LEAF ABSCISSION INDUCED BY ETHYLENE IN WATER-STRESSED INTACT SEEDLINGS OF CLEOPATRA MANDARINE REQUIRES PREVIOUS ABSCISIC-ACID ACCUMULATION IN ROOTS

The involvement of abscisic acid (ABA) in the process of leaf abscissi on induced by 1-aminocyclopropane-1-carboxylic acid (ACC) transported from roots to shoots in Cleopatra mandarin (Citrus reshni Hort. ex Tan .) seedlings grown under water stress was studied using norflurazon (N F). Water stress induced both ABA (24-fold) and ACC (16-fold) accumula tion in roots and arrested xylem flow. Leaf bulk ABA also increased (8 -fold), although leaf abscission did not occur. Shortly after rehydrat ion, root ABA and ACC returned to their prestress levels, whereas shar p and transitory increases of ACC (17-fold) and ethylene (10-fold) in leaves and high percentages of abscission (up to 47%) were observed. N F suppressed the ABA and ACC accumulation induced by water stress in r oots and the sharp increases of ACC and ethylene observed after rewate ring in leaves. NF also reduced leaf abscission (7-10%). These results indicate that water stress induces root ABA accumulation and that thi s is required for the process of leaf abscission to occur. It was also shown that exogenous ABA increases ACC levels in roots but not in lea ves. Collectively, the data suggest that ABA, the primary sensitive si gnal to water stress, modulates the levels of ethylene, which is the h ormonal activator of leaf abscission. This assumption implies that roo t ACC levels are correlated with root ABA amounts in a dependent way, which eventually links water status to an adequate, protective respons e such as leaf abscission.