Changes of Cd content in chloroplasts are mirrored by the activity of photosystem I, but not by photosystem II.

Abstract

Cd is one of the most toxic heavy metals and widespread pollutant. We searched for a direct Cd action on the photosynthetic electron transport chain using induced chlorophyll fluorescence and P700 light absorption. Young barley and maize plants were treated with Cd in toxic (80 M) and nearly lethal (250 M) concentrations. The maximal and relative photochemical activities of PSI, its major limitation at the donor side, and partially acceptor-side limitation of PSII changed in agreement with Cd accumulation in the corresponding chloroplasts. Probably, acceptor-side limitation of PSII increased with a direct Cd action under 80 M that was overcome with an indirect Cd action under 250 M. These alterations can be explained by Cd/Cu substitution in plastocyanin. The photochemical and non-photochemical quenching by PSII varied diversely that cannot be explained unambiguously by any mechanism. The limitations of PSI (Y(ND), Y(NA)) and PSII (qC) were compared for the first time. They were ranged as follows: Y(NA) < qC < Y(ND). Short segments of qC and Y(ND) dynamics varied proportionally to each other. This implies the existence of an unknown mechanism adjusting limitations at the acceptor side of PSII (qC) and at the donor side of PSI (Y(ND)). HighlightsO_LIPSI activity changed in agreement with the changes of Cd content in chloroplasts C_LIO_LIThe data on PSII activity cannot be clearly explained by Cd action C_LIO_LIPSII acceptor-side limitation qC was governed by opposed direct and indirect Cd actions C_LIO_LIPSI and qC changes can be explained by Cd/Cu substitution in plastocyanin C_LIO_LILimitations qC of PSII and Y(ND) of PSI changed proportionally for a short time C_LI