MEASUREMENT OF THE LEAKAGE OF CO2 FROM BUNDLE-SHEATH CELLS OF LEAVES DURING C-4 PHOTOSYNTHESIS

During C-4 photosynthesis, CO2 is released in bundle-sheath cells by decarboxylation of C-4 acids and then refixed via ribulose-1,5-bisphosphate carboxylase. In this study we examined the efficiency of this process by determining the proportion of the released CO2 that diffuses back to mesophyll cells instead of being refixed. This leak of CO2 was assessed by determining the amount of (CO2)-C-14 released from leaves during a chase in high [(CO2)-C-12] following a 70-s pulse in (CO2)-C-14. A computer-based analysis of the time-course curve for (CO2)-C-14 release indicated a first-order process and provided an estimate of the initial velocity of (CO2)-C-14 release from leaves. From this value and the net rate of photosynthesis determined from the (CO2)-C-14 fixed in the pulse, the CO2 leak rate from bundle-sheath cells (expressed as a percentage of the rate of CO2 production from C-4 acids) could be deduced. For nine species of Gramineae representing the different subgroups of C-4 plants and two NAD-malic enzyme-type dicotyledonous species, the CO2 leak ranged between 8 and 14%. However, very high CO2 leak rates (averaging about 27%) were recorded for two NADP-malic enzyme-type dicotyledonous species of Flaveria. The results are discussed in terms of the efficiency of C-4 photosynthesis and observed quantum yields.