Possible Role of Pectic Enzymes in Abscission.

.Abscission has been of interest to botanists since Mohl described it as early as 1860 (20). Many of the early workers assumed that enzymes are involved in the dissolution of the middle lamella as abscission occurs (11, 12, 16, 17). As Addicott and Lynch indicate in their review of the physiology of abscission, however, little has been done with the identification of specific enzymes which may be involved in the dissolution reactions (2). Much of the current research concerned with abscission deals with chemical substances which affect the process with little attention to specific chenmical reactions that may be involved. Osborne has shown recently that the activity of pectin methylesterase (PME) decreases when the time for abscission of leaves in Phaseolus vulgaris occurs normally or when abscission is induced by chemical factors (23). Since the activity of PME is affected by growth substances (4, 21, 26) which also affect abscission (1, 25, 27), a role for PME in the reactions involved in abscission is suggested. The middle lamella is generally considered to be pectic in nature (6, 13, 18). The middle lamella of the abscission zone dissolves as floral abscission occurs in Nicotiana (12, 28). In 1950 Facey reported the results of experiments with Fraxinus cuttings which identified water soluble pectin as the end product of a series of reactions which occur as the calcium pectate of the middle lamella is dissolved during abscission (10). As early as 1918 Kendall reported the appearance of pectin as abscission occurred in tobacco flowers (12). The problem arises as to whether the shifts from pectate to pectin as the middle lamella dissolves is simply caused by a decrease in PME activity or whether additional pectic enzymes are involved. The literature connected with pectic enzymes other than PME. is conflicting, including their actual occurrence in higher plants. Pectin-polygalacturonase is the best known of these other enzymes. Kertesz is doubtful of its occurrence in higher plants although he cites several reports of its occurrence (15). Demain and Phaff recently reported that instead of a single pectin-polygalacturonase, there are several exopolygalacturonases specific for pectins of different degrees of esterification and size of polymer and a single endo-galacturonase (7) (8). Schubert (24), Dingle et al (9), and Ayres et al (3) have reported that endo-polygalacturonase is a complex of enzymes. This complexity in the identification of specific pectinpolygalacturonases may explain some of the conflicting reports. McClendon and Somers have reported the occurrence of two pectin-glycosidases with different conditions required for activity (19). Kertesz has postulated the occurrence of additional pectic enzymes in general and describes one that he has named pectic acid depolymerase (15). Although there is general lack of information concerning pectic enzymes, it is probable that there are one or more enzymes present in abscission tissue which could act to dissolve the cementing middle lamella. The purpose of this investigation was to examine the occurrence of PMAE in the abscission zone of tobacco flowers and to note effects that various substances known to affect abscission might have upon its activity (292, 29, 30). In addition, attempts were made to duplicate in vitro the macerating effects presumably caused by pectic enzymes as abscission occurs normally. These same substances which affect abscission were studied as to their effect upon the macerating activity of certain pectic enzymes.