Pattern and Trigger of Fruit Self-thinning in Japanese Pears

In fruit production, fruit thinning is required to maximize fruit quality and to protect the mother trees. However, thinning is troublesome and laborious work. Fruit self-thinning is the spontaneous elimination of pollinated flowers or fruits within a week to a month after pollination. Since the fruit self-thinning trait has the potential to improve fruit trees, a number of studies using fruit crops such as apple, orange, and tomato have been conducted to clarify the underlying mechanisms. The Japanese pear accession ‘Chojuro’ and its descendants ‘Niitaka’, 162-29, and ‘Akiakari’ have this trait. To identify the pattern and trigger of thinning in the accessions, we pollinated all flowers on the flowering day and recorded the numbers of retained and abscised fruits and the order of flowering in the cluster. The number of retained flowers/fruits in a cluster was widely variable in ‘Chojuro’ and ‘Niitaka’, but was uniform at 3 to 6 fruits per cluster in 162-29 and ‘Akiakari’. In 162-29 and ‘Akiakari’, the earlier the flower opened, the more likely it was to be retained in the cluster, similar to previous observations in apple. In contrast, ‘Chojuro’ and ‘Niitaka’ fruits abscised independently of the flowering order. Therefore, the pattern of fruit self-thinning in pear depends on the accession. To identify the trigger for fruit self-thinning, we analyzed changes in the levels of endogenous auxins and abscisic acid. The results implicate auxin and, to a lesser extent, abscisic acid in fruit self-thinning. Retained fruits showed temperature-dependent transient auxin accumulation, which may trigger self-thinning in pear.