The specific radioactivity of glycolic acid in relation to the specific activity of carbon dioxide evolved in light in photosynthesizing sunflower leaves.

Attached leaves of sunflower (Helianthus annuus L.) were exposed to (14)CO2 during steady-state photosynthesis for 2 to 30 min in 345 μl/l CO2 and 21% O2 at 29° C and a light intensity of 1300 μE m(-2)s(-1). Glycolic acid was extracted with water and diethyl ether, and was determined in the aqueous residue by high-pressure liquid column chromatography. The relative specific radioactivity of the glycolic acid synthesized during photosynthesis reached about 100% after 30 min of photosynthesis and was almost equal to that of the CO2 evolved during photorespiration, their ratio at all times being nearly one. These results provide strong in-vivo evidence that the glycolic acid is the substrate for CO2 evolved by sunflower leaves in light.