Senescence Factor & Abscission of Coleus Leaves.

In 1955 Osborne reported that petiole sections cut from senescent leaves of any one of many species could speed the abscission of excised and debladed bean petioles (5). The sections from senescent leaves speeded abscission if directly attached or if one attached an agar block on which they had stood for 24 hours. She "proposed that leaf abscissions may be controlled not only by endogenous auxins but also by some substance or substances which are produced as the leaf matures, and which reach a maximum at senescence." Similar effects were later obtained with tropical species (6). Coleus has had its normal auxin-abscission relations worked out more exactly than has any other genus, and on the basis of current information the normal abscission pattern of the clone of Coleous blumnei used by Jacobs and co-workers seems to be fully explained in terms of auxin alone ( 1-4 7. 8). The evidence for auxin being the normal limiting factor for the inhibition of abscission is as follows (8). Leaves on the intact plant abscise at intervals directly related to their age. Diffusible auxin is produced by leaf blades of different ages in amounts which are significantly correlated with the normal pre-abscission intervals of the leaves. If the leaf blade is excised, the remaining petiole abscises in 4 to 6 days, irrespective of its age. And if auxin in the form of synthetic indole-acetic acid (IAA) is substitutedl for the leaf blade, then the abscission interval is restored to that of the intact leaf. Similar evidence has been obtained for the role of auxin in speeding the abscission of leaves which are either naturally aged (3), or artificially aged by deblading (1,7). Despite the evidence cited above, however. there are a number of unclarified points. Chief among them is the fact that we do not know the chemical nature of the native auxin in Coleus. When we de-