¹⁴CO₂ fixation in leaf discs of <i>Phaseolus vulgaris</i>

Abstract

Leaf discs of Phaseolus vulgaris L. were exposed to 14 CO 2 in light, and transient changes in the radioactivity of photosynthetic intermediates were determined when illumination was interrupted or the CO 2 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 14 C-sucrose and in the total 14 C-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 CO 2 concentration resulted in a transient rise in 14 C-glycolate which was followed by a temporary increase in 14 C-serine-glycine and in the ambient CO 2 concentration. This supports the hypothesis that glycolate produced in photosynthesis is a substrate for glycine and serine synthesis as well as for CO 2 evolution.