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.     

Partial shoot reiteration in Wollemia nobilis (Araucariaceae) does not arise from 'axillary meristems'

Background and Aims

Conifers are characterized by the paucity of axillary buds which in dicotyledonous trees usually occur at every node. To compensate, conifers also produce ‘axillary meristems’, which may be stimulated to late development. In juvenile material of Wollemia nobilis (Araucariaceae: Massart''s model) first-order (plagiotropic) branches lack both axillary buds and, seemingly, axillary meristems. This contrasts with orthotropic (trunk) axes, which produce branches, either within the terminal bud or as reiterated orthotropic axes originating from axillary meristems. However, plagiotropic axes do produce branches if they are decapitated. This study investigated how this can occur if axillary meristems are not the source.

Methods

The terminal buds of a series of plagiotropic branches on juvenile trees were decapitated in order to generate axillary shoots. Shoots were culled at about weekly intervals to obtain stages in lateral shoot development. Serial sections were cut with a sliding microtome from the distal end of each sample and scanned sequentially for evidence of axillary meristems and early bud development.

Key Results

Anatomical search produced no clear evidence of pre-existing axillary meristems but did reveal stages of bud initiation. Buds were initiated in a group of small starch-rich cortical cells. Further development involved de-differentiation of these small cells and the development of contrasting outer and inner regions. The outer part becomes meristematic and organizes the apex of the new branch. The inner part develops a callus-like tissue of vacuolated cells within which vascular cambia are developed. This kind of insertion of a branch on the parent axis seems not to have been described before.

Conclusions

Axillary meristems in Wollemia characterize the leaf axils of trunk axes so that the origin of reiterated shoots is clear. Plagiotropic axes seemingly lack axillary meristems but still produce axillary branches by distinctive developmental processes. These observations demonstrate limited understanding of branch initiation in trees generally.     

SOME CASES OF DELAYED OR INDUCED DEVELOPMENT OF AXILLARY BUDS FROM PERSISTING DETACHED MERISTEMS IN CONIFERS

In the apparently “empty” axils of the needles of Taxus baccata, Sequoia sempervirens, Sequoiadendron giganteum, Cryptomeria japonica, Thuja occidentalis, and Thujopsis dolabrata persisting detached meristems were found, which are derived from superficial layers of the apical eumeristem. In T. baccata delayed development of minute axillary buds occurs from these meristems after 1–4 yr on the intact plant. In the other conifers, development of additional axillary buds from these meristems was induced by natural frost damage or by artificial pruning and disbudding. The discovery of these detached meristems is discussed with regard to the regenerative capacity of the conifers in comparison to other plants.     

Seasonal culture of dormant reproductive buds ofSalix tetrasperma: Analysis of the flowering process

The flowering process in a female tree ofSalix tetrasperma was analysed by culturing its reproductive buds at different developmental stages during the dormant period on a chemically defined medium and examining the nature of sprouts produced by them. Buds at the upper eight nodes of the actively growing shoots developing in an acropetal sequence were cultured in separate lots. While all the buds collected from the 1st and 2nd nodes of the branches from the top downwards were vegetative and produced shoots, a considerable number of those collected from the 3rd and 4th nodes were reproductively determined and produced catkins. All the buds obtained from the 5th node and below were reproductive. Reproductive buds were cultured at regular time intervals during the dormant period. Freshly formed buds cultured in March during the spring growth flush produced catkins and were therefore reproductively determined. However, such a determination was not tantamount to flowering, as the floral meristems present in the axils of catkin bracts remained quiescent. Floral meristems of the buds cultured during April to August developed into small vegetative shoots. This was followed by the crucial period during September to December when the hitherto vegetative sprouts of the floral meristems showed a gradual transition into ovaries (female flowers) resulting in fertile catkins. Catkins produced from buds cultured in January and February produced well-developed ovaries.     

Survival and growth of eastern white pine shoot apical meristemsin vitro

Shoot apical meristems of seedling and mature eastern white pine trees were excised and grownin vitro. Placing the meristems on filters instead of directly on agarose-solidified nutrient medium enhanced survival of both juvenile and mature meristems. Applying forcing treatments to mature branches improved survival and growth of dissected meristems compared with meristems from non-forced branches in experiments conducted over two years. No consistent differences were observed among 2-, 4-, and 6-week forcing treatments. Including 5.37 nM (0.001 mg l^-1) l-naphthaleneacetic acid in the culture medium did not affect meristem survival or growth. Some meristems from seedlings grew rapidly, produced primary leaves, underwent internode elongation, and in three cases, produced adventitious roots. Meristems from mature trees did not grow as rapidly as seedling meristems. The leaves produced by mature meristems appeared to be scale leaves and a few of these had brachyblast primordia in the axils. The shoots derived from mature meristems did not produce adventitious roots.     

An excellent source of vegetative buds for use in plant hormone studies on apical dominance

When studying the role of plant hormones in the control of growth at apical meristems, it is often difficult to obtain needed amounts of physiologically uniform buds. A source and method are described for obtaining sufficient quantities of large, uniform buds and for the treatment of the buds with indoleacetic acid and kinetin. Buds from the root system of Euphorbia esula L. were grown in Petri plates, with agar suspending the short root sections from which they emanate. Plant hormones are applied by their incorporation in the agar. The effect of various concentrations of indoleacetic acid and kinetin on bud growth was examined.     

棉花花芽分化及部分内源激素变化规律的研究

棉花（Ｇｏｓｓｙｐｉｕｍ ｈｉｒｓｕｔｕｍ）的腋芽原基,有的将来发育成叶枝;有的将来发育成果枝。这２种不同命运的腋芽,在其刚分化的初期就表现出了不同的解剖学特征。将来发育为叶枝的腋芽,其生长锥呈圆锥形或扁圆球形,体积较小,原套层数为１－２层;而将来发育为果枝的腋芽,其生长锥为圆柱形,顶端表面平坦,体积较大,原套层数为２－３层。从子叶展平后到肉眼可见花芽（现蕾）,连续测茎尖的内源ＡＢＡ及ＩＡＡ的含量     

实生成年苹果树外植体的快速微型繁殖

试验结果表明:以实生成苹果树上的芽作为外植体,培养在添加1—2mg/l 6-BA的SH或MS培养基上,在继代培养时芽的萌发率达92％以上,50天内芽数增殖5—6.8倍;同期内,液体旋转培养比琼脂静置培养芽条长度要增加3倍左右;芽条在0.2—0.4mg/lIBA中生根率80％以上;试管苗的移栽成活率85％以上。     

The Morphogenesis of Apple Buds: III. The Inception of Flowers

The early stages in the change from vegetative to reproductivedevelopment of apple spur terminal buds were followed by dissectionof buds from untreated trees, and from trees defoliated at differenttimes in the season. A change in the development of the leafprimordia occurred when there were approximately eight in thebud. This was followed by the development of bracts, which appearedto be necessary for the formation of actual flower parts. Leafprimordia tend to inhibit this process. Whereas their effectupon the apical meristem was subsequently reduced by the formationof bracts, so that eventually a terminal flower formed, theireffect upon the lower lateral meristems was unaltered. Thesemeristems therefore remained in a vegetative state. In addition to the number of leaf primordia in the bud, thedegree of dormancy may be an important factor in determiningthe onset of flowering. Since the number of leaf primordia invegetative buds at the end of the season is eight, the spatialdistribution of primordia on the main axis of the bud and theirvascular connexions might have a decisive effect on bud development.This was related to the effect of older primordia in the budupon the development of younger ones. In buds in which theseolder primordia were inhibited by foliage, etc., i.e. thosewith a long plastochrone, no effects were observed upon thedevelopment of younger primordia and the buds remained vegetative. Whilst correlative inhibition of buds thus affected their abilityto form flowers, there is no evidence of a critical leaf areafor flowering. Flowering in apple buds is more likely to bedue to the removal of factors inhibiting reproductive developmentthan to the synthesis of a specific flower inducing substanceas such.     

ONTOGENETIC STUDY OF FLORAL ORGANS OF PEACH (PRUNUS PERSICA) UTILIZING CYTOCHIMERAL PLANTS

Cell lineages were followed throughout floral ontogeny in cytochimeral peaches [Prunus persica (L.) Batsch] by observations of chromosome number and nuclear size. The contribution of the three apical cell layers to the organs of the flowers was determined. In addition to the epidermal tissue, L-I produced several layers of cells at the suture of the ovary wall, seven or eight cell layers of the nucellus at the micropylar end of the ovule, and almost all of the integuments. L-II gave rise to extensive internal tissue in the calyx and corolla tubes and to all internal tissue of the petal, anther, and ovule except for a small region at the base of the latter two organs. L-III contributed significantly only to the central region of the calyx and corolla tubes and the ovary wall. A single apical layer gave rise to several different tissues, and at times a single tissue was made up of cells from 1–3 different apical layers. Within the limits imposed by their genotype the final form of differentiated cells was determined by their position in the mature organ and not by the apical layer from which they were derived. The corolla tube was shown to be a single structure, congenitally fused, and the ovary to be ontogenetically fused at the suture.     

Increased susceptibility of black-currant bushes to the gall-mite vector (Phytoptusribis Nal.) following infection with reversion virus

Healthy black-currant bushes (var. Wellington XXX) and others infected systemically with a virulent strain of reversion virus were exposed equally to infestation by the gall-mite vector (Phytoptus ribis Nal.). Shoots with malformed leaves caused by mites feeding at the stem apices were virtually restricted to the virus-infected bushes. Buds infested with mites became rounded galls, which were far fewer and more localized on shoots of healthy bushes than on infected ones. Similar results were obtained in further experiments with Wellington XXX and seven other varieties. Bushes infected systemically with an avirulent strain of reversion virus developed more galls than healthy bushes, but fewer galls than bushes infected with a virulent strain. The young buds of virus-infected bushes were 170 times more susceptible to dispersing mites than those of healthy bushes. The axillary buds of healthy bushes resisted infestation and the apical meristems were virtually inaccessible to mites, whereas the axillary and apical buds of virus-infected bushes were invaded readily. An experiment with bushes var. Cotswold Cross that were either healthy or partially or completely infected with reversion virus was retained for two years. Each year there was a relationship between symptom expression and mite infestation; only shoots with chronic virus symptoms developed mite-affected leaves and numerous galls. Virus infection increased the vulnerability and accessibility of the apical and axillary buds by decreasing the density of hairs on the stems and leaves. Infected bushes also presented a greater catchment area to dispersing mites and more shoots and buds were available for colonization than on healthy bushes. The interactions between virus, host and vector are discussed, together with their implications in nature, in experimental design and in disease control.     

The Dormancy of Buds of Citrus sinensis (L.) Osbeck Inserted into Rootstock Stems: Factors Intrinsic to the Inserted Bud

Buds of sweet orange, harvested from shoots of different timeof flushing and from different positions along the shoot, wereused to examine whether lack of burst of inserted buds was acharacteristic of the bud. Bursting of inserted buds was significantlyslower in buds taken from (a) older branches (b) shoots producedunder winter conditions, and (c) basal rather than apical budson the same shoot. The slowness to burst when transferred matched a tendency todormancy in buds on shoot segments grown in vitro, suggestingthat the variation in budburst was intrinsic to the bud. Budburstwas correlated with the extent of secondary bud development;the majority of buds from apical regions of the shoot had developeda secondary bud by the time of implantation, but basal budshad not. Adequate vascular connections with the host tissueswere found in both burst and unburst buds. Citrus sinensis (L.) Osbeck, sweet orange, buds, endodormancy, budding     

Branch development in Lupinus angustifolius L.#I. Not all branches have the same potential growth rate

Although the basal and uppermost lateral branches of Lupinus angustifolius L. frequently grow and contribute to yield, buds formed in the axils of leaves 6-12 (referred to as middle buds) rarely grow. This may be due to an inherent limitation of these buds, or some form of apical dominance or competition imposed by the plant. The hypothesis that middle buds have the full capacity to grow, but remain suppressed on intact plants was tested. The main stem apex and buds from the axils of leaves 1 and 8 (bud 1 and bud 8) were excised and cultured on sterile agar. The buds were removed from culture and weighed every 2-3 d for 21 d. The growth rate of apices from the main stem was approximately 5.8 mg d^-1, compared to 2.4 mg d^-1 for bud 1 and 0.9 mg d^-1 for bud 8. Buds in the axils of leaves 6-10 on intact plants were painted six times with a synthetic cytokinin, benzylaminopurine, from 40 d after sowing. This promoted rapid elongation and thickening of these buds, visible as early as 5 d after painting began. The rapid growth of these branches was associated with a reduction in the length of the remaining branches on the plant. However, excision of lower branches did not increase the growth of the middle buds. It is concluded that buds 6-12 of Lupinus angustifolius L. have a partial potential to grow. This potential appears to be limited by innate factors in the bud, and may be structural and/or hormonal. The limitation appears to develop very early in the plant, and potential growth is not modified by subsequent nutrition of the plant.     

Molecular role of cytokinin in bud activation and outgrowth in apple branching based on transcriptomic analysis

Key message

Axillary bud activation and outgrowth were dependent on local cytokinin, and that bud activation preceded the activation of cell cycle and cell growth genes in apple branching.

Abstract

Cytokinin is often applied to apple trees to produce more shoot branches in apple seedlings. The molecular response of apple to the application of cytokinin, and the relationship between bud activation and cell cycle in apple branching, however, are poorly understood. In this study, RNA sequencing was used to characterize differential expression genes in axillary buds of 1-year grafted “Fuji” apple at 4 and 96 h after cytokinin application. And comparative gene expression analyses were performed in buds of decapitated shoots and buds of the treatment of biosynthetic inhibitor of cytokinin (Lovastatin) on decapitated shoots. Results indicated that decapitation and cytokinin increased ZR content in buds and internodes at 4–8 h, and induced bud elongation at 96 h after treatment, relative to buds in shoots receiving the Lovastatin treatment. RNA-seq analysis indicated that differential expression genes in auxin and cytokinin signal transduction were significantly enriched at 4 h, and DNA replication was enriched at 96 h. Cytokinin-responsive type-A response regulator, auxin polar transport, and axillary meristem-related genes were up-regulated at 4 h in the cytokinin and decapitation treatments, while qRT-PCR analysis showed that cell cycle and cell growth genes were up-regulated after 8 h. Collectively, the data indicated that bud activation and outgrowth might be dependent on local cytokinin synthesis in axillary buds or stems, and that bud activation preceded the activation of cell cycle genes during the outgrowth of ABs in apple shoots.

     

Relative developmental, environmental, and tree-to-tree variability in buds from field-grown apple trees

High-throughput genomic technologies are becoming more accessible to nonmodel plant species, and therefore, tissue collected outside controlled environments is being increasingly used for microarray analyses. In this study, we present a 15,720-feature apple microarray analysis of the variability of gene expression in buds from field-grown apple trees. Tree-to-tree and day-to-day variances were assessed during two different seasons: summer, when the meristems in the buds were undergoing the first stages of floral development, and autumn, when the buds were undergoing transition to winter dormancy. We found that apple trees with the same scion and rootstock cultivars, grown in a standard orchard environment, had small tree-to-tree variation. Gene expression differences caused by season was the dominant cause of variance while using false discovery rate control with a threshold of α* = 0.01 to select significantly different expression between genes. At this threshold, the environmental and location effects accounted for less than 10% of the genes selected. Consequently, we have shown that field microarray experiments are a viable approach for measuring seasonal changes in gene expression during apple bud development. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Induction of Adventitious Buds on Buds of Norway Spruce (Picea abies) Grown in vitro

Resting vegetative buds of Norway spruce, Picea abies (L.) Karst., were induced to form adventitious bud primordia when cultured on medium -containing cytokinin. After transfer of the induced buds to medium lacking cytokinin, adventitious buds developed. The adventitious buds arose from meristems formed de novo in the needle primordia. No differences were found in the ability to form adventitious buds among buds collected from trees ranging from 5–50 years old.     

Execution of the auxin replacement apical dominance experiment in temperate woody species

The classic Thimann-Skoog or auxin replacement apical dominance test of exogenous auxin repression of lateral bud outgrowth was successfully executed in both seedlings and older trees of white ash, green ash, and red oak under the following conditions: (1) decapitation of a twig apex and auxin replacement were carried out during spring flush, (2) the decapitation was in the previous season's overwintered wood, and (3) the point of decapitation was below the upper large irrepressible lateral buds but above the lower repressible lateral buds. Although it has been suggested that neither auxin, the terminal bud, nor apical dominance have control over the outgrowth of the irrepressible buds during spring flush, there is evidence in the present study that indicates that such control over the repressible buds exists. In seedlings, second-order branching, which resulted from decapitation of elongating current shoots, was also inhibited by exogenous auxin in the three species. Hence, the auxin replacement experiments did work on year-old proleptic buds (of branches of older trees) that would have entered the bud bank and also on current buds of seedlings. Cytokinin treatments were ineffectual in promoting bud growth.     

The development of Archaeopteris: new evolutionary characters from the structural analysis of an Early Famennian trunk from southeast Morocco

A 5 m long trunk of a young Archaeopteris/Callixylon erianum tree from the Late Devonian of Morocco shows new branching patterns for early lignophytes. This progymnosperm tree produces a helical pattern of traces that we infer belonged to reduced, short-lived, primary (apical) branches (type A) as well as two types of adventitious traces (types B and H). We infer that type-B traces supplied branches that initiate close to the site of attachment on the trunk of some, but not all type-A branches in an irregular but nonrandom pattern. Unlike ephemeral type-A branches, those of type B persist and become long-lived, potentially permanent units of the architecture of Archaeopteris trees. Type-H adventitious traces are also short-lived and occur singly or in serial groups, but differ from traces of either type A or B branches by lacking differentiation into a readily identifiable organ category. We interpret type-H traces as supplying latent primordia that could develop into either adventitious roots or shoots depending on extrinsic factors. Our new data suggest that Archaeopteris had a wide range of branch primordium amplitude. Type-B branches compare with axillary lateral branch buds of some Early Carboniferous spermatophytes (Calamopitys) and are a major developmental departure from the strictly apical, pseudomonopodial shoot branching of older aneurophyte progymnosperms. Type-H traces suggest that Archaeopteris trees had some potential for formation of adventitious roots or shoots in response to environmental factors, such as partial burial by overbank sedimentation. Collectively, these novel methods of tree branching may partly explain the extraordinary success and worldwide dominance of Archaeopteris forests on fluvially dominated, Late Devonian floodplains.     

Polyp morphogenesis in a scyphozoan: Evidence for a head inhibitor from the presumptive foot end in vegetative buds ofCassiopeia andromeda

Summary Buds ofCassiopeia andromeda have been transected into fragments of various sizes. Depending on their original position in the organism, on their size and on the age of the dissected buds, the fragments either regenerated or developed to a solitary polyp's head without stalk and peduncle. Generally, basal fragments tended to regenerate complete buds, young apical parts mostly differenciated polyp heads whereas apical and middle parts of progressively older buds regenerated buds with increasing frequency. To explain the alteration of the developmental capacities a head inhibitor is postulated which originates from the basal end of the buds and which expands towards the apical pole with increasing age of the buds.     

Elimination of five viruses from sugarcane using in vitro culture of axillary buds and apical meristems

Procedures were developed for the in vitro elimination of Sugarcane mosaic virus (SCMV), Sorghum mosaic virus (SrMV), Sugarcane streak mosaic virus (SCSMV), Sugarcane yellow leaf virus (SCYLV) and Fiji disease virus (FDV) from infected sugarcane. In vitro shoot regeneration, elongation and virus elimination through meristem tissue culture originating from both apical and axillary shoots were compared. The average rates of regeneration and elongation from apical meristem tissues were 91 and 66%, respectively, with the virus-free rate among elongated shoots ranging from 61–92%. Mature axillary buds were cultivated in vitro to produce axillary shoots, from which meristem tissues were excised and cultured. These meristem tissues regenerated (77–100%) and elongated (55–88%) in culture medium at approximately the same rate as the apical meristems. The average virus elimination rate was 90% among elongated shoots derived from mature axillary buds. All five viruses can be eliminated by meristem tissue culture from both apical and axillary shoots using a standardized procedure. The overall average efficiency of virus-free plant production was 45 and 58% from apical and axillary shoots, respectively. There were no significant differences for shoot induction or virus elimination when the meristems were harvested from either the apical or the axillary shoots. This is the first report of SrMV or SCSMV elimination from sugarcane, as well as elimination of any mixed virus infections. This new method of harvesting meristems from axillary buds greatly expands the amount of material available for therapeutic treatments and thereby increases the probability of eliminating viruses from infected sugarcane.