ABSTRACT

The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) as well as phosphoenolpyruvate (PEP) carboxylase along with photosynthetic carbon metabolism in cotton plants ( Gossypium hirsutum L.) was studied in the run of greening process, of plants' and leaves' ontogenesis in different parts of cotton stem. We proved that C3 carbon metabolism is active during all stages of cotton leaf's development and that PGA is the main primary product. At the same time the amount of C₂-, C₃, C₄-metabolites varied in the run of leaf's aging. The highest values of the activity of photosynthesis as well as the activity of carboxylation enzymes were observed in the middle leaves (the 3d and the 4th true leaves) at the moment of study. Specific activity of Rubisco - from 0,29 mk mol ¹⁴CO2 /mg Rubisco min in the juvenile leaf to 3,18 mk14CO2/mg Rubisco min in the 3d true leaf carries evidence on the most pronounced difference between the leaves. The index of PEP carboxylase activity (total one, in particular) proved to be in positive correlation with the rate of CO2 assimilation in photosynthesis/ However the highest PEP carboxylase activity was observed in juvenile leaves which are characterized by very weak level of photosynthetic CO2 assimilation. So the rate of C₂-, C₃, C₄-products' synthesis varied significantly in the leaves located in different levels of the cotton stem. The index of phosphoglyceric acid's (PGA) labeling increased from the first to fifth cotton leaves in synchronous manner with its simultaneous decrease in the 6-7th leaves. The volume of ¹⁴C incorporation into C4 dicarboxylic acids was insignificant in all leaves except juvenile ones, where the labeling of malate, aspartate, citrate and glycolate pathway intermediates was much greater than that found in other leaves. However, even in this case, the bulk of ¹⁴C level was incorporated into Calvin cycle metabolites. Thus, the most significant differences observed in the relationship between the early-labeling C₂-, C₃, C₄- products indicate a strong depends of ¹⁴C distribution among the photosynthetic products in the run of leaf aging.