14CO2 fixation in leaf discs of Phaseolus vulgaris

Abstract

Leaf discs of Phaseolus vulgaris L. were exposed to 14CO2 in light, and transient changes in the radioactivity of photosynthetic intermediates were determined when illumination was interrupted or the CO2 concentration lowered. The responses of phosphoglyceric acid, hexose monophosphate, and ribulose diphosphate showed that the Calvin cycle operates in this tissue. Turning off the light caused a rapid drop in 14C-sucrose and in the total 14C-content of the tissue, with a simultaneous rise in radioactive alanine, glutamate, malate, and aspartate, suggesting the rapid entry of sucrose carbon into respiratory pathways. Lowering the CO2 concentration resulted in a transient rise in 14C-glycolate which was followed by a temporary increase in 14C-serine-glycine and in the ambient CO2 concentration. This supports the hypothesis that glycolate produced in photosynthesis is a substrate for glycine and serine synthesis as well as for CO2 evolution.