Chromium effects on photosynthetic electron transport in pea (Pisum sativum L.).

Components of the photosynthetic electron transport chain in pea (Pisum sativum L.) leaves under in vivo conditions showed the following sensitivity to the inhibitory action of chromium(VI): intersystem electron transport > photosystem I > photosystem II. Inhibitory effects of chromium (VI) (KCrO, Cr) on the light reactions of photosynthesis were studied in vivo in Pisum sativum L. by using Multi-function Plant Efficiency Analyser (M-PEA-2). Photosynthetic parameters related to photosystem (PS) II, PSI 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). We showed that the I step of DF induction is sensitive to inhibition of the Q site of the cytochrome bf complex. Such parameters as δ of the JIP test related to the functional state of photosynthetic reactions beyond the PQ pool, V of the MR induction assigned to the overall rate of P and plastocyanin reduction, and I step of the DF induction were significantly altered in the presence of low-dose Cr(VI). Moderate doses of Cr affected mainly PSI-related parameters including V and ΔMR parameters of the MR induction, whereas high-dose treatment influenced JIP test parameters φ(= F/F) and ψ related to PSII. The obtained results showed that the earliest Cr(VI) effect on the photosynthetic electron transport chain manifests itself by inhibition of the intersystem electron transport, rather, at the level of the cytochrome bf complex. Inhibitory effects of Cr on PSI were more pronounced than those on PSII. Sensitivity of the used kinetic parameters toward the functional state of photosynthetic reactions makes this approach suitable for early diagnostics of toxic action of pollutants on plants.