In the C4plant,Amaranthus graecizans, increasing O2 from 2 up to 100 inhibited photosynthesis, quantum yield, and the carboxylation efficiency, and increased the CO2compensation point (Γ) from 2 to about 12 μl/l. The O2inhibition of photosynthesis was fully reversible. When changing from 2.5 to 40 O2and vice versa, about 1 h was required for full equilibration with an O2inhibition of 18; whereas in wheat, a C3species, inhibition of photosynthesis and its reversal occurs within minutes after changing O2, resulting in 63 inhibition of photosynthesis by 45 O2. These differences in O2inhibition between a C4and C3species can be explained by high diffusive resistance across bundle-sheath cells of C4plants and the increased CO2/O2ratio in bundle-sheath cells which is the consequence of the C4cycle. InA. graecizans, Γ increased with increasing O2 but tended to reach a maximum at relatively high O2levels. The lack of a linear increase in Γ as previously observed for C3species indicates that a considerable amount of photorespired CO2may be re-fixed with increasing levels of O2. In comparison to previous reports with other C4species, photosynthesis ofA. graecizansshows greater sensitivity to O2, with a noticeable inhibition occurring with shifts from 2 to 21 O2.A. graecizanshas characteristics of other C4species with respect to Kranz anatomy, localization of PEP carboxylase in mesophyll cells and RuBP carboxylase in bundle-sheath cells, and little fractionation among carbon isotopes during CO2fixation. The basis for the higher sensitivity of photosynthesis ofA. graecizansto O2may be based upon a lower diffusive resistance of gases across bundle-sheath cells than in some other C4spec
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