Pathways of CO2 Fixation in the CAM Plant Kalanchoe daigremontiana . I Patterns of 14CO2 Fixation in the Light

Leaves of K. daigremontiana from plants grown under controlled conditions were exposed to 14CO2 in the light for 15 s followed by a chase in 12CO2. During steady-state photosynthesis in the late light period the pulsechase labelling of photosynthetic intermediates was very similar to that observed in the C3 plant Atriplex patula subsp. hastata. Labelling of C4 acids and the distribution of 14C within [14C]malate isolated from leaves of K. daigremontiana in these experiments suggest that these acids arise by a secondary carboxylation of labelled phosphoenolpyruvate. During the initial burst of photosynthesis following the night period, pulse-chase labelling of photosynthetic intermediates was quite similar to that observed in A. spongiosa, a C4 species. Degradation of labelled malate suggests that the C4 acids are formed largely by the primary carboxylation of phosphoenolpyruvate during the initial burst of photosynthesis and that these intermediates mix with the pool of malate formed in previous dark fixation. Growth conditions which substantially alter the proportion of CO2 fixed during the clay and night did not alter the pulse-chase behaviour of photosynthetic intermediates.