Ontogeny of proventitious epicormic buds in Quercus petraea. I. In the 5 years following initiation

 The persistence of large epicormic shoots is one of the main factors that reduces timber quality and value in Quercus petraea. The early phases of epicormic shoot formation, i.e. the initiation of the epicormic buds, their survival and their proliferation over the years, are not clearly understood. In the present work, we studied the initiation of the axillary buds giving rise to epicormic buds and shoots, and followed their behaviour during the first 5 years using both scanning electron microscopy and light microscopy. Two types of proventitious epicormic buds have been identified. The first type has small axillary buds associated with the rings of bud-scale scars which are found at the base and tip of each growth unit. These buds are made of a terminal meristem surrounded only by scales; no leaf primordium is detected. During the second and third years of epicormic life, meristematic areas appear in the scale axil. Progressively, the meristematic areas organize into secondary bud primordia composed solely of the terminal meristem surrounded by scales. The second type of epicormic bud has secondary buds produced by a large axillary bud when this large bud either developed into a shoot or partially abscised. The epicormic potential in Q. petraea is characterized by a balance between the epicormic buds in apparent rest, enclosing meristematic areas and secondary bud primordia, and their mortality over the years. Received: 22 January 1998 / Accepted: 8 May 1998     

Axillary meristems and the development of epicormic buds in wollemi pine (Wollemia nobilis)

Intact trees of Wollemia nobilis Jones, Hill and Allen (Araucariaceae) routinely develop multiple coppice shoots as well as orthotropic epicormic shoots that become replacement or additional leaders. As these are unusual architectural features for the Araucariaceae, an investigation was made of the axillary meristems of the main stem and their role in the production of epicormic and possibly coppice shoots. Leaf axils, excised from the apex to the base of 2-m-high W. nobilis plants (seedling origin, ex situ grown), were examined anatomically. Small, endogenous, undifferentiated (no leaf primordia, no vascular or provascular connections) meristems were found in the axils from near the shoot apex. In the more proximal positions about half the meristems sampled did not differentiate further, but became tangentially elongated to compensate for increases in stem diameter. In the remaining axils the meristems slowly developed into bud primordia, although these buds usually developed few leaf primordia and their apical 'domes' were wide and flat. Associated vascular development was generally restricted to provascular dedifferentiation of the cortical parenchyma, with the procambium usually forming a 'closed loop' that did not extend back to the secondary vascular tissues. Development of the meristems was very uneven with adjacent axils often at widely differing stages of development into buds. The study shows that, unlike most conifers, W. nobilis possesses long-lived meristematic potential in most, if not all, leaf axils. Unlike other araucarias that have been investigated, many of the meristems in the orthotropic main stem will slowly develop into bud primordia beneath the bark in intact plants. It appears likely that this slow but continued development provides a ready source of additional or replacement leaders and thus new branches and leaves.     

RADIATION DAMAGE IN APPLE SHOOT APICES

Pratt , Charlotte , John Einset , and Mohammad Zahur . (N. Y. S. Agric. Expt. Sta., Geneva.) Radiation damage in apple shoot apices. Amer. Jour. Bot. 46(7): 537–544. Illus. 1959.—Pattern of shoot growth and anatomy of the shoot apex of ‘Golden Delicious’ apple trees on ‘East Malling IX’ rootstock are compared in normal trees and those growing under chronic gamma irradiation (average doses of 17–48 r per 20-hr. day) at Brookhaven National Laboratory. Bud damage in 6 varieties of apple trees is compared. Irradiated ‘Golden Delicious’ formed lateral buds on the current year's shoot, but the following year these buds grew into spurs which failed to form a terminal bud (“budless” spurs) and enlarged to form “club tips” and “swollen spurs” in this and subsequent years. Cells with thick walls and lightly stained cytoplasm occurred in shoot apices of irradiated lateral buds in mid-June. The first tunica layer was more resistant to radiation than the inner tunica layers and the corpus; pith rib meristem was still more resistant. Inflorescence and floral meristems were rarely found, but once formed, continued development. One-year-old budless spurs had a few leaves but neither an organized apical meristem nor leaf primordia. Surface of the apex was often folded. Periderm and, later, deep-lying wound cambium developed. Expansion of pith, vascular tissue, cortex, wound cambium and periderm caused enlargement of club tips and swollen spurs. Many lateral buds from the gamma field which were propagated without irradiation in early August grew into long shoots with terminal buds. Scions removed from irradiation in November and inserted into normal trees showed lower survival and many of their shoots were budless. This suggests that the capacity for normal growth of a bud damaged by chronic irradiation is greater in mid-summer than later in the year. With reference to percentage of budless shoots, ‘Delicious,’ ‘Golden Delicious’ and ‘McIntosh’ were more sensitive to an average dose of 24 r per day and lower doses than were ‘Cox,’ ‘Macoun’ and ‘Spy.’ Symptoms of radiation damage in apple buds during the first year were similar following acute or chronic irradiation. Degree of radiation damage, as expressed by death of apical meristems, was concluded to vary with stage of development of the bud, structure of the apical meristem, and genetic constitution.     

THE ORIGIN AND DEVELOPMENT OF INDUCED BRANCHES IN HELIANTHUS

The development of buds and their vascular connections are described for Helianthus annuus and H. bolanderi. Bud meristems of H. annuus usually become isolated by parenchymatization of the bud traces in the cortical zone. If the buds are induced to grow as a result of decapitation of the terminal meristem, a continuous range between typical primary connections and pseudo-adventitious connections are made between the main axis and the lateral buds. Considerable growth of the branches occurs within 48 hours after decapitation. Axillary buds of H. bolanderi grow continuously, and both the meristem and vascular system of the buds are derived directly from the apical meristem of the shoots.     

Shoot morphogenesis associated with flowering in Populus deltoides (Salicaceae)

Temporal and spatial formation and differentiation of axillary buds in developing shoots of mature eastern cottonwood (Populus deltoides) were investigated. Shoots sequentially initiate early vegetative, floral, and late vegetative buds. Associated with these buds is the formation of three distinct leaf types. In May of the first growing season, the first type begins forming in terminal buds and overwinters as relatively developed foliar structures. These leaves bear early vegetative buds in their axils. The second type forms late in the first growing season in terminal buds. These leaves form floral buds in their axils the second growing season. The floral bud meristems initiate scale leaves in April and begin forming floral meristems in the axils of the bracts in May. The floral meristems subsequently form floral organs by the end of the second growing season. The floral buds overwinter with floral organs, and anthesis occurs in the third growing season. The third type of leaf forms and develops entirely outside the terminal buds in the second growing season. These leaves bear the late vegetative buds in their axils. On the basis of these and other supporting data, we hypothesize a 3-yr flowering cycle as opposed to the traditional 2-yr cycle in eastern cottonwood.     

DIFFERENTIATION OF LATERAL SHOOTS AS THORNS IN ULEX EUROPAEUS

Ulex europaeus is a much-branched shrub with small, narrow, spine-tipped leaves and axillary thorn shoots. The origin and development of axillary shoots was studied as a basis for understanding the changes that occur in the axillary shoot apex as it differentiates into a thorn. Axillary bud primordia are derived from detached portions of the apical meristem of the primary shoot. Bud primordia in the axils of juvenile leaves on seedlings develop as leafy shoots while those in the axils of adult leaves become thorns. A variable degree of vegetative development prior to thorn differentiation is exhibited among these secondary thorn shoots even on the same axis. Commonly the meristems of secondary axillary shoots initiate 3–9 bracteal leaves with tertiary axillary buds before differentiating as thorns. In other cases the meristems develop a greater number of leaves and tertiary buds as thorn differentiation is delayed. The initial stages in the differentiation of secondary shoot meristems as thorns are detected between plastochrons 10–20, depending on vigor of the parent shoot. A study of successive lateral buds on a shoot shows an abrupt conversion from vegetative development to thorn differentiation. The conversion involves the termination of meristematic activity of the apex and cessation of leaf initiation. Within the apex a vertical elongation of cells of the rib meristem initials and their immediate derivatives commences the attenuation of the apex which results in the pointed thorn. All cells of the apex elongate parallel to the axis and proceed to sclerify basipetally. Back of the apex some cortical cells in which cell division has persisted longer differentiate as chlorenchyma. Although no new leaves are initiated during the extension of the apex, provascular strands are present in the thorn tip. Fibrovascular bundles and bundles of cortical fibers not associated with vascular tissue differentiate in the thorn tip and are correlated in position with successive incipient leaves in the expected phyllotactic sequence, the more developed bundles being related to the first incipient leaves. Some secondary shoots displayed variable atypical patterns of meristem differentiation such as abrupt conversion of the apex resulting in sclerification with limited cell elongation and small, inhibited leaves. These observations raise questions concerning the nature of thorn induction and the commitment of meristems to thorns.     

The mode of origin of root buds and root sprouts in the clonal tree Sassafras albidum (Lauraceae)

The developmental anatomy of root buds and root sprouts was examined in the clonal tree Sassafras albidum. Root samples from 13 clones that varied widely in age and vigor were sectioned and two types of buds were found, "additional" buds and "reparative" buds. Additional buds form during the early growth of uninjured roots and they perennate by growing outwards in concert with the vascular cambium such that bud traces are produced in the secondary xylem. Reparative buds form de novo in response to senescence, injuries, or other types of disturbance. Reparative buds were found on the roots of seven of the clones, whereas additional buds were found on the roots of all 13 clones. The reparative buds had originated in the proliferated pericycle, where they were subtended by sphaeroblasts, or spherical nodules of wood. Few of the reparative buds were vascularized and none were connected with the vasculature of their parent roots. In contrast, most of the additional buds were vascularized, and the leaf traces of several of the additional buds appeared to be contiguous with the conducting xylem of their parent roots. To determine whether both bud types were functional, 82 field-collected root sprouts and 44 incubation-induced sprouts were sectioned at the root-sprout junction and examined for evidence relating to their mode of origin. None of the sprouts were subtended by sphaeroblasts, but 98% were subtended by bud traces, which indicated that they had originated from additional buds. Although reparative buds were more common than additional buds on some of the root samples, they appear to be dysfunctional at sprouting. Additional buds, on the other hand, are able to sprout both as a normal part of clonal spread and from root cuttings.     

Herbivory could unlock mutations sequestered in stratified shoot apices of genetic mosaics

Many higher plants have shoot apical meristems that possess discrete cell layers, only one of which normally gives rise to gametes following the transition from vegetative meristem to floral meristem. Consequently, when mutations occur in the meristems of sexually reproducing plants, they may or may not have an evolutionary impact, depending on the apical layer in which they reside. In order to determine whether developmentally sequestered mutations could be released by herbivory (i.e., meristem destruction), a characterized genetic mosaic was subjected to simulated herbivory. Many plants develop two shoot meristems in the leaf axils of some nodes, here referred to as the primary and secondary axillary meristems. Destruction of the terminal and primary axillary meristems led to the outgrowth of secondary axillary meristems. Seed derived from secondary axillary meristems was not always descended from the second apical cell layer of the terminal shoot meristem as is expected for terminal and primary shoot meristems. Vegetative and reproductive analysis indicated that secondary meristems did not maintain the same order of cell layers present in the terminal shoot meristem. In secondary meristems reproductively sequestered cell layers possessing mutant cells can be repositioned into gamete-forming cell layers, thereby adding mutant genes into the gene pool. Herbivores feeding on shoot tips may influence plant evolution by causing the outgrowth of secondary axillary meristems.     

A novel function of TDIF-related peptides: promotion of axillary bud formation

Small peptides derived from the CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) gene family play a key role in various cell-cell communications in land plants. Among them, tracheary element differentiation inhibition factor (TDIF; CLE41/CLE44 peptide) and CLE42 peptide of Arabidopsis have almost identical amino acid sequences and act as inhibitors of tracheary element differentiation. In this study, we report a novel function of TDIF and CLE42. We found by the GUS (β-glucuronidase) reporter gene assay that while CLE41 and CLE44 are expressed preferentially in vascular bundles, CLE42 is expressed strongly in the shoot apical meristem (SAM) and axillary meristems. Overexpression of CLE42 and CLE41 enhanced axillary bud formation in the leaf and cotyledon axils. Before floral transition, the emergence of axillary buds in these plants occurred in an acropetal order. Exogenous supply of either TDIF or CLE42 peptide to the wild type induced similar excess bud emergence. In vascular bundles, the TDIF RECEPTOR (TDR) acts as the main receptor for TDIF. The axillary bud emergence of tdr mutants was little affected by either of the peptides. It was confirmed by scanning electron microscopy that peptide-treated wild-type plants form an axillary meristem-like structure earlier than non-treated plants. SHOOT MERISTEMLESS (STM), a marker gene for meristems, was up-regulated in peptide-treated plants before the axillary meristem becomes morphologically distinguishable. These results indicate that CLE42 peptide and TDIF have an activity to enhance axillary bud formation via the TDR. Judging from its expression pattern, CLE42 may play an important role in the regulation of secondary shoot development.     

CYTOKININ- AND GIBBERELLIC ACID-INDUCED EFFECTS ON THE STRUCTURE AND METABOLISM OF SHOOT APICAL MERISTEMS IN OPUNTIA POLYACANTHA (CACTACEAE)

The dormant axillary buds of Opuntia polyacantha can be activated by either cytokinins or gibberellic acid. Under the influence of benzylaminopurine (BAP), the axillary bud meristem increases greatly in size and becomes mitotically active. The primordia produced by the meristem develop as normal photosynthetic leaves. Gibberellic acid (GA) also causes the meristem to become mitotically active, but the meristem does not increase in size. The primordia produced under the influence of GA develop as normal cactus spines. Leaf-producing meristems and spine-producing meristems have the same zonation, despite the differences in size. The meristems are composed of a uniseriate tunica, a central mother cell zone, peripheral zone, and a pith rib meristem. The mitotic activity of each of the zones in the leaf-producing meristem differs significantly from the mitotic activity of the corresponding zones in the spine-producing meristem.     

THE RELATIONSHIP BETWEEN THE PRIMARY THICKENING MERISTEM AND THE SECONDARY THICKENING MERISTEM IN YUCCA WHIPPLEI TORR. I. HISTOLOGY OF THE MATURE VEGETATIVE STEM

Anatomical observations were made on 1-, 2-, and 3-yr-old plants of Yucca whipplei Torr, ssp. percursa Haines grown from seed collected from a single parent in Refugio Canyon, Santa Barbara, California. The primary body of the vegetative stem consists of cortex and central cylinder with a central pith. Parenchyma cells in the ground tissue are arranged in anticlinal cell files continuous from beneath the leaf bases, through the cortex and central cylinder to the pith. Individual vascular bundles in the primary body have a collateral arrangement of xylem and phloem. The parenchyma cells of the ground tissue of the secondary body are also arranged in files continuous with those of the primary parenchyma. Secondary vascular bundles have an amphivasal arrangement and an undulating path with frequent anastomoses. Primary and secondary vascular bundles are longitudinally continuous. The primary thickening meristem (PTM) is longitudinally continuous with the secondary thickening meristem (STM). Axillary buds initiated during primary growth were observed in the leaf axils. The STM becomes more active prior to and during root initiation. Layers of secondary vascular bundles are associated with root formation.     

Ontogeny of the vascular system ofBotrychium multifidum (S. G. Gmelin) Rupr.(Ophioglossaceae) and its bearing on stellar theories

Basipetal to the shoot apex, a procambial ring with parenchymatous gaps is present. The protoxylem poles are endarrh in both the ectophloic siphonostele and the collateral vascular bundle which comprises the leaf trace. Each leaf trace has an anastomosing system of protoxylem poles that decreases in number basipetally from five to three to two. Differentiation of the leaf trace procambium and protoxylem is bidirectional, that is the differentiation first occurs near the base of the leaf and acropetally in the leaf and basipetally in the stem. Then a fascicular cambium differentiates betweem the primary xylem and phloem in the leaf. This vascular cambium which is also present in the stem is unidirectional and only produces secondary xylem centripetally. Limited secondary growth also occurs in roots. Medullary tracheids when present are longitudinally continuous with the vascular system. The stele of the stem is interpretated as a sympodium of leaf traces and the pith is considered to be fundamental tissue enclosed by the anastomosing of leaf traces.     

DEVELOPMENT AND PHYLLOTAXIS OF THE VEGETATIVE AXILLARY BUD OF MICHELIA FUSCATA

Tucker, Shirley C. (Northwestern U., Evanston, III.) Development and phyllotaxis of the vegetative axillary bud of Michelia fuscata . Amer. Jour. Bot. 50(7): 661–668. Illus. 1963.—The vegetative axillary buds of Michelia fuscala are dorsiventrally symmetrical with 2 ranks of alternately produced leaves. The direction of the ontogenetic spiral in each of these buds is related both to the symmetry of the supporting branch and to the position of the bud along the branch. On a radially symmetrical branch, all the axillary buds are alike—all clockwise, for example. But in a dorsiventrally organized branch the symmetry alternates from clockwise in 1 axillary bud to counterclockwise in the next bud along the axis. Leaf initiation and ontogeny of the axillary apical meristem conform with those of the terminal vegetative bud. The axillary bud arises as a shell zone in the second leaf axil from the terminal meristem. During this process the axillary apex develops a zonate appearance. The acropetally developing procambial supply of the axillary bud consists wholly of leaf traces. At the nodal level the bud traces diverge from the same gap as the median bundle trace of the subtending leaf. Only the basal 1–2 axillary buds which form immediately after the flowers elongate each year, while the majority remains dormant with 3 leaves or fewer.     

The Structure,Elongation and Thickening of Rhizome in Nelumbo nucifera Gaertn

The seedling of Nelurnbo nucifera is erect and its internodes are very short with four Alternately arranged floating leaves. During the juvenile stage, the shoot elongates remarkably and forms the horizontal rhizome. Each leaf grows out from the dorsal side of the node of the rhizome. There are two kinds of terminal buds in the juvenile shoot. (1) vegetative bud and (2) mixed bud. The axillary scale is the derivative part of the leaf. It forms an ochrea around the terminal bud. The winter buds on the annual shoot are all mixed buds. The vessels are absent in the rhizome and no cambium exists. During tile early growth of the rhizome, the rib meristems contribute mainly to the internode elongation. Later however, divisions are seen to commence in the parenchymatous tissue of the internode. As a result of these divisions the internode becomes elongated. The tuberization of the rhizome is built up from cell divisions of three kinds of tissues: (1) primary thickening meristems, (2) cells of the vascular bundles and (3) parenchyma of cortex. But, the growth in thickness of the rhizome seems to be chiefly due to the enlargement of parenchymatous cells.     

莲的根茎构造,伸长与增粗

莲 (Nelumbo nucifera)种苗的茎短而直立,叶互生。幼苗期茎延伸成横卧根茎,其上生有营养芽及混合芽。腋生鳞片为叶的衍生部分,形如叶鞘状,包着预芽。年苗上的冬芽内全为混合芽。根茎内的维管束分散排列,无导管及形成层存在。节间延长通过肋状分生组织及节间内的薄壁组织细胞分裂与增长来完成。根茎可由初生加厚分生组织,维管束细胞,皮层薄壁细胞等的细胞分裂,使层次增加,但增粗主要是由皮层薄壁细胞体积显著增大而引起的。     

Buds buried in bark: the reason why <Emphasis Type="Italic">Quercus suber</Emphasis> (cork oak) is an excellent post-fire epicormic resprouter

Key message

Cork oak has buds protected by the full thickness of its substantial phellem, thus explaining why it is the only European tree that can epicormically resprout after higher intensity fire.

Abstract

Epicormic resprouting has various ecological advantages over basal resprouting. However, after higher intensity fires epicormic resprouting is rare as it is difficult for trees and shrubs to keep both their buds and vascular cambia alive. Quercus suber (cork oak) is the only European tree that can resprout epicormically after higher intensity fires. Q. suber develops very thick bark and it has been assumed, without anatomical evidence, that the bark protects the epicormic buds. We investigated if developmental anatomy could explain why Q. suber is an excellent post-fire epicormic resprouter. We examined buds from mature Q. suber trees, macroscopically using a stereo microscope and microscopically using semi-thin microtome sections. Q. suber produced buds in the foliage leaf axils and the bud scale axils. With the commencement of extensive phellem (cork) production the base of the epicormic buds remained at, or just below, the level of the phellogen and thus cork began to bury the buds, although a narrow tube connected each bud to the bark surface. Q. suber epicormic buds became deeply buried in the phellem and would be protected from heat by the full phellem thickness. With its rapid and substantial development of phellem Q. suber had well-protected epicormic buds even in relatively small diameter stems. These results provide the anatomical evidence to show why Q. suber is a noted epicormic resprouter after crown fire.

     

Lateral meristems of higher plants: Phytohormonal and genetic control

Lateral meristems (pericycle, procambium and cambium, phellogen) are positioned in parallel to the lateral surface of the organ, where they are present, and produce concentric layers of undifferentiated cells. Primary lateral meristems, procambium and pericycle, arise during embryogenesis; secondary lateral meristems, cambium and phellogen, — during post embryonic development. Pericycle is most pluripotent plant meristem, as it may give rise to a variety of other types of meristems: lateral meristems (cambium, phellogen), apical meristems of lateral roots, and also shoot meristems during plant in vitro regeneration. Procambium and cambium developing from it give rise to the vascular tissues of the stems and roots, ensuring their thickening. The review considers the genetic control of lateral meristem development and the role of phytohormones in the control of their activities.