COMPARATIVE MEASUREMENTS OF CHLOROPHYLL-A FLUORESCENCE, ACID ACCUMULATION AND GAS-EXCHANGE IN EXPOSED AND SHADED PLANTS OF CLUSIA-MINOR L AND CLUSIA-MULTIFLORA HBK IN THE FIELD

Changes in chlorophyll a fluorescence during the day and diurnal-chang es of net CO2-exchange and organic acid contents were determined in tw o species of the genus Clusia during the dry season in Venezuela. The investigations included plants of the C-3/CAM intermediate species Clu sia minor and the C-3 species C. multiflora growing at exposed and sha ded sites. Both species showed a C-3 pattern of net CO2-exchange at th e exposed site. In the shade under extreme drought stress C. minor sho wed a weak expression of CAM without CO2-uptake during the afternoon ( phase IV of CAM). C. multiflora growing in the shade exhibited a C-3-p attern of net CO2-exchange and a small but significant nocturnal accum ulation of citrate. Shaded plants of C. minor were able to double thei r light utilisation for electron transport and to reduce non-photochem ical quenching during phase IU compared to phase II of CAM. Furthermor e, increase of electron transport rate through photosystem II in phase III of CAM is correlated to decarboxylation of malate. At the exposed site C. multiflora was less negatively affected by high PPFD than C. minor. This was shown by a lower reduction of potential electron quant um yield (F-v/F-m) and higher light utilisation of electron transport of C. multiflora compared to C. minor. At the exposed site C. minor di d not make use of the CAM option to increase light utilisation of elec tron transport and to reduce non-photochemical quenching as did the pl ants growing in the shade.