Multiple in vivo Effects of Cadmium on Photosynthetic Electron Transport in Pea Plants

The inhibitory effects of cadmium (CdSO4) on the primary photosynthetic processes were studied in vivo in Pisum sativum by using Multi-function Plant Efficiency Analyser (M-PEA-2). Photosynthetic parameters related to photosystem (PS) II, PS I and intersystem electron carriers were calculated from the light-induced kinetics of prompt chlorophyll a fluorescence (OJIP transient), delayed chlorophyll a fluorescence (DF), and 820 nm modulated reflection (MR). Low-dose exposure to cadmium (20 mu m CdSO4 for 48 h) reduced probability of electron transfer from plastoquinones to the terminal electron acceptors of PSI (delta(Ro)) accompanied by a decrease in the rate of P-700(+) and PC reduction (V-red) and the magnitude of the I-2 step on the DF kinetics. Electron transport through PSI remained unaltered. The obtained results allowed us to propose existence of the potential site of inhibition of photosynthetic electron flow by cadmium between PSII and PSI. We propose to use parameters delta(Ro), V-red, and I-2/I-1 as sensitive indicators of an early contamination by heavy metals.