Senescence Factor & Abscission of Coleus Leaves

Abstract

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 attacheed 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 Coleus blumxei 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 substitute(l 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 determined the amount of diffusible auxin coming from Coleus leaves (8), operationally what we did was to find the amount of curvature obtained in the standard Avena curvature bioassay from our so called unknown blocks of agar, calibrated the assay with known amounts of synthetic IAA, and then expressed the unknown curves as if they were due to IAA alone. It is obvious that the observed decline in auxin activity per leaf could be due to a steadily increasing amount of senescence factor. This would be the more likely if senescence factor acted through interfering with the auxin metabolism of the leaf. This interpretation is supported by Osborne's data showing that senescence factor counteracted the abscission-inhibiting action of synthetic IAA on bean abscission (table 3, 5). Accordingly, the purpose of the present paper is to try to get evidence as to whether Osborne's senescence factor is present in Coleus leaves, and if so whether it increases in amount with age as auxin activity decreases. An allied interest is to see if senescence factor normally causes senescence of intact Coleus leaves, or whether it has the more restricted role of being produced in a leaf after senescence starts.