COMPARATIVE MEASUREMENTS OF GAS-EXCHANGE, ACID ACCUMULATION AND CHLOROPHYLL-A FLUORESCENCE OF DIFFERENT SPECIES OF CLUSIA SHOWING C-3 PHOTOSYNTHESIS, OR CRASSULACEAN ACID METABOLISM, AT THE SAME FIELD SITE IN VENEZUELA

Four different Clusia species showing C-3 photosynthesis (C. multiflor a) or crassulacean acid metabolism (CAM) (C. rosea, an unidentified Cl usia sp. and C. alata) co-occur in the same habitat in the northwest o f Venezuela. The aim of this field study was to correlate the adaptive changes in chlorophyll a fluorescence with gas exchange patterns and deacidification of nocturnally accumulated organic acids. The results indicate that in C. rosea and Clusia sp. a lowering of the potential q uantum yield, measured as F-v/F-m after 10 min of dark adaptation, is restricted to the early morning hours. With the onset of deacidificati on there was no further decline in F-v/F-m, although the irradiance up on the leaves remained high. For the C-3-plant, C. multiflora, changes in F-v/F-m were correlated with changes in irradiance incident on the leaves before predarkening for 10 min for F-v/F-m measurements. The f ast relaxation of F-v/F-m under low light indicates that C. multiflora was not suffering photoinhibitory damage. Effective quantum yield as determined by Delta F/F-m', was low under high irradiances for C. rose a and Clusia sp. and did not change significantly for C. rosea during phase III of CAM. During the time of low incident irradiance C. alata and C. multiflora reached an effective quantum yield that was close to the optimal quantum yield of electron transport. It is noteworthy tha t apparent electron transport rates through photosystem II (PS II) ((D elta F/F-m') x PPFD) were highest for C. rosea, Clusia sp. and C. alat a during phase III of CAM. Electron transport rates in C. multiflora w ere lower under high irradiance and did not differ from those found in phases II and IV of CAM. The possible role of CO2 evolution from acid decarboxylation with respect to photosynthetic efficiency is discusse d.