Fruits of orange-fleshed and green-fleshed muskmelon (Cucumis melo L.) were harvested at different times throughout development to evaluate changes in metabolism which lead to sucrose accumulation, and to determine the basis of differences in fruit sucrose accumulation among genotypes. Concentrations of sucrose, raffinose saccharides, hexoses and starch, as well as activities of the sucrose metabolizing enzymes sucrose phosphate synthase (SPS) (EC 2.4.1.14), sucrose synthase (EC 2.4.1.13), and acid and neutral invertases (EC 3.2.1.26) were measured. Sucrose synthase and neutral invertase activities were relatively low (1.7 ± 0.3 micromole per hour per gram fresh weight and 2.2 ± 0.2, respectively) and changed little throughout fruit development. Acid invertase activity decreased during fruit development, (from as high as 40 micromoles per hour per gram fresh weight) in unripe fruit, to undetectable activity in mature, ripened fruits, while SPS activity in the fruit increased (from 7 micromoles per hour per gram fresh weight) to as high as 32 micromoles per hour per gram fresh weight. Genotypes which accumulated different amounts of sucrose had similar acid invertase activity but differed in SPS activity. Our results indicate that both acid invertase and SPS are determinants of sucrose accumulation in melon fruit. However, the decline in acid invertase appears to be a normal function of fruit maturation, and is not the primary factor which determines sucrose accumulation. Rather, the capacity for sucrose synthesis, reflected in the activity of SPS, appears to determine sucrose accumulation, which is an important component of fruit quality.Midway through development, muskmelon (Cucumis melo L.) fruits undergo a metabolic transition marked by both physical and compositional changes such as netting of the exocarp, mesocarp softening, and the onset of sucrose accumulation (1,7,10,12

(6,25,27). Initially the predominant sugar is sucrose. Hexose sugars appear after sucrose, and ultimately exceed sucrose concentration.

1991. Sucrose phosphate synthase and other sucrose metabolizing enzymes in fruits of various species Recent reports have suggested that sucrose phosphate synthase (EC 2.4.1.14), a key enzyme in sucrose biosynthesis in photosynthetic "source" tissues, may also be important in some sucrose accumulating "sink" tissues. These experiments were conducted to determine if sucrose phosphate synthase is involved in sucrose accumulation in fruits of several species. Peach (Prunus persica NCT 516) and strawberry (Fragaria x ananassa cv. Chandler) fruits were harvested directly from the plant at various stages of fruit development. Kiwi (Actinidia chinensis), papaya (Carica papaya), pineapple (Ananas comosus) and mango (Mangifera indica) were sampied in postharvest storage over a period of several days. Carbohydrate concentrations and activities of sucrose phosphate synthase, sucrose synthase (EC 2.4.1.13), and acid and neutral invertases (EC 3.2.1.26) were measured. All fruits contained significant activities of sucrose phosphate synthase. Moreover, in fruits from all species except pineapple and papaya, there was an increase in sucrose phosphate synthase activity associated with the accumulation of sucrose in situ. The increase in sucrose concentration in peaches was also associated with an increase in sucrose synthase activity and, in strawberries, with increased activity of both sucrose synthase and neutral invertase. The hexose pools in ail fruits were comprised of equimolar concentrations of fructose and glucose, except in the mango. In mango, the fructose to glucose ratio increased from 2 to 41 during ripening as sucrose concentration more than doubled. The results of this study indicate that activities of the sucrose metabolizing enzymes, including sucrose phosphate synthase, within the fruit itself, are important in determining the soluble sugar content of fruits of many species. This appears to be true for fruits which sweeten from a starch reserve and is fruits from sorbito! translocating species, raffinose saccharide translocating species, and sucrose translocating species.

No abstract

Muskmelon (Cucumis melo L.) fruit lack a stored starch reserve and therefore depend on translocated photoassimilate from the leaf canopy for sugar accumulation during ripening. The influence of canopy photosynthesis on sucrose' accumulation within muskmelon fruit mesocarp was examined. Canopy photosynthetic activities were estimated in a sweet and a nonsweet genotype. Photosynthetic rate of the nonsweet genotype, on a per-plant basis, was only 56% of that of the sweet genotype. The effect of limiting leaf area of the sweet genotype on carbohydrate concentrations and sucrose metabolizing enzymes within the fruit was evaluated. A 50% reduction of leaf area 8 days before initiation of fruit sucrose accumulation resulted in canopy photosynthesis similar to that of the nonsweet genotype. Reduced photosynthetic activity resulted in slightly lower soluble-carbohydrate concentration in the fruit; however, fruit sucrose concentration was three times higher than that reported previously for the nonsweet genotype. The extent to which `fruit sucrose phosphate synthase (SPS) activity increased during maturation was diminished by leaf removal. Acid invertase activity declined in all fruit in a similar manner irrespective of defoliation. A reduction of leaf area of a sweet genotype reduced sucrose accumulation within the fruit. Lower fruit sucrose concentration was associated with lower concentration of raffinose saccharides and lower SPS activity within the fruit. Additionally, insufficient assimilate supply was judged not to be the factor responsible for low sucrose accumulation in a nonsweet genotype.