Emilie Wientjes

Summary Theoretically, the PEP‐CK C 4 subtype has a higher quantum yield of CO 2 assimilation () than NADP‐ME or NAD‐ME subtypes because ATP required for operating the CO 2 ‐concentrating mechanism is believed to mostly come from the mitochondrial electron transport chain (mETC). However, reported is not higher in PEP‐CK than in the other subtypes. We hypothesise, more photorespiration, associated with higher leakiness and O 2 evolution in bundle‐sheath (BS) cells, cancels out energetic advantages in PEP‐CK species. Nine species (two to four species per subtype) were evaluated by gas exchange, chlorophyll fluorescence, and two‐photon microscopy to estimate the BS conductance ( g bs ) and leakiness using a biochemical model. Average g bs estimates were 2.9, 4.8, and 5.0 mmol m −2 s −1 bar −1 , and leakiness values were 0.129, 0.179, and 0.180, in NADP‐ME, NAD‐ME, and PEP‐CK species, respectively. The BS CO 2 level was somewhat higher, O 2 level was marginally lower, and thus, photorespiratory loss was slightly lower, in NADP‐ME than in NAD‐ME and PEP‐CK species. Differences in these parameters existed among species within a subtype, and g bs was co‐determined by biochemical decarboxylating sites and anatomical characteristics. Our hypothesis and results partially explain variations in observed , but suggest that PEP‐CK species probably use less ATP from mETC than classically defined PEP‐CK mechanisms.