PHOTOINHIBITION OF PHOTOSYNTHESIS IN PLANTS GROWING IN NATURAL TROPICAL FOREST GAPS - A CHLOROPHYLL FLUORESCENCE STUDY

Photoinhibition of photosynthesis was monitored by means of chlorophyl l a fluorescence in leaves of plants growing in 60-80 m(2) light gaps in a moist tropical lowland forest located on Barro Colorado Island in central Panama. In these forest gaps, photon flux density was low (le ss than 100 mu mol photons m(-2) s(-1)) during most of the day, but in creased on clear days to 1.7 - 1.8 mmol photons m(-2) s(-1) for 1 - 2 h during midday. Nine species representing different taxa and life-for ms were examined. Leaves of all species exhibited substantial photoinh ibition in situ during high light exposure, as manifested by a decreas e in the ratio of variable to maximum fluorescence emission, F-V/F-M. Recovery (reversion of fluorescence quenching) took place in the shade following high light exposure. The major part of recovery occurred in a fast phase within about 1 h after the high light period. A slow pha se of recovery proceeded for another 4-5 h until sunset. After 30-60 m in of recovery in the shade, calculated rates of PSII electron transpo rt remained significantly (5-15%) reduced in comparison to rates obtai ned prior to high light exposure; after about 2 h of recovery, inhibit ion was negligible. All species responded to the high light periods an d following shade periods in a very similar manner. It is concluded th at photoinhibition and recovery exhibited by these gap leaves reflect a dynamic regulatory mechanism of thermal energy dissipation that allo ws plants of different life-forms to cope with periods of high light i n tropical forest gaps.