Roles for leakiness and O₂ evolution in explaining lower-than-theoretical quantum yields of photosynthesis in the PEP-CK subtype of C₄ plants

Theoretically, the PEP-CK C-4 subtype has a higher quantum yield of CO2 assimilation (Phi CO2) than NADP-ME or NAD-ME subtypes because ATP required for operating the CO2-concentrating mechanism is believed to mostly come from the mitochondrial electron transport chain (mETC). However, reported Phi CO2 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 CO2 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 Phi CO2, but suggest that PEP-CK species probably use less ATP from mETC than classically defined PEP-CK mechanisms.