Longevity of Plant Organs: Internal Factors Controlling Abscission

Abstract

The research of many men during the last twenty years has gone toward elucidating the internal factors which control the longevity of two importan t types of plant organs-leaves and fruits. This work, which one would expect to have general interest for developmental biologists, has not been as familiar to them as it might be because most of the papers which could be considered to treat longevity have emphasized the "abscission" side of the research. The emphasis on abscission is quite understandable: there are important economic aspects of leaf abscission and of fruit abscission (e.g., inducing leaf abscission in cotton plants for the mechanical picker, and preventing fruit abscission in some of the tree crops like apples and figs) . Consequently, many of the papers have been published in agricultural research journals , where the practical aspects of the research were emphasized.This review will consider the gradual development of our current under standing as to the internal factors in the plant which control the longevity of leaves and fruits. The extensive literature on external agents which will a rtific ally induce abscission will not be discussed, since it has recently been authoritatively reviewed by Addicott & Lynch (2), and since it is not par ticularly informative concerning the internal controlling factors, despite its value in field practice. Special emphasis will be placed on relating a given piece of research to previous research, both because of general scientific obligation, and because abscission researchers have been unusually lax in doing so. In particular, the early German authors (44,45,46,67) have, in the reviewer's opinion, not been given adequate credit by English-speaking authors. [No "holier-than-thou" attitude is suggested: the author has erroneously attributed to Myers credit that should have gone to Laibach and Mai (99), and undoubtedly will have overlooked some proper references in this review. But the aim and the obligation is to be as fair as human failings allow.]Among the papers published since the last review that are not herein considered in detail are those of Hall et at. (26,27,51,52, 53) and Haccius & Nies (23,24,25). Also omitted is consideration of the effects of position with respect to gravity, fully treated by Terpstra (86) and Vendrig (93, 94) . Of papers published before the last review, DostaJ's 1951 paper on correlative effects on leaf abscission in woody plants should be noted (14) . There are no 1 The survey of literature pertaining to this review was concluded in July 1961. 2 The following abbreviations are used: IAA (indole-3-acetic acid); NAA (1-naphthaleneacetic acid).

The Manipulation of Auxin in the Abscission Zone Cells of Arabidopsis Flowers Reveals That Indoleacetic Acid Signaling Is a Prerequisite for Organ Shedding

“…Prior to this discovery, it had been reported that the attachment of orchid (Dendrobium spp.) pollinia, known to be rich in auxin, to excised coleus tissue dramatically slowed abscission (Laibach, 1951) and that application of indoleacetic acid (IAA) to the distal end of bean (Phaseolus vulgaris) leaf explants delayed cell separation (Jacobs, 1962). These observations led to the hypothesis that the timing of shedding was regulated by the auxin-ethylene balance within the AZ tissues, with a high flux of IAA into these cells, from the subtending organ, preventing abscission from taking place (Taylor and Whitelaw, 2001).…”

In Vivo Measurement of Indole-3-acetic Acid Decarboxylation in Aging Coleus Petiole Sections

“…IAA is usually found in highest concentration in the juvenile parts of the plant, and at lower levels in regions removed from the growing points (10) and in the older leaves (4). In some cases, the senescence process may, in fact, be regulated by the changing levels of this hormone.…”

Abscission

“…Equally well documented (see "Reviews" 17,18,29,31,48) is the ability of auxin to retard or prevent abscission. The amount of auxin reaching the abscission zone and the distribution of auxin both proximal and distal to the abscission zone appear to play an important role in this preventive action of auxin (8,17,29,48).…”