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

We searched for a direct Cd action on the photosynthetic electron transport chain using induced chlorophyll fluorescence and P light absorption. Young barley and maize plants were treated with Cd in concentrations of toxic (80 μM) and nearly lethal (250 μM). 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. Acceptor-side limitation of PSII increased with a direct Cd action at 80 μM that was overcome with an indirect Cd action at 250 μM. These alterations can be explained by Cd/Cu substitution in plastocyanin. The photochemical and nonphotochemical quenching by PSII varied diversely which cannot be explained unambiguously by any mechanism. The limitations of PSI [Y, Y] and PSII (q) were compared for the first time. They were ranged: Y < q < Y.