Sites of Action of Copper in the Photosynthetic Apparatus of Maize Leaves: Kinetic Analysis of Chlorophyll Fluorescence, Oxygen Evolution, Absorption Changes and Thermal Dissipation as Monitored by Photoacoustic Signals

Abstract

Fluorescence, absorbance and photoacoustic methods were used to examine in vivo various functional aspects of the photochemical apparatus of maize leaves exposed to different Cu concentrations. The primary photochemistry of a dark adapted leaf was less affected in the presence of Cu, while in a light-adapted leaf the photochemical events were severely impaired by Cu. Analysis of the characteristics of chlorophyll fluorescence induction revealed that the primary target of Cu stress involved the PSII reaction centre in its ability to adapt to high light conditions. Denaturation of PSII occured at 80 µM Cu resulting in a significant loss of PSII-mediated electron transport under continuous light and a strong inhibition of O₂ evolution. It was also observed that PSI photochemistry, as probed by the photochemical energy storage in far-red light and the kinetics of P₇₀₀ photooxidation by strong far- red light, was more tolerant to Cu compared to the PSII activity. Moreover, in Cu-exposed leaves irradiated with high intensity light, the in vivo heat emission yield increased due to the Cu deactivation of photosynthetic energy conservation. Heat release was well correlated with changes in non- photochemical quenching. Copper gradually prevents the adaptation process from a dark-adapted to a light-adapted state with the consequence that all observed photosynthetic activity criteria under steady state conditions in the light become more inefficient.