Characterization of phloem exudation from castor-bean cotyledons

Exudate was collected fromRicinus communis L. cotyledons after cutting the hypocotyl. It contained high levels of sucrose and potassium, a low level of calcium, and a pH of approx. 7.5. After application of [¹⁴C] sucrose to the cotyledons, radioactivity could be recovered from the exudate, indicating that the exudate was derived from the phloem. Using data from a number of individual seedlings, correlations between loading rates of sucrose, translocation rates, and sucrose and potassium contents were analyzed. A positive correlation was found between the rate of sucrose loading and the rate of sucrose exudation, whereas a negative correlation existed between the contents of sucrose and potassium in the phloem.     

Sucrose transport by seedlings of Ricinus communis L: the export of sucrose from the cotyledons to the hypocotyl as a function of sucrose concentration in the cotyledons

Abstract Using seedlings of Ricinus communis L. sucrose export from the cotyledons to the hypocotyl and roots was measured at different levels of sucrose concentration in the cotyledons. Sucrose export followed Michaelis-Menten kinetics with a half-saturation of export at 35 mM sucrose in the cotyledons. A maximal export flux of 90 μmol h^?l g^?1 fresh weight of the cotyledons was obtained. Both these figures coincide with those obtained for sucrose uptake into the cotyledons. It is postulated that sucrose uptake and sucrose export occurs by the same mechanism and possibly by the same cells which then would have to be part of the phloem. Since sucrose uptake has been shown to proceed as proton-sucrose co-transport, phloem loading might also be energized by the protonmotive potential difference. The data, furthermore, are difficult to reconcile with the symplastic route of phloem loading.     

Transport of hexoses by the phloem of Ricinus communis L. seedlings

The sieve-tube sap of Ricinus communis L. seedlings has been analysed to determine whether or not hexoses can be taken up by the phloem. Under natural conditions, i.e. with the endosperm attached to the cotyledons, glucose and fructose occurred only in trace amounts in the sieve-tube sap. Incubation of the cotyledons with hexoses in the concentration range 25–200 mM caused a rapid and substantial uptake of hexoses into the phleom, where they appeared eventually in the sieve-tube sap at the same concentration as in the incubation medium. Phloem loading of glucose, 3-O-methyl-glucose and sorbitol occurred easily, whereas fructose was less well loaded. glucose and to a larger extent fructose were also transformed to sucrose, which was loaded into the phloem. The loading of hexoses into the sieve tubes as observed in the experimental exudation system also occurred in the intact seedling, but transloction in the latter soon came to a standstill, probably because of lack of consumption by the sink tissues. These results indicate that the virtual absence of hexoses in the sievetube sap under in-vivo conditions is not because of the inability of the phloem-loading system to transport the monosaccharides but because of the absence of sufficiently high concentrations in the apoplast.     

The cellular pathway of sucrose transport in developing cotyledons ofVicia faba L. andPhaseolus vulgaris L.: a physiological assessment

The cellular pathway of sugar uptake in developing cotyledons of Vicia faba L. and Phaseolus vulgaris L. seed was evaluated using a physiological approach. The cotyledon interface with the seed coat is characterised by a specialised dermal cell complex. In the case of Vicia faba cotyledons, the epidermal component of the dermal cell complex is composed of transfer cells. Sucrose is the major sugar presented to the outer surface of both cotyledons and it is taken up from the apoplasm unaltered. Estimated sucrose concentrations within the apparent free space of Vicia and Phaseolus cotyledons were 105 and 113 mM respectively. Rates of in-vitro uptake of [¹⁴C]sucrose by cotyledon segments or by whole cotyledons following physical removal or porter inactivation of the outer cells demonstrated that, for both Vicia and Phaseolus cotyledons, the dermal cell complexes are the most intense sites of sucrose uptake. Accumulation of [¹⁴C]sucrose in the storage parenchyma of whole cotyledons was directly affected by experimental manipulation of uptake by the outer cell layers and plasmolytic disruption of the interconnecting plasmodesmata. These findings indicated that sucrose accumulated by the dermal cell complexes is transported symplasmically to the storage parenchyma. Overall, it is concluded that the dermal cell complexes of the developing legume embryo, irrespective of the presence or absence of wall ingrowths, are the major sites for the uptake of sucrose released from the maternal tissues to the seed apoplasm. Thereafter, the accumulated sucrose is transported radially inward through the symplast to the storage parenchyma.Abbreviations AFS apparent free space - CF 5-(6)-carboxyfluorescein - CFDA 5-(6)-carboxyfluorescein diacetate - Mes 2-(N-morpholino)ethanesulfonic acid - PCMBS p-chloromercuribenzenesulfonic acid - SRG sulphorhodamine G The investigation was supported by funds from the Research Management Committee, The University of Newcastle and the Australian Research Council. One of us, R. McDonald, gratefully acknowledges the support of an Australian Postgraduate Research Award. We are grateful to Stella Savoury for preparing the photomicrographs.     

Phloem loading in Ricinus cotyledons: sucrose pathways via the mesophyll and the apoplasm

Ricinus communis cv. Carmencita seedlings with their cotyledons incubated in sucrose solution and their hypocotyls cut to induce exudation of phloem sap, constitute a system of sucrose fluxes into and out of the cotyledons. This system was characterized with respect to quasi-steady-state conditions of sucrose uptake and export and then used to investigate the pathways of sucrose during phloem loading. The redistribution of ¹⁴C-labelled internal sucrose between the three compartments, cotyledons (mesophyll), exudate (sieve tubes) and incubation medium (cell-wall space), was measured in the presence or absence of external nonlabelled sucrose. It was found that mesophyll-derived labelled and external sucrose compete at uptake sites in the apoplasm. On the basis of the specific radioactivity of sucrose which reflects the proportionate intermixture of mesophyll-derived and external sucrose in the three compartments, it was determined that about 50% of the sucrose exported is loaded directly from the apoplasm, while the other half takes the route via the mesophyll. It was confirmed that mesophyll-derived sucrose is released into the apoplasm, so that the existence of an indirect apoplasmic loading pathway is established. Calculations depending on the concentration gradients of labelled and non-labelled sucrose in the cell-wall space are presented to quantify tentatively the proportions of direct and indirect apoplasmic as well as symplasmic loading.This work was supported by the Deutsche Forschungsgemeinschaft (SFB 137). We thank Walter Köckenberger and Ernst Steudle (Bayreuth, FRG) for discussions on the water flow in the exuding Ricinus seedling, and Dietrich Samoray (Bayreuth, FRG) for the conceptual discussions throughout this work.     

Comparison of gibberellin-like substances from cotyledons and phloem exudate collected from cotyledons of Pharbitis nil in relation to photoperiodic flowering

Gibberellin (GA)-like substances were analyzed in extracts from cotyledons and phloem exudate collected from cotyledons in photoinduced and vegetative seedlings of the short-day plant Pharbitis nil Chois. var. Violet, using high performance liquid chromatography (HPLC) and the dwarf rice bioassay, to see whether any specific GA-like substances were transported from the photoinduced cotyledons via phloem. Cotyledon extracts exhibited five peaks of free GA-like activity in HPLC, whereas only one or two active peaks were detected in phloem exudate extracts. The level of free GA-like activity was considerably lower in phloem exudate than in the cotyledons. In five out of six analyses of cotyledons and phloem exudate, there were substantially higher levels of free GA-like substances in photoinduced plants. Conjugated GA-like substances were present in much higher levels than free GA-like substances in the cotyledon extracts but the levels were not influenced by daylength. In phloem exudate extracts there was no conjugated GA-like substances. The free GA-like substances that are transported via phloem cochromatographed with GA_5/20 and GA₁₉ on HPLC. These were significantly higher in photoinduced plants and thus could have some influence on the photoperiodically-induced flowering in P. nil.     

Sulphate uptake and xylem loading of young pea (Pisum sativum L.) seedlings

Sulphate uptake and xylem loading was analysed in young pea (Pisum sativum) seedlings. The rate of sulphate uptake into intact 8-days-old pea seedlings (determined by a 1 h exposure to radiolabelled sulphate in the nutrient solution) was 585 nmol sulphate g^–1 root fresh weight h^–1. When the cotyledons were removed on day 6 the 8-days-old seedlings took up only 7% of the controls. Interruption of the phloem transport by steam girdling of the stem or the root (1 h before incubation with radiolabelled sulphate) diminished sulphate uptake by approximately 50%. The addition of sucrose to the nutrient solution during incubation did not restore sulphate uptake rates indicating that the decrease was not due to a lack of energy. Apparently, a signal from the shoot and/or the cotyledons is necessary to stimulate sulphate uptake into the roots of pea seedlings. Glutathione fed to the roots for 3 h prior to incubation with radiolabelled sulphate diminished sulphate uptake by approximately 50%. The relative proportion of the sulphate taken up that was loaded into the xylem remained unchanged (between 7 and 9% of total uptake), even when the stem was girdled above the cotyledons or when the seedlings were pre-exposed to glutathione. Only removal of the cotyledons or girdling of the root below the cotyledons increased the proportion of sulphate loaded into the xylem to 13–15% of total uptake upon exposure to glutathione. Apparently, a signal from the cotyledons represses xylem loading to some extent.     

A symplasmic flow of sucrose contributes to phloem loading in Ricinus cotyledons

External sucrose, supplied by the endosperm in vivo, is the physiological source of sucrose for Ricinus communis L. seedlings. It is taken up by the cotyledons and exported via the sieve tubes to the growing hypocotyl and root. Two parallel pathways of external sucrose to the sieve tubes, directly via the apoplasm and indirectly after transit through the mesophyll, have already been established (G. Orlich and E. Komor, 1992). In this study, we analysed whether a symplasmic flow of sucrose contributes to phloem loading. Uptake of external sucrose into the mesophyll and into the sieve tubes, and export of total sucrose were measured with intact and exuding seedlings in the presence of p-chloromercuribenzenesulfonic acid (PCMBS). Sucrose uptake into the mesophyll and into the sieve tubes was inhibited by 80–90%. Consequently, export of total sucrose slowed down. However, after the addition of PCMBS, sucrose was transiently exported in such a high amount that could not be accounted for by the residual uptake activity nor by the amount of sucrose confined to the sieve element-companion cell complex (seccc). From the results, we conclude that most of the sucrose exported transiently had moved to the sieve tubes from a symplasmic domain larger than the seccc, comprising at least all the cells of the bundle including the bundle sheath. We suggest that the symplasmic flow of sucrose observed is a mass flow driven by a turgor pressure. As a structural prerequisite for a symplasmic flow, plasmodesmata interconnect all the cells from the bundle sheath to the sieve tubes and also occur between the bundle sheath and the mesophyll. The phloem loading pathway of Ricinus cotyledons can thus be classified as a combination of three different routes. Received: 17 October 1997 / Accepted: 9 March 1998     

Sucrose transport into the phloem of Ricinus communis L. seedlings as measured by the analysis of sieve-tube sap

Careful cutting of the hypocotyl of Ricinus communis L. seedlings led to the exudation of pure sieve-tube sap for 2–3 h. This offered the possibility of testing the phloem-loading system qualitatively and quantitatively by incubating the cotyledons with different solutes of various concentrations to determine whether or not these solutes were loaded into the sieve tubes. The concentration which was achieved by loading and the time course could also be documented. This study concentrated on the loading of sucrose because it is the major naturally translocated sieve-tube compound. The sucrose concentration of sieve-tube sap was approx. 300 mM when the cotyledons were buried in the endosperm. When the cotyledons were excised from the endosperm and incubated in buffer, the sucrose concentration decreased gradually to 80–100 mM. This sucrose level was maintained for several hours by starch breakdown. Incubation of the excised cotyledons in sucrose caused the sucrose concentration in the sieve tubes to rise from 80 to 400 mM, depending on the sucrose concentration in the medium. Thus the sucrose concentration in the sieve tubes could be manipulated over a wide range. The transfer of labelled sucrose to the sieve-tube sap took 10 min; full isotope equilibration was finally reached after 2 h. An increase of K⁺ in the medium or in the sieve tubes did not change the sucrose concentration in the sievetube sap. Similarly the experimentally induced change of sucrose concentration in the sieve tubes did not affect the K⁺ concentration in the exudate. High concentrations of K⁺, however, strongly reduced the flow rate of exudation. Similar results were obtained with Na⁺ (data not shown). The minimum translocation speed in the sieve tubes in vivo was calculated from the growth increment of the seedling to be 1.03 m·h^-1, a value, which on average was also obtained for the exudation system with the endosperm attached. This comparison of the in-vivo rate of phloem transport and the exudation rate from cut hypocotyls indicates that sink control of phloem transport in the seedlings of that particular age was small, if there was any at all, and that the results from the experimental exudation system were probably not falsified by removal of the sink tissues.Abbreviations PTS 3-hydroxy-5,8, 10-pyrenetrisulfonate     

Loading and translocation of various cytokinins in phloem and xylem of the seedlings of Ricinus communis L.

The phloem sap of Ricinus seedlings was analyzed for cytokinins and the concentration was compared with that in cotyledons and xylem sap. The dominant cytokinin in the phloem sap was isopentenyladenine (70 nM) when the endosperm was attached to the cotyledons; zeatin, dihydrozeatin and cytokinin-ribosides were present at relatively low concentrations (1–2 nM). Removal of the endosperm and incubation of the cotyledons in buffer led to a sharp decrease in the level of isopentenyladenine in the phloem sap, down to the value for zeatin, namely 1–2 nM. Similar low cytokinin concentrations were found in the xylem sap, too, whereas in the cotyledons the cytokinin content was at least 10-fold higher. Incubation of the cotyledons with various cytokinins (isopentenyladenine, zeatin and their ribosides) led to an increase of each of the applied cytokinins in the phloem sap, including also the metabolically closely related cytokinins. Zeatin was especially well loaded. It is concluded that the phloem translocates most free bases and ribosides of the various cytokinin species, if they are offered to the phloem. The data also show that the cytokinin levels in the phloem, which may be far higher than in the xylem, are subject to strong fluctuations depending on the physiological situation.This work was supported by the Deutsche Forschungsgemeinschaft (SFB 137). The experimental assistance by P. Geigenberger and the help in cytokinin analysis by Dr. A. Fußeder, Dr. B. Wagner, W. Peters (all Bayreuth) and by Prof. E. Weiler (Bochum) is gratefully acknowledged. Also the constructive discussions with Profs. E. Weiler (Bochum) and E. Beck (Bayreuth) are much appreciated.     

Transport of magnesium ions in the phloem of Ricinus communis L. seedlings

During growth of Ricinus communis seedlings, magnesium ions are mobilized in the endosperm, taken up by and accumulated to very high levels (150 μmol·g FW^?1) in the cotyledons, and translocated to hypocotyl and roots. The magnesium gain from days 6 to 7 in the cotyledons and the seedling axis necessitates a total up-take rate of 600 nmol·h^?1-seedling^?1 and the phloem translocation rate must amount to 200 nmol·h^?1. seedling^?1. The phloem loading of magnesium and the regulatory properties of this process were investigated, making specific use of the ability to collect pure phloem sap from the cut hypocotyl of 6-d-old Ricinus seedlings. The concentration of magnesium in sieve-tube sap (5 mM) was fairly constant under many incubation conditions, e.g. incubation in magnesium-free buffer, incubation with different cations (K⁺, Na⁺, NH ₄ ⁺ ) or anions (Cl^?, NO ₄ ^- , SO ₄ ^2- ), or incubation with sucrose and amino acids. Even addition of magnesium chloride to the cotyledons did not enhance phloem loading of magnesium ions. Therefore the high magnesium content of the cotyledons was sufficient for continuous phloem loading of magnesium, irrespective of external ionic conditions. Also, the flow rate of sieve-tube sap did not influence the magnesium concentration in the sap. Only the incubation with sulfate and phosphate ions increased the magnesium-ion concentration in the phloem. Magnesium sulfate offered to the cotyledons caused a threefold increase of magnesium ions in the sieve-tube sap, which was inhibited by Na⁺, NH ₄ ⁺ and Ca²⁺ in rising order, but not by K⁺. Incubation with phosphate for a prolonged period (8 h) led to an increased mobilization of intra-cotyle-donary magnesium and an enhanced phloem loading of mobilized magnesium. It is concluded that phosphate availability is a decisive factor for mobilization and translocation of magnesium ions within the plant.     

Inorganic ions modulate the path of phloem loading of sucrose in Ricinus communis L. seedlings

The impact of inorganic ions on sucrose fluxes in the cotyledons and on the pathway of phloem loading was studied in Ricinus communis L. seedlings. The cotyledons were incubated in defined solutions which contained either potassium, sodium, magnesium or calcium as chloride salts, or the sodium salts of sulphate or phosphate. Sucrose uptake from the medium into the cotyledons was only slightly affected by the salts. Sucrose efflux to the medium was increased by phosphate and sulphate and to a lesser extent by sodium and potassium. Phloem loading from the apoplasm and the symplasm was analysed by addition of labelled sucrose to the medium, determination of the specific radioactivity of sucrose in sieve-tube exudate and quantification of export into the seedling axis. Potassium and sodium stimulated the apoplasmic route of phloem loading of sucrose, mostly at the expense of loading from cotyledon sucrose pools. In contrast, sulphate and phosphate strongly inhibited the apoplasmic route whereas the (small) symplasmic flux from the cotyledon sucrose pools was less affected. Magnesium ions inhibited phloem loading by both pathways. The potential of ions in modulating the pathways of sucrose export in day to day operation of plants is discussed.     

Sucrose is metabolised by sucrose synthase and glycolysis within the phloem complex of Ricinus communis L. seedlings

Metabolites and enzyme activities were measured in the phloem sap exuding from a cut hypocotyl of germinating castor-bean (Ricinus communis L.) seedlings. The sap contained considerable quantities of adenine nucleotides, uridine nucleotides, uridine diphosphoglucose (UDPGlc), all the major phosphorylated metabolites required for glycolysis, fructose-2,6-bisphosphate and pyrophosphate. Supplying 200 mM glucose instead of sucrose to the cotyledons resulted in high concentrations of glucose in the sap, but did not modify the metabolite levels. In contrast, when 200 mM fructose was supplied we found only a low level of fructose but a raised sucrose concentration in the sap, which was accompanied by a three-to fourfold decrease of UDPGlc, and an increase of pyrophosphate, UDP and UTP. The measured levels of metabolites are used to estimate the molar mass action ratios and in-vivo free-energy change associated with the various reactions of sucrose breakdown and glycolysis in the phloem. It is concluded that the reactions catalysed by ATP-dependent phosphofructokinase and pyruvate kinase are removed from equilibrium in the phloem, whereas the reactions catalysed by sucrose synthase, UDPGlc-pyrophosphorylase, phosphoglucose mutase, phosphoglucose isomerase, aldolase, triose-phosphate isomerase, phosphoglycerate mutase and enolase are close to equilibrium within the conducting elements of the phloem. Since the exuded sap contained negligible or undetectable activities of the enzymes, it is concluded, that the responsible proteins are bound, or sequesterd behind plasmodesmata, possibly in the companion cells. It is argued that sucrose mobilisation via a reversible reaction catalysed by sucrose synthase is particularily well suited to allow the rate of sucrose breakdown in the phloem to respond to changes in the metabolic requirement for ATP, and for UDPGlc during callose production. It is also calculated that the transport of nucleotides in the phloem sap implies that there must be a very considerable uptake or de-novo biosynthesis of these cofactors in the phloem.     

The kinetics of sucrose concentration in the phloem of individual vascular bundles of the Ricinus communis seedling measured by nuclear magnetic resonance microimaging

The sucrose concentration was measured at 70-min intervals in the phloem of individual bundles of the hypocotyl of Ricinus seedlings by ¹H nuclear magnetic resonance (NMR) spectroscopic imaging. The sucrose concentration stayed fairly constant in all bundles for more than 7 h if the cotyledons were embedded in the endosperm or excised and incubated in 100 mM sucrose. If, however, the sucrose solution was replaced by sucrose-free buffer solution, the sucrose levels in the phloem decreased with a kinetic depending on the seedling: in some cases there was a smooth decline, in some a decline followed by a slight recovery and in some cases a clear-cut oscillation. The sucrose concentration was often not identical in the phloem of the individual bundles. The oscillations were larger in the phloem at the apex of the hypocotyl than in the phloem at the base of the hypocotyl. Cutting the petiole of one cotyledon led to a decrease in sucrose not only in the four bundles directly connected to the severed petiole but in all eight bundles of the hypocotyl. Cutting the petiole and dividing the vascular ring at the cotyledonary node and at the root crown did not prevent the decline of sucrose in all eight bundles. Therefore, a functional equilibration of translocated solutes between the eight bundles may occur within the 1-h measuring interval by radial diffusion through the parenchyma of the hypocotyl. Received 4 July 1997 / Accepted: 4 October 1997     

Subcellular concentrations of sugar alcohols and sugars in relation to phloem translocation in <Emphasis Type="Italic">Plantago major,Plantago maritima</Emphasis>, <Emphasis Type="Italic">Prunus persica</Emphasis>, and <Emphasis Type="Italic">Apium graveolens</Emphasis>

Sugar and sugar alcohol concentrations were analyzed in subcellular compartments of mesophyll cells, in the apoplast, and in the phloem sap of leaves of Plantago major (common plantain), Plantago maritima (sea plantain), Prunus persica (peach) and Apium graveolens (celery). In addition to sucrose, common plantain, sea plantain, and peach also translocated substantial amounts of sorbitol, whereas celery translocated mannitol as well. Sucrose was always present in vacuole and cytosol of mesophyll cells, whereas sorbitol and mannitol were found in vacuole, stroma, and cytosol in all cases except for sea plantain. The concentration of sorbitol, mannitol and sucrose in phloem sap was 2- to 40-fold higher than that in the cytosol of mesophyll cells. Apoplastic carbohydrate concentrations in all species tested were in the low millimolar range versus high millimolar concentrations in symplastic compartments. Therefore, the concentration ratios between the apoplast and the phloem were very strong, ranging between 20- to 100-fold for sorbitol and mannitol, and between 200- and 2000-fold for sucrose. The woody species, peach, showed the smallest concentration ratios between the cytosol of mesophyll cells and the phloem as well as between the apoplast and the phloem, suggesting a mixture of apoplastic and symplastic phloem loading, in contrast to the herbal plant species (common plantain, sea plantain, celery) which likely exhibit an active loading mode for sorbitol and mannitol as well as sucrose from the apoplast into the phloem.     

Quantitative aspects of latex lipid synthesis in Euphoribia lathyris L. seedlings

Changes in the dry weight of the endosperm of Euphorbia lathyris L. seedlings showed that 2 mg material was taken up by the cotyledons after 10 d germination. A similar amount of sucrose could be taken up by these seedlings after removal of the endosperm. The maximum yield of latex triterpenes synthesized from this exogenously supplied substrate was in the same order of magnitude as the daily latex lipid increase in 19 g per seedling. Cotyledons and adjacent 1–2 cm segment of the hypocotyl were the most active tissues in latex trieterpene synthesis. Excised cotyledons were able to accumulate 1–1.5 mg sucrose in 48 h from a sugar concentration higher than 0.1 mol l^-1. In this period a maximum amount of 8–10 g latex triterpenes could be synthesized from this substrate. [¹⁴C]Mevalonic acid was rapidly taken up by excised cotyledons but not metabolized by the laticifers. This exogenously supplied precursor was rapidly converted to squalene and triterpenes by the adjacent tissue, and after 48 h incubation most of the ¹⁴C in the nonsaponifiable fraction was traced in the phytosterolds.     

The differential transport of amino acids into the phloem of Ricinus communis L. seedlings as shown by the analysis of sieve-tube sap

The cotyledons of castor bean (Ricinus communis L.) act as absorption organs for amino acids, which are supplied to the medium. The analysis of the sieve-tube sap, which exudes from the cut hypocotyl, demonstrated the ability of the cotyledons to load particular amino acids into the phloem and to reject the loading of others. The sieve-tube sap of cotyledons, which were embedded in the endosperm, contained 150 mM amino acids, with 50 mM glutamine as the major amino acid, and 10–15 mM each of valine, isoleucine, lysine and arginine. Removal of the endosperm led to a drastic decline in the amino-acid content of sieve-tube sap down to 16 mM. Addition of single amino acid species to the medium increased the amino acid concentration in the sieve-tube sap in specific manner: glutamine caused the largest increase (up to 140 mM in exudate), glutamate and alanine smaller increases (up to 60 mM), and arginine the smallest. In addition, the amino acid composition of the sieve-tube sap changed, for instance, glutamine or alanine readily appeared in the sieve-tube sap upon incubation in glutamine or alanine, respectively, whereas glutamate was hardly discernible even in the case of incubation with glutamate; arginine was loaded into the sieve tubes only reluctantly. In general, glutamine and alanine accumulated four- to tenfold in the sieve tubes. The uptake of amino acids and of sucrose into the sieve tubes was interdependent: the loading of sucrose strongly reduced the amino acid concentration in the sieve-tube exudate and loading of amino acids decreased the sucrose concentration. Comparison of the concentrations of various amino acids on their way from the endosperm via the cotyledon-endosperm interface, through the cotyledons and into the sieve tubes showed that glutamine, valine, isoleucine and lysine are accumulated on this pathway, whereas glutamate and arginine are more concentrated in the cotyledons than in the sieve tubes. Obviously the phloem-loading system has a transport specificity different from that of the amino acid uptake system of the cotyledon in general and it strongly discriminates between amino acids within the cotyledons.     

Phloem transport of abscisic acid in Ricinus communis L. seedlings

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2–0.5 mmol m^?3abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceeded that in root pressure exudate under a wide range of water supply. Exudation of sieve tube sap from the cut hypocotyls caused water loss, and this induced ‘water shortage’ in the cotyledons which resulted in the ABA concentration in the cotyledons increasing by 3-fold and that in the sieve tube sap increasing by up to 50-fold within 7h. The wounded surface of the cut hypocotyl was not responsible for the ABA increase. Incubation of the cotyledons of endosperm-free seedlings in various ABA concentrations (up to 100 mmol m^?3) increased the ABA concentration in sieve tube sap. The concomitant increase in ABA, both in cotyledons and in sieve tube sap, had no effect on the phloem loading of sucrose, K⁺ and Mg²⁺ within the experimental period, i.e. up to 10h. It can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem-mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA concentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.