ABSTRACrSupr content and composition are major criteria used in judging the quality of netted muskmelon (Cwucmis melo L. var reticulatus) fruit. Sugar composition and four enzymes of sucrose metabolism were determined in 'MNum 45' muskmelon fruit at 10-day intervals binig 10 days after pollination (DAP) until full-slip (35 DAP). Sugar content increased in both outer (green) mesocarp and inner (orange) mesocarp between 20 and 30 DAP. The major proportion of total increase in sugar was attributed to sucrose accumulation. The large increase in sucrose relative to glucose and fructose was accompanied by a dramatic decrease in acid invertase activity, which was highest in both tissues at 10 and 20 DAP, and increases in sucrose phosphate synthase (SPS) and sucrose synthase activities. The green tissue had a lower propordto of total sugar as sucrose, greater invertase activity, and less SPS activity than the orange tissue. Changes in relative sucrose content were highly correlated with changes in enzyme activity. The results strongly suggest that increases in the proportion of sucrose found in melon fruit were associated with a decline in acid invertase activity and an increase in SPS activity approximately 10 days before full-slip. Therefore, these enzymes apparently play a key role in determining sugar composition and the quality of muskmelon fruit. Ethylene. Tissue cylinders were taken along the equatorial plane of each fruit with a 0.8 cm diameter cork borer and cut to a uniform length of 2.5 cm. The tissue cylinders were divided into a 0.5 cm portion including the rind and the remaining 2.0 cm segment made up of the inner orange tissue. Each segment was placed in a 12 x 75 mm glass vial and capped with a rubber serum cap. The headspace gas was allowed to accumulate for 0.5 to 2 h; 1 ml was then analyzed for ethylene by FID-gas chromatography on an alumina column (2.46 m x 0.32 cm).Sugar extraction and analysis. Four cylinders of tissue were taken from each melon as desibed above using a 1.9 cm diameter cork borer. The netting was removed, and the cylinders of mesocarp were divided into the outer 0.5 cm (green) and the inner 2.0 cm (orange). Tissue was weighed and frozen at -80°C

Development of sugarcane (Saccharum spp. hybrids) genotypes with higher sucrose concentrations is desirable. Activities of sucrosemetabolizing enzymes during development of internodal storage tissue may determine the rate of total sugar accumulation and final sucrose concentration. Internodes of seven commercial cultivars were tagged prior to elongation and sampled at intervals from July until December during two growing seasons. Internode elongation was completed by about 380°C d (base temperature 18°C). Dry matter accumulated until about 800°C d. Water content decreased from about 920 to about 720 g kg−1 fresh weight (FW). Soluble acid invertase and sucrose synthase (sucrose cleavage direction) had peaks of activity during elongation, then declined to lower levels. Neutral invertase and sucrose‐phosphate synthase activities increased during development. Sugar accumulated, and the sucrose to total sugar ratio approached 1.0 as internodes developed, although the combined sucrose cleavage activity of the invertases and sucrose synthase in most internodes was greater than the synthetic activity of sucrose‐phosphate synthase. Sugar accumulation rate was not consistently correlated with activity of any enzyme assayed. Sucrose content during internode development was correlated with sucrose‐phosphate synthase activity and the difference between sucrose‐phosphate synthase and acid invertase activities. Sucrose content of mature internodes was not correlated with any enzyme activity. The data do not support the hypothesis that sucrose synthase activity is related to the rate of sugar accumulation, but do support the hypothesis that sucrose‐phosphate synthase and acid invertase play key roles in determining sucrose concentration during maturation in sugarcane internodes.

Sweet sorghum (Sorghum bicolor (L.) Moench] accumulates large amounts of sugar in culm parenchyma after anthesis, but the biochemical basis of this accumulation is not understood. This study was to identify the enzymes responsible for the pattern of sucrose accumulation in sweet sorghum. Sugars and four enzymes of sucrose metabolism were extracted from whole primary culms and individual internodes of primary culms of ‘Rio’ sweet sorghum at intervals during reproductive development. In whole culms, sucrose concentration increased seven‐fold between the boot and mid‐grain filling stages. This increase was accompanied by a decline in soluble acid invertase (pH optimum 5.0) and sucrose synthase activities. Soluble neutral invertase (pH optimum 7.0) activity was evident throughout, although its activity declined between soft dough and physiological maturity. Very low activity (<13 μmole·kg−1 FW·s−1) of sucrosephosphate synthase was also detected throughout development. At the flag leaf stage, the two uppermost internodes were still elongating. These internodes had the highest activities of acid and neutral invertase and sucrose synthase, and virtually no accumulation of sucrose. In succeeding growth stages, sucrose accumulated most rapidly in internodes below the top two. At anthesis, the top six internodes had similar activities, on a fresh weight basis, of all enzymes assayed. The onset of sucrose accumulation in sweet sorghum was associated with the onset of the reproductive phase of growth and the decrease in acid invertase activity, but sucrose concentration was not correlated with the activity of any enzyme. In this regard, sweet sorghum appears to be biochemically different from sugarcane (Saccharum spp.).

Application of [14CJfructosyl sucrose was used to determine whether sucrose cleavage was necessary for sucrose uptake by sugarcane (Saccharum spp.) intemode tissue. Although approximately 25% of 14C in the apoplast was present as fructose, indicating some sucrose cleavage, less than 15% of the label was randomized in the sucrose that remained in the tissue after a 30 minute osmoticum rinse. This is insufficient to support cleavage and resynthesis as the sole sucrose transport scheme. The lack of randomization of label between the glucose and fructose moieties of the sucrose molecule was taken as presumptive evidence that sucrose does not have to be cleaved prior to uptake by parenchyma cells in sugarcane intemode tissue.