The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) as well as phosphoenolpyruvate (PEP) carboxylasealong with photosynthetic carbon metabolism in cotton plants ( Gossypium hirsutum L.) was studied in the run of greeningprocess, of plants' and leaves' ontogenesis in different parts of cotton stem. We proved that C3 carbon metabolism is activeduring all stages of cotton leaf's development and that PGA is the main primary product. At the same time the amount ofC_2-, C_3-, and C4–metabolites varied in the run of leaf's aging. The highest values of the activity of photosynthesis as well asthe activity of carboxylation enzymes were observed in the middle leaves (the 3d and the 4th true leaves) at the moment ofstudy. Specific activity of Rubisco - from 0,29 mk mol 14CO_ /mg Rubisco min in the juvenile leaf to 3,18 mk14CO_/mgRubisco min in the 3d true leaf carries evidence on the most pronounced difference between the leaves. The index of PEPcarboxylase activity (total one, in particular) proved to be in positive correlation with the rate of CO_ assimilation in photosynthesis/However the highest PEP carboxylase activity was observed in juvenile leaves which are characterized by veryweak level of photosynthetic CO_ assimilation. So the rate of C_2-, C3, C4-products' synthesis varied significantly in the leaveslocated in different levels of the cotton stem. The index of phosphoglyceric acid's (PGA) labeling increased from the first tofifth cotton leaves in synchronous manner with its simultaneous decrease in the 6-7th leaves. The volume of 14C incorporationinto C4 dicarboxylic acids was insignificant in all leaves except juvenile ones, where the labeling of malate, aspartate, citrateand glycolate pathway intermediates was much greater than that found in other leaves. However, even in this case, the bulkof 14C level was incorporated into Calvin cycle metabolites. Thus, the most significant differences observed in the relationshipbetween the early-labeling C_2-,C_3-,and C4- products indicate a strong depends of 14C distribution among the photosyntheticproducts in the run of leaf aging.
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