Photosynthetic and Photorespiratory Carbon Metabolism in Mesophyll Protoplasts and Chloroplasts Isolated from Isogenic Diploid and Tetraploid Cultivars of Ryegrass (Lolium perenne L.).

Photosynthetic (14)CO(2) fixation, [(14)C]glycolate formation, and the decarboxylation of [1-(14)C]glycolate and [1-(14)C]glycine by leaf mesophyll protoplasts isolated from isogenic diploid and tetraploid cultivars of ryegrass (Lolium perenne L.) were examined. The per cent O(2) inhibition of photosynthesis in protoplasts from the tetraploid cultivar was less than that of the diploid line at both 21 and 49% O(2). Kinetic studies revealed that the K(m) (CO(2)) for photosynthesis by the diploid protoplasts was about twice that of the tetraploid line. In contrast, the K(i) (O(2)) for protoplast photosynthesis was similar in both cultivars, as was the potential for oxidizing glycolate and glycine to CO(2) via the photorespiratory carbon oxidation cycle. Although the maximal rates of glycolate accumulation by the isolated protoplasts in the presence of 21% O(2) and a glycolate oxidase inhibitor were similar in the two cultivars, the percentage of total fixed (14)C entering the [(14)C]glycolate pool and the ratio of the rate of [(14)C]glycolate formation to (14)CO(2) fixation at 21% O(2) and low pCO(2) were about two times greater in protoplasts and intact chloroplasts isolated from the diploid line compared to the tetraploid. These results fully support the recent observation that a doubling of ploidy in various ryegrass cultivars reduced the K(m) (CO(2)) of purified ribulose bisphosphate carboxylase-oxygenase by about one-half without affecting the K(i) (O(2)) (Garrett 1978 Nature 274: 913-915).