Contribution of photosynthetic assimilates to development of flower and spring flush in Citrus unshiu Marc

Carbon-14C dioxide was administered to a bearing tree (an unsprayed control) and a non-bearing tree (fruit bud fromation inhibited by application of a gibberellic acid spray) during the bud formation stage (April 14-15), when the photosynthetic activity in old leaves resumed in early spring. Samples were taken from various parts of the experimental trees one week (April 21) and 4 weeks (May 12, full bloom stage) after the 14CO2 was supplied, and the distribution of radioactivity in the harvested trees was examined. 1. The newly emerging growth organs (floral-, and new shoot-organs) of the bearing tree contained 18.9% of the total 14C activity one week after application, and 35.4% four weeks after; those of the non-bearing tree contained 5.4%, increasing to 43.8%. The percentage of 14C in the floral varied little, from 16-17% at week 1 to 15% at week 4, but in the new shoots it increased greatly, from 2-3% a tweek 1 to 20-43.8% at week 4. 2. The decrease in the amount of 14C in the available carbohydrate fraction of old leaves in the non-bearing tree from the first week to the fourth week was much larger than that of the bearing tree. It was concluded that reserved carbohydrate is applied principally to development of the floral organ, and that carbohydrate produced by old leaves in spring is applied principally to the development of new shoots during the period from the sprouting stage (April 21) to the full bloom stage (May 12).