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.