HETEROPHYLLY AND NEOFORMATION OF LEAVES IN SUGAR MAPLE (ACER SACCHARUM)

Variation in leaf form and timing of leaf initiation were investigated in vigorous leader shoots of open-grown saplings and larger forest trees of sugar maple (Acer saccharum Marsh.). Winter buds of leader shoots usually contained 6 or 8 leaf primordia and embryonic leaves, whereas 12 to 18 leaves typically expanded along the shoots each year. Preformed (early) leaves differ in form from neoformed (late) leaves. As in some other Acer species, the first-formed late leaves have large angles of secondary lobe divergence and deeply indented sinuses. This pattern of heterophylly contributes to the multilayered nature of open-grown saplings and leader shoots of forest trees of sugar maple. SUGAR MAPLE (Acer saccharum Marsh.) has frequently been characterized as a species in which shoots are fully preformed (Critchfield, 1971; Marks, 1975; Bormann and Likens, 1979; Kramer and Kozlowski, 1979). That is, the leaves which are borne along the shoot one year are all initiated the previous year and overwinter in the bud as embryonic leaves and leaf primordia. The contrasting condition, leaf neoformation, occurs when some primordia mature immediately after they are initiated, without any intervening period of rest (Halle, Oldeman and Tomlinson, 1978). Unless growth is continuous, shoots which exhibit neoformation also produce some leaves by preformation. In such a shoot, the preformed leaves mature rapidly at the beginning of a growth flush, while the neoformed leaves mature later in the flush. Hence, the two types of leaves are referred to as "early" and "late" leaves (Critchfield, 1960; Clausen and Kozlowski, 1965; Kozlowski, 1971). If early and late leaves differ markedly in their form, shoots with both leaf types are referred to as seasonally heterophyllous. Preliminary observations indicated that the leader shoots of open-grown sugar maple saplings described previously (Steingraeber, Kascht and Franck, 1979) contained more 1 Received for publication 16 June 1981; revision accepted 21 January 1981. I am grateful to Don Waller, Dan Franck, Larry Kascht, Gary Breckon, George Diggs, and Robert Kowal for their assistance in this study. I also thank Jack Fisher for reviewing the manuscript. Portions of this work are contained in a dissertation submitted to the Graduate School of the University of Wisconsin. Work at F.T.G. was supported by NSF Grant DEB79-14635. 2 Present address: Department of Botany and Plant Pathology, Colorado State University, Fort Collins, Colorado 80523. leaves than other sugar maple shoots. Leader shoots often had 12 to 24 leaves (and, in some cases, as many as 52), while most other shoots had, at most, eight leaves. Based upon these observations, I hypothesized that leader shoots of vigorous saplings produce both preformed (early) and neoformed (late) leaves. The purpose of this study is to determine whether vigorous leader shoots of sugar maple exhibit neoformation of leaves and heterophylly. MATERIALS AND METHODs-Timing of leaf initiation-During the autumn of 1976, comparable leader shoots on approximately 65 open-grown saplings (from populations previously described by Steingraeber, Kascht and Franck, 1979) were tagged for further study. In November of 1976, 40 of these shoots were randomly collected. For each shoot, the number of leaf scars from 1976 leaves was counted and the terminal bud was then dissected to determine the number of embryonic leaves and leaf primordia (and, hence, the number of leaves preformed for expansion in 1977). In November of 1977, the remaining tagged shoots were sampled to determine the number of leaves per shoot during 1977. The mean number of leaves in the winter bud, and the mean numbers of leaves produced in 1976 and 1977 were then compared using t-tests pairwise, with the level of significance determined using Tukey's (1977) modification. During the autumn of 1979, 21 additional leader shoots were collected from the same populations of saplings. The number of leaves on each shoot in 1979 was recorded, and the terminal bud of each shoot was then dissected to determine the number of leaves preformed for expansion in 1980. A t-test with paired data