Studies of intermediary metabolism in radish cotyledons. Turnover of photosynthetic products in 14CO2 pulse-chase experiments
Ability of 11-day-old radish cotyledons to carry out photosynthesis has been examined by supplying 14CO2 to cotyledon disks for periods ranging from 30 s to 20 min. Carbon-14 was rapidly incorporated into malate, citrate, sugar phosphates, and a number of amino acids. The kinetics of 14C incorporation suggest that carbon initially enters components of the organic acid and sugar phosphate fraction, being then rapidly transferred to the amino acids and sugars. In pulse-chase experiments, illuminated cotyledon disks were incubated with 14CO2 for 30 min then transferred to air containing 12CO2 for periods up to 240 min with illumination. During such postpulse incubations, considerable losses of 14C occurred from most of the principal compounds labeled during the chase incubation. As these losses were not entirely accounted for by losses of label to the incubation medium they can largely be attributed to metabolic turnover of these compounds. It is concluded that, in general, the photosynthesis of radish cotyledons is similar to that of mature leaves and of physiological significance as a route for synthesis of several important cellular constituents required for seedling growth and development.