Photochemical activity of primary leaves in cadmium stressed Phaseolus coccineus depends on their growth stages

Runner bean plants (Phaseolus coccineus L., cv. Piękny Jaś) grown hydroponically were treated with Cd (2.5 • 10-5 M Cd in the form of 3CdSO4 • 8H2O) at different stages of primary leaves development. Ten days after the metal treatment, the changes in leaf growth parameters and their PSII photochemistry were investigated. The younger were the plants exposed to Cd (at the initial growth stage of primary leaves) the stronger was the leaf area reduction, fresh weight decrease and increase of plastid pigment level. These changes were correlated with lower level of photosynthetic 02 evolution (75% of control) but chlorophyll a fluorescence measurements suggested an inhibitory effect of Cd on the acceptor side of PSII. With the development of primary leaves the sensitivity of plants to Cd increased. Cd treatment of plants at the final growth stage of the primary leaves caused slight changes in growth parameters compared with control, whereas the chlorophyll content and PSII activity (48% of control) significantly decreased. Analyses of fast chlorophyll a fluorescence kinetics revealed changes suggesting Cd-induced alterations of PSII complex extending on to their acceptor side and the reaction centre. Differential response of primary leaves to Cd depending on their growth stages indicated alterations of the adaptation mechanism in leaf cells of runner bean plants. The decrease in younger Cd-treated plants of photosynthetic activity was connected with disturbances in growth processes, however in plants treated with Cd at the final growth stage of the primary leaves it was resulted from destruction of the photosynthetic apparatus