PHOSPHORUS EFFECTS ON OLIVE LEAF ABSCISSION

When continuously stem-fed with 75 mm NaH2PO4, 'Manzanillo' olive explants showed significant leaf abscission after 48 hours; by that time 1.042 mg.g-1 fresh weight P had accumulated in the abscission zone (AZ). The potential contribution of ethylene to phosphate-enhanced abscission was investigated using aminooxyacetic acid (AOA), an ethylene-synthesis inhibitor, and by measuring ethylene evolution in phosphate-treated explants. In combination with NaH2PO4, AOA did not affect leaf abscission. Though ethylene evolution from explants increased as leaf abscission was initiated, it was about two orders of magnitude less than the concentration necessary to induce leaf abscission as judged by exogenous treatments. Based on leaf-abscission kinetics, we have concluded that the mechanism of P-induced abscission is independent of gross measurement of evolved ethylene, but we cannot rule out ethylene confined to the AZ itself. When evaluated for P-induced leaf abscission, leaves of 'Manzanillo' and 'Sevillano' abscised earlier than 'Ascolano' and 'Mission'.