Ecophysiology of the C3-CAM intermediate Clusia minor L. in Trinidad: seasonal and short-term photosynthetic characteristics of sun and shade leaves

The seasonal changes in crassulacean acid metabolism (CAM) activity in response to daily integrated photon flux density (PFD) and precipitation were compared in sun and shade leaves of the C3-CAM intermediate tree Clusia minor L. Measurements of CAM activity (H⁺) showed that maximum leaf acidity consistently occurred 4 h after dawn, suggesting that new sampling procedures need to be adopted in order to quantify CAM in Clusia species. Whilst exposed leaves responded to intermittent dry conditions, shaded leaves showed a clear induction of CAM activity as conditions became drier. The magnitude of CAM activity correlated well with daily integrated PFD, such that the extent of decarboxylation of organic acids was consistently associated with increased acidification during the subsequent dark period. Over two sampling days, both sun and shade leaves exhibited the four phases of CAM, although PEPc remained active throughout phase II with the result that 50% of the maximum leaf acidity in shade leaves was accumulated during this time. During phase III, internal CO2 supply was augmented by substantial citrate decarboxylation, in addition to malic acid. Chlorophyll fluorescence characteristics were dominated by high rates of PSII electron transport, together with an extremely high potential for thermal dissipation, such that excess light was maintained within safe limits at times of maximum PFD. Photochemical stability was maintained by matching supply and demand for internal CO2: in the morning, C3 and C4 carboxylation processes were regulated by extended PEPc activity, so that decarboxylation was delayed until temperature and light stress were highest at midday.