Carbon partitioning during sucrose accumulation in sugarcane internodal tissue

The temporal relationship between sucrose (Suc) accumulation and carbon partitioning was investigated in developing sugarcane internodes. Radiolabeling studies on tissue slices, which contained Suc concentrations ranging from 14 to 42% of the dry mass, indicated that maturation coincided with a redirection of carbon from water-insoluble matter, respiration, amino acids, organic acids, and phosphorylated intermediates into Sue. It is evident that a cycle of Suc synthesis and degradation exists in all of the internodes. The decreased allocation of carbon to respiration coincides with a decreased flux from the hexose pool. Both the glucose and fructose (Fru) concentrations significantly decrease during maturation. The phosphoenolpyruvate, Fru-6-phosphate (Fru-6-P), and Fru-2,6-bisphosphate (Fru-2, 6-P-2) concentrations decrease between the young and older internodal tissue, whereas the inorganic phosphate concentration increases. The calculated mass-action ratios indicate that the ATP-dependent phosphofructokinase, pyruvate kinase, and Fru-1,6-bisphosphatase reactions are tightly regulated in all of the internodes, and no evidence was found that major changes in the regulation of any of these enzymes occur. The pyrophosphate-dependent phosphofructokinase reaction is in apparent equilibrium in all the internodes. Substrate availability might be one of the prime factors contributing to the observed decrease in respiration.