Effect of bearing angle of satsuma mandarin fruit on rind roughness, pigmentation, and sugar and organic acid concentrations in the juice

Abstract

The effect of the bearing angle of satsuma mandarin (Citrus unshiu Marc.) fruit to the branch (upward or sideward) on rind roughness, pigmentation, and sugar and organic acid concentrations in the juice was investigated. The results obtained were compared with those from different crop loads reported earlier (Kubo et al. 1996). The rind surface of sideward fruit became smoother as the fruit grew, whereas upward fruit developed a rough skin until late Sept.; a marked difference between them was observed from mid-July to early Sept. The time course curve of 'roughness value', which is a plot of the numerical score of rind roughness transformed with an image analyzer, was examined among each treatment. Although upward fruit and those from lightly cropped trees have rough surfaces, they differed physiologically because the former became smooth between early Aug. and late Sept., whereas fruit from the latter remained rough into late Oct. Observation of rind roughness by paraffin section showed that the rind involved 2 types of convex protrusions; one was a projection made just above an oil gland and another was between the oil glands. The former types are detected frequently on mature fruit surface, whereas latter occur on immature ones. Similar convexities were observed on upward bearing fruit and on fruit from trees with a low crop load. Compared to the rough rind fruit, that with smooth rinds contained a higher concentration of sugars from mid-Sept. to harvest, and a lower concentration of organic acids from early Aug. to early Sept. Just before the rind became smooth (Sept. 12), chlorophyll in the skin reached a maximum level and then decreased rapidly. Carotenoid synthesis began when chlorophyll concentration attained its maximum level; the carotenoids continued to increase until harvest on Dec. 5. From these results, we conclude that the growth of satsuma mandarin fruit can be divided into 3 stages as follows: stage I; (sideward fruit, flowering to mid-July; upward fruit, flowering to early Aug.), the rind becomes rough and the sugar and organic acid concentrations in the juice are considerably low. stage II; (sideward fruit, mid-July to mid-Sept.; upward fruit, early Aug. to late Sept.), the rind becomes smooth and organic acid accumulates rapidly in the juice sacs. stage III; (sideward fruit, mid-Sept. to harvest; upward fruit, late Sept. to harvest), the rind becomes even more smooth and thin; the juice sacs accumulate sugars rapidly while organic acids decrease significantly. The onset of stage III, when the skin roughness disappears, represents a significant stage when fruit maturation in mandarin begins.