The Influence of Leaf Development on the Expression of C4 Metabolism in Flaveria trinervia, a C4 Dicot

The initial products of ¹⁴CO₂ assimilation were determined understeady state illumination of leaves of Flaveria trinervia, aC₄ dicot of the NADP-mialic enzyme subgroup. Leaf age influencedthe partitioning of ¹⁴CO₂ between the C₄ cycle and the reductivepentose phosphate (RPP) pathway. An estimated 10 to 12%of theCO₂ entered the RPP pathway directly in leaves about 20% fullyexpanded, whereas CO₂ was apparently fixed entirely throughthe C₄ pathway in leaves 75% or more expanded. This partitioningpattern was attributed to the bundle sheath compartment in youngleaves having a relatively high conductance to CO₂ (i.e., beingsomewhat leaky). Of the initially labelled C₄ acids, the proportion that wasmalate, relative to aspartate, increased continuously duringleaf expansion (from 60 : 40 to 87 : 13 at full expansion).Concurrently, there was an increase in the whole leaf activityof NADP malate dehydrogenase and a decrease in the activitiesof aspartate and alanine aminotransferases. Low chlorophylla/b values were observed in young leaves, which may coincidewith an enhanced capacity for non-cyclic electron transportin the bundle sheath chloroplasts of such tissue. Both enhancedaspartate metabolism and direct fixation of CO₂ in the bundlesheath could provide a greater sink for utilization of photochemicallyderived NADPH in the bundle sheath of young leaves. Such metabolicchanges are discussed in relation to a possible decrease inCO₂ conductance of the bundle sheath during leaf development. (Received March 4, 1986; Accepted June 25, 1986)