Michael S. Greenwood

Is early life cycle success a determinant of the abundance of red spruce and balsam fir

Red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea (L.) Mill.) are sympatric in much of the Acadian Forest, but their relative regeneration success during the changing climates of the H...

Separating the effects of tree size and meristem maturation on shoot development of grafted scions of red spruce (Picea rubens Sarg.)

In order to separate the effects of size and meristem maturation on age-related changes in shoot growth behaviour, a reciprocal grafting experiment was conducted involving juvenile (J), mid-age (MA) and old-growth (OG) red spruce (Picea rubens Sarg.) trees as both scion donors and rootstock. The effects of rootstock and scion age on vegetative growth, foliar morphology and reproductive development were assessed over 7 years after grafting. Vegetative growth potential declines with meristem maturation, but the high growth potential of J meristems on J rootstock cannot be expressed when J scions are grafted on MA and OG rootstock. Branch density decreases with meristem maturation. The tendency for high J branch density is expressed across all rootstock ages, but is minimally expressed on MA rootstock where elongation of terminal shoots is significantly greater than on OG rootstock. Both needle width and leaf mass area increase with meristem maturation and increasing tree size. Reproductive competence is mainly a function of meristem maturity, and rootstock had little effect on strobilus development, although the more fecund rootstocks did promote some flowering on J scions. Developmental decline in height growth does not appear to be a function of decreased meristem vigour, but reduced terminal long shoot elongation and decreased apical control in OG trees will reduce height growth.

Regulation of Ontogeny in Temperate Conifers

Through their life stages, long-lived forest trees must adapt to challenges resulting from vast changes in size and external environment. Trees accomplish this by producing new tissues and growth habits adapted to specific life stages by pluripotent meristems. The morphology and physiology of these new tissues are determined by complex interactions of the external and internal environments of the tree, and gene expression both within meristems and during differentiation of cells produced by meristems. The trajectories of various age-related changes are both inter-related and independent at various scales, and, for the majority of species, are not purely physio-mechanical responses to increased tree height. Understanding the relationships between tree developmental state, tree size and its environment requires both a whole-tree approach integrated through time to encompass life stage strategies, and a molecular approach to understand the cues, transduction pathways and epigenetic mechanisms that regulate whole-life ontogeny.

Regulation of foliar plasticity in conifers: developmental and environmental factors.

Foliar plasticity is widespread among woody plants, and historically most observations have been made in angiosperms. This review concentrates on examples from gymnosperms, particularly the Pinaceae, since there have been a number of recent studies on coniferous forest species. Foliar plasticity can be defined as variation in the morphology and physiological function of foliage produced over time and space within a single individual. For example, leaves produced in the sun have a higher leaf mass area (LMA) than leaves produced in the shade, and differ in their photosynthetic capacities. Sun and shade leaves are examples of heterophylly, or variation in leaf morphology in response to environmental variation in the immediate space surrounding the apical meristem producing the leaf primordia. The LMA of coniferous foliage also tends to increase with age, independently of the external environment, and this variation, called heteroblasty, appears to be a result of maturation of the apical meristem, which occurs over time. The regulation of variation due to heterophylly and heteroblasty appears to be very different. Heterophylly in response to light appears to vary linearly with available light. More massive sun foliage produced by a given apical meristem may be a response to available Photosynthetically Active Radiation (PAR) and its effects on net photosynthesis, or may be a photomorphogenetic response to the ratio of red to far red light. In contrast, heteroblastic variation can occur independently of available light, and may be the result of the developmental age of the apical meristem. It appears to vary curvilinearly with age, with the most rapid change occurring during the earliest life stages of the plant. Heteroblastic variation appears to be less plastic than heterophyllic, since grafted scions from mature or juvenile donors retain LMA characteristics of donor whether grafted onto juvenile or mature rootstocks.

Molecular approaches to gene expression during conifer development and maturation

Abstract Our understanding of the processes, and the control of those processes that lead to the growth, development and maturation of conifers has been severely limited owing to their complex nature. Newer technologies now provide us with alternative approaches to study these important developmental phenomena. Some of the literature that has lead to current models for the control of development in a variety of plant and animal systems are reviewed here. We use these models to suggest ways to study maturation in conifers and some of our own results in this area of research are described.

Differential Gene Expression During Maturation-Caused Decline in Adventitious Rooting Ability in Loblolly Pine (Pinus taeda L.)

Loss of adventitious rooting ability occurs very quickly in loblolly pine seedlings, where hypocotyl cuttings from 50-day-old seedlings root readily in response to auxin, while epicotyl cuttings from the same seedling do not, despite their anatomical similarity (Diaz-Sala et al., 1996a). In hypocotyl cuttings, auxin causes root meristems to organize in the cambial region centrifugal to the former primary xylem poles over a 12-day period. In epicotyl cuttings, the cambium dedifferentiates in response to auxin, but roots rarely form, and then only after 50 or more days. A brief exposure (a 5 min pulse) to auxin is sufficient to promote rooting. While both types of cutting transport auxin in a polar manner, a pulse of N-(l-napthl)phthalamic acid (NPA), which inhibits auxin efflux, also delays rooting in hypocotyls only if applied within the first 3 days after the auxin pulse; thereafter it has no effect. These observations show that key events that determine root meristem formation occur in the first 72 hours, and that auxin binding to the efflux carrier that mediates polar auxin transport, may be necessary for rooting (Diaz-Sala et al., 1996a).

The Molecular Genetics of Maturation in Eastern Larch ( Larix Laricina [Du Roi] K. Koch)

Maturation in woody plants has recently received much attention because of the maturation-related decrease in the ability to clone selected individuals using expiants from mature plants (Hackett, 1985; Greenwood, 1987). Furthermore, the phenotypic changes that accompany maturation make it difficult for the tree breeder to select superior genetic families or individuals at the seedling stage (e.g., Lambeth, 1980). The changes associated with maturation include growth rate, branching characteristics and growth habit, reproductive behavior, and the morphology and physiology of foliage. The significance of the latter will be emphasized in this brief report, with emphasis on the role of gene expression in the maturational process. Changes in foliage associated with maturation have been discussed (e.g., Zimmerman et al., 1985; Bauer and Bauer, 1980) and do not follow a completely consistent pattern among species.

Molecular Approaches To Maturation-Caused Decline In Adventitious Rooting Ability in Loblolly Pine (Pinus Taeda L.)

New approaches towards understanding the control of adventitious root formation include studies at the level of gene expression. We discuss attempts to determine the identity of as yet unknown genes which are specific to formation of roots, as well as studies on the expression of genes known to be involved in the wound response. A number of genes appear to be specific to adventitious root formation in several species including loblolly pine, and the expression of these genes changes with maturation. Keywords: adventitious rooting, gene expression, loblolly pine

Gene Expression during Growth and Maturation

The expression of cab and rbcS gene families were measured in RNA extracted from needles from larch trees ranging in age from 1 y to 75 y. Steady state cab mRNA levels are relatively higher (~40%) in newly expanding short shoot foliage from juvenile plants compared with mature plants. Later in the season no consistent difference in cab expression between juvenile and mature plants was detected. Unlike cab gene expression, the expression of the rbcS gene family did not seem to vary with age. These data show that the maturation-related changes in morphological and physiological phenotypes are associated with changes in gene expression. No causal relationship has been established, however. Indeed, we conclude that the faster growth of juvenile scions is not due to increased net photosynthesis or cab expression.

Effect of maturation on loblolly pine breeding and seed orchards

Abstract Since virtually all loblolly pine ( Pinus taeda L.) seed production and breeding orchards are established by grafting, and since advanced generation breeding of this species is being accelerated, the ability of scions from young trees to flower must be considered. The results of experiments on the effect of scion age on flowering are discussed. One-year-old scions are initially almost incompetent reproductively, but reproductive competence increases rapidly between ages 1 and 4. More rapid vegetative growth by young scions will result in larger tree size, which might offset lack of early flowering in a long-lived seed production orchard. But scions from young ortets could result in a substantial delay in breeding progress. Flower stimulation treatments may only partially offset reproductive incompetence due to juvenility.