THE FATE OF THE DWARF SHOOT APEX IN BRISTLECONE PINE (PINUS LONGAEVA)

The fate of the pine dwarf shoot (DS) apex after needle initiation has been controversial. Dwarf shoot primordia of Pinus longaeva were examined to determine the developmental basis for DS with and without interfoliar buds. Interfoliar buds are microscopic buds derived from the original terminal apex of the DS. In October, all the DS primordia are similar in size and appearance. However, as the needles elongate in the following June the apices of more proximal DS decrease in size, such that by July there is a clear diminishing size gradient of apical domes in going from the most distal to the most proximal positions. The distal DSs start to form bud scales in July and have fully formed interfoliar buds by mid-August. In contrast, those DS apices lacking protective bud scales at needle maturity become suberized and can never proliferate into long shoots. The distal placement of interfoliar buds may be due to a group effect, where each developing DS inhibits the more proximal DSs in the long shoot terminal bud.     

Auxin transport and the regulation of petiole abscission in cotton (Gossypium hirsutum L.): A comparison of indol-3yl-acetic acid and phenylacetic acid

Distal applications of indol-3yl-acetic acid (IAA) to debladed cotyledonary petioles of cotton (Gossypium hirsutum L.) seedlings greatly delayed petiole abscission, but similar applications of phenylacetic acid (PAA) slightly accelerated abscission compared with untreated controls. Both compounds prevented abscission for at least 91 h when applied directly to the abscission zone at the base of the petiole. The contrasting effects of distal IAA and PAA on abscission were correlated with their polar transport behaviour-[1-¹⁴C]IAA underwent typical polar (basipetal) transport through isolated 30 mm petiole segments, but only a weak diffusive movement of [1-¹⁴C]PAA occurred.Removal of the shoot tip substantially delayed abscission of subtending debladed cotyledonary petioles. The promotive effect of the shoot tip on petiole abscission could be replaced in decapitated shoots by applications of either IAA or PAA to the cut surface of the stem. Following the application of [1-¹⁴C]IAA or [1-¹⁴C]PAA to the cut surface of decapitated shoots, only IAA was transported basipetally through the stem. Proximal applications of either compound stimulated the acropetal transport of [¹⁴C]sucrose applied to a subtending intact cotyledonary leaf and caused label to accumulate at the shoot tip. However, PAA was considerably less active than IAA in this response.It is concluded that whilst the inhibition of petiole abscission by distal auxin is mediated by effects of auxin in cells of the abscission zone itself, the promotion of abscission by the shoot tip (or by proximal exogenous auxin) is a remote effect which does not require basipetal auxin transport to the abscission zone. Possible mechanisms to explain this indirect effect of proximal auxin on abscission are discussed.     

Morphogenesis in dormant embryonic shoots of Picea abies: Influence of the crown and cold treatment

Retention of the crown, a plate of homogeneously thickened collenchymatous cells at the base of the embryonic shoot of Picea abies (L.) Karst., has marked effects on the induction of adventitious buds from the needle primordia. These effects were observed on embryonic shoots of three clones which were cold-treated at 4 ± 1°C for nil, two, four and six months; and then excised either with or without the crown, before subculturing either directly on a medium lacking plant growth regulators or after application of 125 μM benzyladenine (BA) as a 3-h pulse. Embryonic shoots retaining the crown and treated with BA responded reproducibly better than those lacking it. This was true both for the number of shoots that produced adventitious buds and for the numbers of adventitious buds per shoot. Differentiation of as many as five adventitious buds in a single needle primordium was observed, with development occurring nearly twice as early (7 as compared with 12 weeks) in the presence as in the absence of the crown. The response decreased progressively with increasing exposure to cold treatment.
A number of anatomical changes occurred during culture in vitro. After 3 to 4 weeks a band of sclerenchymatous cells commenced differentiation at the centre of the crown. In embryonic shoots without the crown, a tissue which was identified as a wound periderm formed at the excision surface. Vascular unions were observed between the tissues of the newly — formed shoots induced from the needle primordia and those of the original shoot explants.     

栝楼不定根尖分化不定芽过程中的细胞组织学研究

将栝楼（Trichosanthes kirilowii)长约0.5-1cm不定根尖（连同原外植体茎段或根段一起,或不连）培养在MS附加6－BA5mg/L的培养基上光照培养15d,可在根尖分化出大量不定芽。不定根尖培养过程中每隔2－3d取材,用FAA固定液固定1次,通过石蜡切片观察,将栝楼的不定根尖端分化不定芽的细胞组织学变化分为4个时期。1．启动期（0-3d),根尖分生组织细胞、中柱鞘细胞起动分裂。2“根茎转变区”原形成层形成期（4－6d0。起动细胞分裂后形成2－3层体积小、核大、质浓、近似扁平形的细胞层,组成“杯形”的“根茎转变区”原形成层。3．“根茎转变区”形成期（7－10d)。原形成层不同部位加速分裂使根尖膨大成半球形、球形或梭形,并在膨大区进行维管组织的转变。4．芽分化形成期（11－15d)。原形成层在不同部位向外形成“突起”即分生细胞团,每个“突起”发育为1个芽原基。作者还讨论了栝楼与其它植物根芽产生的异同。     

Morpho-Histological Investigation of Plantlet Formation In Vitro in Taiwania flousiana Gaussen

Histological events during adventitious shoot formation in cultured shoot apex of 10–12-day-old seedlings and adventitious root formation in the elongated shoot of Taiwania floudana Gaussen were examined. Ceils of the peripheral subsurface layers of the shoot apex responded to cytokinin and divided into meristematic cells from which the shoot primordia were proliferated. A few bud primordia also originated from the epidermis and hypodermis of the adaxial surface of the cotyledon. The parenchyma of leaf gap of the shoots cultured in rooting medium dedifferentiated to regain the capacity of division and form adventitious root. Besides, cells that had relatively low potential of differentiation, such as the cortex parenchyma, pith ray, phloem parenchyma and cambium zone, albeit initiated to divide, but seldom formed root primordium. The origin of the adventitious roots in the leaf gap facilitated the establishment of the vascular connection between the shoot and root.     

Shoot-tip abscission and adventitious abscission of internodes in mulberry (Morus alba)

In mulberry ( Morus alba L. cv. Shin-ichinose), shoot-tip abscission following the cessation of apical growth could be induced in different internodes, depending on the vigour of the shoot and its apex and other internal and external factors. In the lateral, short shoots of 1-year-old stems of low-pruned trees, the apical growth cessation and shoot-tip abscission (May–June) resulted primarily from the dominance of the upper, long shoots and intense competition among laterals along the stem. Decapitation of the laterals, before abortion of their apices took place (early May), readily caused adventitious abscission of the distal internode. Similar decapitation-induced, adventitious abscission of the distal internode of the upper, long shoots of 1-year-old stems of pruned trees also occurred (May–September), demonstrating that the abscission itself is not directly associated with photoperiod. In May and June, decapitation induced abscission primarily in parallel with or after sprouting of lateral buds and shoot elongation, while in July, August and September, the abscission was induced by decapitation and independently of sprouting. Shoot (stem) orientation positively affected the abscission, which is related to gravimorphic effects on buds and shoots on the lower and lateral sides of the horizontally trained stem. These results suggest that the vigour of shoots and apices is an important determinant of growth and apex abscission in mulberry.     

<Emphasis Type="Italic">In vitro</Emphasis> organogenesis of chestnut (<Emphasis Type="Italic">Castanea sativa</Emphasis> Mill.) cotyleodon explants: responses to growth regulators and developmental aspects

Summary Adventious root and shoot formation was obtained from cotyledon fragments of chestnut (Castanea sativa Mill.) and these processes followed two phases. In a first stage after detachment of the embryonic axis, the cotyledon fragments in culture formed a cotyledon petiole, which elongated for about 6d. Thereafter, root primordia arose at the tip of the cotyledon petioles, followed by normal root development. In some cases, the cotyledon, petioles showed adventitious shoot regeneration from a nodular structure previously formed at the end of the petioles. The presence or absence of growth regulators did not significantly influence root regeneration, whereas cytokinins stimulated shoot formtion. The processes of root and shoot differentiation were studied also at the histological level. Observation with a light microscope showed that the developing root apical meristems were connected with a vascular bundle of the cotyledon petiole. Similarly, shoot bud meristem connections were observed with vascular tissue inside the nodular structure.     

Factors promoting efficient in vitro regeneration from de-embryonated cotyledon explants of <Emphasis Type="Italic">Arachis hypogaea</Emphasis> L.

Highly efficient (>90%) protocols were developed for in vitro regeneration from de-embryonated cotyledon explants of peanut (Arachis hypogaea L.). Phytohormone combinations and concentrations, explant source and orientation, period of incubation and the response of genotypes were examined for optimization of the regeneration efficiency. Adventitious shoot primordia could be induced from de-embryonated cotyledon explants when (1) the proximal end of the explant was kept in contact with the shoot induction medium-I supplemented with 5 mg l⁻¹ BAP + 2 mg l⁻¹ 2,4-D for 4 weeks, or (2) the distal end was kept in contact with shoot induction medium-II supplemented with only BAP at 20 mg l⁻¹ for the first 2 weeks, followed by subculture in the same medium containing 15 mg l⁻¹ BAP for the next 2 weeks. Orientation of placing the explant on the above media was critical for in vitro regeneration. The factors affecting the morphogenic responses like, repetitive organogenesis, shoot elongation, in vitro flowering and rhizogenesis were examined. Shoot bud formation was genotype independent. Histological studies showed multicellular origin of adventitious shoot primordia. The protocols gave healthy and fertile plants within 4 months.     

Adventitious shoot formation in decapitated dicotyledonous seedlings starts with regeneration of abnormal leaves from cells not located in a shoot apical meristem

Regeneration of new shoots in plant tissue culture is often associated with appearance of abnormally shaped leaves. We used the adventitious shoot regeneration response induced by decapitation (removal of all preformed shoot apical meristems, leaving a single cotyledon) of greenhouse-grown cotyledon-stage seedlings to test the hypothesis that such abnormal leaf formation is a normal regeneration progression following wounding and is not conditioned by tissue culture. To understand why shoot regeneration starts with defective organogenesis, the regeneration response was characterized by morphology and scanning electron and light microscopy in decapitated cotyledon-stage Cucurbita pepo seedlings. Several leaf primordia were observed to regenerate prior to differentiation of a de novo shoot apical meristem from dividing cells on the wound surface. Early regenerating primordia have a greatly distorted structure with dramatically altered dorsoventrality. Aberrant leaf morphogenesis in C. pepo gradually disappears as leaves eventually originate from a de novo adventitious shoot apical meristem, recovering normal phyllotaxis. Similarly, following comparable decapitation of seedlings from a number of families (Chenopodiaceae, Compositae, Convolvulaceae, Cucurbitaceae, Cruciferae, Fabaceae, Malvaceae, Papaveraceae, and Solanaceae) of several dicotyledonous clades (Ranunculales, Caryophyllales, Asterids, and Rosids), stems are regenerated bearing abnormal leaves; the normal leaf shape is gradually recovered. Some of the transient leaf developmental defects observed are similar to responses to mutations in leaf shape or shoot apical meristem function. Many species temporarily express this leaf development pathway, which is manifest in exceptional circumstances such as during recovery from excision of all preformed shoot meristems of a seedling.     

SHOOT TIP ABORTION IN ULMUS AMERICANA

Millington , W. F. (Marquette U., Milwaukee, Wis.) Shoot tip abortion in Ulmus americana. Amor. Jour. Bot. 50(4): 371–378. Illus. 1963.—Phenological observations of American elm have shown that the phenomenon of shoot tip abortion in which the distal several plastochrons of each shoot turn yellow and abort, inducing sympodial growth, occurs over a period 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 abscission, 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.     

In vitro phyllomorphic regeneration of shoot buds and shoots in Picea abies

Needles (10–15 mm) of frost-hardened 20–22-week-old (physiological age equivalent to 1 year) plants of Picea abies L. excised just after flushing, were induced to form adventitious shoot buds and shoots on media supplemented with BAP (6-benzylaminopurine) and NAA (1-napthaleneacetic acid). The addition of nanomolar concentrations (0.5–50) of NAA combined with 1–10 μM BAP considerably stimulated formation of pseudobulbils on the basal to mid-part of the needle axis, as well as their subsequent development into shoot buds and shoots. On a medium containing 10 μM BAP, pseudobulbils that formed at the needle base did not develop further, but became necrotic and died with the omission of NAA. With 5 μM BAP + 50 nM NAA the initial phase of development was slow, but later showed good response and up to 22% of the needles produced shoot buds. Two to three shoots per needle could be excised and subcultured individually onto fresh media. It is concluded that the level of endogenous auxin decreases progressively from the needle's base to its tip, so that that concentration of exogenous auxin (50 nM NAA) which promotes pseudobulbil and shoot-bud formation part-way along the needle axis, simultaneously inhibits their induction at the needle base.     

Cells within the nodal region of carnation shoots exhibit a high potential for adventitious shoot formation

Adventitious shoot formation was studied with leaf, stem and axillary bud explants of carnation (Dianthus caryophyllus L.). The shoot regeneration procedures were applicable for a wide range of cultivars and shoot regeneration percentages were high for all explant types. Using axillary bud explants, shoot regeneration efficiency was independent of the size of the bud and of its original position in the plant. In contrast, shoot regeneration from stem and leaf explants was strongly dependent on their original position on the plant. The most distal explants (just below the apex) showed the highest level of shoot regeneration. The adventitious shoot primordia developed at the periphery of the stem segment and at the base of leaf explants. In axillary bud, stem and leaf explants, shoot regeneration originated from node cells, located at the transition area between leaf and stem tissue. Moreover, a gradient in shoot regeneration response was observed, increasing towards the apical meristem.Abbreviations BA benzyladenine - NAA naphthaleneacetic acid     

Peroxidases in Tobacco Abscission Zone Tissue: II. Time Course Studies of Peroxidase Activity during Ethylene-induced Abscission

Ethylene-induced abscission in flower pedicels of Nicotiana tabacum L. cv. Little Turkish causes a progressive increase in peroxidase activity during the first 4 hours of a 5-hour time course ethylene treatment period, with decrease in peroxidase activity occurring between 4 hours and 5 hours, when the supernatant extracts of abscission zone segments are tested spectrophotometrically for peroxidase activity, using guaiacol and hydrogen peroxide. Nonethylene-treated tissue has a much lower level of peroxidase activity over the same time course period. In ethylene-treated tissue the decline in break-strength correlates with the beginning of increase in peroxidase activity (3 hours). When the abscission zone area of the pedicel is further divided into proximal, abscission zone, and distal portions, respectively, the ethylene-treated tissue has the highest peroxidase activity in the abscission zone portion, with the maximum peak occurring at 4 hours and decreasing between 4 hours and 5 hours. Acrylamide gel electrophoresis of enzyme breis from ethylene-treated aand nonethylene-treated plants reveals that no new peroxidase isozymes are formed in response to ethylene, indicating an increase in the amount of one or in both of the two already existing isozyme banding patterns. The measurement of protein in the proximal, abscission zone, and distal segments, over a 5-hour ethylene treatment period, indicates that it is being translocated in a distal to proximal direction in the abscission zone pedicel. The possible participatory role for peroxidase in ethylene-induced tobacco flower pedicel abscission are discussed.     

Periods of organogenesis in shoots of Nothofagus dombeyi (Mirb.) oersted (Nothofagaceae)

The organogenetic cycle of main-branch shoots of Nothofagus dombeyi (Nothofagaceae) was studied. Twelve samples of 52-59 parent shoots were collected from a roadside population between September 1999 and October 2000. Variations over time in the number of nodes of terminal and axillary buds, and the length, diameter and number of leaves of shoots derived from these buds (sibling shoots) were analysed. The number of nodes of buds developed by parent shoots was compared with the number of nodes of buds developed, I year later, by sibling shoots. The length, diameter and number of leaves of sibling shoots increased from October 1999 to February 2000 in those shoots with a terminal bud. However, extension of most sibling shoots, including the first five most distal leaf primordia, ceased before February due to abscission of the shoot apex. Axillary buds located most distally on a shoot had more nodes than both terminal buds and more proximal axillary buds. The longest shoots included a preformed part and a neoformed part. The organogenetic event which initiated the neoformed organs continued until early autumn, giving rise to the following year's preformation. The absence of cataphylls in terminal buds could indicate a low intensity of shoot rest. The naked terminal bud of Nothofagus spp. could be interpreted as a structure less specialized than the scaled bud found in genera of Fagaceae and Betulaceae.     

Formation of adventitious buds in needle primordia isolated from embryonic shoots of adult picea abies

Needle primordia were isolated from embryonic shoot expiants of 88 year oldPicea abies (Norway spruce) in order to develop a method for micropropagation using adult mother trees. The isolation of needle primordia in combination with frequent subculture, stimulated the formation of peripheral meristematic tissue in expiants cultured on a modified Murashige & Skoog medium supplemented with cytokinin. Transfer of isolated needle primordia with peripheral meristematic tissue to medium without growth regulators, and with a reduced ratio of NH+4/ NO?3, resulted in the formation of adventitious buds.     

GIBBERELLIN AND AUXIN RELATIONSHIPS IN ABSCISSION

Gibberellic acid (GA) has no effect on abscission when applied proximally or distally to the abscission zones of debladed petioles of Coleus. Application of GA to the stem apex increases the rate of abscission of debladed petioles. The effect on abscission is accompanied by an increase in the level of endogenous auxin in the stem. Correspondingly proximal applications of indoleacetic acid (IAA) accelerate abscission, whereas the longevity of the debladed petiole approaches that of the intact leaf only in the presence of a continuous distal supply of IAA. No correlation is found between petiole elongation and its longevity. The experimental data support the view that auxin acts at the abscission zone in regulating separation processes and that the effect of GA is through its effect on the level of endogenous auxin.     

A HISTOCHEMICAL STUDY OF THE SEEDLING SHOOT APICAL MERISTEM OF PINUS LAMBERTIANA

Vernalized seeds of Pinus lambertiana were scarified and planted in perlite. At 5, 8, 10, 13 and 16 days after planting, seedlings were selected for morphological examination and histochemical study. The shoot apical meristem consisted of a relatively homogeneous population of cells at 5 days. Cytohistological zonation was observed in the meristem by the eighth day and needle primordia initiation began at this time. Acid phosphatase (AP) activity was high in the extreme tip of the apex at 5 days. At 8 days AP activity was intense in the peripheral zone but weak in the apical initial and central mother cell zones. The apical meristem of the 10–16-day-old seedlings exhibited high AP activity in the peripheral zone only during the early stages of needle primordia initiation. The distribution of cytoplasmic and nuclear protein-bound SH was correlated with cytohistological zonation. Protein-bound SH was distributed relatively uniformly at 5 days, but by the eighth day the 4 cytohistological zones contained differential quantities. Succinic dehydrogenase (SD) activity was observed throughout the apex at 5 days, but by the eighth day the apical initial and central mother cell zones exhibited differentially greater levels of SD activity. Irradiation with 500 R of X-rays at 7 days after planting completely inhibited needle primordia initiation and disrupted the cytohistological zonation of the apex. Correlated with the inhibition of needle primordia initiation was the loss of SD activity in the apical initial and central mother cell zones. Irradiation also resulted in the gradual loss of protein-bound SH from the cytoplasm of the apical initial, central mother cell and peripheral zone.     

Efficient procedures for callus induction and adventitious shoot organogenesis in sugar beet (<Emphasis Type="Italic">Beta vulgaris</Emphasis> L.) breeding lines

Summary Improved in vitro tissue culture systems are needed to facilitate the application of transgene technology to the improvement of sugar beet germplasms. Several commercially important sugar beet breeding lines (SDM, 3, 5, 8, 9, 10, 11, HB 526, and CMS 22003) and commercial varieties (Roberta and Gala) were tested for their regeneration capacity through adventitious shoot organogenesis from cotyledons, hypocotyls, root/hypocotyl/shoot transition zone tissues, and leaf lamina and petiole via an intervening callus phase. Callus induction and adventitious shoot regeneration was dependent on genotype and combinations of plant growth regulators. With cotyledon or hypocotyl explants, SDM 3 and 10 showed a better response on adventitious shoot regeneration in medium containing benzyladenine (BA) and 2,3,5-triiodobenzoic acid or 1-naphthaleneacetic acid (NAA) than SDM 11, 5, and 9. Shoot regeneration was obtained from hypocytyl-root or hypocotyl-shoot transition zone tissue in SDM 9, 10, and HB 526 grown on PGo medium supplemented with BA to induce callus, and the regeneration frequency was 25%. Adventitious shoots were also regenerated from leaf explants of SDM 3 and 9 cultured on medium containing NAA for callus induction and BA and NAA to induce shoot regeneration, and in SDM 10 and CSM 22003 cultured on medium containing BA for callus induction and to induce shoot regeneration.     

Plant regeneration through multiple adventitious shoot differentiation from callus cultures of slash pine (Pinus elliottii)

A plant regeneration system through multiple adventitious shoot differentiation from callus cultures has been established in slash pine (Pinus elliottii). Influences of seven different basal media on callus induction, adventitious shoot formation, and rooting were investigated. Among the different basal media, B5, SH, and TE proved to be suitable for callus induction and plantlet regeneration. Multiple adventitious shoot formation was obtained from callus cultures of slash pine on B5, SH, and TE media containing indole-3-butyric acid, N6-benzyladenine, and thidiazuron. Scanning electron microscopy demonstrated the early development of adventitious shoots derived from callus cultures. These results indicate that an efficient plant regeneration protocol for micropropagation of slash pine had been established. This protocol could be most useful for future studies on genetic transformation of slash pine.     

Shoot tip culture and cryopreservation for eradication of Apple stem pitting virus (ASPV) and Apple stem grooving virus (ASGV) from apple rootstocks ‘M9’ and ‘M26’

This study attempted to eradicate Apple stem pitting virus (ASPV) and Apple stem grooving virus (ASGV) from virus‐infected in vitro shoots of apple rootstocks ‘M9’ and ‘M26’ using shoot tip culture and cryopreservation. In shoot tip culture, shoot tips (0.2 mm in length) containing two leaf primordia failed to show shoot regrowth. Although shoot regrowth rate was the highest in the largest shoot tips (1.0 mm in length) containing four leaf primordia, none of the regenerated shoots was virus‐free. Shoot tips (0.5 mm in length) containing two and three leaf primordia produced 100% and 10% of ASPV‐free shoots, respectively, while those (1.0 mm) containing four leaf primordia were not able to eradicate ASPV. ASGV could not be eradicated by shoot tip culture, regardless of the size of the shoot tips tested. In cryopreservation, shoot tips (0.5 mm in length) containing two leaf primordia did not resume shoot growth. Although 1.0‐mm and 1.5‐mm shoot tips gave similarly high ASPV‐free frequencies, the latter had much higher shoot regrowth rate than the former. Very similar results of shoot regrowth and virus eradication by shoot tip culture and cryopreservation were observed in both ‘M9’ and ‘M26’. Histological observations showed that only cells in upper part of apical dome and in leaf primordia 1–3 survived, while other cells were damaged or killed, in shoot tips following cryopreservation. Virus immunolocalization found ASPV was not detected in upper part of apical dome and leaf primordia 1 and 2, but was present in lower part of apical dome, and in leaf primordium 4 and more developed tissues in all samples tested. ASPV was also detected in leaf primordium 3 in about 16.7% and 13.3% samples tested in ‘M9’ and ‘M26’. ASGV was observed in apical dome and leaf primordia 1–6, leaving only a few top layers of cells in apical dome free of the virus. Different abilities of ASPV and ASGV to invade leaf petioles and shoot tips were also noted.