Effects of Applied IAA on the Position of Abscission Sites Induced in Wounded Explants from Impatiens sultani Internodes

Previous work established that if segments of Impatiens sultaniinternodes are explanted and incubated on a suitable medium,they tend to undergo abscission by a transverse separation layerthat differentiates a short distance above the explant base.The present study has shown that the position of the abscissionsite can be modified experimentally. When an explant was splitdown to midlength and auxin (IAA) was applied to the top ofone of the two arms, abscission often occurred at or near thebase of the other arm. Again, when IAA was applied to the explantlaterally midway along its length, abscission often occurredjust above the application point. These two modifications ofabscission sites had been predicted by a hypothesis statingthat separation layers tend to be positioned where auxin concentrationdecreases in the morphologically upward direction. Studies with[¹⁴C]IAA confirmed that the separation layers above the explantbase, and in the two experimentally modified sites, did indeedarise where the concentration decreased upwards. Also, woundingaltered the position of abscission in these explants in waysthat can be interpreted in terms of the above hypothesis coupledwith the destruction of auxin that occurs at wound surfaces.In this system, auxin is acting as a morphogen: its concentrationgradients provide positional information. Impatiens sullani Hook., abscission, auxin, IAA, morphogen, positional control, separation layer, wounding     

The Role of Basipetal Auxin Transport in the Positional Control of Abscission Sites Induced in Impatiens sultani Stem Explants

If segments of Impatiens sultani stem are explanted and incubated,separation layers often form across them and lead to abscission.To test the suggested role of auxin concentration in controllingthe position of abscission sites, explants were labelled byapplying [¹⁴C]IAA to the shoot tip 4 h prior to explanting;transport of auxin applied in this way seems to resemble thatof endogenous auxin. During subsequent incubation of explantsfor 20 h, basipetal transport resulted in ¹⁴C accumulating justabove the base of the explants (nearly 80 % in the bottom 4mm of 24 mm explants). In internodal explants that had beenwounded at explanting by incising one side so as to sever avascular bundle, and in nodal explants with the leaf removed,the ¹⁴C also accumulated just above the wound or node to abouttwice the concentration otherwise expected; this accumulationwas probably due to basipetal transport being impeded by vasculardiscontinuity at the wound or node. Accumulation just abovethe base, or above a wound or node, resulted in gradients of¹⁴C concentration (presumably reflecting endogenous auxin concentration)decreasing in the morphologically upward direction at each ofthese three positions where abscission sites tend to occur. Impatiens sultani, abscission, auxin, IAA, node, polarized transport, positional control, separation layer, wounding     

Auxin Concentrations in Nodes and Internodes ofImpatiens sultani

Greater concentrations of auxin at nodes than in internodes,resulting from some nodal barrier to basipetal transport, havelong been postulated as the cause of early differentiation ofinitially isolated xylem and cambium at the nodes. However,this study, using [¹⁴C] indole-3-acetic acid (IAA) applied apicallyand gas chromatography-mass spectrometry, found that in stemsofImpatiens sultanithe IAA concentrations (per unit f. wt) atnodes were similar to those in adjacent internodes, though alittle greater at nodes if expressed per unit length of stemand a little less per unit d. wt. By contrast, in decapitatedshoots and in stem explants of dicotyledons, loss of the apicalsource of basipetally flowing auxin can result in auxin drainagewith some auxin retention in the uppermost remaining nodes.When [¹⁴C]IAA was applied apically to shoots for 4 h and stemexplants were excised, the explants had no nodal accumulationinitially whereas comparable explants incubated for 20 h revealedsignificant nodal accumulation. If decapitation leads both tonodal auxin accumulation and to adventitious abscission justabove the node, this fits the hypothesis that abscission sitesare positioned where auxin concentration decreases locally inthe apical direction. Difficulties in quantifying nodal auxindynamics are discussed, and some crude estimates of metabolicrates and locations of the auxin are presented.Copyright 1999Annals of Botany Company Abscission, auxin,Impatiens sultani, indole-3-acetic acid, node.     

Morphogenetic, Inductive and Inhibitory Effects of IAA Conjugates on Abscission in Stem Explants of Impatiens sultani

IAA-L-alanine and IAA-DL-aspartate, when applied to internodalexplants of Impatiens sultani, mimic the effects of IAA bothin inhibiting abscission (when applied at the apical end) andin promoting abscission and rooting, and displacing the siteof induced abscission (when applied basally). These IAA conjugatesare less active than IAA at similar concentrations, and theirpromotory effects are slower. Their activity is interpretedin terms of their diffusion into the explants, where they become‘slow-release’ sources of free IAA through enzyme-catalyzedhydrolysis. Abscission, auxin, IAA, IAA conjugates, IAA-L-alanine, IAA-DL-aspartate, inhibition, morphogenesis, separation layer     

The Role of Cell Expansion in the Abscission of Impatiens sultani leaves

The histological events occurring during the latter stages ofabscission were followed continuously in longitudinal slicesthrough the petiole base of Impatiens sultani Hook. It appearsthat the middle lamella of the cortical parenchyma cells isdegraded first. This is followed by an expansion of these cellsand a concomitant stretching and separation of the collenchymaand vascular trace. The parenchyma cells continue to enlargeuntil they become virtually spherical, a process which finallyruptures the last restraining xylem vessels. The increased volumeof the parenchyma cells appears to be principally due to a conformationalchange in cell shape from a near regular hexagonal prism toa sphere of similar surface area. The dimensions of the prismaticcells are such that most of them change into spheres whose diametersare the same as the transverse distance between the oppositesides of the six axially orientated faces of the prisms. Cellularexpansion is thus entirely directed along the axis of the petiole.The significance of these observations to the general anatomyand mechanism of fracture of abscission zones is discussed. Impatiens sultani Hook., abscission, cell expansion, cell wall degradation, cell shape     

The Role of Auxin in the Apical Regulation of Leaf Abscission in Cotton (Gossypium hirsutum L.)

The petiole abscission induced by deblading cotyledonary leavesof cotton (Gossypium hirsutum L. cv. Delta Pine) was acceleratedby the presence of the intact shoot apex or, in decapitatedplants and explants, by application to the stem (proximal application)of indol-3yl-acetic acid (IAA) or 1-aminocyclopropane-l-carboxylicacid (ACC). IAA and ACC accelerated the abscission of debladedpetioles whether applied above or below the cotyledonary node.Transport of IAA to the node was not required for the responseto proximal IAA. [2,3-¹⁴C]ACC was readily transported to thenodal region whether applied to the stem above or below thenode. Application of IAA or ACC to the stem did not induce theabscission of intact leaves or of debladed petioles treateddistally with IAA The acceleration of abscission by proximal IAA, but not thatcaused by ACC, was prevented if explants were treated with a-aminooxyaceticacid (AOA), an inhibitor of ACC-synthase. AOA also preventedthe acceleration of abscission caused by the shoot apex. Theprogress of abscission in debladed explants was greatly delayedby silver thiosulphate (STS—an inhibitor of ethylene action),whether or not the explants were treated with IAA or ACC. Itis suggested that the speeding effects of the shoot apex andof proximal auxin on the abscission of debladed petioles requiresauxin-induced ACC synthesis. The possibility is discussed thatACC may function as a mobile abscission promoter Key words: Abscission, ACC, ACC-synthase, cotton (Gossypium), proximal auxin     

Mechanism of Action of Abscission Accelerators

Abscission zone explants of Gossypium hirsutum L., Cassia fistula L., and Coleus blumei Benth. were used to investigate correlations between endogenous rates of ethylene evolution and time of abscission. Additions of 0.1 nl/ml ethylene to the explants markedly accelerated abscission; continuous aeration of the explants, to prevent accumulation of small amounts of endogenously produced ethylene, inhibited abscission compared with that of sealed controls. Substances that stimulated abscission simultaneously accelerated ethylene evolution on all three species and at any position of application. The positional effects of auxin are explained as being due to differences in transport in the explant. Thus, distally applied auxin inhibits abscission, regardless of the accelerated rate of ethylene evolution, by being rapidly transported to the abscission zone. Auxin applied proximally stimulates abscission because it is unable to move as rapidly to the abscission zone and the ethylene effect becomes dominant. Ethylene was found to be most effective on aged tissues, and it is concluded that abscission rates are determined by an increase in sensitivity of the tissue to the ethylene that is already being produced.     

Abscisic Acid, Auxin, and Ethylene in Explant Abscission

Experiments with explants of Phaseolus vulgaris L., cv. CanadianWonder, show that abscission and the associated rise in oarboxymethyl-cellulaseactivity in the separation zone are initiated by a peak in ethyleneproduction during senescence of pulvinar tissue distal to thezone. Distal applications of abscisic acid (ABA) induce an earlierpeak in ethylene production, increase cellulase activity, andpromote abscission. ABA is more effective in these ways if treatmentis delayed from 0 to 24 h after excision. With increasing concentrations of ABA the maximum rate of ethylene production is achievedsooner. Indol-3yl-acetic acid (IAA) and ABA are antagonisticin this system and have opposing effects. IAA retards the timeof peak ethylene-production and delays abscission. Explantsmay be retained for long periods without abscinding if incubatedin an ethylene-free atmosphere: the addition of ethylene forany one 24-h period (except the first 24 h after excision) willinduce abscission. The initial period of insensitivity to ethyleneis extended by distal applications of IAA. Ethylene-inducedabscission can be inhibited by IAA applied up to 72 h afterexcision provided the ethylene is not applied first. It is proposedthat abscission in the explant is controlled at two levels:(1) an auxin-dependent stage determining the duration of insensitivityto ethylene; (2) the timing of a rise in ethylene productionin senescing tissue distal to the separation zone. An auxin-ethylenebalance-mechanism at the separation zone is discussed.     

Effect of Protein Synthesis Inhibitors, Auxin, and Gibberellic Acid on Abscission

Chloramphenicol, actinomycin D, and other inhibitors of protein synthesis promote abscission in several plant genera. Abscission is accelerated in species where an abscission layer is present, as well as in tissue where no abscission layer develops prior to abscission. The inhibitors promote abscission in species where cell division is reported to precede the separation processes as well as in tissues where no cell division is associated with the initiation of abscission. Indoleacetic acid (IAA) or auxin precursors, when applied with chloramphenicol and aclinomycin D, overcome the promotive effects of the inhibitors on abscission. These inhibitors apparently do not promote abscission through their effects on auxin precursor conversion, IAA transport, and IAA destruction in the petiole. IAA increases the incorporation of leucine-1-¹⁴C into a trichloroacetic acid precipitable fraction of the abscission zone under conditions where abscission is retarded. A low concentration of IAA which accelerates abscission, decreases incorporation of leucine into protein. Other promoters of abscission — chloramphenicol, d-aspartic acid, and gibberellic acid —also decrease the incorporation of leucine into the protein of the abscission zone. The data indicate that enzymes required for the degradative processes associated with abscission are already present in the abscission zone whereas a continuous synthesis of protein is required for the retention of the leaf.     

Abscission in Coleus: Light and Phytohormone Control

Light control of leaf abscission in Coleus (Coleus blumei Benthcv. Ball 2719 Red) appears to be regulated by the quantity ofendogenous auxin transported from the leaf blade to the abscissionzone. Gas chromatographic—mass spectrophotometric analysisindicated that diffusate collected from leaf tissue treatedwith red light contained significantly higher levels of auxinthan dark and far-red light-treated leaf tissue. In addition,diffusate from red light-treated tissue inhibited abscissionof leafless petioles while diffusate from far-red light-treatedtissue promoted abcission when compared with diffusate fromdark-treated tissue. The effect of red light on abscission couldbe mimicked by IAA, but not by other phytohormones. An auxintransport inhibitor, 2, 3, 5-triiodobenzoic acid (TIBA), appliedeither as a lanolin ring around the petiole or vacuum infiltratedinto tissue, could completely eliminate any red light effecton abscission. The data are consistent with a phytochrome-mediatedlight regulation of endogenous auxin level in the leaf whichthen controls abscission. Key words: Abscission, Coleus, IAA, plant hormones, red (far-red) light, TIBA     

Role of IAA-Oxidase in Abscission Control in Cotton

The potential role of indoleactic acid (IAA)-oxidase as an in vivo abscission regulating system in the cotton (Gossypium hirsutum L.) cotyledonary explant was investigated. Phenols (usually monophenols), which are cofactors of cotton IAA-oxidase in vitro, accelerated abscission. Phenols (usually orthodihydroxyphenols), which inhibit cotton IAA-oxidase in vitro, inhibited abscission. Inhibition or stimulation of abscission was accomplished by phenols both with and without IAA. Results were similar when treatments were applied as lanolin pastes to the cut petiole ends or as solutions in which explants were submerged. An abscission accelerating phenol stimulated the decarboxylation of IAA-1-¹⁴C by explants and an abscission inhibiting phenol inhibited the decarboxylation of IAA-1-¹⁴C.     

Abscission of Phaseolus and Impatiens Explants: Effects of Ionizing Radiation upon Endogenous Growth Regulators and de Novo Enzyme Synthesis

Stem-petiole explants from the lower pulvinus of the primary leaves of Phaseolus vulgaris L. cv. Red Kidney and from Impatiens sultani Hook cv. Scarlet Baby were exposed to varying dosages of γ-radiation. With bean, irradiation of 175 to 525 kiloroentgens (kR) significantly accelerated the onset of abscission with a maximum response at 175 to 280 kR. Higher dosages (beginning at 600-700 kR) usually prevented abscission. With Impatiens, 18 to 35 kR significantly accelerated both the onset of abscission and possibly the initial abscission rate; 350 kR cut the time to 100% abscission in half and substantially accelerated the initial abscission rate. Inhibition of abscission in Impatiens was not possible with the available dose rate (35 kR/hour).     

ABSCISIN,AUXIN, AND GIBBERELLIN EFFECTS ON THE DEVELOPMENTAL ASPECTS OF ABSCISSION IN COTTON (GOSSYPIUM HIRSUTUM)

The effects of accelerating and retarding amounts of abscisin (Ab II), auxin (IAA), and gibberellin (GA₃) on abscission in explants of 14-day-old cotton (Gossypium hirsutum L.) seedlings were studied. Applications of Ab II, a potent accelerant (0.025 μg/abscission zone), resulted in a lysigenous breakdown of cells in a weakly defined separation layer in contrast to GA₃, an accelerant (0.01 μg/abscission zone), and IAA, a retardant (0.125 μg/abscission zone), which resulted in a schizogenous type of breakdown of cells in a well-defined separation layer, three or more rows of cells wide. Separation usually commenced adaxially with GA₃, abaxially with IAA and in the controls, and either ad- or abaxially with Ab II. Cell division preceded abscission, the number of cells increasing greatly within 24 hr after GA₃ treatment. Tyloses formed in vessel elements throughout the explant, both distal and proximal to the plane of separation in all treatments and in the controls. The retardant, IAA, appeared to stimulate tyloses formation. Tylosis development was not causal but was secondarily related to abscission.     

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.     

Optimal and Spatial Analysis of Hormones,Degrading Enzymes and Isozyme Profiles in Tomato Pedicel Explants During Ethylene-Induced Abscission

Activities of degrading enzymes, hormones concentration and zymogram patterns were investigated during control and ethylene-induced abscission of tomato pedicel explants. Exogenous ethylene accelerated abscission of pedicel explants. It was showed that IAA concentration in abscission zone tended to decline at first and then was reduced before separation in control and ethylene-treatment. Moreover, IAA (indole acetic acid) and ABA (abscise acid) concentrations were elevated in each segment when exposing to ethylene, but GA_{1 + 3} (gibberellin1 + gibberellin3) concentration was decreased in abscission zone and the proximal side. Activities of cellulase, polygalacturonase and pectinesterase in the explants were induced in the separating process and strengthened by ethylene. However, comparing with the proximal side, cellulase and polygalacturonase activities in abscission zone and distal side were higher. Electrophoresis of isozymes revealed that at least three peroxidase and three superoxidase isozymes appeared in the explants, respectively. One peroxidase isozyme exhibited differentially among the three positions in control and ethylene-treatment. One esterase isozyme weakened or disappeared in the following hours, but three novel esterase isozymes were detectable from beginning of the process. The data presented support the hypothesis that the distal side, together with abscission zone of explants plays a more important role in separation than does the proximal side. The possible roles of degrading enzymes, hormones and isozymes in three segments during ethylene-induced abscission of tomato pedicel explants are discussed.     

The positional differentiation of ethylene-responsive cells in rachis abscission zones in leaves of Sambucus nigra and their growth and ultrastructural changes at senescence and separation

Summary In leaves of S. nigra, fragmentation of the rachis follows the autumnal abscission of leaflets and the high levels of ethylene produced by the senescing blades. Fragmentation is accompanied by cell growth and ultrastructural changes in a zone of cells precisely differentiated at the separation zone. Studies with explants from the rachis show that those that contain an abscission zone increase in freshweight by as much as 50% before and during cell separation. Cell growth changes are induced by ethylene but not by auxin, and are restricted to explants that contain the separation zone cells. In ethylene, enlarging cells of the zone show cytoplasmic activation indicated by dilated dictyosomes, enhanced production of Golgi vesicles, elongated profiles of rough endoplasmic reticulum, a crenellated plasmalemma, and the apparent discharge and accumulation of cytoplasmic vesicles within the desmotubules of the branched plasmodesmata. Degradation of the middle lamella and cell wall matrix could be associated with the release of hydrolytic enzymes on the disruption of the vesicles. Although ultrastructural changes of a similar but limited nature occur in all cells of the rachis in response to ethylene, only those that are morphologically delimited as zone cells exhibit the growth and separation that leads to rachis fragmentation. It is proposed that abscission can occur only at the sites of the positional differentiation of these special ethylene-responsive target cells.Abbreviation IAA indole-acetic acid (auxin)     

The influence of thidiazuron on shoot regeneration from leaf explants of fifteen cultivars of Rhododendron

The influence of cytokinin thidiazuron (TDZ) and auxin indole-3-acetic acid (IAA) on in vitro shoot organogenesis of fifteen Rhododendron genotypes was investigated and a protocol for high frequency adventitious shoot regeneration from leaf explants was developed. High genotypic variation was observed and regeneration frequencies ranged from 0 to 100 %. Genotype Ovation had the highest number of shoots (26.4 per explant) after 12 weeks on medium with 0.57 μM IAA and 1.20 μM TDZ, but only 65 % of explants regenerated. Catawbiense Grandiflorum had 17.7 shoots per explant and 75 % regeneration on medium with 5.70 μM IAA and 0.45 μM TDZ and Van Werden Poelman had 14.3 shoots per explant and 100 % regeneration on medium with 0 57 μM IAA and 0.45 μM TDZ.     

Effect of Peroxydisulfate on Vigna radiata Abscission

K₂S₂O₈, applied to the basal end of cuttings of Vigna radiatastimulated leaf abscission in the light or dark. Because inhibitionof leaf sbscission in the dark by IAA was completely abolishedby K₂S₂O₈, and IAA decreased stimulation of abscission by K₂S₂O₈,destruction of IAA in the cuttings by K₂S₂O₈ is indicated. K₂S₂O₈had no effect on leaf abscission when applied as a foliar sprayor when roots of undisturbed seedlings were treated. When appliedproximally or distally to leafless explants, K₂S₂O₈ inhibitedpetiole abscission, and neither IAA nor ethylene had an effecton the inhibition. Although K₂S₂O₈ destroyed IAA in vitro, ithad no effect on abscission inhibitors in macerates of Vignaleaves and corn roots, nor did it destroy the biological activityof IAA added to such macerates. Substances liberated by macerationmay interfere with the ability of K₂S₂O₈ to destroy IAA. (Received May 2, 1981; Accepted August 24, 1981)     

Transport and Accumulation of IAA-14C in Tumour-forming Nicotiana Hybrids

The polar transport of indol-3yl-acetic acid (IAA-2-¹⁴C) instem explants and decapitated shoots of tumour-prone Nicotianahybrids (2n, 3n, and 4n) was compared with that in the normal,non-tumorous parent species N. glauca and N. langsdorffii. Thetotal uptake of the auxin from donor blocks was greatest inthe hybrids and N. glauca. The velocity of the basipetal movementof IAA-¹⁴C was the same in all species tested, i.e. 8 mm/h.The transport capacity for the hormone, however, was decreasedin the three tumour-prone hybrids. Gas chromatography showedthat between 70 and 90 per cent of the transported auxin waspresent in the form of IAA, between 10 and 30 per cent in theform of indol-3yl-aldehyde (IAld). The basipetal transport exceeded the acropetal transport inyoung (third) intemodes of all plants studied, whereas in olderstem segments (tenth intenodes) the reverse was found. The polarity of auxin transport was less well expressed in thetumorous hybrids. Blocking the active transport by pre-treatment of stem cuttingswith 2,4-dinitrophenol (2,4-DNP) caused a drastic reductionin the polar IAA-¹⁴C movement; in all plants tested the auxintransport was reduced to the same low level. The accumulation of auxin at the base of cuttings was higherin N. glauca and the 2n hybrid than in N. langsdorffii, i.e.about seven times higher after 1-h and three times higher after12-h transport experiments. The release of ¹⁴C from the cuttinginto an agar receiver block, however, was markedly reduced inthe 2n hybrid, whereas in N. glauca the labelled substancesmoved more freely into the receiver blocks. Differences in the capacity for the accumulation and the releaseof IAA-¹⁴C in hybrid and N. glauca stem tissues were studiedusing decapitated greenhouse plants wounded by incision abovethe fourth internode. Accumulation of the auxin occurred onlyabove the wound-cut in hybrid plants. This observation is consistentwith the view that tumour formation on hybrid stems occurs atsites of wounding. Our data suggest an elevated auxin levelto be present during tumour initiation at these sites. These results on polar transport and accumulation of IAA-¹⁴Cin tumorous Nicotiana plants together with our previous dataon various endogenous auxins suggest that the induction of neoplasticgrowth in tobacco plants is correlated with increased auxinlevels and an accumulation of the hormone at sites of wounding.     

Effect of Applied and Endogenous Auxin on Callus and Root Formation of In Vitro Shoots of the Apple: Rootstocks M26 and A2

The influence of exogenous IBA (indol-3yl-butyric acid) on rootand callus formation was studied in shoots of the apple rootstocksA2 and M26. The shoots grown in vitro were derived originallyfrom meristems of both juvenile and adult trees. Endogenousindol-3yl-acetic acid (IAA) concentrations in leaves and stemswere correlated with the responses to applied IAA. After 30 subcultures shoots from A2 and M26 rooted easily, butA2 did so more readily and even without IBA. Treatment withIBA improved percentage rooting and number of roots in bothrootstocks. Ex-adult and ex-juvenile shoots of A2 formed rootsto the same extent. However, ex-adult shoots of A2 showed ahigher IBA optimum for root number than ex-juvenile A2 and werealso less sensitive to supra-optimal IBA concentrations. Incontrast, in M26, there were no differences between ex-adultand ex-juvenile shoots. The results imply that rooting ability is associated more withdifferences between cultivars than with the origin of the explants.The best rooting occurred in ex-adult shoots of A2 which hadthe lowest endogenous IAA concentration, while callus formationwas correlated with high endogenous auxin concentration. Ex-adultA2 produced almost no callus even after exposure to high IBAconcentrations (25µM) whereas ex-adult M26 formed muchmore callus at 1/10 of the IBA concentration. Malus sylvestris (L.) Mill. var. domestica Borkh., Malus pumila Mill., apple rootstocks A2 and M26, in vitro culture, root and callus formation, HPLC analyses of IAA