CHANGES IN LABELLING PATTERNS AFTER FEEDING BRYOPHYLLUM TUBIFLORUM WITH 14CO2 AT DIFFERENT TIMES DURING LIGHT/DARK PERIOD .I. 14CO2-FIXATION IN LIGHT

Detached phyllodia of Bryophyllum tubiflorum were fed under illumination with14CO2 at different times during the light/dark period (12:12 hours). After photosynthesis in presence of14CO2 during the intrinsic dark period the greatest part of soluble radioactivity was found in malate. When the same experiment was repeated during the light period, radioactivity was incorporated mainly into sucrose in the first hours while malate was labelled rather weakly. In the late afternoon (last third of the light period), malate became most heavily labelled again during photosynthesis with14CO2. Our results indicate that the synthesis of malate by PEP-carboxylase/malate dehydrogenase is inhibited at certain times during the night/day period by end product inhibition of PEP-carboxylase, as was demonstrated by Queiroz (1967, 1968) and Ting (1968) in vitro. During inhibition of the PEP-carboxylase there is no competition between the synthesis of malate and CO2-fixation by the Calvin cycle. Thus radioactivity can flow into sucrose via the Calvin cycle during this time. When the malate content of the phyllodia is low, CO2-fixation by PEP-carboxylase is not inhibited. Now this pathway dominates over photosynthesis via the Calvin cycle, for PEP-carboxylase has a higher affinity for CO2 than carboxydismutase. Therefore malate now becomes more labelled than sucrose. © 1969 Springer-Verlag.