Relationship between fruit enlargement and seed development with respect to physiological fruit drop in 'Shimizu Hakuto' peach

The relationships between fruit enlargement and development of seed components, particularly embryo were investigated in 2000 and 2001 with respect to physiological fruit drop by using 'Shimizu Hakuto' peach (Prunus persica Batsch) trees subjected to different fruit thinning levels. As the standard fruit thinning (SFT), the first thinning was conducted in early May to 2,3-fold levels as final crop load (about 70 leaves per fruit at harvest), followed by the second thinning in early June to the final level. For heavy fruit thinning (HFT), fruit density was adjusted in early May to the final crop load and for light fruit thinning (LFT), in early June to 1.5-fold of the final crop load. The percentage of physiological fruit drop was significantly higher (P < 0.05) in trees under HFT than in LFT and SFT in both years. The percentage of split-pit fruit was significantly higher (P < 0.05) in HFT than LFT in 2000 but there was no significant difference between HFT and SFT in 2001. At the end of Growth Stage 2, fruit weight was proportional to seed size, but it was inversely related to embryo size in 2000 under both HFT and LFT. The mean embryo length of HFT fruit was two-thirds of that of LFT, although the different thinning levels had no significant effect on endosperm length. There was no difference in embryo length between those of split-pit and non-split-pit fruit at early Growth Stage 2. However, at the late Growth Stage 2 in HFT trees, the embryos diverted into two types: normal and very small (< 3 mm); this diversion was not observed in SFT. In HFT trees, there was an inverse relationship between fruit weight and dry matter accumulation of seed components. The larger the fruit, the smaller was the dry matter accumulation in the nucellus and endosperm. Thus, the increased rate of physiological fruit drop in HFT trees might be attributed to the reduction of photosynthates partitioned to the seed because of rapid fruit enlargement which leads to inferior embryo development.