A COMPARATIVE-STUDY ON THE REGULATION OF C-3 AND C-4 CARBOXYLATION PROCESSES IN THE CONSTITUTIVE CRASSULACEAN ACID METABOLISM (CAM) PLANT KALANCHOE-DAIGREMONTIANA AND THE C-3-CAM INTERMEDIATE CLUSIA MINOR

A comparison of carbon metabolism in the constitutive crassulacean aci d metabolism (CAM) plant Kalanchoe daigremontiana Hamet et Perr. and t he C-3-CAM intermediate Clusia minor L. was undertaken under controlle d environmental conditions where plants experience gradual changes in light intensity, temperature and humidity at the start and end of the photoperiod, The magnitude of CAM activity was manipulated by maintain ing plants in ambient air or by enclosing leaves overnight in an atmos phere of N-2 to suppress C-4 carboxylation. Measurements of diel chang es in carbon-isotope discrimination and organic acid content were used to quantify the activities of C-3 and C-4 carboxylases in vivo and to indicate the extent to which the activities of phosphoenolpyruvate ca rboxylase (PEPCase), ribulose-1,5-bisphosphate carboxylase-oxygenase ( Rubisco) and decarboxylation processes overlap at the start and end of the photoperiod. These measurements in vivo were compared with measur ements in vitro of changes in the diel sensitivity of PEPCase to malat e inhibition. The results demonstrate fundamental differences in the d own-regulation of PEPCase during the day in the two species. While PEP Case is inactivated within the first 30 min of the photoperiod in K. d aigremontiana, the enzyme is active for 4 h at the start and 3 h at th e end of the photoperiod in C. minor. Enclosing leaves in N-2 overnigh t resulted in a two- to threefold increase in PEPCase-mediated CO2 upt ake during Phase II of CAM in both species. However, futile cycling of CO2 between malate synthesis and decarboxylation does not occur durin g Phase II in either species. In terms of overall carbon balance, C-4 carboxylation accounted for = 20% of net daytime assimilation in both species under control conditions. increasing to 30-34% after a night i n N-2. Although N-2-treated leaves of K. daigremontiana took up 25% mo re CO2 than control leaves during the day this was insufficient to com pensate for the loss of ed? taken up by CAM the previous night, In con trast, in N-2-treated leaves of C. minor, the twofold increase in dayt ime PEPCase activity and the increase in net CO2 uptake by Rubisco dur ing Phase III compensated for the inhibition of C-4 carboxylation at n ight in terms of diel carbon balance.