Sucrose is the most important factor determining the sweetness of melon fruit. The rate of sucrose accumulation in melon fruit increases during the latter half of the fruit development as the result of cell enlargement (Kano, 2002). In addition, sucrose accumulation has been demonstrated to occur in response to cellular enlargement if cell size is increased by auxin treatment during early fruit development (Kano, 2002) as well as in response to heating fruits (Kano, 2006). Conversely, sucrose accumulation in melon fruit can be suppressed by mechanical restriction resulting from relative decreases in the number of large cells associated with such treatment (Kano, 2004a) or by treatment with plant growth inhibitors (Kano, 2004b). Sucrose accumulation is suppressed in Japanese pears as a result of an increase in the number of small cells that result from 2-chloro-4-pyridyl-N-phenylurea treatment (Kano, 2003). Thus, it is considered that increased sucrose accumulation is associated with a higher number of large cells relative to small cells in the fruit. Most of the imported sugars accumulate in the vacuole of sink-tissue storage cells (Leigh et al., 1979; Yamaki and Ino, 1992). Cell size during the latter stage of fruit development is mostly equal to that of the vacuole because vacuoles in peaches force the cytoplasm to the outside of the mesocarp cells in the middle stages of fruit development (Ishida et al., 1973). Taken together, these findings suggest that the occurrence of lots of large cells is associated with increased sucrose content. The treatment of seeds with maleic hydrazine (MH) was found to increase cell size and vacuolation in wheat seedlings (Mendhulkar, 2000), and low concentrations of MH have been shown to increase cell length in algae (Gupta and Kumar, 1970). Furthermore, the inhibition of sprouting in onion has been demonstrated by MH treatment (Benkeblia, 2004; Benkeblia et al., 2002; El-Otmani et al., 2003) as a result of its effects on inhibiting cell division (Marcano, et al., 2004; Zilkah, et al.,1981), Thus, MH treatment has the effect of both accelerating cell enlargement as well as inhibiting cell division. Consequently, I hypothesized that sucrose content increases in melon fruit with early cell enlargement and with a cessation in cell division after an inflexion point.
I therefore treated melon fruit with MH to clarify the relationship between the size and the number of cells and sugar accumulation in the fruit.
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