Leaf succulence determines the interplay between carboxylase systems and light use during Crassulacean acid metabolism in Kalanchoe species

The photosynthetic physiology of Crassulacean acid metabolism was investigated in two Kalanchoe species with differing leaf succulence. The magnitude of CAM was higher for the more succulent leaves of K. daigremontiana, compared to the less succulent leaves of K. pinnata. High succulence was related to low mesophyll conductance: K. pinnata was able to maximize diurnal carbon gain by the C-3 pathway, whereas increased succulence is associated with a higher commitment to the CAM cycle in K. daigremontiana. The Rubisco specificity factor, tau, determining selectivity for CO2 over O-2, was similar for both species (similar to 88), and lower than that of Spinacea (similar to 95), but in contrast to C-4 plants, the Rubisco K-mCO2 ( determined independently) was also lower in Kalanchoe spp. than in spinach. Differences in light use were related to the nature of the sink strength in each Phase of CAM, with PEPC activity resulting in low electron transport rates. Decarboxylation was marked by high, non-saturated rates of electron transport, with Rubisco activity and activation state increasing in both species during the course of the light period. The degree of succulence, and extent of CAM activity, was associated with a progressive inhibition of PSII photochemistry and potential Rubisco activity during the night in both species. Rubisco could be 'woken up' more rapidly in K. pinnata, whereas 45 min light acclimation was required for full recovery of electron transport and Rubisco activity in K. daigremontiana. Leaf morphology therefore seems to alter the expression of and dependence on CAM, but also the extent of co-regulation of carboxylase networks and light use capacity.