Induction of tension wood by 2,4-dinitrophenol and auxins

Summary The effect of DNP and auxins on the development of the secondary xylem in erect stems ofAcer rubrum was studied. DNP affected the development of the secondary xylem only locally in the treated internode. Tension wood is formed in the stem below the DNP treatment site whereas above the application site the development of tracheary elements is altered. InAcer rubrum seedlings that were treated with auxin, especially at low concentrations, a thick ring of tension wood is developed in the erect stem below the treatment site. Previous suggestions that the formation of tension wood in arborescent angiosperms is a developmental response to auxin deficiency are discussed in terms of the induction of tension wood inAcer rubrum by DNP and auxins.The following abbreviations will be used TIBA (2,3,5-tri-iodobenzoic acid) - IAA (indole-3-acetic acid) - GA (gibberellic acid) - NAA (naphthaleneacetic acid) - 2,4-D (2,4-dichlorophenoxyacetic acid) - DNP (2,4-dinitrophenol) This material was included in a doctoral thesis submitted by P. R.Morey to the graduate school of Yale University, New Haven.     

Developmental changes in the secondary xylem ofAcer rubrum induced by gibberellic acid,various auxins and 2,3,5-tri-iodobenzoic acid

Summary The effect of GA on the development of the secondary xylem ofAcer rubrum was studied. GA was applied to seedlings (1) separately, (2) simultaneously with TIBA, and (3) simultaneously with TIBA and an auxin. GA alone is without effect on the development of the secondary xylem. In seedlings treated with TIBA-GA or TIBA-GA-auxin, the differentiation of the xylem elements is in general similar to that of seedlings treated with TIBA or TIBA-auxin respectively. GA however, does stimulate cambial activity in a localized region of the stem if the endogenous rate of cambial division is low.The following abbreviations will be used TIBA (2,3,5-tri-iodobenzoic acid) - IAA (indole-3-acetic acid) - GA (gibberellic acid) - NAA (naphthaleneacetic acid) - 2,4-D (2,4-dichlorophenoxyacetic acid) This material was included in a doctoral thesis submitted by P. R.Morey to the graduate school of Yale University, New Haven.     

Developmental changes in the secondary xylem ofAcer rubrum induced by various auxins and 2,3,5-tri-iodobenzoic acid

Summary TIBA has been applied laterally to actively growing stems of uprightAcer rubrum seedlings. The frequency of initiation of tracheary elements is reduced and a complete ring of tension wood is developed in the stem locally below the TIBA application site. Rings of tension wood were never formed above the TIBA treatment site. In regard to anatomy, lignin distribution and peroxidase activity, the tension wood fibers formed as a result of TIBA treatment are identical to those which can be induced by bending.In the region of the stem above the site of TIBA application there is a particularly strong alteration in the development of tracheary elements.Application of IAA, NAA, or 2,4-D to the TIBA treatment site suppresses the capacity of TIBA to induce the development of tension wood and at the same time generally increases the frequency of initiation of tracheary elements.The effect of auxin alone on theAcer rubrum system has been studied. The secondary xylem formed during treatment with auxins (especially 2,4-D and NAA) at the stated concentrations is generally characterized by large groups of tracheary elements with a conspicuous angular outline in transverse section.The evidence suggests that auxins are involved in the regulatory systems which bring about the orderly development of the secondary xylem in arborescent angiosperms.This material was included in a doctoral thesis submitted by P. R.Morey to the graduate school of Yale University, New Haven.     

Response of cytokinin concentration in the xylem exudate of bean (Phaseolus vulgaris L.) plants to decapitation and auxin treatment,and relationship to apical dominance

When xylem exudate of previously untreated Phaseolus vulgaris plants was analysed for cytokinins by radioimmunoassay, a low concentration (about 5 ng · ml^–1) was found. However, when the plants were decapitated about 16 h before the xylem exudate was collected, an almost 25-fold increase in cytokinin concentration was observed. Twenty-four hours after decapitation this increase even reached 4000 compared to control plants. Applying naphthaleneacetic acid (NAA) to the shoot of decapitated plants almost eliminated the effect of shoot tip removal on cytokinin concentration, suggesting that cytokinins in the xylem exudate of intact plants are under the control of the polar auxin transport system. Other xylem constituents, such as potassium or free amino acids did not show this strong increase after decapitation and did not respond to NAA application. It is concluded that the observed auxin/cytokinin interaction has an important regulatory role to play, not only in apical dominance but in many other correlative events as well.Abbreviations AD apical dominance - CKs cytokinin(s) - iAde/iAdo isopentenyladenine/iospentenyladenosine - NAA naphthaleneacetic acid - Z/ZR zeatin/zeatin riboside     

The specificity of auxin transport in intact pea seedlings (Pisum sativum L.)

Summary When eight ¹⁴C-labelled auxin and non-auxin compounds were applied to the apical buds of intact dwarf pea seedlings (Pisum sativum L.), only [1-¹⁴C]indoleacetic acid ([¹⁴C]IAA) and -[1-¹⁴C] naphthaleneacetic acid ([¹⁴C]NAA) underwent appreciable basipetal transport during the first 24 h; over a longer period (72 h) considerable basipetal transport of the auxin [1-¹⁴C]2,4-dichlorophenoxyacetic acid ([¹⁴C]2,4-D) also occurred, but at a very much lower velocity (ca. 1.4–2.2 mm·h^-1). The movement of 2,4-D possessed many of the characteristics of a typical auxin transport. During uptake and transport IAA and NAA were extensively metabolised to the corresponding aspartates, and to ethanol-insoluble/NaOH-soluble compounds; little metabolism of 2,4-D was observed. None of the non-auxin compounds applied (sorbose, sucrose, leucine, adenine and kinetin) underwent appreciable basipetal transport from the apical bud. All but sorbose were extensively metabolised by the apical tissues. Little metabolism of sorbose itself was detected.The results suggest that the long-distance basipetal auxin transport system from the apical bud of intact plants is specific for auxins; the specificity may result from the affinity of auxins for specific transport sites.     

Growth Hormones and Propagation of Cymbidium in vitro

Protocorms of Cymbidium (Orchidaceae) were grown on solid or liquid medium with macro-nutrients according to Wimber (van Raalte 1967) and iron, micro-nutrients and vitamins according to Nitsch (1968) the medium also contained 2% sucrose. The effects of 1) the auxins; indol-3yl-acetic acid (IAA), α-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D); 2) the cytokinins; 6-furfurylaminopurine (kinetin) and benzyladenine (BA) and 3) the gibberellin; gibberellic acid (GA) were examined alone or in combinations. IAA had no effect alone. NAA resulted in optimal fresh weight at 10 μM and the protocorms were vigorous, but lighter green than usual. 2,4-D caused a high weight increase at 1 μM, but the protocorms were abnormal. Higher concentrations of NAA and 2,4-D inhibited chlorophyll synthesis. On solid medium kinetin (100 μM) induced a growth of many small shoots, but had no effect on the fresh weight. In liquid medium, kinetin promoted a callus formation and fresh weight increase. BA had effects similar to kinetin, but at lower concentrations. GA alone promoted shoot and leaf growth. Combinations of kinetin and NAA resulted in a maximal fresh weight increase at kinetin concentrations one tenth of the NAA concentrations. The optimal growth and the best development occurred at 10 μM NAA and 1 μM kinetin. NAA and kinetin together could limit the shoot and leaf growth induced by GA.     

Effect of apex excision and replacement by 1-naphthylacetic acid on cytokinin concentration and apical dominance in pea plants

As known from literature lateral buds from pea ( Pisum sativum ) plants are released from apical dominance when repeatedly treated with exogenous cytokinins. Little is known, however, about the endogenous role of cytokinins in this process and whether they interact with basipolar transported IAA, generally regarded as the main signal controlling apical dominance. This paper presents evidence that such an interaction exists.
The excision of the apex of pea plants resulted in the release of inhibited lateral buds from apical dominance (AD). This could be entirely prevented by applying 1-naphthylacetic acid (NAA) to the cut end of the shoot. Removal of the apex also resulted in a rapid and rather large increase in the endogenous concentrations of zeatin riboside (ZR), isopentenyladenosine (iAdo) and an as yet unidentified polar zeatin derivative in the node and internode below the point of decapitation. This accumulation of ZR and iAdo, was strongly reduced by the application of NAA. The observed increase in cytokinin concentration preceded the elongation of the lateral buds, suggesting that endogenous cytokinins play a significant role in the release of lateral buds from AD. However, the effect of NAA on the concentration of cytokinins clearly demonstrated the dominant role of the polar basipetally transported auxin in AD. The results suggest a mutual interaction between the basipolar IAA transport system and cytokinins obviously produced in the roots and transported via the xylem into the stem of the pea plants.     

The effect of plant growth substances on the development of tension wood in horizontally inclined stems ofAcer rubrum seedlings

Summary In horizontally placedAcer rubrum seedlings the development of tension wood is inhibited by auxin especially when applied along the upper side of the axis. Treatment separately with GA and K does not alter the normal pattern of tension wood formation. The development of tension wood appears to be correlated with a reduced auxin level on the upper side of the stem.Geotropic reorientation of the woody stem ofAcer rubrum seedlings that are placed in the horizontal position is related closely to the amount of tension wood that is formed on the upper side of the axis. Evidence is discussed which indicates that tension wood itself participates actively in the reorientation mechanism.The following abbreviations will be used TIBA (2,3,5-tri-iodobenzoic acid) - IAA (indole-3-acetic acid) - GA (gibberellic acid) - NAA (naphthaleneacetic acid) - 2,4-D (2,4-dichloro-phenoxyacetic acid) - K (kinetin) This material was included in a doctoral thesis submitted by P. R.Morey to the graduate school of Yale University, New Haven.     

Effects of temperature and sink activity on the transport of 14C-labelled indol-3yl-acetic acid in the intact pea plant (Pisum sativum L.)

The velocity and intensity of basipetal transport of ¹⁴C-labelled indol-3yl-acetic acid (IAA) applied to the apical bud of the intact pea plant were influenced by the temperature to which the stem was exposed and were not influenced by changes in the temperature of the root system when this was controlled independently between 5°C and 35°C. The velocity of transport increased steadily with temperature to a maximum in excess of 35°C and then fell sharply with further increase in temperature. The Q₁₀ for velocity, determined from Arrhenius plots, was low (ca. 1.3). Transport intensity increased to a maximum at about 25°C (Q₁₀=2.2) and then declined gradually with further increase in temperature. It is suggested that transport velocity and transport intensity are controlled independently.The characteristics of auxin transport through the stem were not affected by removal of the root system, or by the withdrawl of root aeration. Labelled IAA did not pass a region of the stem cooled to about 1.0°C, or through a narrow zone of stem tissue killed by heat treatment. In the latter case the heat treatment was shown not to interfere with the upward transport of water in the xylem. Labelled IAA continued to move into, and to accumulate in, the tissues immediately above a cooled or heat-killed region of the stem. It was concluded that the long-distance basipetal transport of auxin through the stem of the intact plant is driven by the transporting cells themselves and is independent of the activity of sinks for the transported auxin.The fronts of the observed tracer profiles in the stem were closely fitted by error function diffusion analogue curves. However, diffusion of IAA alone could not account for the observed characteristics of the transport and it is suggested that the curvilinear fronts of the profiles resulted from a diffusive mixing of exogenous IAA (or IAA-carrier complexes) with endogenous IAA already in the transport pathway.Abbreviations IAA indol-3yl-acetic acid - IAAsp indol-3yl-acetyl aspartic acid - CFM methyl 2-chloro-9-hydroxyfluorene-9-carboxylate (morphactin) - TIBA 2,3,5-triiodobenzoic acid - ABA abscisic acid     

Effects of auxins and cytokinins on growth and rosmarinic acid formation in cell suspension cultures of Anchusa officinalis

Cell suspension cultures of Anchusa officinalis required exogenous phytohormones for their normal growth. Cell lysis was observed at the third passage in a hormone-free medium. Using hormone — depleted cells, the effects of auxins (2,4-D, NAA, IAA and CFP) and cytokinins (BA, kinetin, and zeatin) on cell growth and RA production were investigated. All auxins tested could maintain growth and integrity of the cells whereas cytokinins alone could not, suggesting that this culture is auxindependent. Among the auxins tested, NAA had a pronounced effect on RA production. The total RA content obtained at optimum NAA concentration (0.25 mg/l) reached 1.7 g/l (12% of dry weight). The kinetics of growth and RA production suggested that the increase in final RA content was due to both an increase in the rate of RA synthesis and initiation of the period of synthesis in the exponential rather than the linear growth phase.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - IAA indoleacetic acid - CFP 2-chloro-4-fluorophenoxyacetic acid - BA 6-benzyladenine - RA rosmarinic acid     

Another evidence for inhibitory effect of auxin in adventitious bud formation of decapitated flax (Linum usitatissimum L.) seedlings

Adventitious buds were formed on the hypocotyls of decapitated flax seedlings. Scanning electron and light microscopic examinations of hypocotyls showed that epidermal cells divided to produce meristematic spots from which several leaf primordia were formed. Between leaf primordia and the original vascular tissues of hypocotyls, new xylem cells were formed which connected them. About 10, 30 and 60% of adventitious buds were formed on upper, middle and basal parts of hypocotyls of decapitated seedlings, respectively. Removal of apical meristem together with longer hypocotyl zero to four cm long below the apical meristem) induced higher percentage of adventitious bud formation in the remaining hypocotyl. When the entire hypocotyl was cut into 16 segments (0.25 cm each) and these segments were cultured on MS medium containing 3% sucrose and 0.8% agar, adventitious buds were mainly formed in the lowest five segments. These results suggested that there was a gradient of inhibitory factor(s) from apical to basal part of hypocotyl with respect to adventitious bud formation. Auxin transport inhibitors, morphactin and TIBA induced adventitious bud formation on intact seedlings by suppressing the basipetal movement of auxin.     

Auxin transport in intact pea seedlings (Pisum sativum L.): The inhibition of transport by 2,3,5-triiodobenzoic acid

Summary The application of 2,3,5-triiodobenzoic acid (TIBA, 10 mg·g^-1 in lanolin) to the stem of intact pea seedlings (Pisum sativum L.) inhibited the basipetal transport of ¹⁴C from indoleacetic acid-1-¹⁴C (IAA-1-¹⁴C) applied to the apical bud, but not the transport of ¹⁴C in the phloem following the application of IAA-1-¹⁴C or sucrose-¹⁴C to mature foliage leaves. It was concluded that fundamentally different mechanisms of auxin transport operate in these two pathways.When TIBA was applied at the same time as, or 3.0 h after, the application of IAA-1-¹⁴C to the apical bud, ¹⁴C accumulated in the TIBA-treated and higher internodes; when TIBA was applied 24.0 h before the IAA-1-¹⁴C, transport in the stem above the TIBA-treated internode was considerably reduced. TIBA treatments did not consistently influence the total recovery of ¹⁴C, or the conversion of free IAA to indoleaspartic acid (IAAsp). These results are discussed in relation to the possible mechanism by which TIBA inhibits auxin transport,.Attention is drawn to the need for more detailed studies of the role of the phloem in the transport of endogenous auxin in the intact plant.Abbreviations TIBA 2,3,5-triiodobenzoic acid - IAAsp indoleaspartic acid     

The specificity of carrier-mediated auxin transport by suspension-cultured crown gall cells

1. The specificity of the auxin transport system of suspension-cultured crown gall cells from Parthenocissus tricuspidata Planch- is examined with regard to 2,4-Dichlorophenoxyacetic acid (2,4 D), l-Naphthylacetic acid (NAA) and Benzoic acid (BA) as well as for indole-3-acetic acid (IAA). — 2. All four weak acids can be accumulated by the cells from a medium more acidic than the cytoplasm. This is by virtue of non-specific passive diffusion of their lipid-soluble protonated forms down a concentration gradient. The corresponding anionic species are much less permeant. The extent of the accumulation is dependent on the pH difference that is maintained by the cells between their cytoplasm and the incubation medium. Studies of the concentration dependence of BA and NAA net uptake at a series of external pHs suggest that an acidification of the cytoplasm can be eventually brought about by the entry of weak acid into the cells. — 3. The uptake of 2,4 D, as well as that of IAA, has a saturable carrier-mediated component in addition to the passive diffusion of the undissociated acid. These saturable components probably represent anion uptake and appear to be mediated by a common carrier. The kinetic studies provided no evidence for the participation of carriers in the transport of BA or NAA. — 4. It was shown that the efflux of 2,4 D also has a carrier-mediated component and it is suggested that both the influx and efflux of IAA and 2,4 D occur on a common carrier. — 5. The inhibitor of polar auxin transport, 2,3,5-triiodobenzoic acid (TIBA), stimulates the net uptake of IAA by inhibiting carrier-mediated efflux of IAA from the cells. However, TIBA could not be demonstrated to have a significant effect on 2,4 D transport and any perturbation that occurs is very small in comparison with its effect on IAA movement. To account for this, the proposed common carrier could exhibit some difference in its internal binding characteristics betweend 2,4 D and IAA. An alternative explanation is that a second carrier is present, which mediates IAA efflux only, and which is inhibited by TIBA. — 6. TIBA has no significant effect on the transport of either BA or NAA, except to bring about an inhibition of net uptake, and a corresponding stimulation of efflux, when it is present at concentrations sufficient to acidify the cytoplasm. —7. The crown gall cells are compared to intact plant tissues capable of polar auxin transport with regard to the specificities exhibited for the transport of the auxins IAA, 2,4 D and NAA and the non-auxin BA.Abbreviations IAA indol-3-yl acetic acid - 2,4 D 2,4-Dichlorophenoxyacetic acid - NAA 1-Naphthylacetic acid - BA Benzoic acid - TIBA 2,3,5-triiodobenzoic 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     

Effect of Some Auxins and Cytokinins on Growth and Gametangial Formation in the Liverwort Riccia gangetica Ahmad

The present work deals with the effect of some auxins [2,4-D,IAA, NAA and ß-naphthoxyacetic acid (NOA)] and cytokinins[6-benzylaminopurine (BAP), 6-(,-dimethylallylamino)purinc (2iP),KN and 6-(4-hydroxy-3-methylbut-2-enylaimno)purine (zeatin)]on growth and gametangial formation in the liverwort Ricciagangetica. Of these, 2,4-D and IAA stimulate vegetative growth,induce the formation of rhizoids on dorsal surface of thallusand shorten the time required for gametangial initiation. NAAand NOA inhibit growth, and do not affect the time of gametangialproduction. The auxins increase archegonial number without affectingantheridial induction and, among those tested, IAA is the mosteffective. Archegonial number is enhanced by the cytokinins,but antheridial production decreases. The maximum number ofarchegonia is initiated in response to 2iP, and this effectappears to be specific. On the other hand, thallus growth, stimulatedby the other cytokinins, is inhibited by 2iP. Riccia gangetica Ahmad, liverwort, auxins, cytokinins, growth, gametangial, formation     

Evaluation of the role of hormonal factors in secondary growth of dicots

The present investigation was carried out with a view to finding out the role of hormonal factors in secondary growth in stems ofCapsicum annuum andSolanum melongena. Seedlings were treated with aqueous solutions of growth substances and observations were made in respect to initiation of cambium, magnitude of secondary growth and differentiation of the secondary vascular tissues. Enhancement of secondary growth by auxins (IBA, IAA and NAA) and its partial or complete inhibition by auxin inhibitors (MH, PMN and DCA) clearly establishes the controlling nature of auxins on the process. Evidence has been presented demonstrating that auxins are more involved with differentiation of xylem, whereas GA is more involved with phloem. The investigation may be of economic value in improving the quality and quantity of wood.     

Trijodbenzoesäure und die Stoffleitung bei höheren Pflanzen

Summary The effect of 2,3,5-triiodobenzoic acid (TIBA) on the translocation of various substances within etiolated pea plants was tested by applying the substances to different places on the decapitated plants and measuring the effect of the substances on the growth of lateral buds after placing a lanolin ring containing TIBA around the stem between the place of application of the substance and the buds. In control experiments the TIBA-ring was placed in such a way, that TIBA and the other substance reached the bud from opposite directions.TIBA blocked the basipetal translocation of indole-3-acetic acid, -4-chlorophenoxyisobutyric acid and -(1-naphthylmethylsulfid) propionic acid, it did not block the acropetal translocation of sucrose, potassium nitrate and indole-3-acetic acid. It did howevers block the acropetal translocation of native inhibitors (correlation inhibitors) extracted from pea plants. It is concluded that TIBA is a general blocker of the energy requiring translocation of many substances in plants. Moreover TIBA produces various different effects.Intensity of translocation of TIBA was equal in both directions, acropetal and basipetal. Application of TIBA to more distal points of the stem may cause greater effects because of a faster migration through younger tissues into the stem.Mit 2 Textabbildungen.     

Short-term Effects of Some Chemicals on Cambial Activity

Aqueous solutions of indol-3yl-acetic acid (IAA), 1-naphthyl-aceticacid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), gibberellicacid (GA), 6-furfuryl-aminopurine (FAP), myo-inositol, and sucrosewere applied singly and in mixtures to the apical ends of disbuddedstem segments of willow. After 4 weeks all substances had hadsome effect on differentiation of xylem from cambial derivatives.The production of potential xylem cells, as well as their differentiationwere most markedly enhanced when IAA, GA, and FAP were appliedtogether, although the response was further augmented by additionof inositol or sucrose. The action of the substances when appliedas mixtures was often synergistic. This means that it is difficultto assess the role of different chemicals in xylem productionby extrapolation from experiments involving the applicationof single substances.     

Influence of 2,3,5-triiodobenzoic acid on the transport and metabolism of IAA in lupin hypocotyls

The influence of 2,3,5-triiodobenzoic acid (TIBA) on the transport and metabolism of indolyl-3-acetic acid (IAA) was studied in etiolated lupin (Lupinus albus L) hypocotyls. Double isotope-labeled IAA [(5-³H)-IAA plus (1-¹⁴C)-IAA] was applied to the cut surface of decapitated seedlings. This confirmed that the species mobilized was unaltered IAA and permitted us to measure the in vivo decarboxylation of applied IAA. A pretreatment with TIBA applied to the cut surface produced a partial or drastic inhibition in the basipetal IAA movement at 0.5 or 100 M, respectively. Since TIBA inhibits auxin polar transport by interfering with the efflux carrier, the above results suggest that 100 M TIBA is sufficient to saturate the binding sites in the transporting cells. Compared to the control plants, in vivo decarboxylation of IAA was enhanced in 0.5 M TIBA-treated plants, while no decarboxylation was detected after treatment with 100 M TIBA. The in vitro decarboxylation of (1-¹⁴C)-IAA catalyzed by purified peroxidase was moderately activated by 100 M and unaffected by 0.5 M TIBA. The paradoxical effect of TIBA in vivo vs in vitro assays suggests that the in vivo effect of TIBA on IAA oxidation might be the consequence of the action of TIBA on the auxin transport system. Thus, transport reduction by 0.5 M TIBA caused a temporary accumulation of IAA in that apical region of the hypocotyl which has the highest capacity to decarboxylate IAA. In the presence of 100 M TIBA, a concentration which presumably saturates the efflux carriers, most of the added IAA can be expected to be located in the transporting cells where, according to the present data, IAA decarboxylation cannot take place.     

Micropropagation of <Emphasis Type="Italic">Metrosideros excelsa</Emphasis>

Multiple shoots were induced on stem segments of an 8-y-old plant of Metrosideros excelsa Sol ex Gaertn. “Parnel”. Axillary shoots produced on uncontaminated explants were excised, segmented, and recultured in the same medium to increase the stock of shoot cultures. The Murashige and Skoog (MS) medium, augmented with different concentrations of 2- isopenthenyladenine (2iP) and indole-3-acetic acid (IAA), either singly or in combinations, as potential medium for shoot multiplication by nodal segments was tested. In the following experiment, equal molar concentrations of four cytokinins [2iP, kinetin, zeatin, and N ⁶-benzyladenine (BA)] in combination with equal molar concentrations of three auxins [IAA, α-naphthaleneacetic acid (NAA), and indole-3-butyric acid (IBA)] were tested for ability to induce axillary shoot development from single-node stem segments. The highest rate of axillary shoot proliferation was induced on MS agar medium supplemented with 1.96μM 2iP and 1.14μM IAA after 6 wk in culture. Different auxins (IAA, IBA, and NAA) were tested to determine the optimum conditions for in vitro rooting of microshoots. The best results were accomplished with IAA at 5.71μM (89% rooting) and with IBA at 2.85 or 5.71μM (86% and 86% rooting, respectively). Seventy and 90 percent of the microshoots were rooted ex vitro in bottom-heated bench (22 ± 2°C) after 2 and 4 wk, respectively. In vitro and ex vitro rooted plantlets were successfully established in soil.