Plant and Cell Physiology, 1986, Vol. 27, No. 6 1159-1167
© 1986
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The Influence of Leaf Development on the Expression of C4 Metabolism in Flaveria trinervia, a C4 Dicot
1Department of Botany, Washington State University Pullman, Washington, U.S.A. 99164-4230
2Institute of Biological Chemistry, Washington State University Washington, U.S.A. 99164-4230
3 To whom reprint requests should be addressed
The initial products of 14CO2 assimilation were determined under steady state illumination of leaves of Flaveria trinervia, a C4 dicot of the NADP-mialic enzyme subgroup. Leaf age influenced the partitioning of 14CO2 between the C4 cycle and the reductive pentose phosphate (RPP) pathway. An estimated 10 to 12%of the CO2 entered the RPP pathway directly in leaves about 20% fully expanded, whereas CO2 was apparently fixed entirely through the C4 pathway in leaves 75% or more expanded. This partitioning pattern was attributed to the bundle sheath compartment in young leaves having a relatively high conductance to CO2 (i.e., being somewhat leaky).
Of the initially labelled C4 acids, the proportion that was malate, relative to aspartate, increased continuously during leaf expansion (from 60 : 40 to 87 : 13 at full expansion). Concurrently, there was an increase in the whole leaf activity of NADP malate dehydrogenase and a decrease in the activities of aspartate and alanine aminotransferases. Low chlorophyll a/b values were observed in young leaves, which may coincide with an enhanced capacity for non-cyclic electron transport in the bundle sheath chloroplasts of such tissue. Both enhanced aspartate metabolism and direct fixation of CO2 in the bundle sheath could provide a greater sink for utilization of photochemically derived NADPH in the bundle sheath of young leaves. Such metabolic changes are discussed in relation to a possible decrease in CO2 conductance of the bundle sheath during leaf development.
(Received March 4, 1986; Accepted June 25, 1986)
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