The Fate of Exogenously Applied Indoleacetic Acid in Light Grown Stems

Elongating segments from light grown pea (Pisum sativum L. cv. Alaska) and bean (Pbaseolus vulgaris L. cv. Red kidney) stems were incubated in 10^-5M indoleacetic acid (IAA)-1-¹⁴C,and -5-³H in the light. Radoactive derivatives, extracted in water, ethanol or ether, and 1 N sodium hydroxide at three different times were chromatographed in three separate systems and the different metabolites identified by their labeling and chromatographic characteristics. Major metabolites included indoleacetyl aspartate, possibly indoleacetyl glucoside, hydroxymethyloxindole, and in bean a further major unidentified compund. Other compounds occurred in lesser amounts. Indole aldehyde was present only in very small quantities. IAA breakdown commenced between 1 and 6 h of incubation, following which IAA decreased and most metabolites increased, though IAA was still present after 24 h. Alkaline hydrolysates contained mainly IAA at a level which changed little between 6 and 24 h.     

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.     

Cell length,light and14C-labelled indol-3yl-acetic acid transport inPisum satisum L. andPhaseolus vulgaris L

The putative auxin-transporting cells of the intact herbaceous dicotyledon are the young, differentiating vascular elements. The length of these cells was found to be considerably greater in dwarf (Meteor) than in tall (Alderman) varieties ofPisum sativum L., and to be greater in etiolated than in light-grown plants ofP. sativum cv Meteor andPhaseolus vulgaris L. cv Mexican Black. Under given light conditions during transport these large differences in cell length did not influence the shapes of the transport profiles or the velocity of transport of¹⁴C-labelled indol-3yl-acetic acid (IAA) applied to the apical bud. However, in both etiolated and light-grown bean and dwarf pea plants the velocity of transport in darkness was ca. 25% lower than that in light. Under the same conditions of transport velocities in bean were about twice those observed in the dwarf pea. Exposure to light during transport increased the rate of export of¹⁴C from the labelled shoot apex in green dwarf pea plants but not in etiolated plants. The light conditions to which the plants were exposed during growth and transport had little effect on the rates of uptake of IAA from the applied solutions. The results indicate that the velocity of auxin transport is independent of the frequency of cell-to-cell interfaces along the transport pathway and it is suggested that in intact plants auxin transport is entirely symplastic.     

Auxin transport in roots

Summary The reasons underlying the initial increase and subsequent decrease in the amount of radioactivity in the receiver block at the apical end of a Zea root segment supplied with a basal donor block containing labelled IAA have been investigated.The phenomenon was observed in segments supplied with IAA-1-¹⁴C, IAA-2-¹⁴C and IAA-5-³H. An acropetal polarity in the movement of radioactivity into the receiver blocks was observed using donor blocks containing IAA-5-³H at concentrations as low as 10^-10M.The decrease in the amount of radioactivity in the receiver block begins after 6–8 h of transport at 25° C, and is unaffected by renewal of the donor block every 2 h, or the presence of 2% sucrose in the donor and receiver blocks.The net export of radioactivity into the receiver block at the apical end of the segment virtually ceases after 6–8 h of transport at 25° C, and is not prolonged by the presence of 2% sucrose in the donor and receiver blocks. At 10° C, net export of radioactivity continues for at least the first 50 h of transport, and the amount of radioactivity in a continuously applied receiver block continues to increase over this period.Receiver blocks removed from the apical end of segments after 8 h of transport and placed on planchettes show little or no decrease in the amount of radioactivity they contain as a function of time, in marked contrast to those left in contact with the segment.There is a marked, and metabolically dependent, resorption of radioactivity from the receiver block at the apical end of the segment after about 8 h of transport at 25° C; most of the resorbed radioactivity remains in the apical 2–4 mm of the segment.There is a loss of radioactive CO₂ from segments supplied with a basal donor block containing 10^-6M IAA-1-¹⁴C at 25° C, the emission beginning after 6–8 h of transport. Segments similarly supplied with 10^-6M IAA-2-¹⁴C did not begin to lose radioactive CO₂ until after about 10–12 h of transport.The ability of the segments to transport radioactivity in a polar manner declines with time after they are excised from the root, regardless of whether their cut ends are kept in the intervening period in contact with plain agar blocks, or ones containing unlabelled IAA at 10^-6M. By the 6th h after excision at 25° C no transport of radioactivity through the segments and into the receiver blocks could be detected in either the aropetal or basipetal direction.The decrease in radioactivity in the receiver block after transport periods of 6–8 h at 25° C is therefore due to (1) a cessation of net export of radioactivity into the block, and (2) the onset of a metabolically-dependent, net resorption of radioactivity. At this time substantial amounts of radioactive CO₂ begin to be evolved from segments supplied with IAA-1-¹⁴C, whereas with IAA-2-¹⁴C radioactive CO₂ is not evolved for a further 4–6 h.     

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     

Abscisic Acid Action on Basipetal Auxin Transport

Several experiments have been performed to analyse the ABA effects on the basipetal transport of IAA-2-¹⁴C, using sections of epicotyls prepared from etiolated Lens seedlings. The sections were incubated in an ABA solution or ABA was applied in the donor blocks containing IAA. For each type of assay, the uptake (analyses of the donor blocks) and the movement of IAA-C¹⁴ (analyses of the receiver blocks) were inhibited by ABA. The distribution of continuous decrease of the radioactivity, along the sections' axis, showed a ¹⁴C level from the apical towards the basal segments. ABA caused a decrease in the ¹⁴C concentration for the total sections, but a relative increase for the basal segment. When ABA was applied simultaneously with IAA in the donor blocks, the transport velocity of IAA, through the sections, was not changed significantly, while an ABA pretreatment caused a significant decrease.     

Auxin transport in roots

Summary The net uptake and movement of radioactivity by 12-mm root segments of Zea mays have been studied as a function of time at 5, 15 and 25° C. Segments were supplied with an agar donor block containing 1 M IAA-1-¹⁴C or IAA-2-¹⁴C continuously or for a limited period of time (pulse-labelling). In the latter case the original donor block was replaced either by a plain agar block or by one containing 1 M unlabelled IAA. Receiver blocks were placed at the other end of the segments.The net uptake of radioactivity from the donor block at 15° C was greater at the basal end than at the apical end of the segment. At 5 and 15° C, the net uptake from a basal donor was virtually linear with time but at 25° C the rate of net accumulation decreased after about 10 h. Decarboxylation of IAA undoubtedly occurred at 15 and 25° C when the concentration in the tissue attained a high value.An acropetally polarised movement of radioactivity into the receiver blocks occurred regardless of whether the results were based on the actual amounts of radioactivity in the receiver block, or on the amounts in the receiver block expressed as a percentage of the net total radioactivity accumulated from the donor block. Only one radioactive substance was present in the receiver block and it ran to the same R_f as IAA in the isopropanol: ammonium: water solvent system.The amounts of radioactivity moving into that part of the root segment at least 6 mm distant from the end in contact with either an apical or a basal donor block were assessed. An acropetal polarity in the movement of radioactivity was observed on the basis of the actual amounts of radioactivity in these distal parts of the segments, but no such polarity was evident when the amounts of radioactivity were expressed as a percentage of the net total accumulated from the donor block. At least 3 radioactive substances were present in the tissue in addition to the substance running to the same R_f as IAA. The distribution of radioactivity in the segment cannot therefore be used to assess the distribution of IAA.Acropetal movement of radioactivity into an apical receiver block is not dependent upon the continued uptake of IAA at the basal end of the segment. No distinct pulses of radioactivity were detected moving through the root segments.Only a small part of the radioactivity in the root segment appears to be located in the polar transport system, while the bulk is not. The polarity found in the movement of the bulk radioactivity within the segment seems to be related to the polarity in IAA uptake from the donor blocks.     

The Effect of Morphactin (Methyl 2-Chloro-9-hydroxyfluorene-9-carboxylate) on Basipetal Transport of Indol-3-ylacetic Acid in Hypocotyl Sections of Phaseolus vulgaris L.

The effect of morphactin (methyl 2-chloro-9-hydroxyfluorene-9-carboxylate)on basipetal transport of auxin (Indol-3-ylacetic acid-2-¹⁴C)was studied in bean (Phaseolus vulgaris) hypocotyl with thedonor-receiver block method. Morphactin (5 x 10^–6m) reduced IAA (5 x 10^–6m) transportintensity by an average of 83 per cent and auxin transport capacityby 90 per cent, but transport velocity was not affected. Morphactin did not inhibit uptake of IAA into hypocotyl tissue,but it did prevent transfer of IAA from the tissue into receiverblocks. Chromatographic analysis of the tissue after 4 h IAA-2-¹⁴Ctransport showed that 54 per cent of the total activity wasin the form of IAA in the control and 42 per cent in the morphactintreated tissue. No difference was found in the rate of decarboxylationof IAA-1-¹⁴C between control and morphactin treated tissue sections.Nor could any difference between control and morphactin be shownin the radioactivity associated with a TCA ppt fraction. Ina study of the transportable auxin pool, morphactin decreasedthe size of the pool and increased the half-life of decay ofauxin transport from 1•22 h to 3•85 h. In a kineticanalysis of the reversal of morphactin (5 x 10^–6m) inhibitionby increasing concentration of IAA-2-14C (5 x 10^–6m to2 x 10^–5m), it was shown that IAA transport resemblesMichaelis-Menten enzyme reaction kinetics, and that inhibitionby morphactin fitted a ‘mixed type’ model. IAA hada dissociation constant of 8•5 x 10^–6m and morphactinthat of 4•3 x 10^–7m with a K_m for the transport processof 8•5 x 10^–6m.     

Decarboxylation and Transport of Auxin in Segments of Sunflower and Cabbage Roots. II. A Chromatographic Study Using IAA-1-14C and IAA-5-3H

The polar movement of IAA has been examined in 5-mm root segments of Brassica oleracea and Helianthus annum. The movement was studied partly with IAA-1-¹⁴C and partly with IAA-5-³H. In both plants a slight acropetal flux of ¹⁴C and IAA-³H was found through the segments. The recovered radioactivity in the agar receiver blocks and in the receiver end of the segments increased as a function of time. A large portion of the applied IAA was converted on the cut surfaces and in the tissues of the segments. Chromatographic analysis indicated different destruction products when estimated by scintillation counting and by spraying with in-dole reagent (DMCA). Chromatograms run in isopropanol: ammonia: water, 8:1:1, yielded three different substances, one spot near the starting line and one near the front, neither of which has been identified. Finally there was a spot with R_f 0.4–0.6, probably representing IAA.     

Effects of phenolic substances on metabolism of exogenous indole-3-acetic acid in maize stems

Four-day-old stem segments of Zea mays L. cv. Seneca 60 were treated sequentially with phenolic substances and indole-3-acetic [2-¹⁴C] acid ([2-¹⁴C]IAA). Formation of bound IAA was rapid, but a pretreatment with p-coumaric acid, ferulic acid or 4-methylumbelliferone decreased the level of bound IAA. The decrease is not likely related to the effect of the phenolics on enzymic oxidation of IAA, since the level of free IAA was not limiting and the activity of ferulic acid in enzymic oxidation of IAA is different from that of p-coumaric acid and 4-methylum-belliferone. Apparently these compounds inhibited the formation of bound IAA and consequently caused an accumulation of free IAA. In contrast, caffeic acid, protocatechuic acid and 2,3-dihydro-2, 2-dimethyl-7-benzofuranol had little effect. After the uptake of IAA there was a slow but steady incorporation of the radioactivity into the 80% ethanol-insoluble, 1 M NaOH-soluble fraction. Phenolic substances also affected this process. The compounds which are cofactors of IAA-oxidase increased the incorporation while those which are inhibitors of IAA-oxidase decreased the incorporation. Results suggest that the phenolics also affected the enzymic oxidation of IAA in vivo in the same way as in vitro.     

Influence of Gamma Radiation on the Transport of Indolyl-3-Acetic Acid in Bark Tissue from Apple Trees

The influence of gamma radiation on the polarity of IAA translocation was investigated. Pieces of apple tree shoots, taken from 3-year-old trees, were irradiated in a dark irradiation chamber BK-⁶⁰Co source, 10 kCi. Doses from 20 to 150 krad were given. Strips of bark were peeled off both irradiated and control shoots. The middle parts of the bark strips were placed on agar blocks loaded with IAA-1-¹⁴C. The radioactivity of basal and acropetal segments was determined after about 20 h and results expressed as polarity quotient. It has been shown that irradiation with 20 krad (200 Gy) decreases the polarity quotient remarkably (polarity quotient of control equals 7; for 20 krad—2.9; 100 krad—1.6). The decrease in polarity results from inhibition of the basipetal translocation of IAA. Irradiated tissue shows a decrease in IAA uptake. Possible mechanisms of gamma irradiation effects on polar translocation of IAA are discussed.     

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.     

Studies on soluble RNA binding indoleacetic acid in etiolated mung bean hypocotyl sections

Nucleic acid was extracted by the SLS-phenol method from Phaseolusaureus hypocotyl treated with IAA-2-¹⁴C. Radioactivity in thenucleic acid fraction was found at the positions of sRNA andrRNA on an MAK column. IAA-¹⁴C was released from the radioactivecompound(s) in the sRNA fraction, by alkaline hydrolysis, butnot by ethanol extraction, or by dialysis to 2 M NaCl, 8 M urea,and 0.1 M EDTA. When the radioactive compound at the positionof sRNA on an MAK column was further re-chromatographed on aDEAE-cellulose column and on a BD-cellulose column, it was alwayslocalized only in a settled part of the fraction of each column.From this fraction IAA-¹⁴C was released by alkaline hydrolysis.Also, IAA-¹⁴C was released from the radioactive compound insRNA fraction, by RNase digestion, but not by pronase treatment.Results of these experiments suggest the existence of some kindsof sRNA binding IAA. The genesis of this sRNA binding IAA-¹⁴Cwas observed within 30 min after the supply of IAA-¹⁴C, andthe sRNA became saturated with IAA-¹⁴C about 2 hr after thebeginning of incubation. The behavior of sRNA binding IAA, representedby sRNA binding IAA-¹⁴C, may have a role in IAA induced growthof mung bean hypocotyl sections. (Received July 6, 1971; )     

Distribution of labeled auxin in geotropically stimulated steins of cucumber and pea

The distribution of IAA-2-¹⁴C or IAA-5-³H applied to the apexin the upper and lower (with respect to gravity) halves of geotropicallystimulated stems of cucumber and pea was examined and the resultsobtained are as follows: 1. A larger amount of IAA-2-¹⁴C or IAA-5-³H was detected inthe lower than upper half of cucumber hypocotyls with 3-hr geostimulation. 2. A larger amount of IAA-2-¹⁴C was distributed in the lowerthan upper epidermis of pea epicotyls with 1-hr geostimulation. 3. Freezing autoradiography revealed that IAA-2-¹⁴C was concentratedin the vascular bundles and epidermis of cucumber hypocotyls,the distribution being affected by geostimulation only in thelatter. 4. Application of 1% TIBA in lanolin inhibited the distributionof IAA-2-¹⁴C in the lower epidermis of pea epicotyls, causingsuppression of geotropic curvature. 5. From these results, we concluded that IAA which accumulatedin the lower epidermis of the stem upon geostimulation causedthe negative geotropic curvature of the stem. (Received October 13, 1975; )     

Metabolism of tyramine and feruloyltyramine in TMV inoculated leaves of Nicotiana tabacum

TMV inoculation is known to stimulate tyramine N-feruloyl-CoA transferase activity in Nicotiana tabacum cv Xanthi n.c. leaves during the hypersensitive reaction. When [2-¹⁴C]-tyramine is fed for 2 hr to TMV inoculated leaf discs or detached leaves, ca 1 % of the supplied radioactivity is integrated into cinnamoyl-, p-coumaroyl- and feruloyltyramine and up to 14 % is integrated into the cell wall residue. [2-¹⁴C]-tyramine can only be partially released from this residue by acid hydrolysis. After nitrobenzene oxidation, 97 % of the radioactivity found in the cell walls is made soluble but only 13 % is recovered in p-hydroxybenzaldehyde. Feruloyltyramine is very rapidly metabolised, ca 20 % of the administrated radioactivity is found after 2 hr feeding in unindentified methanoi soluble metabolites. Acid hydrolysis of the cell wall fraction, which hydrolyses the amide bond of feruloyltyramine, releases labelled tyramine, while radioactivity is still detected in the acid insoluble residue. Label from [¹⁴C]-feruloyltyramine is integrated into this residue more quickly than from free [2-¹⁴C]-tyramine.     

Localization and identification of auxin in roots of Zea mays

Summary Roots of 3.5-day-old seedlings of Zea mays cv. Giant White Horsetooth contain an extractable auxin which has chromatographic properties and reactions to chromogenic sprays identical with those of indole-3-acetic acid (IAA). By separating stele from cortex (and root tips) before extraction it was shown that the auxin is localized predominantly in the stele, with little being found in the cortex. Whole roots, isolated cortices and isolated steles accumulate and metabolize exogenously applied IAA-1-¹⁴C. The stelar tissue is distinguished from whole roots and cortical tissue in having a different pattern of IAA metabolism.     

Auxin Transport in Tendril Segments of Passiflora caerulea

The movement of auxin through tendril segments of Passiflora caerulca L. has been investigated using IAA-2-¹⁴C. It has been shown that (1) flux of IAA through the segments is strongly polarized basipetally: (2) the amount of ¹⁴C recovered in the basal receiver blocks increases linearly within a transport period of 6 h; (3) velocity of basipetal transport is 14.5 mm h^?1; (4) at least 70% of the radioactivity in the receiver blocks is confined to the IAA molecule: approximately 55% of ¹⁴C from methanolic extracts of the segments is IAA: (5) at low temperatures (2–4°C) the basipetal transport is abolished; (6) white light promotes basipetal transport, and this effect is abolished in a CO₂-free atmosphere; (7) no difference could be detected in ¹⁴C content between dorsal and ventral halves of tendril segments nor among individual dorsal and ventral receiver blocks.     

The Relationship between Growth and Proline Incorporation after Auxin Treatment of Mung Bean Hypocotyls

Indoleacetic acid (IAA) stimulates the incorporation of ¹⁴C-proline into both the cyloplasmic and the cell wall fractions of the hypocotyl of mung bean (Phaseolus aureus Roxb. cv. Black). It neither stimulates the transfer of ¹⁴C-proline from the cyloplasmic fraction into the cell wall fraction, nor the retention of ¹⁴C-proline in the wall or cytoplasmic fractions. Moreover, the stimulation of growth caused by IAA parallels the stimulation of the incorporation of proline into the cytoplasmic fraction, but does not parallel the stimulation into the cell wall fractions. The stimulation of the incorporation into the cyloplasmic fraction seems to appear within 30 minutes after auxin treatment, at about the same time the increase in the growth is observed in response to IAA, suggesting a connection between these effects. On the other hand, the stimulation of the proline incorporation into the cell wall fraction seems to require more than 90 minutes after auxin treatment, suggesting no close connection between growth and proline incorporation into the cell wall fraction.     

Effect of ethylene on [C]indole-3-acetic Acid metabolism in leaf tissues of woody plants

The effect of ethylene on [¹⁴C]indole-3-acetic acid (IAA) metabolism was investigated in defoliation sensitive leaf tissues of citrus (Citrus sinensis) and resistant leaf tissues of eucalyptus (Eucalyptus camaldulensis). IAA metabolites were fractionated into 80% ethanol-soluble, H₂O-soluble, NaOH-soluble, and insoluble components. In citrus, pretreatment with 25 microliters per liter ethylene for 24 hours significantly increased the amount of ethanol- and H₂O-extractable conjugates during the first hour of incubation in [¹⁴C]IAA and increased 3- to 4-fold the formation of NaOH-extractable conjugates during the entire 6-hour incubation period. However, induction of the IAA-aspartate conjugation system was inhibited by ethylene. In eucalyptus, ethylene pretreatment only slightly stimulated the formation of IAA metabolites. Increased formation of ethanol-extractable conjugates in ethylene-pretreated eucalyptus tissues was observed only after 6 hours of incubation. Chromatographic analysis indicated that the ethanol and H₂O extracts of both species contained various low molecular weight conjugates, whereas in citrus leaf tissues high molecular weight conjugates accounted for most of the greater radioactivity detected in the NaOH extracts as a result of ethylene-pretreatment. It is suggested that ethylene may reduce the level of endogenous IAA in citrus leaf tissues by stimulating IAA conjugation.     

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