Fruit quality of melon is determined primarily by its sugar content (Yamaguchi et al., 1977). The mature fruit sugar content in sweet melons is comprised of the disaccharide sucrose and its two hydrolysis products, the hexoses glucose and fructose. However, the increase in total sugar content during fruit maturation is due particularly to the accumulation of sucrose during the final stages of fruit development (Burger et al., 2000; Hubbard et al., 1989, 1991; Lingle and Dunlap, 1987; McCollum et al., 1988; Rosa, 1928; Schaffer et al., 1987, 1996). Variation in sucrose levels also accounts for the genetic differences in total sugar contents (Burger et al., 2000; Stepansky et al., 1999) and for the natural variability within a particular cultivar as a result of environmental and developmental differences (Burger et al., 2000).
Although source primary photosynthate production will play a role in determining the availability of assimilate supply, the accumulation of sucrose appears to be controlled by the metabolism of carbohydrates in the fruit sink itself (Hubbard et al., 1989; Lester et al., 2001, Schaffer et al., 1996, 2000). Sucrose and the galactosyl–sucrose oligosaccharides, raffinose and stachyose, are translocated from the source to fruit sink in the Cucurbitaceae family, including Cucumis melo (Chrost and Schmitz, 1997; Mitchell et al., 1992). The near absence of raffinose and stachyose in the melon fruit flesh points to the rapid hydrolysis and metabolism of these translocated sugars in the fruit, or adjacent to it (Chrost and Schmitz, 1997; Hubbard et al., 1989; Hughes and Yamaguchi, 1983; Pharr and Hubbard, 1994).
Previous studies have shown that the melon fruit undergoes a metabolic transition from the stage of fruit growth to that of sucrose accumulation, characterized by a developmental loss of soluble acid invertase (AI) activity (Hubbard et al., 1989; Iwatsubo et al., 1992; Lester et al., 2001; McCollum, et al., 1988; Ranwala et al., 1991; Schaffer et al., 1987). The same phenomenon has been described for other sucrose-accumulating fruit (Hubbard et al., 1990, 1991; Schaffer et al., 1989), including species of tomato (Solanum lycopersicun L.) (Miron and Schaffer, 1991; Stommel, 1992; Yelle et al., 1991).
A key role for sucrose phosphate synthase (SPS) activity in sucrose-accumulating melon fruit was proposed by Hubbard et al. (1991), who showed that sucrose accumulation in melon fruit was characterized by a developmental increase in SPS activity, in addition to the developmental loss of AI activity. Lester et al. (2001) confirmed the importance of the loss in AI activity and the increase in SPS activity in two sweet melon cultivars and emphasized particularly the necessity for SPS activity to be higher than that of AI. An increase in SPS activity was similarly reported for other sucrose-accumulating fruit (Irving et al., 1997; Langenkamper et al., 1998; Miron and Schaffer, 1991; Yelle et al., 1991). The increase in SPS activity implies physiological significance and indicates the importance of sucrose synthesis in the fruit, perhaps as part of a hydrolysis–resynthesis scheme of sucrose accumulation. As such, it was hypothesized to be a potential key controlling point in determining sugar accumulation and content (Miron and Schaffer, 1991). However, Klann et al. (1993) reported no significant differences in SPS activity between fruit of sucrose-accumulating and hexose-accumulating genotypes of tomatoes and, similarly, Stommel (1992) did not observe an increase in SPS activity concomitant with sucrose accumulation in the sucrose-accumulating wild tomato species Lycopersicon peruvianum (L.) Mill. (Solanum peruvianum L.).
Both sucrose synthase (Giaquinta, 1979; Moriguchi, et al., 1990, 1992; Schaffer et al., 1987; Suzuki et al., 1996) and neutral, or alkaline, invertase (Glasziou and Gayler, 1972; Kato and Kubota, 1978; Ricardo and Rees, 1970) activities have also been implicated in sucrose accumulation. However, the role of these enzymes in sucrose accumulation is not obvious because SuSy is generally associated with sucrose cleavage rather than synthesis, and neutral invertase (NI) catalyzes sucrose hydrolysis. Accordingly, Hubbard et al. (1989) related to SuSy and NI as cleavage enzymes, together with AI, in their calculations of the relationship between sucrose metabolism enzyme activities and sucrose accumulation. In the sugarcane (Saccharum officinarum L.) stem (Zhu et al., 1997), sucrose accumulation was associated with the developmental loss of AI activity and an increase in SPS activity; however, SuSy and NI activities were unrelated to the increase in sucrose content.
The putative roles of the aforementioned sucrose metabolizing enzymes have been hypothesized primarily from developmental correlative studies of a small number of genotypes, frequently only one or two in each study. Comparative and correlative studies based on small numbers of genotypes may be misleading. The purpose of the current research was to expand the study of the contributions of the sucrose metabolizing enzymes to sucrose accumulation, in light of the conflicting evidence for their role in sucrose-accumulating fruit. We took an approach that we feel minimizes the limits of correlative studies and compared, developmentally, a large number of genotypes that span the spectrum of genetic variability of sucrose accumulation.
The C. melo species comprises a spectrum of germplasm with a very broad range of sugar levels (Pitrat et al., 2000; Stepansky et al., 1999). At the one extreme are those primitive types with low sugar in the mature fruit, consisting primarily of only hexoses, glucose and fructose. At the other extreme are the cultivated high-sugar sweet dessert melons, characterized by high sucrose levels, in addition to the hexoses. Sugar levels in the species span the continuum from low to high, but the major component contributing to this continuum in total sugar concentration is specifically the sucrose levels (Burger et al., 2000; Stepansky et al., 1999). We took advantage of this genetic resource and compared sucrose accumulation and sucrose metabolism among C. melo genotypes ranging from low to high sucrose accumulation. In addition, we included also a single Cucumis sativus L. genotype, which does not accumulate sucrose.
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