical quenching (q(P) ), nonphotochemical quenching (q(NP)), and excitation capture efficiency of PSII (Phi(exc)).