Differential Carbon Monoxide Sensitivity of Cytochrome c Oxidase in the Leaves of C3 and C4 Plants

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Abstract
CO sensitivity of cytochrome a3 in the leaves of a number of C3 and C4 plants was monitored by the nitrate reductase assay under differing CO to O2 ratios. All the C3 plants were relatively insensitive to CO and required a high CO to O2 ratio of 40 to promote significant nitrate reductase activity. However, when treated with 2 millimolar 2,4-dinitrophenol, these leaves readily responded to CO even at low CO to O2 ratios of 10 or less. On the other hand, the leaves of all C4 plants tested, belonging to the three subgroups, were highly sensitive to CO, even at CO to O2 ratios of 5 or less. In these leaves, the uncoupler was without any effect, probably because the mitochondria, either from mesophyll or bundle sheath cells or both, lacked tight respiratory control.