Abstract
‘Akiou’ is a nonaploid (2n=9x=135) seedless pollination-constant non-astringent (PCNA) type of Japanese persimmon characterized by a large fruit size, high sugar content, and excellent taste, and has been introduced in many regions. However, ‘Akiou’ shows early physiological fruit drop, which is due to the plant being anucleate and containing neither strong monosperms nor pseudo-monosperms. In this study, we investigated the effect of duration of sunshine and shading on the physiological fruit drop of ‘Akiou’ to elucidate a factor causing physiological fruit drop. The results showed that the fruit set rate was lower than 25% when the total duration of sunshine was less than 1 h on two or more consecutive 3-day periods from full bloom to 70 days after full bloom. The fruit set under 50% shading was significantly lower than that in the control. The decrease in photosynthetically available photosynthetic photon flux density (PPFD), due to the decrease in total integrated duration of sunshine for 3 consecutive days and shading, was found to be a factor in the physiological fruit drop of ‘Akiou'. Therefore, to establish a stable technique for fruit drop control, we also investigated the relationship between girdling position and physiological fruit drop. The results of girdling on branches from different parts of the same tree revealed that the smaller the branch circumference at the girdling position, the higher the fruit set rate. These results suggested that the physiological drop of ‘Akiou’ is caused by a decrease in assimilate production due to reduced duration of sunshine and PPFD, leading to insufficient supply of assimilates to the fruit due to competition for nutrients between the fruit and the branch trunks. Additionally, girdling closer to the fruit was found to help avoid nutrient competition with the branch trunk and suppress physiological drop.
