A Study of Abcission Zone Development In Leaves of Ginkgo Biloba L

The development of the leaf abscission zone in Ginkgo biloba trees and seedlings was studied. Leaves from Ginkgo seedlings were debladed to enhance anatomical changes in the abscission region. The development of an abscission layer was observed six days after deblading. The formation of the zone was not complete, but an abaxial to adaxial pattern of development was apparent. No evidence of separation was noted. Debladed Ginkgo petioles remained green and did not abscise during the six weeks period of study. The development of an abscission layer was observed in leaves from a (;inkgo tree one week prior to separation. The zone occupied an area approximately eleven cells wide at the junction of the petiole and stem. Separation of the petiole occurred through an apparent dissolution of the middle lamella. The separation was observed in the distal region of the zone between the ninth and tenth cell layers. The pattern of leaf abscission zone development is similar in Ginkgo trees and seedlings. Abscission refers to the natural separation of any part of a plant. It is most noticeable as the means by which deciduous plants shed their leaves prior to dormancy. Other examples are the separation of flowers, fruit and bark. The process could also be considered a self-pruning device utilized for the removal of injured or senescent organs ( 1). The abscission zone is a general term for the region at the base of the abscising organ through which separation eventually occurs. Separation denotes the physical separation of accessory organs from the main axis of the plant. Those cells involved in separation are referred to as the separation layer. Separation is usually within or distal to the tiers of cells formed during abscission. The primary studies on abscission were anatomical. Von Mohl ( 10) reported an abscission layer where separation occurred. Further, he recognized two independent phases of the process: the separation of the organ and development of protective tissue. This early work has led to investigations of the abscission zone and changes taking place in this region prior to separation. Plants such as Coleus, maple and oak have a structurally distinguishable zone where abscission is favored. Rubenstein and Leopold ( 12) reported Gymnocladus to have ninety zones in one compound leaf. Yager ( 19) reported that the abscission zone consisted of twelve tiers of cells in tobacco flowers. He distinguished the cells of the separation layer as being smaller than surrounding cells. Yager 1 10908 St. Boniface Lane, St. Ann, Missouri 63074 'Department of Biology, University of N. Iowa, Cedar Falls, Iowa 50613