Release of lateral buds from apical dominance by glyphosate in soybean and pea seedlings

Application of a sublethal dose of glyphosate (N-[phosphonomethyl]glycine) to the seedlings of soybean (Glycine max L. Merr. cv. Evans) and pea (Pisum sativum L. cv. Alaska) promoted growth of the cotyledonary and other lateral buds. The pattern of the glyphosate-induced lateral bud growth was different from that induced by decapitation. Under the experimental condition, glyphosate did not kill the apical buds. Feeding stem sections of the seedlings with radiolabeled indole-3-acetic acid ([2¹⁴C]IAA) and subsequent analysis of free [2-¹⁴C]IAA and metabolite fractions revealed that the glyphosate-treated plants had higher rates of IAA metabolism than the control plants. The treated pea plants metabolized 75% of [2-¹⁴C]IAA taken up in the 4-h incubation period compared to 46.5% for the control, an increase of 61%. The increase was small but consistent in soybean seedlings. As a result, the glyphosate-treated plants had less free IAA and ethylene than the control plants. The increase of IAA metabolism induced by glyphosate is likely to change the auxin-cytokinin balance and contribute to the release of lateral buds from apical dominance in these plants.     

Effect of glyphosate on indole-3-acetic acid metabolism in tolerant and susceptible plants

A comparison study was conducted on the effect of glyphosate (N-[phosphonomethyl]glycine) on indole-3-[2-¹⁴C]acetic acid (IAA) metabolism, ethylene production, and growth of 7-day-old seedlings of different plants. The plants tested were American germander (Teucrium canadense L.), soybean (Glycine max L. Merr.), pea (Pisum sativum L. cv. Alaska and Little marvel), mungbean (Vigna radiata L.), and buckwheat (Fagopyrum esculentum Moench). A spray with 2 mM glyphosate affected IAA metabolism to a varied degree. The induced increase of IAA metabolism was greater in buckwheat, Alaska pea, and mungbean than soybean, Little marvel pea, and American germander. The increased IAA metabolism was correlated with the inhibition of growth and with the decrease of ethylene production. The natural rate of IAA metabolism was markedly different among the plant species and cultivars tested and appeared to be related to the sensitivity of the plants to glyphosate. American germander and Little marvel pea with high rates of IAA metabolism were more tolerant to glyphosate than buckwheat and Alaska pea, which had low rates of IAA metabolism. Plants with a high natural rate of IAA metabolism were probably less dependent on IAA and thus less susceptible to glyphosate.     

Effect of glyphosate on indole-3-acetic acid metabolism in tolerant and susceptible plants

A comparison study was conducted on the effect of glyphosate (N-[phosphonomethyl]glycine) on indole-3-[2-¹⁴C]acetic acid (IAA) metabolism, ethylene production, and growth of 7-day-old seedlings of different plants. The plants tested were American germander (Teucrium canadense L.), soybean (Glycine max L. Merr.), pea (Pisum sativum L. cv. Alaska and Little marvel), mungbean (Vigna radiata L.), and buckwheat (Fagopyrum esculentum Moench). A spray with 2 mM glyphosate affected IAA metabolism to a varied degree. The induced increase of IAA metabolism was greater in buckwheat, Alaska pea, and mungbean than soybean, Little marvel pea, and American germander. The increased IAA metabolism was correlated with the inhibition of growth and with the decrease of ethylene production.The natural rate of IAA metabolism was markedly different among the plant species and cultivars tested and appeared to be related to the sensitivity of the plants to glyphosate. American germander and Little marvel pea with high rates of IAA metabolism were more tolerant to glyphosate than buckwheat and Alaska pea, which had low rates of IAA metabolism. Plants with a high natural rate of IAA metabolism were probably less dependent on IAA and thus less susceptible to glyphosate.     

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.     

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     

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.     

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 Benzyladenine and Gibberellin A(1) from Roots in Relation to the Dominance between the Axillary Buds of Pea (Pisum sativum L.) Cotyledons

In etiolated, 5-day-old pea (Pisum sativum L.) seedlings a significantly more intensive growth of buds situated in the axil of the excised cotyledons was observed as early as 4 hours after decapitation and excision of one cotyledon of each pair. If [8-¹⁴C]benzyladenine ([¹⁴C]BA) was applied to roots of intact plants 10 hours prior to such decapitation and excision, significantly higher both total and specific ¹⁴C activities were observed in buds situated on the side of the excised cotyledons as early as 4 hours after decapitation and excision. Although the removal of a substantial part of the root system carried out simultaneously with decapitation and excision of one cotyledon resulted in a decrease in total ¹⁴C activity of buds, nevertheless a higher accumulation of ¹⁴C activity was maintained in buds situated on the side of excised cotyledon. If [¹⁴C]BA was applied to roots of seedlings after they were decapitated and deprived of one cotyledon, both total and specific ¹⁴C activities of buds situated on the side of excised cotyledons were significantly higher as early as the end of uptake of [¹⁴C]BA by roots, i.e. after 10 hours. On the other hand, [1,2-³H]gibberellin A₁ applied to roots of intact and/or decapitated and one-cotyledon-deprived seedlings in the same way as [¹⁴C]BA did not appear in the buds until very much later and only in negligible amounts (i.e.³H activity). This indicates that the release of buds from apical dominance represents an active and selective process which can result from the ability of buds to utilize and/or synthesize only certain growth substances within a certain time interval.     

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     

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     

Accumulation of 14C from exogenous labelled auxin in lateral root primordia of intact pea seedlings (Pisum sativum L.)

When [¹⁴C]indol-3yl-acetic acid was applied to the apical bud of 5-day old dwarf pea seedlings which possessed unbranched primary roots, a small amount of ¹⁴C was transported into the root system at a velocity of 11–14 mm h^-1. Most of the ¹⁴C which entered the primary root accumulated in the young lateral root primordia, including the smallest detectable (20–30 mm from the primary root tip). In older (8-d old) seedlings in which the primary root bore well-developed lateral roots, ¹⁴C also accumulated in the tertiary root primordia. In contrast, little ¹⁴C was detected in the apical region of the primary root or, in older plants, in the apices of the lateral roots.Abbreviations IAA indol-3yl-acetic acid     

Changes after Decapitation in Concentrations of Indole-3-Acetic Acid and Abscisic Acid in the Larger Axillary Bud of Phaseolus vulgaris L. cv Tender Green

Early changes in the concentrations of indole-3-acetic acid (IAA) and abscisic acid (ABA) were investigated in the larger axillary bud of 2-week-old Phaseolus vulgaris L. cv Tender Green seedlings after removal of the dominant apical bud. Concentrations of these two hormones were measured at 4, 6, 8, 12 and 24 hours following decapitation of the apical bud and its subtending shoot. Quantitations were accomplished using either gas chromatography-mass spectrometry-selected ion monitoring (GS-MS-SIM) with [¹³C₆]-IAA or [²H₆]-ABA as quantitative internal standards, or by an indirect enzyme-linked immunosorbent assay, validated by GC-MS-SIM. Within 4 hours after decapitation the IAA concentration in the axillary bud had increased fivefold, remaining relatively constant thereafter. The concentration of ABA in axillary buds of decapitated plants was 30 to 70% lower than for buds of intact plants from 4 to 24 hours following decapitation. Fresh weight of buds on decapitated plants had increased by 8 hours after decapitation and this increase was even more prominent by 24 hours. Anatomical assessment of the larger axillary buds at 0, 8, and 24 hours following decapitation showed that most of the growth was due to cell expansion, especially in the intermodal region. Thus, IAA concentration in the axillary bud increases appreciably within a very few hours of decapitation. Coincidental with the rise in IAA concentration is a modest, but significant reduction in ABA concentration in these axillary buds after decapitation.     

Ontogenetic Changes in the Transport of Indol-3yl-acetic Acid into Maize Roots from the Shoot and Caryopsis

The quantities of endogenous indol-3yl-acetic acid (IAA) in endosperms and scutella of 6-day-old maize seedlings (Zea mays L. cv Giant White Horsetooth) were determined by a fluorimetric method. Endosperms were found to contain 33.4 nanograms IAA per plant, and scutella 7.5 nanograms IAA per plant. [5-³H]IAA applied to endosperms of 6-day-old seedlings moved into the roots and radioactivity accumulated at the apex of the primary root within 8 hours. Two to 7-day-old seedlings were treated simultaneously with [5-³H]IAA in the endosperm and [2-¹⁴C] IAA on the shoot apex. The patterns of transport into the root were found to change during ontogeny: in successively older plants, transport from the shoot into the roots increased relative to transport from the endosperm into the roots. The auxin required for the growth of maize roots could, therefore, partially be contributed by the shoot and endosperm. Ontogenetic changes in the relative importance of these two supplies could be of significance for the integration of growth and development between shoot and root.     

Transfer of exogenous auxin from the phloem to the polar auxin transport pathway in pea (Pisum sativum L.)

When [1-¹⁴C]indol-3yl-acetic acid ([1-¹⁴C]IAA) was applied to the upper surface of a mature foliage leaf of garden pea (Pisum sativum L. cv. Alderman), ¹⁴C effluxed basipetally but not acropetally from 30-mm-long internode segments excised 4 h after the application of [1-¹⁴C]IAA. This basipetal efflux was strongly inhibited by the inclusion of 3.10^–6 mol· dm³ N-1-naphthylphthalamic acid (NPA) in the efflux buffer. In contrast, when [¹⁴C] sucrose was applied to the leaf, the efflux of label from stem segments excised subsequently was neither polar nor sensitive to NPA. The [1-¹⁴C]IAA was initially exported from mature leaves in the phloem — transport was rapid and apolar; label was recovered from aphids feeding on the stem; and label was recovered in exudates collected from severed petioles in 20 mM ethylenediaminetetraacetic acid. No ¹⁴C was detected in aphids feeding on the stems of plants to which [1-¹⁴C]IAA had been applied apically, even though the internode on which they were feeding transported considerable quantities of label. Localised applications of NPA to the stem strongly inhibited the basipetal transport of apically applied [1-¹⁴C]IAA, but did not affect transport of [1-¹⁴C]IAA in the phloem. These results demonstrate for the first time that IAA exported from leaves in the phloem can be transferred into the extravascular polar auxin transport pathway but that reciprocal transfer probably does not occur. In intact plants, transfer of foliar-applied [1-¹⁴C]IAA from the phloem to the polar auxin transport pathway was confined to immature tissues at the shoot apex. In plants in which all tissues above the fed leaf were removed before labelling, a limited transfer of IAA occurred in more mature regions of the stem.Abbreviations IAA indol-3yl-acetic acid - EDTA ethylenediaminetetraacetic acid - NPA N-1-naphthylphthalamic acid We are grateful to the Nuffield Foundation for supporting this research under the NUF-URB95 scheme and for the provision of a bursary to A.J.C. We thank Professor Dennis A. Baker for constructive comments on a draft of this paper and Mrs. Rosemary Bell for her able technical assistance.     

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     

The effect of temperature on the velocity of exogenous auxin transport in intact chilling-sensitive and chilling-resistant plants

The velocity of exogenous indol-3yl-acetic acid ([1-¹⁴C]IAA) transport from the apical buds of intact pea, sunflower and cotton plants was determined from 0.5° C to 47° C. The minimum temperature at which transport occurred varied from 2° C (pea and sunflower) to 7° C (cotton). Above these temperatures the velocity of transport increased steadily to maxima near 44° C in all three species. Further increase in temperature resulted in a complete cessation of transport, suggesting a sudden high-temperature breakdown of the auxin transport system. Temperature coefficients (Q₁₀) for transport velocity calculated from Arrhenius plots were low (1.36 to 1.41 between 15° C and 30° C).Arrhenius plots for the chilling-sensitive cotton and sunflower plants exhibited abrupt discontinuities at 14.6° C and 8.7° C respectively. An Arrhenius plot for the chilling-resistant pea exhibited no such discontinuity over the whole temperature range at which transport occurred.Abbreviation IAA indol-3yl-acetic acid     

Translocation and Metabolism of Endosperm-Applied [2-C] Indoleacetic Acid in Etiolated Avena sativa L. Seedlings

The role of free indole-3-acetic acid (IAA) in the endosperm of Avena sativa L. seedlings was investigated to determine its contribution to free IAA in the shoot. [2-¹⁴C]IAA was injected into the endosperm of darkgrown seedlings and the transport and metabolism of the [¹⁴C]-labeled compounds determined. It was concluded that translocation of free IAA directly from the endosperm is probably not a significant source of free IAA in the shoot, mainly because even small amounts of [¹⁴C]IAA introduced into the endosperm were rapidly metabolized. This suggested that, in Avena, free IAA does not normally exist in the liquid endosperm.     

Pathways of auxin transport in the intact pea seedling (Pisum sativum L.)

Summary When small colonies of the pea aphid [Acyrthosiphon pisum (Harris)] were established on the stem of Meteor Dwarf Pea seedlings (Pisum sativum L.), ¹⁴C was found in the honeydew 4.5 h after applying IAA-1-¹⁴C to a fully-expanded foliage leaf. In contrast, no activity was found in the honeydew or aphids 4.5 h after the application of IAA-1-¹⁴C to the intact apical bud even though the internode upon which the aphids were feeding contained high levels of ¹⁴C. The lack of radio-activity in aphids feeding on stems to which IAA-1-¹⁴C was applied via the apical bud was found not to be influenced by the internode position or by the transport interval allowed (up to 24 h).Radioactivity derived from either foliar or apical applications of IAA-1-¹⁴C was not transported through stem tissues killed by heat treatment. Xylem function was shown not to be impared by the heat treatment employed.It was concluded that the long-distance transport of IAA from the apical bud of intact pea seedlings does not take place in the phloem sieve tubes involved in the transport of metabolites from foliage leaves, or in the non-living tissues of the xylem.     

Lateral diffusion of polarly transported indoleacetic acid and its role in the growth of Lupinus albus L. hypocotyls

The transport and metabolism of indole-3-acetic acid (IAA) was studied in etiolated lupin (Lupinus albus L, cv. Multolupa) hypocotyls, following application of dual-isotope-labelled indole-3-acetic acid, [5-³H]IAA plus [1-¹⁴C]IAA, to decapitated plants. To study the radial distribution of the transported and metabolized IAA, experiments were carried out with plants in which the stele was separated from the cortex by a glass capillary. After local application of labelled IAA to the cortex, radioactivity remained immobilized in the cortex, near the application point, showing that polar transport cannot occur in the outer tissues. However, following application of IAA to the stele, radioactivity appeared in the cortex in those hypocotyl sections below the first 1 cm (in which the capillary was inserted), and the basipetal IAA movement was similar to that observed after application of IAA to the complete cut surface. In both assays, longitudinal distribution of ¹⁴C and ³H in the stele outside the first 1 cm was positively correlated with that of cortex, indicating that there was a lateral migration of IAA from the transport pathway (in the stele) to the outer tissues and that this migration depended on the amount of IAA in the stele. Both tissues (stele and cortex) exhibited intensive IAA metabolism, decarboxylation being higher in the stele than in the cortex while IAA conjugation was the opposite. Decapitation of the seedlings caused a drastic reduction of hypocotyl growth in the 24 h following decapitation, unless the hypocotyls were treated apically with IAA. Thus, exogenous IAA, polarly transported, was able to substitute the endogenous source of auxin (cotyledons plus meristem) to permit hypocotyl growth. It is proposed that IAA escapes from the transporting cells (in the stele) to the outer tissues in order to reach the growth-responsive cells. The IAA metabolism in the outer tissues could generate the IAA gradient necessary for the maintenance of its lateral flow, and consequently the auxin-induced cell elongation.     

The effect of zeatin and iso-pentenyladenine on IAA transport from the shoot to the root of Pinus pinea seedlings

The tips of the tap roots of Pinus pinea seedlings were dipped in zeatin or iso-pentenyladenine solutions. Immediately after cytokinin application to the root tip or after a 24 h lag phase, [2-¹⁴C]IAA was applied to the shoot apex. Treating with zeatin resulted in an increase in [2-¹⁴C]IAA transport from the shoot to the root. Iso-pentenyladenine also caused a slight increase in transport of radioactivity to the root but this was less pronounced compared to the results obtained with zeatin. With zeatin treatment increasing amounts of radioactivity accumulated in the lateral root emerging zone of the tap root (Section III). This was in sharp contrast to the treatment with iso-pentenyladenine where little radioactivity accumulated in this section of the root. Recovery of radioactivity 48 h after applying [2-¹⁴C]IAA showed that 33% of the recovered radioactivity co-chromatographed with authentic IAA. The implications of the effect of different cytokinins on the distribution of radioactivity along the tap root of Pinus pinea following [2-¹⁴C]IAA application to the shoot are discussed.Abbreviations Z zeatin - iP iso-pentenyladenine - TCL thin-layer chromatography