The Growth of Citrus Fruits, Satsuma Mandarin and Hassaku, in Relation to the Development of Tissue Systems in the Leaf and Fruit Stalk

Abstract

The development of citrus fruits, satsuma mandarin cv. Miyagawa (Citrus unshiu Marc.) and Hassaku (Citrus hassaku hort. ex Tanaka), was analyzed by deriving the relative growth rate (RGR) and the growth rate from diameter and weight measurements throughout the fruit growing season. The rates were related to the development of tissue systems in the leaf and fruit stalk.
In satsuma mandarin and Hassaku, the maximum peak of RGR in fruit diameter and fruit weight appeared 3 weeks after full bloom, although a very brief but higher peak occurred immediately after flowering. Cumulative diameter and weight increases have a single sigmoid pattern corresponding with changes in growth rate. In both cultivars, the rising in the growth rate of fruit diameter commenced at an early developmental stage and ceased earlier than the onset and completion of fruit weight. High values in the growth rate of fruit diameter lasted for approximately 12 weeks between mid-June and mid-September in satsuma mandarin, and for 15 weeks mid-June and early-October in Hassaku. On the other hand, the high values of growth rate of fruit weight were marked for 8 weeks between mid-August and late-October in satsuma mandarin, and for 13 weeks between mid-July and late-October in Hassaku. The pattern of the rate in the fresh weight increase was almost similar to the dry weight increase throughout the fruit growing season.
In both cultivars, mesophyll development was almost completed by early-June. In satsuma mandarin, the development of palisade, spongy tissues, and intercellular spaces of the mesophyll was almost completed during the early stages of fruit development, being between late-May and early-June. In Hassaku, the tissue system of mesophyll differentiated and the differentiation was completed between mid-May and early-June. The xylem tissues in the petiole, especially xylem vessels, were formed as early as the first week of May, developed markedly by late-May, and were almost completed by mid-June in satsuma mandarin. In Hassaku, the tissue systems in the petiole began one week earlier than that of the satsuma mandarin, developed strikingly by mid-May, and was almost completed by early-June.
The xylem vessel differentiation in the fruit stalk had begun about one month before anthesis. These tissues developed strikingly approximately 4 weeks after blooming. The development of xylem tissue lasted until mid-July, and slightly progressed by early-September. There appeared to be a correlation between the onset of fruit weight increase and the completion of xylem tissue development in the stalk. Stalk diameter and xylem tissue development determined on the basis of number and diameter of xylem vessels were greater in Hassaku fruits than that in satsuma mandarin.