The role of auxin efflux carriers in the reversible loss of polar auxin transport in the pea (Pisum sativum L.) stem

Correlatively inhibited pea shoots (Pisum sativum L.) did not transport apically applied ¹⁴C-labelled indol-3yl-acetic acid ([¹⁴C]IAA), and polar IAA transport did not occur in internodal segments cut from these shoots. Polar transport in shoots and segments recovered within 24 h of removing the dominant shoot apex. Decapitation of growing shoots also resulted in the loss of polar transport in segments from internodes subtending the apex. This loss was prevented by apical applications of unlabelled IAA, or by low temperatures (approx. 2° C) after decapitation. Rates of net uptake of [¹⁴C]IAA by 2-mm segments cut from subordinate or decapitated shoots were the same as those in segments cut from dominant or growing shoots. In both cases net uptake was stimulated to the same extent by competing unlabelled IAA and by N-1-naphthylphthalamic acid. Uptake of the pH probe [¹⁴C]-5,5-dimethyloxazolidine-2,4-dione from unbuffered solutions was the same in segments from both types of shoot. Patterns of [¹⁴C]IAA metabolism in shoots in which polar transport had ceased were the same as those in shoots capable of polar transport. The reversible loss of polar IAA transport in these systems, therefore, was not the result of loss or inactivation of specific IAA efflux carriers, loss of ability of cells to maintain transmembrane pH gradients, or the result of a change in IAA metabolism. Furthermore, in tissues incapable of polar transport, no evidence was found for the occurrence of inhibitors of IAA uptake or efflux. Evidence is cited to support the possibility that the reversible loss of polar auxin transport is the result of a gradual randomization of effluxcarrier distribution in the plasma membrane following withdrawal of an apical auxin supply and that the recovery of polar transport involves reestablishment of effluxcarrier asymmetry under the influence of vectorial gradients in auxin concentration.Abbreviations DMO 5,5-dimethyloxazolidine-2,4-dione - IAA indol-3yl-acetic acid - NPA N-1-naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid This work was supported by grant no. GR/D/08760 from the U.K. Science and Engineering Research Council. We thank Mrs. R.P. Bell for technical assistance.     

Correlative inhibition of lateral bud growth in Phaseolus vulgaris L. timing of bud growth following decapitation

Summary On intact, 3-week-old plants of Phaseolus the larger bud in the axils of the primary leaves shows slow, continuous elongation growth. Release from correlative inhibition can be detected within 30 min following decapitation. When 0.1% indoleacetic acid in lanolin is applied to the decapitated stem stump, the lateral bud shows slow growth during the first 7 h, then stops completely for a further 15 h but after 2 days a further gradual increase in length is observed.The movement of ¹⁴C-labelled assimilates from the subtending primary leaf into the lateral bud increases following removal of the shoot apex. When indole acetic acid is applied to decapitated plants the ability of the buds to import ¹⁴C increases for 5–7 h and then declines to a negligible amount. Little or no radioactivity from tritiated indoleacetic acid is transported into the lateral buds of decapitated plants during the first 48 h following removal of the apex and it appears that rapid metabolism of the compound occurs in the stem tissues.     

Transport of benzyl-8-14C-adenine in pea seedlings in relation to stem apical dominance

In intact, decapitated and decapitated indole-3-acetic acid (IAA) treated pea seedlings the translocation of benzyl-8-^l4C-adenin (¹⁴C-BA) from the roots was studied with regard to the release of lateral buds from apex-induced inhibition. In intact plants (controls) a substantial part of the activity was found in the apical part of the epicotyl. Decapitation resulted in the initiation of growth of lateral buds. As early as 24 h after decapitation and application of¹⁴C-BA a significantly higher activity was found in growing lateral buds (cotylars) of decapitated plants than in inhibited ones of intact or IAA-treated decapitated plants. The accumulation of¹⁴C-activity in stump tops of decapitated plants treated with IAA was associated with the thickening growth.     

Translocation of14C-abscisic acid from roots into the aboveground part of pea (pisum sativum L.) seedlings

The translocation of¹⁴C-ABA from roots into other parts of the plant was followed in intact and decapitated pea seedlings. In intact plants ABA from roots was translocated above all into the apical part of epicotyl. In decapitated plants the regulative ability of intact apex can be partly simulated by exogenous IAA. The growth of lateral buds occurring after decapitation was associated with an intensive flow of¹⁴C-ABA from roots into released lateral buds as late as 72 h after decapitation,i.e. in the stage of intensive elongation growth of buds.     

Transport of exogenous auxin in two-branched dwarf pea seedlings (Pisum sativum L.)

Dwarf pea plants bearing two cotyledonary shoots were obtained by removing the epicotyl shortly after germination, and the patterns of distribution of ¹⁴C in these plants was investigated following the application of [¹⁴C]IAA to the apex of one shoot. Basipetal transport to the root system occurred, but in none of the experiments was ¹⁴C ever detected in the unlabelled shoot even after transport periods of up to 48 h. This was true both of plants with two equal growing shoots and of plants in which one shoot had become correlatively inhibited by the other, and in the latter case applied whether the dominant or subordinate shoot was labelled. In contrast, when [¹⁴C]IAA was applied to a mature foliage leaf of one shoot transfer of ¹⁴C to the other shoot took place, although the amount transported was always low. Transport of ¹⁴C from the apex of a subordinate shoot on plants bearing one growing and one inhibited shoot was severely restricted compared with the transport from the dominant shoot apex, and in some individual plants no transport at all was detected. Removal of the dominant shoot apex rapidly restored the capacity of the subordinate shoot to transport apically-applied [¹⁴C]IAA, and at the same time led to rapid cambial development and secondary vascular differentiation in the previously inhibited shoot. Applications of 1% unlabelled IAA in lanolin to the decapitated dominant shoot maintained the inhibition of cambial development in the subordinate shoot and its reduced capacity for auxin transport. These results are discussed in relation to the polarity of auxin transport in intact plants and the mechanism of correlative inhibition.Abbreviations IAA Indol-3-yl-acetic acid - TIBA 2,3,5-triiodobenzoic acid - 2,4D 2,4-dichlorophenoxyacetic acid - IAAsp Indol-3-yl-acetyl aspartic acid     

Applicability of the chemiosmotic polar diffusion theory to the transport of indol-3yl-acetic acid in the intact pea (Pisum sativum L.)

The transport of exogenous indol-3yl-acetic acid (IAA) from the apical tissues of intact, light-grown pea (Pisum sativum L. cv. Alderman) shoots exhibited properties identical to those associated with polar transport in isolated shoot segments. Transport in the stem of apically applied [1-¹⁴C]-or [5-³H]IAA occurred at velocities (approx. 8–15 mm·h^-1) characteristic of polar transport. Following pulse-labelling, IAA drained from distal tissues after passage of a pulse and the rate characteristics of a pulse were not affected by chases of unlabelled IAA. However, transport of [1-¹⁴C]IAA was inhibited through a localised region of the stem pretreated with a high concentration of unlabelled IAA or with the synthetic auxins 1-napthaleneacetic acid and 2,4-dichlorophenoxyacetic acid, and label accumulated in more distal tissues. Transport of [1-¹⁴C]IAA was also completely prevented through regions of the intact stem treated with N-1-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid.Export of IAA from the apical bud into the stem increased with total concentration of IAA applied (labelled+unlabelled) but approached saturation at high concentrations (834 mmol·m^-3). Transport velocity increased with concentration up to 83 mmol·m^-3 IAA but fell again with further increase in concentration.Stem segments (2 mm) cut from intact plants transporting apically applied [1-¹⁴C]IAA effluxed 93% of their initial radioactivity into buffer (pH 7.0) in 90 min. The half-time for efflux increased from 32.5 to 103.9 min when 3 mmol·m^-3 NPA was included in the efflux medium. Long (30 mm) stem sections cut from immediately below an apical bud 3.0 h after the apical application of [1-¹⁴C]IAA effluxed IAA when their basal ends, but not their apical ends, were immersed in buffer (pH 7.0). Addition of 3 mmol·m^-3 NPA to the external medium completely prevented this basal efflux.These results support the view that the slow long-distance transport of IAA from the intact shoot apex occurs by polar cell-to-cell transport and that it is mediated by the components of IAA transmembrane transport predicted by the chemiosmotic polar diffusion theory.Abbreviations IAA indol-3yl-acetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - NPA N-1-naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid     

Relationship of Apical Dominance to the Nutrient Accumulation Pattern in Pisum sativum var. Alaska

Decapitation of the pea plant resulted in the growth of all the lateral shoots. The initial growth of all lateral buds was somewhat similar. The differential growth rates developed later on. The pattern of growth of lateral shoots varied with the age of the plant when decapitation was performed. The basal shoots dominated when the plants were decapitated at the 2-leaf stage. At 3-leaf stage decapitation resulted in the dominance of shoot 5. Decapitation at 4-or-more-leaf stage resulted in the eventual dominance of the suhterminal lateral shoot. As a rule P-32 moved to the most actively growing part of the plant, i.e. apex in intact vegetative plant, the growing lateral shoots in a decapitated plant, the elongating subapical parts of the stem and the roots. The various metabolic sinks seemed to compete actively for this nutrient, therefore P-32 accumulation in any particular growing region of the plant was taken as an indicator of nutrient utilization potential of that part. The stem apex of an intact plant seemed to loose its dominance with the increasing age of the plant. The loss of apical dominance was almost complete during the reproductive phase of the plant, during which the upper lateral shoots initiated growth. Their growth, however, was inhibited soon because of competition with the other developing sinks, viz., the flower and the fruit. The amount of soluble carbohydrates in various parts of the pea plant followed essentially the same pattern as did P-32 accumulation. These distribution patterns were apparently correlated with the growth of the plant.     

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.     

Effect of decapitation on absorption,translocation, and phytotoxicity of imazamethabenz in wild oat (Avena fatua L.)

The release of apical dominance by the physical destruction in situ of the apical meristem and associated leaf primordia (decapitation) promoted the growth of tillers in non-herbicide-treated wild oat plants, as indicated by increased tiller lengths and fresh weights. At 96 h after [¹⁴C] herbicide treatment following decapitation, the absorption of [¹⁴C]imazamethabenz and total translocation of radioactivity were respectively increased by 28% and 49%. By 96 h after [¹⁴C]imazamethabenz application, the radioactivity detected in the roots of decapitated plants was 45% higher than that in the roots of nondecapitated plants while the radioactivity in tillers of decapitated plants was 2.6-fold that in tillers of intact plants. Decapitation together with foliar spraying of imazamethabenz at 200 g ha^–1 further reduced tiller fresh weight, greatly decreased the total tiller number, and thereafter significantly increased overall phytotoxicity by 32% as measured by total shoot fresh weight. The results of this study support the hypothesis that main shoot apical dominance limits translocation of applied imazamethabenz to lateral shoots, rendering tillers less susceptible to growth inhibition by the herbicide.     

Differential Responses of Buds along the Shoot to Factors Involved in Apical Dominance

Intact and decapitated 6-node shoots of Hygrophila sp. weregrown aseptically immersed in liquid half-strength Knop's solutionwith microelements and 2% (w/v) sucrose (control medium), andin medium with 0.1 mg l^–1 benzyladenine (BA). In intactshoots grown in control medium apical dominance suppressed outgrowthof the lateral buds; in decapitated shoots buds grew out atseveral of the most apical nodes, increasing in size acropetally.There was a lag in outgrowth of the bud at the most apical node,attributable to its initially smaller size. Lateral shoots grewout first at basal nodes of intact shoots in BA medium, decreasingin size acropetally; in decapitated shoots in BA medium lateralshoots of approximately equal size grew out at all nodes. Differentialeffects of decapitation and cytokinin treatment on lateral shootoutgrowth along the shoot could be interpreted by postulatinga basipetally decreasing gradient of endogenous auxin concentrationin the intact shoot. Application of 20 mg l^–1 indoleaceticacid (IAA) in agar to decapitated shoots completely preventedbud outgrowth for at least 7 d in control medium, inhibitingit thereafter, and inhibited bud outgrowth in BA medium, thussupporting the hypothesis. Comparison of lateral shoot outgrowthin whole decapitated shoots and severed decapitated shoots (isolatednodes) lent no support to the alternative hypothesis that theremight be an acropetally decreasing concentration gradient ofa bud-promoting substance in the intact shoot, and demonstratedmuch greater lateral shoot growth in isolated nodes. The resultsemphasize important correlative relationships between the partsof a shoot with several nodes.     

Abscisic acid and apical dominance in Phaseolus coccineus L.

Abscisic acid (ABA) in lanolin, applied to the internode of decapitated runner bean plants enhances the outgrowth of lateral buds. The optimum concentration of the paste is 10^-5 M. The effect of ABA is counteracted by indoleacetic acid (IAA) but not by gibberellic acid (GA₃). There is no effect when ABA is applied to the apical bud or lateral buds of intact plants. However, 13.2 ng given to the lateral buds of decapitated plants stimulate their growth, whereas higher concentrations are inhibitory. Consequently, ABA enhances growth of lateral buds directly, but only when apical dominance is already weakened. The growth of the decapitated 2^nd internode was not affected by ABA. Radioactivity from [2-¹⁴C] ABA, applied to nonelongating 2^nd internode stumps of decapitated runner bean plants moves to the lateral buds, whereas [1-¹⁴C]IAA-and [³H]GA₁-translocation is much weaker. ABA transport is inhibited if IAA or [³H]GA₁ is applied simultaneously. In elongating internodes [¹⁴C]ABA is almost completely immobile. [¹⁴C]IAA-and [³H]GA₁-translocation is not affected by ABA. The amount of radioactivity from labelled ABA, translocated to the lateral buds, is highest during the early stages of bud outgrowth.Abbreviations ABA 2,4-cis, trans-(+)-abscisic acid - GA gibberellic acid - IAA indoleacetic acid - p.l. plain lanolin     

Effects of shoot bending of apple trees on accumulation and translocation of14C-labelled assimilates

The translocation of¹⁴C-labelled assimilates from a single leaf in bent and intact apple shoots was studied in varying stages of shoot development. In actively growing shoots¹⁴C-labelled assimilates translocated from the treated leaf and accumulated mainly in the shoot apex. In moderately growing apple shoots radioactive assimilates were translocated from the treated leaf in both directions towards and down the shoot. In apple shoots showing only slight growth activity the¹⁴C-labelled assimilates were transported from the treated leaf mainly to the base of the shoot, stem and roots. Bending shoots changed the pattern of distribution of radioactive assimilates. Bending actively and moderately growing shoots resulted in higher concentration of 14-carbon in the shoot apex than in controls. In slowly growing and non-growing apple shoots bending caused a higher accumulation of radioactive assimilates in the bent section than in an equal section of control shoots.     

The indirect role of 2,4-D in the maintenance of apical dominance in decapitated sunflower seedlings (Helianthus annuus L.)

When sufficient 2,4-D to maintain apical dominance for at least 21d was applied to the cut stem interface of sunflower seedlings which had been decapitated in the epicotyl, it could not be detected in the vicinity of the inhibited axillary buds 7d after application. Rather the 2,4-D concentrated at the stump apex where it was associated with formation of meristematic tissue. The results indicate an indirect role for 2,4-D in the maintenance of apical dominance in this system, possibly involving the induced meristematic activity.Abbreviations 2,4-D 2,4-Dichlorophenoxyacetic acid - [¹⁴C]2,4-D 2,4-dichlorophenoxy[2-¹⁴C]acetic acid - IAA indol-3-yl-acetic acid     

Regulation of auxin transport in pea (Pisum sativum L.) by phenylacetic acid: inhibition of polar auxin transport in intact plants and stem segments

The transport of [¹⁴C]phenylacetic acid (PAA) in intact plants and stem segments of light-grown pea (Pisum sativum L. cv. Alderman) plants was investigated and compared with the transport of [¹⁴C]indiol-3yl-acetic acid (IAA). Although PAA was readily taken up by apical tissues, unlike IAA it did not undergo long-distance transport in the stem. The absence of PAA export from the apex was shown not to be the consequence of its failure to be taken up or of its metabolism. Only a weak diffusive movement of PAA was observed in isolated stem segments which readily transported IAA. When [1-¹⁴C]PAA was applied to a mature foliage leaf in light, only 5.4% of the ¹⁴C recovered in ethanol extracts (89.6% of applied ¹⁴C) had been exported from the leaf after 6.0 h. When applied to the corresponding leaf, [¹⁴C]sucrose was readily exported (46.4% of the total recovered ethanol-soluble ¹⁴C after 6.0 h). [1-¹⁴C]phenylacetic acid applied to the root system was readily taken up but, after 5.0 h, 99.3% of the recovered ¹⁴C was still in the root system.When applied to the stem of intact plants (either in lanolin at 10 mg·g^-1, or as a 10^-4 M solution), unlabelled PAA blocked the transport through the stem of [1-¹⁴C]IAA applied to the apical bud, and caused IAA to accumulate in the PAA-treated region of the stem. Applications of PAA to the stem also inhibited the basipetal polar transport of [1-¹⁴C]IAA in isolated stem segments. These results are consistent with recent observations (C.F. Johnson and D.A. Morris, 1987, Planta 172, 400–407) that no carriers for PAA occur in the plasma membrane of the light-grown pea stem, but that PAA can inhibit the carrier-mediated efflux of IAA from cells. The possible functions of endogenous PAA are discussed and its is suggested that an important role of the compound may be to modulate the polar transport and-or accumulation by cells of IAA.Abbreviations IAA indol-3yl-acetic acid - NPA N-1-naphthylphthalamic acid - PAA phenylacetic acid - IIBA 2,3,5-triiodobenzoic acid     

An assessment of auxin-promoted transport in decapitated stems and whole shoots of Phaseolus vulgaris L.

¹⁴C-photosynthate transfer in decapitated stems of P. vulgaris plants, treated with IAA (indol-3yl-acetic acid), appeared, as ascertained by microautoradiography, to be restricted to cells of sieve-element appearance. The IAA-induced promotion of photosynthate transport was found not to depend on any artifacts caused by the decapitation procedure. Rather, decapitation primarily served the purpose of removing photosynthate sources above the point of hormone application which otherwise suppressed the expression of the IAA effect on acropetal photosynthate transport. Furthermore, by manipulating stem levels of endogenous auxins with the inhibitor of polar auxin transport, 1-(2¹-carboxyphenyl)-3-phenylpropane-1,3-dione (ACP1.55), evidence was obtained indicating that photosynthate transfer to the shoot apex depended, at least in part, on endogenous levels of auxins at site(s) remote from the apical sink (i.e. shoot apex).Abbreviations ACP1.55 1-(2¹-carboxyphenyl)-3-phenylpropane-1,3-dione - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - IAA indol-3yl-acetic acid     

Correlative Inhibition in the Shoot of Agropyron repens ( L.) Beauv

Correlative inhibition was investigated in plants of Agropyronrepens at two temperatures. Reciprocal inhibition ocrurred betweenthe main shoot apex and the outgrowing axillary shoots, withthe balance of inhibition varying with temperature. Apical dominancewas stronger at 10 °C than at 20 °C , but even at 10°C release of apical dominance by decapitation had onlyminor effects on the timing of outgrowth, growth pattern andrate of dry weight aocumulation of the axillary shoots. Dominanceof the main shoot apex by the axillary shoots was stronger at20 °C than at 10 °C. Removal of axillary buds preventeddecline in size and activity of the main shoot apex ard resultedin increased rates of primordium initiation, leaf emergenceand dry weight accumulation in the main shoot. It is suggestedthat a system of reciprocal dominance provides a mechanism formaintaining the characteristic habit of the grass plant andlimits growth in height of vegetative shoots. Agropyron repens (L.) Beauv, couch grass, correlative inhibition, apical dominance, shoot, apex     

Micropropagation of Rosmarinus officinalis L.

Single-node stem segments of Rosmarinus officinalis L. var. genuina forma erectus proved better explants than shoot tips (ca. 2 cm long) for extablishment of field-grown plants in aseptic cultures. Benzylaminopurine was far more effective than kinetin for shoot induction in shoot tips excised from aseptically-grown plants. Maximum numbers of shoot buds (ca. 14) were formed per explant at 0.2 mgl^-1 benzylaminopurine in 30 days. After further growth of isolated shoots and treatment with 0.25 mgl^-1 indolepropionic acid for 7 days, 80% shoots produced roots. In vitro raised plantlets were successfully grown in soil to plants. About 5,000 plants could be produced from a single nodal segment in 1 year.NBRI Research Publication No. 195 (N.S.)     

CONTROL OF SHOOT-RHIZOME DIMORPHISM IN THE WOODY MONOCOTYLEDON,CORDYLINE (AGAVACEAE)

In Cordyline terminalis negatively geotropic leafy shoots and positively geotropic rhizomes develop from single axillary buds on either shoots or rhizomes. All axillary buds have similar morphogenetic potential when released from apical dominance. Experiments in which the orientation of the apex is changed, organs removed, or growth regulators applied indicate that after a rhizome is initiated, it is maintained as a rhizome by auxin originating in the leafy shoot. When auxin levels are lowered by changes in the orientation of the axis or shoot removal, the rhizome apex becomes a shoot apex, which appears to be the stable state of the actively growing apex. Benzyl adenine when applied exogenously to the apex or lateral buds has the same effect as lowering the auxin level. Gibberellic acid has no effect on the apex or lateral buds. High levels of exogenous naphthaleneacetic acid cause bud release and development of rhizomes from previously inhibited axillary buds of the shoot. However, it was not possible to convert a shoot apex into a rhizome apex by auxin treatment. It is suggested that the release of buds on the lower side of horizontal branches and of buds directly above a stem girdle is caused by high auxin levels on the lower side or distal to the girdle. The experimental results are discussed in relation to naturally occurring shoot-rhizome dimorphism.     

Changes in Phytohormone Status in Stems and Roots after Decapitation of Pea Seedlings

Experiments were performed on the first and second internodes and 4-cm-long apical segments of main roots of pea (Pisum sativum L.) seedlings, grown in the light and decapitated above the second node on the seventh day after seed germination. Endogenous phytohormones were measured by the enzyme-linked immunosorbent assay during three days after decapitation of seedlings. The IAA level in the internodes decreased 2–3 times on the second day after decapitation of seedlings while the cytokinin level increased 5–6 times for zeatin and zeatin riboside (Z and ZR) and 1.5–2 times for isopentenyl adenine and isopentenyl adenosine (IP and IPA). In contrast to internodes, the IP and IPA contents in the roots of decapitated seedlings did not change, but the levels of Z and ZR increased 1.5–2 times compared to intact plant roots. The IAA level in the apical region of root remained almost unchanged after the removal of shoot apex. It was concluded that the apical meristem of the main root is not the site of the cytokinin response to the auxin signal coming from the stem apex and that a slight accumulation of Z and ZR after decapitation is due to upper zones of the root. There was no difference in the content of gibberellin-like substances between the internodes of intact and decapitated seedlings. However, the content of gibberellins (GA) in the root tip decreased after decapitation of seedling, which suggests an essential role of apical bud in supplying the root with GA and/or intermediates for their biosynthesis.     

In vitro differentiation and plant regeneration of Albizia chinesis (Osb.) Merr

Summary Petiolar and distal cotyledonary segments (PCS and DCS) of Albizia chinensis were cultured on Murashige and Skoog's (MS; 1962) medium and induced to form adventitious shoot buds in the presence of either cytokinins 6-benzylamino purine (BAP), kinetin (KN) or thidiazuron (TDZ). Superiority of BAP in inducing shoot bud and differentiation was observed. PCS was more morphogenic to shoot bud differentiation than DCS. TDZ was highly effective in inducing shoot buds, but arrested shoot growth, while KN produced more callus during differentiation of shoots. Rapid and high rate of shoot multiplication per explant was achieved through subculture in MS medium containing BAP (1.0 mg l⁻¹) and indole-3-acetic acid (IAA) (0.5 mg l⁻¹). BAP at low concentration was required to enhance shoot multiplication and elongation. Successful rooting of regenerated shoots was carried out in a two-step culture procedure in MS media with indole-3-butyric acid (IBA) (2.0 mg l⁻¹) and subsequent subculture in IBA-free medium.