Studies on the movement of indole auxins in willow (Salix viminalis L.)

Summary Auxin activity was detected in honeydew obtained from the aphid Tuberolachnus salignus (Gmelin) feeding on willow (Salix viminalis). Active uptake of ¹⁴C-indolyl-3-acetic acid (IAA) into the sieve tubes was demonstrated by irrigating the cambial surface of willow bark with ¹⁴C-IAA solution and assaying aphid stylet exudate. When, however, ¹⁴C-IAA was applied to the peridermal tissues of the bark or to a mature leaf most of the radioactivity (collected in honeydew or stylet exudate) co-chromatographed with indolyl-3-acetyl-aspartic acid (IAAsp). The presence of IAAsp in honeydew was not affected by extraction procedure or by aphid metabolism. Honeydew obtained from willow treated with ¹⁴C-tryptophan contained only ¹⁴C-tryptophan. When ¹⁴C-IAA was applied in agar to the cut end of willow segments the radioactivity was found to move in a basipetally polar manner. The direction of movement of radioactivity in the sieve tubes, however, was found to be influenced by the proximity of the roots. Nevertheless, there was evidence that endogenous auxin in the sieve tubes does move in a predominantly basipetal direction.     

Transport of 14C-IAA in Cucurbita maxima: A Comparative Study of Rapid, Non-Polar Movement and Slow, Polar Auxin Transport

The transport of ¹⁴C-IAA has been studied in Cucurbita maxima.IAA fed to the leaf of an intact plant moves rapidly in a non-polarfashion in the phloem. Collection and analysis of exudate fromsevered sieve tubes showed that there was no metabolic conversionor complexing of IAA for several hours. Polar movement of ¹⁴C-IAA in isolated internode segments occursat rates an order of magnitude slower than movement in the phloem.The importance of discrete and isolated channels of hormonetransport, that vary in direction and rates, is briefly discussed.     

Free tryptophan and indole-3-acetic acid levels in the leaves and vascular pathways of Ricinus communis L.

Levels of free tryptophan in the leaves, phloem and xylem saps of Ricinus communis L. were determined by colorimetric assay. Values of 0.38 g ml^-1 in root pressure sap and 96.0 g ml^-1 in phloem sap were recorded. Tryptophan levels were highest in mature and senescing leaves. Levels of indoleacetic acid (IAA) in the phloem sap and leaves were determined by gas chromatography—mass spectrometry using a deuterated internal standard. A mean value of 13.0 ng ml^-1 was recorded in phloem sap. The distribution in the leaves showed an inverse relationship to that of tryptophan, being highest in young leaves.Abbreviations IAA indoleacetic acid - GC-MS Gas chromatography-mass spectrometry - PFP-derivative pentafluoropropionyl-derivative - TLC thin layer chromatography     

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.     

Phloem translocation of gibberellins in three species of higher plants

Phloem sap was collected from white lupin (Lupinus albus L.), cowpea (Vigna unguiculata L.) and castor bean (Ricinus communis L.) and analysed for gibberellins (GAs) using gas chromatography-mass spectrometry (GC-MS). A large number of GAs were found in the phloem exudate of all three species, particularly where the sap was collected from pods (white lupin and cowpea) and in these legumes GAs representing both the early C-13-hydroxylation and non-hydroxylation pathways of biosynthesis were identified. In the sap collected from the vegetative tissues of castor bean the number of GAs identified was fewer than that in the other species, representing mainly the non-hydroxylation pathway. Data from sap collected from the pedicel and stylar ends of pods and by making feeds of radiolabelled GAs to seeds in situ in white lupin indicate that the GAs present in the phloem are derived mainly from the vegetative tissues of the plant. No evidence for metabolism of GAs in the phloem could be found.     

Phloem loading in peach: Symplastic or apoplastic?

Sorbitol and sucrose are the two main soluble carbohydrates in mature peach leaves. Both are translocated in the phloem, in peach as in other rosaceous trees. The respective role of these two soluble carbohydrates in the leaf carbon budget, and their phloem loading pathway, remain poorly documented. Though many studies have been carried out on the compartmentation and export of sucrose in sucrose-transporting species, far less is known about sorbitol in species transporting both sucrose and sorbitol. Sorbitol and sucrose concentrations were measured in several tissues and in sap, in 2-month-old peach (Prunus persica L. Batsch) seedlings, i.e. leaf blade, leaf main vein, petiole, xylem sap collected using a pressure bomb, and phloem sap collected by aphid stylets. The sorbitol to sucrose molar ratio depended on the tissue or sap, the highest value (about 7) found in the leaf main vein. Sorbitol concentration in the phloem sap was about 560 mM, whereas that of sucrose was about 140 mM. The lowest sorbitol and sucrose concentrations were observed in xylem sap collected from the shoot. The volume of the leaf apoplast, estimated by infiltration with ³H-inulin, represented about 17% of the leaf blade water content. This volume was used to calculate a global intracellular concentration for each carbohydrate in the leaf blade. Following these simplifying assumptions, the calculated concentration gradient between the leaf's intracellular compartment and phloem sap is nil for sorbitol and could thus allow for the symplastic loading of the phloem of this alditol. However, infiltration of ¹⁴C-labelled source leaves with 2 mMp-chloromercuribenzenesulfonic acid (PC-MBS), a potent inhibitor of the sucrose carrier responsible for phloem loading in sucrose-transporting plants, had a significant effect on the exudation of both labelled sucrose and sorbitol from the phloem. Therefore, in peach, which is a putative symplastic loader according to minor vein anatomy and sorbitol concentration gradients, apoplastic loading may predominate.     

TRANSPORT OF IAA AND ACC IN FLORAL ORGANS OF IPOMOEA NIL (CONVOLVULACEAE)

The involvement of the stamens as transporters of plant growth regulators in flowers was examined by measuring the movement of ¹⁴C-indole-3-acetic acid (IAA) and ^l4C-l-aminocyclopropane-1-carboxylic acid (ACC) through floral organs of Ipomoea nil. During the transport of ¹⁴C-IAA through isolated filament segments, the polar accumulation of ¹⁴C-IAA in receiver blocks increased with time during filament development, which correlated with polar efflux rates at older stages of filament development. An inhibitor of polar IAA transport, 2,3,5-triiodobenzoic acid, disrupted the polarity of auxin transport by reducing the movement of ¹⁴C- IAA from filaments into receiver blocks. Transport of both ¹⁴C-IAA and ^l4C-ACC through filaments into other floral organs also was monitored in isolated flower buds in the laboratory and intact buds in the greenhouse. In isolated and intact buds 21 hr before anthesis, substantially higher levels of isotope were recovered in corolla tissue when ¹⁴C-ACC was transported through the filaments than when ¹⁴C-IAA was transported from the filaments. In isolated buds, substantial levels of both isotopes accumulated in the pistil (69 hr and 45 hr before anthesis), but minimal amounts were observed in receptacle and calyx tissues (69 hr to 21 hr before anthesis). In intact buds, high levels of both isotopes were recovered in receptacle, calyx, and pistil tissues (69 hr to 21 hr before anthesis). The results from this study support the hypothesis that Ipomoea stamens are transporters for ACC and IAA to regulate ethylene production in the corolla and other floral tissues.     

Changes in the amino acid composition and protein modification in phloem sap of rice

Pure phloem sap was collected from insects feeding on rice (Oryza sativa L.) leaves by a laser technique similar to the aphid stylet technique. Rapid circulation of nitrogen in the sieve tubes was demonstrated directly using ¹⁵N as a tracer. Application to the roots of the metabolic inhibitors of amino acids, aminooxyacetate and methioninesulfoximine, changed the amino acid composition in the sieve tubes. Feeding methionine to leaf tips resulted in its bulk transfer into the sieve tubes. In vitro experiments confirmed the existence of protein kinases in the pure rice phloem sap. The phosphorylation status of the sieve tube sap proteins was affected by the light regime. The possibility that changes in chemical composition or protein modification such as phosphorylation in the sieve tubes might affect plant growth are discussed.Analysis of pure phloem sap collected from rice plants by insect laser technique has shown dynamic changes in the chemical composition and the quality of proteins in the sap.     

The extrafloral nectaries of cowpea (Vigna unguiculata (L.) Walp.) II. Nectar composition,origin of nectar solutes,and nectary functioning

Nectar was collected from the extrafloral nectaries of leaf stipels and inflorescence stalks, and phloem sap from cryopunctured fruits of cowpea plants. Daily sugar losses as nectar were equivalent to only 0.1–2% of the plant's current net photosynthate, and were maximal in the fourth week after anthesis. Sucrose:glucose:fructose weight ratios of nectar varied from 1.5:1:1 to 0.5:1:1, whereas over 95% of phloem-sap sugar was sucrose. [¹⁴C]Sucrose fed to leaves was translocated as such to nectaries, where it was partly inverted to [¹⁴C]glucose and [¹⁴C]fructose prior to or during nectar secretion. Invertase (EC 3.2.1.26) activity was demonstrated for inflorescence-stalk nectar but not stipel nectar. The nectar invertase was largely associated with secretory cells that are extruded into the nectar during nectary functioning, and was active only after osmotic disruption of these cells upon dilution of the nectar. The nectar invertase functioned optimally (phloem-sap sucrose as substrate) at pH 5.5, with a starting sucrose concentration of 15% (w/v). Stipel nectar was much lower in amino compounds relative to sugars (0.08–0.17 mg g^-1 total sugar) than inflorescence nectar (22–30 mg g^-1) or phloem sap (81–162 mg g^-1). The two classes of nectar and phloem sap also differed noticeably in their complements of organic acids. Xylem feeding to leaves of a range of ¹⁴C-labelled nitrogenous solutes resulted in these substrates and their metabolic products appearing in fruit-phloem sap and adjacent inflorescence-stalk nectar. ¹⁴C-labelled asparagine, valine and histidine transferred freely into phloem and appeared still largely as such in nectar. ¹⁴C-labelled glycine, serine, arginine and aspartic acid showed limited direct access to phloem and nectar, although labelled metabolic products were transferred and secreted. The ureide allantoin was present in phloem, but absent from both types of nectar. Models of nectary functioning are proposed.     

Interactions between Plants and Epiphytic Bacteria Regarding Their Auxin Metabolism

Epiphytic, IAA-producing bacteria strains were fed with ¹⁴C-tryptophan (Try). ¹⁴C-Try absorption and, after transfer to a Try-free medium, ¹⁴C-IAA output were stated. Using 4 different methods, the ¹⁴C-Try containing bacteria were applied to the tips of sterile corn coleoptiles and the ‘diffusible’ auxin collected at the coleoptile bases by means of agar blocks. ¹⁴C-IAA was detected in the agar blocks. Sterile coleoptiles the tips of which were wupplied with ¹⁴C-Try also deliver some ¹⁴C-IAA at their bases, but much less than both sterile coleoptiles supplied with ¹⁴C-Try-containing bacteria and nonsterile supplied with ¹⁴C-Try.     

Auxin biosynthesis and metabolism in isolated roots of Zea mays

The synthesis and metabolism of indole-3-acetic acid (IAA) was investigated in isolated roots of corn, Zea mays L. Roots were cultured aseptically in media supplemented with either ¹⁴C-tryptophan or ¹⁴C-IAA. Exogenously supplied IAA is rapidly and completely metabolized by root tissues. The main site in the root for the synthesis of IAA is in the apex. Removal of either the root cap or the quiescent center, or the root cap and the quiescent center from the apex has no effect on the IAA-synthesizing ability of the apex. Subdividing the terminal 2.1 cm of the root into various segments and culturing them separately stimulates IAA synthesis in these isolated root tissues. Roots in culture maintain relatively constant IAA levels, reflecting the precise controls of the level of this hormone.     

Determinations of indole-3-acetic acid in xylem sap of Ricinus communis L. using mass fragmentography

Mass fragmentography employing a deuterated internal standard was used to make quantitative analyses of indole-3-acetic acid in xylem sap collected from Ricinus communis L. When contamination of the sap by microorganisms was reduced by frequent collection, levels of IAA were found to be less than 0.5 ng ml^-1. It is therefore proposed that the transpiration stream does not play a significant role in the transport of IAA within the plant.Abbreviations IAA indole-3-acetic acid - GC-MS gas chromatography-mass spectrometry - BSTFA bis-Trimethylsilytrifluoroacetamide - TMCS trimethylchlorosilane - BSA bis-Trimethylsilylacetamide - TMS₂-IAA bis-trimethylsilyl derivative of IAA     

Uptake and Utilization of Xylem-borne Amino Compounds by Shoot Organs of a Legume

Amino compounds representative of the major N solutes of xylem sap were pulse-fed (10 to 20 minutes) singly in ¹⁴C-labeled form to cut transpiring shoots of white lupin (Lupinus albus L.). ¹⁴C distribution was studied by autoradiography and radioassays of phloem sap, leaflet tissues, and shoot parts harvested at intervals after labeling. Primary distribution of N by xylem was simulated using a 20-minute labeling pulse followed by a 30-minute chase in unlabeled xylem sap. Shoots fed ¹⁴C-labeled asparagine, glutamine, valine, serine, or arginine showed intense labeling of leaflet veins and marked retention (35 to 78%) of ¹⁴C by stem + petioles. Shoots fed ¹⁴C-labeled aspartic acid or glutamic acid showed heaviest ¹⁴C accumulation in interveinal regions of leaflets and low uptake (11 to 20%) of ¹⁴C by stem + petioles. Departing leaf traces were major sites of uptake of all amino compounds, and the implications of this were evaluated. Fruits acquired only 1 to 5% of the fed label directly from xylem, but more than doubled their intake during the period 30 to 160 minutes after feeding through receipt of ¹⁴C transferred from xylem to phloem in stem and leaves. ¹⁴C-Labeled asparagine and valine transferred directly from xylem to phloem, but the ¹⁴C of ¹⁴C-labeled aspartic acid and arginine appeared in phloem mainly as metabolic products of the fed compound. The labeling of the soluble pool of leaflets reflected these differences. The significance of heterogeneity in distribution and metabolism of xylem amino compounds in the shoot was discussed.     

Adventitious root formation in relation to irradiance and auxin supply

High irradiance during treatment of mung bean cuttings favours root formation in response to supplied auxin, whether the latter is IAA or IBA. On the other hand it is inhibitory towards root formation in the absence of supplied auxin. Light promotes the uptake of¹⁴C-IAA into cuttings and its upward movement into the leaves. When¹⁴C-IAA is applied to leaves of cuttings high irradiance favours movement of radioactivity into the epicotyl and hypocotyl. This movement is also enhanced by concomitant supply of IBA to the base of the cuttings. The irradiance under which stock plants are raised also affects the extent of root formation on cuttings. When cuttings are held in darkness without a supply of exogenous auxin they root best if prepared from seedlings raised under high irradiance. However, transport of¹⁴C-IAA out of leaves of cuttings is favoured when cuttings are prepared from seedlings grown under low irradiance. These observations are discussed in relation to auxin transport, photodestruction and, possibly, metabolism.     

Long-distance translocation of polyamines in phloem and xylem of Ricinus communis L. plants

Polyamine content and enzyme activities in the biosynthetic and degradative pathways of polyamine metabolism were investigated in sieve-tube sap, xylem sap and tissues of seedlings and adult plants of Ricinus communis L. Polyamines were present in tissues and translocation fluids of both seedlings and adult plants in relatively high amounts. Only free polyamines were translocated through the plant, as indicated by the finding that only the free form was detected in the phloem and the xylem sap. Removal of the endosperm increased the polyamine content in the sieve-tube exudate of seedlings. The level and pattern of polyamines in tissue of adult leaves changed during leaf age, but not, however, in the sieve-tube sap. Xylem sap was relatively poor in polyamines. Polyamine loading in the phloem was demonstrated by incubating cotyledons with [¹⁴O]putrescine and several unlabelled polyamines. Feeding cotyledons with cadaverine and spermidine led to a decrease in the level of putrescine in sieve-tube sap, indicating a competitive effect. Comparison of polyamine content in the tissue and export rate showed that the export would deplete the leaves of polyamines within 1–3 d, if they were not replenished by biosynthesis. Polyamine biosynthesis in Ricinus proceeds mostly via arginine decarboxylase, which in vitro is 100-fold more active than ornithine decarboxylase. The highest arginine decarboxylase, ornithine decarboxylase and diamine oxidase activities were detected in cotyledons, while in sieve-tube sap only a slight arginine decarboxylase activity was found. Received: 18 March 1997 / Accepted: 20 August 1997     

Autoradiographic and biochemical evidence for the systemic translocation of systemin in tomato plants

The movement of systemin, the 18-amino-acid polypeptide inducer of proteinase inhibitors in tomato (Lycopersicon esculentum L.) plants, was investigated in young tomato plants following the application of [¹⁴C]systemin to wounds on the surface of leaves. Wholeleaf autoradiographic analyses revealed that [¹⁴C]systemin was distributed throughout the wounded leaf within 30 min, and then during the next several hours was transported to the petiole, to the main stem, and to the upper leaves. The movement of [¹⁴C]systemin was similar to the movement of [¹⁴C]sucrose when applied to leaf wounds, except that sucrose was slightly more mobile than systemin. Analyses of the radioactivity in the petiole phloem exudates at intervals over a 5-h period following the application of [¹⁴C]systemin to a wound demonstrated that intact [¹⁴C]systemin was present in the phloem over the entire time, indicating that the polypeptide was either stable for long periods in the phloem or was being continually loaded into the phloem from the source leaf. The translocation pathway of systemin was also investigated at the cellular level, using light microscopy and autoradiography. Within 15 min after application of [³H]systemin to a wound on a terminal leaflet, it was found distributed throughout the wounded leaf and was primarily concentrated in the xylem and phloem tissues within the leaf veins. After 30 min, the radioactivity was found mainly associated with vascular strands of phloem tissue in the petiole and, at 90 min, label was found in the phloem of the main stem. Altogether, these and previous results support a role for systemin as a systemic wound signal in tomato plants.The authors acknowledge the Washington State University Electron Microscope Center and staff for their technical advice and collaboration. We also thank Greg Wichelns for growing our plants and Dr. Steven Doares for providing [³H]systemin. This research was supported in part by the Washington State College of Agriculture and Home Economics Project No. 1791 and National Science Foundation grants IBN 9117795 and IBN 9104542     

The transport of radiolabeled indoleacetic acid and its conjugates in nodal stem segments of Phaseolus vulgaris L.

The transport of radiolabeled indoleacetic acid (IAA), and some of its conjugates, was investigated in nodal stem segments of Phaseolus vulgaris L. Donor agar blocks containing either [2-acetyl-¹⁴C]-IAA; [2-acetyl-¹⁴C]-indole-3-acetyl-L-aspartate (IAAsp); [2-acetyl-¹⁴C]-indole-3-acetyl-L-glycine (IAGly); or [2-acetyl-¹⁴C]-indole-3-acetyl-L-alanine (IAAla) were placed on either the apical or basal cut surface of stem segments each bearing an axillary bud at the midline. In some experiments, a receiver block was placed on the end opposite to the donor. After transport was terminated, the segments were divided into five equal sections plus the bud, and the radioactivity of donors, receivers and each part of the stem segment was counted.For all four substances tested, the amount of ¹⁴C transported to the axillary bud from the base was the same or greater than that from the apical end. After basipetal transport, the distribution of ¹⁴C in the segment declined sharply from apex to base. The inverse was true for acropetal transport. Transport for the three IAA conjugates did not differ substantially from each other.The IAA transport inhibitor, N-1-naphthylphthalamic acid (NPA), inhibited basipetal ¹⁴C-IAA transport to the base of the stem segment but did not alter substantially the amount of ¹⁴C-IAA recovered from the bud. Transport of ¹⁴C-IAA from the apical end to all parts of the stem segment declined when the base of the section was treated with nonradioactive IAA. Taken together with data presented in the accompanying article [Tamas et al. (1989) Plant Growth Regul 8: 165–183], these results suggest that the transport of IAA plays a role in axillary bud growth regulation, but its effect does not depend on the accumulation of IAA in the axillary bud itself.     

Translocation from leaves to fruits of a legume,studied by a phloem bleeding technique: Diurnal changes and effects of continuous darkness

Summary Diurnal changes in the carbohydrates of leaf laminae and fruits and in the bleeding of sugar and amino acids from fruit phloem were followed by successive sampling from a population of Lupinus albus L. plants. Phloem sap was collected for a standard 5 min period from cut distal tips of attached fruits. Daily fluctuations in leaf dry matter resulted largely from changes in starch and sugar. Leaf sugar rose to a maximum in the afternoon, starch to a maximum at, or shortly after, dusk. Leaves lost sugar and starch from dusk to dawn. Phloem bleeding rate varied little over a daily cycle but sucrose levels fluctuated from a noon maximum of 12–13% (w/v) to a dawn minimum of 9–10%. The rhythm of phloem sugar levels matched closely those of fruit and leaf. Phloem amino acid levels fluctuated in phase with that of sucrose: the relative composition of the amino fraction did not vary significantly over the daily cycle. Pulse feeding of source leaves with ¹⁴CO₂ at different times in the photoperiod allowed study of the pattern of release of labelled photosynthate to the fruit phloem and the build up and depletion of ¹⁴C starch in leaves. Plants transferred to continuous darkness showed a rapid decline in output and concentration of phloem sap solutes, and translocated nitrogen to their fruits at only one quarter of the rate of control plants retained in natural daylight. The combined data from the experiments showed that the rate of output of sugar from cut phloem of a fruit was directly related to the current level of sugar in leaves. When leaf sugar levels were low (5–10 mg ml tissue water^-1) sugar in phloem was 10–11 times more concentrated than in source leaves, but at high leaf sugar levels (25–30 mg ml^-1) this concentration difference was only 3–4 fold.     

Apoplastic Transport of 14C-Photosynthates Measured under Drought and Nitrogen Supply

Using water infiltration of the plant and individual shoots with the subsequent intercellular liquid extraction by the pressure chamber, dynamics of the movement ¹⁴C-photosynthates from cell to apoplast, and ¹⁴C distribution among photosynthetic products in mesophyll cells and apoplast were studied. The relative quantity of ¹⁴C-photosynthetes in leaf apoplast depended on growing conditions; drought increased, and nitrate supply decreased it. When the middle leaves absorbed ¹⁴CO₂, photosynthates moving down in stem phloem appeared in intercellular space, where they were transported up by transpiration stream. ¹⁴C-photosynthates entering to the apex and young leaves were utilized a accumulated, and photosynthates transported to the mature leaves were reloaded into the phloem and reexported. Thus, photosynthates circulated through the plant and were redistributed to the plant organs according to their transpiration. In leaf apoplast photosynthetic sucrose was partly hydrolyzed to glucose and fructose. This increased under high nitrogen supply. The result indicate that apoplast sucrose hydrolysis is the basic cause of the reduction of photosynthate flux from leaves when the nitrate concentration in soil increases.     

Transport of14C-IAA from leaves and shoots to different fruit parts

Transport of¹⁴C-IAA was studied in apple spurs of a 20-year-old McIntosh with one fruit and one shoot. Water solutions of IAA were applied to intact, pricked or scratched leaf blades, to decapitated shoots or to petioles (leaf-blade removed) at the end of June, July and August.¹⁴C-IAA (in an unknown form) was transported from intact leaves and shoots to pedicel, pericarp and seeds. Radioactivity of the pedicels increased every month while that of seeds reached maximum at the end of July and then markedly decreased in August. Total radioactivity of whole fruit doubled, at least, with every month due to enlargement of the pericarp. Pedicels deprived of fruits had their retention prolonged on spurs with leaves or shoots treated with 1% IAA in lanoline. It is assumed that auxin delivered from shoots or still growing leaves at the time of its deficiency in seeds, restrains fruits from premature dropping. At the same time seeds seem to be protected by a regulatory system in pedicel against too massive flow of auxin from outside.