SHOOT TIP ABORTION IN ULMUS AMERICANA

MILLINGTON, W. F. (Marquette U., Milwaukee, Wis.) Shoot tip abortion in Ulmus americana. Amer. Jour. Bot. 50(4): 371-378. Illus. 1963.-Phenological observations of American elm have shown that the phenomencn cf shcot tip abortion in which the distal several plastochrons of each shoot turn yellow and abort, inducing syrnpcdial growth, occurs over a peried of several weeks starting at the time fruits are shed from older trees. Time of abortion varies among plants of different age, among individuals of the same age, and among shoots on the same individual. In the latter case, time of abortion is inversely correlated with the vigor of the shoot, the most vigorous shoots on a branch being the last to abort. Abortion is first evident in the yellowing of the entire shoot tip. Cytohistological studies show that necrosis commences at the sixth to eighth node back of the apex, where it is apparent in autolysis of internal cells of the stipules. Necrosis progresses acropetally in the stipules and young leaves and ultimately involves the leaf primordia at the shoot apex. Mitosis ceases in the shoot apex and its meristematic appearance is lost. These changes follow in basipetal sequence in the axillary buds down to the bud below the abscission site. This bud remains active and will resume growth the following season. The abscission site is evident externally as a green-yellow boundary in the basal part of the internode. No protective layer is present at the time of abscissicn, but it develops after the shoot tip abscises. There is no indication of blocking of vascular tissues before shoot tip abortion and limitation of water supply probably is not a causal factor. Photoperiod studies show that shoot tip abortion is accelerated in short days and delayed but not prevented in long days. Greenhouse experiments show that abortion is delayed also in seedlings, in plants supplied with organic fertilizer, or grown with the roots unconfined. Plants grown in a nutrient solution deficient in nitrogen aborted ahead of controls and plants deficient in calcium. Although shoot tip abortion occurs coincident with fruit drop, there is no indication of a causal relationship. The literature relating to shoot tip abortion is discussed in relation to the above observations. RECENT investigations of the effect of photoperiod on vegetative growth of woody perennials (Nitsch, 1957a,b; Downs, 1958; Wareing and Black, 1958) have directed attention to shoot tip abortion, a morphogenetic phenomenon which until recently has had only limited study. In many woody genera (Ailanthus, Betula, Catalpa, Celtis, Cercidiphyllum, Staphylea, Syringa, Tilia, Ulmus) shoot tip abortion is an inherent characteristic which induces subsequent development from subjacent axillary buds, resulting in the sympodial growth pattern typical of these genera. Under seasonal stimuli, the entire distal portion of the shoot, usually including several nodes and internodes, becomes necrotic and may abscise. While the phenomenon is described in the older botanical literature (Kerner, 1895; Goebel, 1905; Buisgen and Muinch, 1929), and the resultant occurrence of a "pseudoterminal bud" is commonly utilized as a distinguishing characteristic in taxonomic keys to woody plants, little has been written until recently of its mor1 Received for publication September 17, 1962. Supported by a grant from the National Science Foundation (G16735). Gratitude is expressed to Dr. J. Talacko, Department of Mathematics, Marquette University, for assistance with statistical analyses, and to Mr. B. Sparkowski and the Milwaukee County Park Commission for their cooperation in some of the experimental phases of this study. phogenetic aspects (Titman and Wetmore, 1955; Garrison and Wetmore, 1961). Abortion of the shoot tips in American elm, Ulmus americana L., is a natural phenomenon which takes place in spring or early summer rapidly following a decline in growth of the distal portion of each expanding shoot. Shortly the entire shoot tip turns yellow and abscises at the base of an internode several plastochrons back of the shoot apex (Fig. 1). The purpose of this paper is to note some correlations between abortion of the shoot tip and environment, age and vigor in elm, and to follow the process morphologically for clues to its cause. ABORTION IN RELATION TO PHENOLOGY, AGE, AND VIGOR-In an attempt to obtain clues to the factors which induce abortion of the shoot tip in elm, phenological observations of elms of various ages have been made over the past few years in Madison and in Milwaukee, Wisconsin. The timing of shoot tip abortion and of a number of events in the seasonal development of the shoot are shown in Table 1. Note that flower buds expand in advance of vegetative buds in the spring. Fruits begin to abscise just in advance of or parallel with shoot tip abortion, before the leaves and internodes are fully expanded. Shoot tip abortion in older trees, which commences in mid-May, progresses at variable rates depending