Photosynthetic and Photorespiratory Carbon Metabolism in Mesophyll Protoplasts and Chloroplasts Isolated from Isogenic Diploid and Tetraploid Cultivars of Ryegrass (Lolium perenne L.) 1

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Photosynthetic 14CO2 fixation, [14C]glycolate formation, and the decarboxylation of [1-14C]glycolate and [1-14C]glycine by leaf mesophyll protoplasts isolated from isogenic diploid and tetraploid cultivars of ryegrass (Lolium perenne L.) were examined. The per cent O2 inhibition of photosynthesis in protoplasts from the tetraploid cultivar was less than that of the diploid line at both 21 and 49% O2. Kinetic studies revealed that the Km (CO2) for photosynthesis by the diploid protoplasts was about twice that of the tetraploid line. In contrast, the Ki (O2) for protoplast photosynthesis was similar in both cultivars, as was the potential for oxidizing glycolate and glycine to CO2 via the photorespiratory carbon oxidation cycle. Although the maximal rates of glycolate accumulation by the isolated protoplasts in the presence of 21% O2 and a glycolate oxidase inhibitor were similar in the two cultivars, the percentage of total fixed 14C entering the [14C]glycolate pool and the ratio of the rate of [14C]glycolate formation to 14CO2 fixation at 21% O2 and low pCO2 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 Km (CO2) of purified ribulose bisphosphate carboxylase-oxygenase by about one-half without affecting the Ki (O2) (Garrett 1978 Nature 274: 913-915).

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