Histological study on rind roughness of satsuma mandarin fruit

The rind roughness of satsuma mandarin (Citrus unshiu Marc. cv. Okitsu Wase ) fruit from low and high crop load trees was investigated histologically; the former bears rough rind, whereas the latter has smooth rind. Rind roughness seemed to be associated mainly with oil gland development. A t the mature stage, the rough fruit contained various sizes of primary and secondary oil glands, whereas smooth fruit contained relatively uniform siz es. The vigorous development of oil glands from Aug. to Dec. was a main cau se of rind roughness at harvest. In particular, convex protrusions formed j ust above the oil glands, which were frequently detected on mature fruit su rface, appeared to be the result of marked development of secondary oil gla nds. Convex protrusions, independent of oil glands, were also frequently ob served in immature fruit. Rind tissue consists the flavedo, a single layer of pigmented epidermal cel ls, several layers of hypodermal cells, and the albedo, a relatively thick, white parenchymatous tissue below the hypodermis. In a rough fruit, the hy podermal and parenchyma cells were smaller and the alignment of the hypoder mal cells was more irregular than the smooth fruit. Cell division of hypode rmal tissue ceased about Aug.10 in smooth fruit, but it continued until Sep t.10 in a rough rind. Thus, rind roughness of satsuma mandarin fruit seems to be induced by vigor ous and/or abnormal development of both oil glands and hypodermal tissues.