Inflorescence Abscission in Olive: Anatomy and Histochemistry in Response to Ethylene and Ethephon

Naturally occurring postanthesis abscission in olive (Olea europaea L. Manzanillo) results in approximately 99% loss of flowers. Ethylene gas treatment induces sequential abscission of flowers, rachis branches and internodal sections, and peduncles of mature (expanded) inflorescences on shoot explants. Treatment with 2-chloroethyl phosphonic acid (ethephon) triggers sequential abscission of flowers and peduncles of mature inflorescences but only very infrequently rachises and never internodal sections. Immature (not fully expanded) inflorescences on shoot explants abscise intact in response to ethylene gas or ethephon. Sites of floral abscission on trees are limited to bracts, petals, individual flowers, and peduncles. With the exception of bracts, floral organs on trees do not abscise until after pollination. Naturally occurring abscission is preceded by plasmolysis of abscission zone cells, loss of cell wall materials as evidenced by changes in stain intensity, and lacunar cell separation. Abscission of floral organs of trees and of explants treated with ethephon or ethylene gas occurs at localized sites and is preceded by cell wall gelatinization and swelling (in ethephon-treated samples only), loss of cell wall and middle lamella pectins, lacunar cell separation, and senescence of cortical parenchyma cells. Phytotoxic symptoms, plasmolysis and cell senescence throughout all explant tissues occurred with ethephon treatments. All active abscission zone cells exhibit small size, isodiametric shape, dense cytoplasm, and starch accumulation. Cell division does not occur in any abscission zone with any treatment.