DISCRIMINATION PROCESSES AND SHIFTS IN CARBOXYLATION DURING THE PHASES OF CRASSULACEAN-ACID-METABOLISM

The magnitude and extent of Crassulacean acid metabolism (CAM) activit y in two Clusia species was manipulated to investigate the regulation of the distinct CAM phases. First, in response to leaf-air vapor press ure deficit at night, changes in lear conductance altered on-line carb on-isotope discrimination throughout the theoretical range for dark CO 2 uptake during CAM. These ranged from the limit set by phosphoenolpyr uvate carboxylase (PEPc) (-6 parts per thousand, delta(13)C equivalent of -2) to that imposed by diffusion limitation (+4 parts per thousand , delta(13)C equivalent of -12 parts per thousand), but the lowest car bon-isotope discrimination occurred when p(i)/p(a), was only 0.7. Seco nd, when the availability of external or internal sources of CO2 was r educed for both field- and greenhouse-grown plants, CO2 uptake by day via PEPc during phase II largely compensated. Third, by reducing the d ark period, plants accumulated low levels of acidity, and CO2 uptake o ccurred throughout the subsequent light period. Discrimination switche d from being dominated by PEPc (phase II) to ribulose 1,5-bisphosphate carboxylase/oxygenase (phase III), with both enzymes active during ph ase IV. Under natural conditions, photochemical stability is maintaine d by extended PEPc activity in phase II, which enhances acid accumulat ion and delays decarboxylation until temperature and light stress are maximal at midday.