Effects of cadmium on growth of Helianthus annuus seedlings: physiological aspects

Sunflower seedlings (Helianthus annuus hybrid Select) were grown in a complete nutrient solution in the absence or presence of Cd2+ (10 and 20 mu M). Analyses were performed to establish whether there was a differential effect of Cd2+ on mature and young leaves. After 7 d the growth parameters as well as the leaf area had decreased in both mature and young leaves. Accumulation of Cd2+ in the roots exceeded that in the shoots. Seedlings treated with 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 yield of electron transport by PSII (Phi(PSII)), photochemical quenching (q(P)), nonphotochemical quenching (q(NP)), and excitation capture efficiency of PSII (Phi(exc)).