Sucrose phosphate synthase and sucrose synthase activity during maturation of internodal tissue in sugarcane

Sucrose accumulation rates, sucrose-phosphate synthase (SPS, EC 2.4.1.14) and soluble sucrose synthase (SuSy, EC 2.4.1.13) activities were measured in internodal tissue from a sugarcane (Saccharum species hybrids) variety N19. The sucrose accumulation rate sharply increases between internodes 3 to 11. In the older internodes SPS activity was at least three times higher than the SuSy activity. A highly significant positive correlation was found between SPS activity and sucrose content. In contrast, no significant correlation was observed between SuSy and sucrose content. In agreement, when radiolabelled glucose was fed to internodes with a high sucrose accumulation rate, label was equally distributed in the hexose moieties of sucrose. This clearly indicates that SPS is the major sucrose synthesis activity in the culm of sugarcane. Different kinetic forms of SPS apparently exist in the internodal tissue at different stages of development.