C4-Pathway photosynthesis in Portulaca oleracea and the significance of alanine labelling.

Portulaca oleracea L. has some features common to C4 species but it has been suggested from other studies that a substantial proportion of assimilated carbon dioxide is incorporated into pyruvate and alanine by routes not involving C4 acids or 3-phosphoglycerate. The present paper reports that enzyme activities in P. oleracea leaves, and the labelling patterns observed during assimilation of (14)CO2, are entirely consistent with the operation of the C4 pathway, with most of the label appearing initially in the C-4 of C4 acids, followed by the prominent labelling of 3-phosphoglycerate, hexose phosphates and then sucrose and starch after longer periods. In contrast to other recent observations, neither alanine nor pyruvate was a prominent early-labelled product, and the observed labelling of alanine was consistent with it being derived from radioactive carbon initially incorporated into C4 acids.