The ability to concentrate CO2 around Rubisco allows C-4 crops to suppress photorespiration. However, as phosphoenolpyruvate regeneration requires ATP, the energetic efficiency of the C-4 pathway at low photosynthetic flux densities (PFD) becomes a balancing act between primary fixation and concentration of CO2 in mesophyll (M) cells, and CO2 reduction in bundle sheath (BS) cells. At low PFD, retro-diffusion of CO2 from BS cells, relative to the rate of bicarbonate fixation in M cells (termed leakiness phi), is known to increase. This paper investigates whether this increase in phi could be explained by incomplete inhibition of photorespiration. The PFD response of phi was measured at various O-2 partial pressures in young Zea mays plants grown at 250 (LL) and 750 mu mol m-2 s-1 PFD (HL). phi increased at low PFD and was positively correlated with O-2 partial pressure. Low PFD during growth caused BS conductance and interveinal distance to be lower in the LL plants, compared to the HL plants, which correlated with lower phi. Model analysis showed that incomplete inhibition of photorespiration, especially in the HL plants, and an increase in the relative contribution of mitochondrial respiration at low PFD could explain the observed increases in phi.
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