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 μm CdSO₄ for 48 h) reduced probability of electron transfer from plastoquinones to the terminal electron acceptors of PSI (δ_Ro ) accompanied by a decrease in the rate of P₇₀₀ ⁺ and PC reduction (V_red ) and the magnitude of the I₂ 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 δ_Ro , V_red , and I₂ /I₁ as sensitive indicators of an early contamination by heavy metals.