CARBON PARTITIONING DURING SUCROSE ACCUMULATION IN SUGARCANE INTERNODAL TISSUE

The temporal relationship between sucrose (Suc) accumulation and carbo n partitioning was investigated in developing sugarcane internodes. Ra diolabeling studies on tissue slices, which contained Suc concentratio ns ranging from 14 to 42% of the dry mass, indicated that maturation c oincided with a redirection of carbon from water-insoluble matter, res piration, 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 t o respiration coincides with a decreased flux from the hexose pool. Bo th the glucose and fructose (Fru) concentrations significantly decreas e during maturation. The phosphoenolpyruvate, Fru-6-phosphate (Fru-6-P ), and Fru-2,6-bisphosphate (Fru-2, 6-P-2) concentrations decrease bet ween the young and older internodal tissue, whereas the inorganic phos phate concentration increases. The calculated mass-action ratios indic ate that the ATP-dependent phosphofructokinase, pyruvate kinase, and F ru-1,6-bisphosphatase reactions are tightly regulated in all of the in ternodes, and no evidence was found that major changes in the regulati on of any of these enzymes occur. The pyrophosphate-dependent phosphof ructokinase 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.