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

Fluorescence, absorbance and photoacoustic methods were used to examin e in vivo various functional aspects of the photochemical apparatus of maize leaves exposed to different Cu concentrations. The primary phot ochemistry of a dark adapted leaf was less affected in the presence of Cu, while in a light-adapted leaf the photochemical events were sever ely impaired by Cu. Analysis of the characteristics of chlorophyll flu orescence induction revealed that the primary target of Cu stress invo lved the PSII reaction centre in its ability to adapt to high light co nditions. Denaturation of PSII occured at 80 mu M Cu resulting in a si gnificant loss of PSII-mediated electron transport under continuous li ght and a strong inhibition of O-2 evolution. It was also observed tha t PSI photochemistry, as probed by the photochemical energy storage in far-red light and the kinetics of P-700 photooxidation by strong far- red light, was more tolerant to Cu compared to the PSII activity. More over, in Cu-exposed leaves irradiated with high intensity light, the i n vivo heat emission yield increased due to the Cu deactivation of pho tosynthetic energy conservation. Heat release was well correlated with changes in nonphotochemical quenching. copper gradually prevents the adaptation process from a dark-adapted to a light-adapted state with t he consequence that all observed photosynthetic activity criteria unde r steady state conditions in the light become more inefficient.