Energy fluxes and driving forces for photosynthesis in Lemna minor exposed to herbicides

Analysis of fast chlorophyll fluorescence rise OJIP was carried out to assess the impact of diuron, paraquat and flazasulfuron on energy fluxes and driving forces for photosynthesis in Lemna minor. Results showed that diuron and paraquat treatment produced major changes in electron transport in active reaction centres (RCs). However, diuron had a more pronounced effect on the yield of electron transport per trapped exciton (psi(0)) than on the yield of primary electron transport (phi(P0)) showing that dark reactions are more sensitive to diuron than light-dependent reactions. In contrast, paraquat treatment effects were not due to a target-specific action on those dark and light reactions. Paraquat also induced a marked surge in the total absorption of photosystem II (PSII) antenna chlorophyll per active RC displaying a large increase of the dissipation of excess energy through non-photochemical pathways (thermal dissipation processes). Flazasulfuron induced a slight decrease of both the total driving force for photosynthesis and the quantum yield of electron transport beyond Q(A)(-) combined to a small but significant increase of the non-photochemical energy dissipation per RC (DI0/RC). We conclude that energy fluxes and driving force for photosynthesis generate useful information about the behaviour of aquatic plant photosystems helping to localize different target sites and to distinguish heterogeneities inside the PSII complexes. Regardless of the active molecule tested, the DFABS, phi(E0), DI0/RC and/or ET0/RC parameters indicated a significant variation compared to control while phi(P0) (F-V/F-M) showed no significant inhibition suggesting that those parameters are more sensitive for identifying a plant's energy-use efficiency than the maximum quantum yield of primary PS II photochemistry alone. (C) 2008 Elsevier B.V. All rights reserved.