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Caixi Zhang, Kenji Tanabe, Fumio Tamura, Akihiro Itai, and Shiping Wang

The aim of this study was to investigate the roles of spur characteristics and carbon partitioning in regulating cultivar differences in fruit size of two late-maturing japanese pear cultivars, `Atago' and `Shinkou'. The study of spur characteristics showed that the two cultivars displayed different patterns in leaf development, flower characteristics, fruit growth, and shoot type. In contrast to `Atago' with dramatically larger fruit, `Shinkou' is a heavily spurred cultivar with a higher total leaf area and leaf number per spur early in fruit growth, less vegetative shoots, and smaller fruit but larger core. No significant differences were obtained in specific leaf weight, leaf thickness, chlorophyll content, and net photosynthesis of mature leaves, and seed number per fruit between the two cultivars. The results of trace experiment with 13C revealed that on a spur basis, there were no significant differences in the amount of 13C assimilate produced by spur leaves on each labeling date except at 190 days after anthesis, however, there were highly significant differences in the amount of 13C allocated to fruit between cultivars. Moreover, a higher amount of 13C assimilates was allocated to `Atago' flesh (or fruit) than that in `Shinkou'. Analysis of relative sink strength (RSS) indicates that the sink strength of fruit was dominant over those of other organs in the spur measured in both cultivars except at the early stage of fruit growth. `Atago' exhibited a greater RSS of fruit and lower losses of 13C for respiration and export than `Shinkou'. These results suggest that the movement of photosynthates into the fruit was determined by sink strength of the fruit rather than the source strength in the two cultivars.

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Caixi Zhang, Kenji Tanabe, Hiroko Tani, Hiromitsu Nakajima, Minori Mori, and Emi Sakuno

Both ‘Atago’ and ‘Shinkou’ are late-season japanese pear (Pyrus pyrifolia Nakai) cultivars with russet-brown fruit, and the progenies of crosses made between ‘Nijisseiki’ and unknown cultivars, but they display different growth habit, fruit size, and fruit quality. To clarify the difference in fruit development between the two cultivars, the levels of endogenous gibberellins (GA1, GA3, and GA4) and abscisic acid (ABA) in fruit were identified and quantified by gas chromatography/mass spectrometry, and a histological study of fruit was carried out. The results showed that cell number rather than cell size of mesocarp is responsible for the difference in fruit size between the two cultivars. Furthermore, analysis of endogenous bioactive gibberellins revealed that ‘Atago’ fruit has much higher levels of GA1, GA3, and GA4 than ‘Shinkou’ fruit during fruit development, particularly during the early period of fruit growth. However, ‘Shinkou’ has a much higher ABA level than ‘Atago’ during the early period of fruit development. Abscisic acid concentration remained at an extremely low level after the first production peak after anthesis in ‘Atago’ compared with ‘Shinkou’.