In order to establish the proper method in thinning of kaki 'Saijo', a leading variety of astringent type in the Chugoku district, the effects of NAA sprays in different times and concentrations on fruit drop, and also some factors related to the effectiveness of the spray applications were investigated during the period 1968-1970.
1. Although the mature leaves of 'Saijo' were much larger in size than those of 'Fuyu' and 'Hiratanenashi', they were thinner and distinctively lower in the activity of apparent photosythesis per leaf than the others. When leaf number per fruit on girdled blanches were adjusted by hand to the ratios from 5:1 to 25:1 after physiological fruit drop came almost to the end, the ratio of 15:1 was found to be most superior, followed by 20:1 for increasing the weight and soluble solids of fruits.
2. Through the three years' tests, the most desirable thinning effect was gained when NAA was sprayed at the concentrations of 25 and 50 ppm about 4 weeks after fullbloom. In these treatments, the percent of fruit drop deviated from 28.7 to 62.5 year by year, which resulted in adequate or lower leaf-fruit ratios. The applications about 3 weeks after fullbloom at the same concentrations caused appropriate fruit drop (50.3 - 52.8%) in 1969, but over-drop (71.3-77.5%) in 1970. In a single year of 1970, the sprays at 5 and 10 ppm 9 days after fullbloom were added to the treatments, both of which failed to induce appreciable fruit drop.
3. The peak of fruit drop induced by the NAA sprays about 3 or 4 weeks after fullbloom appeared 4 or 5 days after treatments regardless of the concentrations. The spray at 10 ppm 9 days after fullbloom, however, rather supressed fruit drop for several days soon after treatment and the appearance of its peak was delayed by 3 or 4 days comparing with that in the later sprays with higher concentrations.
4. When fruits were divided into several classes according to the cross diameter by 2 mm interval at the time of application, the percent of fruit drop induced by the sprays was higher in the classes of smaller fruit size.
5. When NAA was applied to the limited portions on shoots ; fruits only, foliage only and whole shoot including fruits, the whole shoot treatment was most effective fcr thinning fruits, followed by the foliage only and fruits only in the order. This result indicated that the NAA absorbed by leaves rather than fruit play an important role in inducing fruit drop.