ECOPHYSIOLOGY OF THE C-3-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 precipitat ion were compared in sun and shade leaves of the C-3-CAM intermediate tree Clusia minor L, Measurements of CAM activity (Delta H+) showed th at maximum leaf acidity consistently occurred 4 h after dawn, suggesti ng that new sampling procedures need to be adopted in order to quantif y CAM in Clusia species. Whilst exposed leaves responded to intermitte nt dry conditions, shaded leaves showed a clear induction of CAM activ ity as conditions became drier. The magnitude of CAM activity correlat ed well with daily integrated PFD, such that the extent of decarboxyla tion of organic acids was consistently associated with increased acidi fication during the subsequent dark period. Over two sampling days, bo th sun and shade leaves exhibited the four phases of CAM, although PEP c 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 ci trate decarboxylation, in addition to malic acid. Chlorophyll fluoresc ence characteristics were dominated by high rates of PSII electron tra nsport, together with an extremely high potential for thermal dissipat ion, such that excess light was maintained within safe limits at times of maximum PFD. Photochemical stability was maintained by matching su pply and demand for internal CO2: in the morning, C-3 and C-4 carboxyl ation processes were regulated by extended PEPc activity, so that deca rboxylation was delayed until temperature and light stress were highes t at midday.