LIGHT-DEPENDENT MODIFICATION OF PHOTOSYSTEM-II IN SPINACH LEAVES

In dark-adapted spinach leaves approximately one third of the Photosys tem II (PS II) reaction centers are impaired in their ability to trans fer electrons to Photosystem I. Although these 'inactive' PS II center s are capable of reducing the primary quinone acceptor, Q(A), oxidatio n of Q(A)(-) occurs approximately 1000 times more slowly than at 'acti ve' centers. Previous studies based on dark-adapted leaves show that m inimal energy transfer occurs from inactive centers to active centers, indicating that the quantum yield of photosynthesis could be signific antly impaired by the presence of inactive centers. The objective of t he work described here was to determine the performance of inactive PS II centers in light-adapted leaves. Measurements of PS II activity wi thin leaves did not indicate any increase in the concentration of acti ve PS II centers during light treatments between 10 s and 5 min, showi ng that inactive centers are not converted to active centers during li ght treatment. Light-induced modification of inactive PS II centers di d occur, however, such that 75% of these centers were unable to sustai n stable charge separation. In addition, the maximum yield of chloroph yll fluorescence associated with inactive PS II centers decreased subs tantially, despite the lack of any overall quenching of the maximum fl uorescence yield. The effect of light treatment on inactive centers wa s reversed in the dark within 10-20 mins. These results indicate that illumination changes inactive PS II centers into a form that quenches fluorescence, but does not allow stable charge separation across the p hotosynthetic membrane. One possibility is that inactive centers are c onverted into centers that quench fluorescence by formation of a radic al, such as reduced pheophytin or oxidized P680. Alternatively, it is possible that inactive PS II centers are modified such that absorbed e xcitation energy is dissipated thermally, through electron cycling at the reaction center.