THE FATE OF ASSIMILATED C14O2 IN THE SUGAR BEET LEAF STUDIED BY DISPLACEMENT WITH C12O2

Abstract

Disks of sugar beet leaf that had assimilated C¹⁴O₂ in light for 10 to 30 seconds were exposed to C¹²O₂ in light or darkness, or in combinations of both for periods of 4 to 25 minutes. The changes in distribution of C¹⁴ among the compounds soluble in 80% ethanol were examined as a function of time for each "flush" treatment. In light, most of the C¹⁴ ultimately accumulates in sucrose, while phosphoglycerate, phosphoenolpyruvate, sugar phosphates, glycine, and serine have more transitory roles. In dark, sucrose is formed much more slowly than in light while alanine becomes the main repository of C¹⁴. In light following darkness, the accumulation of C¹⁴ in alanine is reversed in favor of its accumulation in sucrose, with the sugar phosphates acting as intermediates. In dark following light, the converse does not occur; sugar phosphates and alanine lose, rather than gain, C¹⁴, and sucrose shows little tendency to be metabolized. It is suggested that one of the factors directing the course of these interconversions is the level of light-produced phosphate bond energy needed for sucrose synthesis.