ABSCISIC-ACID INCREASES IN NONGROWING AND PACLOBUTRAZOL-TREATED FRUITS OF SEEDLESS MANDARINS

Clementine (Citrus reticulata [Hort.] Ex. Tanaka cv. Oroval) is a self -incompatible mandarin with a slow rate of fruit growth and high perce ntage of fruit abscission. Seedless Satsuma (Citrus unshiu [Mak] Marc. cv. Clausellina) shows natural parthenocarpy and higher fruit set. Ap plication of 25 mu g fruit(-1) of paclobutrazol (PP333), an inhibitor of gibberellin biosynthesis, reduced the rate of growth and accelerate d fruit abscission in both varieties. In contrast, gibberellin A(3) (G A(3)) stimulated fruit growth only in the self-incompatible mandarin. Clementine fruits, in the absence of pollination, showed an approximat ely 2-fold transient increase in the free abscisic acid (ABA) content shortly after petal fall, In Satsuma, a very small accumulation of ABA was detected, Paclobutrazol treatment induced a 3-fold increase in AB A in Satsuma fruits but did not substantially affect the pattern of AB A accumulation in Clementine. In this variety, GA(3) suppressed the AB A increase observed in untreated fruits. These effects were observed 2 4 h after treatment. However, in Satsuma fruits, the effect of GA(3) o n the ABA content was negligible. In addition, a comparative analysis of growing and non-growing fruits of Clementine showed that ABA, on a per unit weight basis, was always higher in the nongrowing fruits. Tre atment with 85 mu M fluridone, an inhibitor of carotenoid biosynthesis and thus indirectly of ABA, delayed fruit abscission in Clementine, b ut also decreased fruit growth. Collectively, these observations indic ate a relationship between high ABA content and a reduced rate of frui t growth and an acceleration of fruit abscission.