DIFFERENTIAL CARBON-MONOXIDE SENSITIVITY OF CYTOCHROME-C-OXIDASE IN THE LEAVES OF C3 AND C4 PLANTS

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 C O to O2 ratios. All the C3 plants were relatively insensitive to CO an d required a high CO to O2 ratio of 40 to promote significant nitrate reductase activity. However, when treated with 2 millimolar 2,4-dinitr ophenol, these leaves readily responded to CO even at low CO to O2 rat ios of 10 or less. On the other hand, the leaves of all C4 plants test ed, belonging to the three subgroups, were highly sensitive to CO, eve n at CO to O2 ratios of 5 or less. In these leaves, the uncoupler was without any effect, probably because the mitochondria, either from mes ophyll or bundle sheath cells or both, lacked tight respiratory contro l.