Inhibition of the circadian rhythm of CO2 metabolism in Bryophyllum leaves by cycloheximide and dinitrophenol.

The circadian rhythm of CO2 output in darkened leaves of Bryophyllum fedtschenkoi R. Hamet and Perrier can be inhibited by cycloheximide (≧10(-6) mol) and 2,4-dinitrophenol (≧10(-5) mol) applied via the transpiration stream. After having been suppressed by 10(-6) M cycloheximide, the rhythm can be reinitiated with a 12-h exposure to light. Experiments using (14)CO2 show that cycloheximide abolishes the rhythm by inhibiting the dark fixation of CO2. Cycloheximide inhibits malate accumulation and acidification of the leaves, but does not affect the amount of the CO2-fixing enzyme phosphoenol-pyruvate carboxylase (PEP-C, EC 4.1.1.31) which can be extracted from the leaves during the 45 h of the experiment. Cycloheximide has no direct effect on the activity of the enzyme as measured in the assay. PEP-C from desalted leaf extracts was inhibited by L-malate (Ki=0.4 mmol). The most likely explanation for the inhibitory effect of cycloheximide and dinitrophenol is that they cause changes in tonoplast properties which result in a redistribution of malate from the vacuole to the cytoplasm. An increase in malate concentration in the cytoplasm will lead to inhibition of PEP-carboxylase, and hence the suppression of the rhythm of CO2 output.