Four different Clusia species showing C3 photosynthesis (C. multiflora) or crassulacean acid metabolism (CAM) (C. rosea, an unidentified Clusia sp. and C. alata) co-occur in the same habitat in the northwest of 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 quantum yield, measured as Fv/Fm after 10 min of dark adaptation, is restricted to the early morning hours. With the onset of deacidification there was no further decline in Fv/Fm, although the irradiance upon the leaves remained high. For the C3-plant, C. multiflora, changes in Fv/Fm were correlated with changes in irradiance incident on the leaves before predarkening for 10 min for Fv/Fm measurements. The fast relaxation of Fv/Fm under low light indicates that C. multiflora was not suffering photoinhibitory damage. Effective quantum yield as determined by ΔF/Fm, was low under high irradiances for C. rosea 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 that apparent electron transport rates through photosystem II (PS II) (ΔF/F'm) × PPFD) were highest for C. rosea, Clusia sp. and C. alata during phase III of CAM. Electron transport rates in C. multiflora were 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 discussed.