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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