Enhancement of Phloem Exudation from Fraxinus uhdei Wenz. (Evergreen Ash) using Ethylenediaminetetraacetic Acid

Ethylenediaminetetraacetic acid (EDTA) enhanced the exudation of ¹⁴C-labeled assimilates from excised leaflets and whole plant specimens of Fraxinus uhdei Wenz. A 2 millimolar EDTA concentration was found to be most effective in promoting exudation from excised leaflets, while 10 millimolar EDTA was most effective in whole plants experiments. Exudation rate reached a maximum after 24 hours in both experiments. The continuous presence of EDTA throughout the treatment period was required for maximum exudation from excised leaflets. Stachyose, raffinose, verbascose, and sucrose were the principal compounds found to occur in exudate samples. These compounds are typically transported in sieve elements of various Fraxinus species suggesting the exudate was of phloem origin. Electron microscope studies of petiolule sieve plate pores from excised leaflets showed substantially less callose appearing after treatment with EDTA than after H₂O treatment. It is suggested that EDTA enhances phloem exudation by inhibiting or reducing callose formation in sieve plate pores. The exudation enhancement technique described for whole plant specimens is suggested as a useful means of collecting phloem sap and studying translocation in woody plants.     

Phloem Transport of Amino Acids in Relation to their Cytosolic Levels in Barley Leaves

A comparison of barley (Hordeum vulgare L.) leaves was made between the cytosolic content of amino acids and sucrose as determined by subcellular fractionation and the corresponding concentration in phloem sap, which was collected continuously for up to 6 days from severed aphid stylets. Because amino acids were found to be almost absent from the vacuoles, and because the amino acid patterns in the stroma and cytosol are similar, whole leaf contents could be taken as a measure of cytosolic amino acid levels for a comparison of data during a diurnal cycle. The results show that the pattern of amino acids in the phloem sap was very similar to the pattern in the cytosol. Therefore, we concluded that the overall process of transfer of amino acids from the cytosol of the source cells into the sieve tubes, although carrier mediated, may be a passive process and that the translocation of amino acids via the sieve tubes requires the mass flow of sucrose driven by the active sucrose transport involved by the phloem loading.     

Regulation of glutamine synthetase 1 and amino acids transport in the phloem of young wheat plants

The possible regulation of amino acid remobilization via the phloem in wheat (Triticum aestivum L.) by the primary enzyme in nitrogen (N) assimilation and re-assimilation, glutamine synthetase (GS, E.C. 6.3.1.2) was studied using two conditions known to alter N phloem transport, N deficiency and cytokinins. The plants were grown for 15 days in controlled conditions with optimum N supply and then N was depleted from and/or 6-benzylaminopurine was added to the nutrient solution. Both treatments generated an induction of GS1, monitored at the level of gene expression, protein accumulation and enzyme activity, and a decrease in the exudation of amino acids to the phloem, obtained with EDTA technique, which correlated negatively. GS inhibition by metionine sulfoximide (MSX) produced an increase of amino acids exudation and the inhibitor successfully reversed the effect of N deficiency and cytokinin addition over phloem exudation. Our results point to an important physiological role for GS1 in the modulation of amino acids export levels in wheat plants.     

Comparison of the contents of sucrose and amino acids in the leaves, phloem sap and taproots of high and low sugar-producing hybrids of sugar beet (Beta vulgaris L.)

The contents of sucrose and amino acids in the leaves, phloemsap and taproots have been analysed in three experimental hybridsof sugar beet and compared with earlier analysed leaf and phloemsap contents in spinach and barley. The three hybrids accumulatedsucrose and amino acids to various extents in the mature rootsas well as in the young taproots (9–12 weeks). The differencesin the sucrose-to-amino acid ratios in the taproots were reflectedin the corresponding ratios in the phloem sap. The leaf contentsof sucrose and amino acids in the three hybrids were found tobe very similar to each other and also to those in spinach andbarley. In contrast, the phloem concentration of sucrose (1.3M) was much higher, and that of amino acids much lower thanin spinach and barley. In the taproots, the overall concentrationof sucrose was about half that in the phloem sap. From thesefindings it is con cluded that the decisive factor in the highsucrose accumulation in sugar beet roots is the very efficientprocess of phloem loading in the leaves. The patterns of theamino acids in the phloem sap and in the taproots resembledthose in the leaves, indicating that there is no special transportform for a-amino nitrogen from the leaves to the roots, butall amino acids which are present in the cytosol are translocated. Key words: Amino acids, Beta vulgaris L., phloem sap, sucrose, tap roots, transport     

Amino Acid and sucrose content determined in the cytosolic, chloroplastic, and vacuolar compartments and in the Phloem sap of spinach leaves

Amino acid and sucrose contents were analyzed in the chloroplastic, cytosolic, and vacuolar compartments and in the phloem sap of illuminated spinach leaves (Spinacia oleracea L.). The determination of subcellular metabolite distribution was carried out by nonaqueous fractionation of frozen and lyophilized leaf material using a novel three-compartment calculation method. The phloem sap was collected by aphid stylets which had been severed by a laser beam. Subcellular analysis revealed that the amino acids found in leaves are located mainly in the chloroplast stroma and in the cytosol, the sum of their concentrations amounting to 151 and 121 millimolar, respectively, whereas the amino acid concentrations in the vacuole are one order of magnitude lower. The amino acid concentrations in the phloem sap are found to be not very different from the cytosolic concentrations, whereas the sieve tube concentration of sucrose is found to be one order of magnitude higher than in the cytosol. It is concluded that the phloem loading results in a preferential extraction of sucrose from the source cells.     

Collection and Chemical Composition of Phloem Sap from Citrus sinensis L. Osbeck (Sweet Orange)

Through utilizing the nutrient-rich phloem sap, sap feeding insects such as psyllids, leafhoppers, and aphids can transmit many phloem-restricted pathogens. On the other hand, multiplication of phloem-limited, uncultivated bacteria such as Candidatus Liberibacter asiaticus (CLas) inside the phloem of citrus indicates that the sap contains all the essential nutrients needed for the pathogen growth. The phloem sap composition of many plants has been studied; however, to our knowledge, there is no available data about citrus phloem sap. In this study, we identified and quantified the chemical components of phloem sap from pineapple sweet orange. Two approaches (EDTA enhanced exudation and centrifugation) were used to collect phloem sap. The collected sap was derivatized with methyl chloroformate (MCF), N-methyl-N- [tert-butyl dimethylsilyl]-trifluroacetamide (MTBSTFA), or trimethylsilyl (TMS) and analyzed with GC-MS revealing 20 amino acids and 8 sugars. Proline, the most abundant amino acid, composed more than 60% of the total amino acids. Tryptophan, tyrosine, leucine, isoleucine, and valine, which are considered essential for phloem sap-sucking insects, were also detected. Sucrose, glucose, fructose, and inositol were the most predominant sugars. In addition, seven organic acids including succinic, fumaric, malic, maleic, threonic, citric, and quinic were detected. All compounds detected in the EDTA-enhanced exudate were also detected in the pure phloem sap using centrifugation. The centrifugation technique allowed estimating the concentration of metabolites. This information expands our knowledge about the nutrition requirement for citrus phloem-limited bacterial pathogen and their vectors, and can help define suitable artificial media to culture them.     

Magnesium deficiency results in accumulation of carbohydrates and amino acids in source and sink leaves of spinach

Accumulation of assimilates in source leaves of magnesium‐deficient plants is a well‐known feature. We had wished to determine whether metabolite concentrations in sink leaves and roots are affected by magnesium nutrition. Eight‐week‐old spinach plants were supplied either with a complete nutrient solution (control plants) or with one lacking Mg (deficient plants) for 12 days. Shoot and root fresh weights and dry weights were lower in deficient than in control plants. Mg concentrations in deficient plants were 11% of controls in source leaves, 12% in sink leaves and 26% in roots, respectively. As compared with controls, increases were found in starch and amino acids in source leaves and in sucrose, hexoses, starch and amino acids in sink leaves, whereas they were only slightly enhanced in roots. In phloem sap of magnesium‐deficient and control plants no differences in sucrose and amino acid concentrations were found. To prove that sink leaves were the importing organs they were shaded, which did not alter the response to magnesium deficiency as compared with that without shading. Since in the shaded sink leaves the photosynthetic production of metabolites could be excluded, those carbohydrates and amino acids that accumulated in the sink leaves of the deficient plants must have been imported from the source leaves. It is concluded that in magnesium‐deficient spinach plants the growth of sink leaves and roots was not limited by carbohydrate or amino acid supply. It is proposed that the accumulation of assimilates in the source leaves of Mg‐deficient plants results from a lack of utilization of assimilates in the sink leaves.     

Free amino Acid composition of leaf exudates and Phloem sap : a comparative study in oats and barley

Comparisons were made between the free amino acid composition in leaf exudates and that in pure phloem sap, using twin samples taken from a single leaf of two oat (Avena sativa L.) and three barley (Hordeum vulgare L.) varieties. Leaf exudate was collected in a 5 mm EDTA-solution (pH 7.0) from cut leaf blades and phloem sap was obtained through excised aphid (Rhopalosiphum padi L.) stylets. Fluorescent derivatives of amino acids were obtained using 9-fluorenylmethyl chloroformate and were separated by means of high performance liquid chromatography. The total concentration of free amino acids varied considerably in the exudate samples. There was no correlation between the total amino acid content in the exudate samples and that of the corresponding phloem sap samples, but the amino acid composition of the corresponding samples was highly correlated (median R²-value 0.848). There was only limited between-plant variation in phloem sap amino acid composition. Nevertheless, in comparisons involving all samples, many of the amino acids showed significant correlations between their relative amounts in exudate and phloem sap. The results presented here indicate that the exudate technique holds great promise as an interesting alternative to the laborious and time-consuming stylet-cutting technique of obtaining samples for comparative studies of phloem sap.     

Assimilation of gaseous ammonia and the transport of its products in barley and spinach leaves

The interactions between the assimilation and transport of nitrogenand carbon were investigated in barley and spinach leaves. Bothplants were fumigated with NH₃ (1 mg m^–3 and the contentof amino acids, sucrose and carbon intermediates of amino acidmetabolism were analysed in the leaves, apoplast and phloemsap. The following changes took place in the C- and N-metabolismof barley leaves during 5 h of fumigation with NH₃ (a) The contentsof amino acids, especially glutamine, largely increased andthe contents of sucrose, 2-oxoglutarate, phosphoenolpyruvate,and glycerate-3-phosphate declined. (b) A decrease in the phophoenolpyruvatecontent was accompanied by an increased activity of phosphoenolpyruvatecarboxylase. (c) The altered cytosolic concentrations of aminoacids and sucrose during NH₃ fumigation correlated with similarchanges in the apoplast and phloem sap. The altered percentageof each amino acid relative to the total amino acid concentrationin the cytosol, caused by NH₃ fumigation, is reflected in theapoplast and the phloem sap. The results indicate that the concentrations of amino acids in the cytosol determine their concentrationsin the phloem. Key words: Amino acids, ammonia fumigation, barley leaves, C: N partitioning, phosphoenolpyruvate carboxylase, phloem sap, spinach leaves     

Export of amino acids to the phloem in relation to N supply in wheat

The effect of different N supply on amino acid export to the phloem was studied in young plants of wheat (Triticum aestivum L. cv. Klein Chamaco), using the exudation in EDTA technique. Plants were grown in a growth cabinet in pots with sand, and supplied with nutrient solutions of different NO₃^? concentrations. When plants were grown for 15 days with nutrient solutions containing 1.0, 3.0, 5.0, 10.0, 15.0 or 20.0 mM KNO₃, the exudation rate of sugars from the phloem was unaffected by N supply, but sugars accumulated in the leaf tissue when the N supply was limiting for growth. On the other hand, the rate of exudation of amino acids was proportional to the NO₃^? concentration in the nutrient solution. When the supply of N to plants grown for 15 days with 15.0 mM NO₃^? was interrupted, the exudation of sugars was again unaffected, but there was a fast decrease in the amount of amino acids exudated, and of the concentration of amino acids and nitrogen in the tissues. Also, when 10-day-old plants grown without N were supplied with 15.0 mM NO₃^?, there was a sharp increase in the exudation of amino acids, without changes in the amount of sugar exudated. The rate of exudation of amino acids to the phloem was independent of the concentration of free amino acids in the leaves in all three types of experiment. Asp was the most abundant amino acid in the leaf tissue, while Glu was the one most abundant in the phloem exudate. Asp and Ala were exported to the phloem at a rate lower than expected from their leaf tissue concentrations, indicating some discrimination. On the contrary, Glu showed a preferential export at low N supply. It is concluded that the rate of amino acid export from the leaf to the phloem is dependent on the N available to the plant. This N is used for synthesis of leaf protein when the supply is low, exported to the phloem when supply is adequate, and accumulated in the storage pool when supply is above plant demand.     

Patterns of Assimilate Production and Translocation in Muskmelon (Cucumis melo L.) : I. Diurnal Patterns

Continuous monitoring of steady-state carbon dioxide exchange rates in mature muskmelon (Cucumis melo L.) leaves showed diurnal patterns of photosynthesis and respiration that were translated into distinct patterns of accumulation and phloem export of soluble sugars and amino acids. Leaf soluble sugar patterns in general followed the pattern of photosynthetic activity observed in the leaf, whereas starch accumulated steadily throughout the light period. Sugar and starch levels declined through the dark phase. Phloem exudate analysis revealed that diurnal levels of the major transport sugars (stachyose and sucrose) in the phloem did not appear to correlate directly with the photosynthetic activity of the leaf but instead were inversely correlated with leaf starch accumulation and degradation. The amino acid pool in leaf tissues remained constant throughout the diurnal period; however, the relative contribution of individual amino acids to the total pool varied with the diurnal photosynthetic and respiratory activity of the leaf. In contrast, the phloem sap amino acid pool size was substantially larger in the light than in the dark, a result primarily due to enhanced export of glutamine, glutamate, and citrulline during the light period. The results indicate that the sugar and amino acid composition of cucurbit phloem sap is not constant but varies throughout the diurnal cycle in response to the metabolic activities of the source leaf.     

Accumulation,selection and covariation of amino acids in sieve tube sap of tansy (Tanacetum vulgare) and castor bean (Ricinus communis): evidence for the function of a basic amino acid transporter and the absence of a γ‐amino butyric acid transporter

Sieve tube sap was obtained from Tanacetum by aphid stylectomy and from Ricinus after apical bud decapitation. The amino acids in sieve tube sap were analyzed and compared with those from leaves. Arginine and lysine accumulated in the sieve tube sap of Tanacetum more than 10‐fold compared to the leaf extracts and they were, together with asparagine and serine, preferably selected into the sieve tube sap, whereas glycine, methionine/tryptophan and γ‐amino butyric acid were partially or completely excluded. The two basic amino acids also showed a close covariation in sieve tube sap. The acidic amino acids also grouped together, but antagonistic to the other amino acids. The accumulation ratios between sieve tube sap and leaf extracts were smaller in Ricinus than in Tanacetum. Arginine, histidine, lysine and glutamine were enriched and preferentially loaded into the phloem, together with isoleucine and valine. In contrast, glycine and methionine/tryptophan were partially and γ‐amino butyric acid almost completely excluded from sieve tube sap. The covariation analysis grouped arginine together with several neutral amino acids. The acidic amino acids were loaded under competition with neutral amino acids. It is concluded from comparison with the substrate specificities of already characterized plant amino acid transporters, that an AtCAT1‐like transporter functions in phloem loading of basic amino acids, whereas a transporter like AtGAT1 is absent in phloem. Although Tanacetum and Ricinus have different minor vein architecture, their phloem loading specificities for amino acids are relatively similar.     

Transport of organic solutes in Phloem and xylem of a nodulated legume

Collections of xylem exudate of root stumps or detached nodules, and of phloem bleeding sap from stems, petioles, and fruits were made from variously aged plants of Lupinus albus L. relying on nodules for their N supply. Sucrose was the major organic solute of phloem, asparagine, glutamine, serine, aspartic acid, valine, lysine, isoleucine, and leucine, the principal N solutes of both xylem and phloem. Xylem sap exhibited higher relative proportions of asparagine, glutamine and aspartic acid than phloem sap, but lower proportions of other amino acids. Phloem sap of petioles was less concentrated in asparagine and glutamine but richer in sucrose than was phloem sap of stem and fruit, suggesting that sucrose was unloaded from phloem and amides added to phloem as translocate passed through stems to sinks of the plant. Evidence was obtained of loading of histidine, lysine, threonine, serine, leucine and valine onto phloem of stems but the amounts involved were small compared with amides. Analyses of petiole phloem sap from different age groups of leaves indicated ontogenetic changes and effects of position on a shoot on relative rates of export of sucrose and N solutes. Diurnal fluctuations were demonstrated in relative rates of loading of sucrose and N solutes onto phloem of leaves. Daily variations in the ability of stem tissue to load N onto phloem streams were of lesser amplitude than, or out of phase with fluctuations in translocation of N from leaves. Data were related to recent information on C and N transport in the species.     

Interaction of phloem-translocated amino compounds with nitrate net uptake by the roots of beech (Fagus sylvatica) seedlings

In the present study two experimental approaches were used to investigate the influence of changes in the allocation of amino compounds in the phloem of beech (Fagus sylvatica L.) seedlings on nitrate net uptake by the roots. In a first set of experiments Gin or Asp were directly fed into the phloem of the epicotyl via bark flaps. These compounds were previously found to be allocated in the phloem of adult beech trees and were shown to inhibit nitrate net uptake when supplied to beech roots. Feeding of solutions containing 100 mM of Gin or Asp plus 10 mM EDTA into the phloem resulted in a significant enrichment of the fine root tissue with the amino compound fed as compared to the roots of control plants supplied with amino acid-free EDTA solutions. Nitrate net uptake by the roots decreased by 61% (Gin) and 79% (Asp) as compared to the controls. In a second approach, shoots of young beech seedlings were exposed to 40g NH3 m^-3. NH3 uptake by shoots, nitrate net uptake by roots, and the contents and composition of total soluble non-protein nitrogen (TSNN) in leaves, phloem, and fine roots were determined and were compared to results gained with control plants exposed to charcoal-filtered air. NH3 fumigation of the shoots of beech seedlings resulted in a 35% reduction of nitrate net uptake by the roots as compared to controls. TSNN contents in leaves and phloem exudate of NH3-fumigated plants increased by 56% and 37%, respectively. This enrichment was mainly due to Arg and Glu in the leaves and Asp, Asn, Glu, and Gin, but not to Arg, in phloem exudate. The TSNN content of the fine roots was not changed by NH3 fumigation, but a significant increase in the Gin content was observed. From these results it is concluded that phloem transport of amino compounds, especially of Gin and Asp, from the shoot to the roots mediates regulation of nitrate net uptake by the roots of beech trees in order to adapt this process to the nitrogen demand of the whole plant.     

Amino acid contents and transport in oilseed rape (<Emphasis Type="Italic">Brassica napus</Emphasis> L.) under different nitrogen conditions

Oilseed rape (Brassica napus L.) needs very high nitrogen fertilizer inputs. Significant amounts of this nitrogen are lost during early leaf shedding and are a source of environmental and economic concern. The objective of this study was to investigate whether the remobilization of leaf amino acids could be limiting for nitrogen use efficiency. Therefore, amino acid concentrations were analyzed in subcellular compartments of leaf mesophyll cells of plants grown under low (0.5 mM NO₃^–) and high (4 mM NO₃^–) nitrogen supply. With high nitrogen supply, young leaves showed an elevated amino acid content, mainly in vacuoles. In old leaves, however, subcellular concentrations were similar under high and low nitrogen conditions, showing that the excess nitrogen had been exported during leaf development. The phloem sap contained up to 650 mM amino acids, more than four times as much than the cytosol of mesophyll cells, indicating a very efficient phloem-loading process. Three amino acid permeases, BnAAP1, BnAAP2, and BnAAP6, were identified and characterized. BnAAP1 and BnAAP6 mediated uptake of neutral and acidic amino acids into Xenopus laevis oocytes at the actual apoplastic substrate concentrations. All three transporters were expressed in leaves and the expression was still detectable during leaf senescence, with BnAAP1 and BnAAP2 mRNA levels increasing from mature to old leaves. We conclude that phloem loading of amino acids is not limiting for nitrogen remobilization from senescing leaves in oilseed rape.     

Response of amino acid changes in Aphis gossypii (Glover) to elevated CO2 levels

Effects of elevated CO₂ levels on the amino acid constituents of cotton aphid, Aphis gossypii (Glover), fed on transgenic Bacillus thuringiensis (Berliner) (Bt) cotton [Cryl A(c)], grown in ambient and double‐ambient CO₂ levels in closed‐dynamics CO₂ chambers, were investigated. Lower amounts of amino acids were found in cotton phloem under elevated CO₂ than under ambient CO₂ levels. However, higher amounts of free amino acids were found in A. gossypii fed on elevated CO₂‐grown cotton than those fed ambient CO₂‐grown cotton, and the contents of amino acids in honeydew were not significantly affected by elevated CO₂ levels. A larger amount of honeydew was produced by cotton aphids feeding on leaves under elevated CO₂ treatment than those feeding on leaves under ambient CO₂ treatment, which indicates that A. gossypii ingests more cotton phloem because of the higher C:N ratio of cotton phloem under elevated CO₂ levels. Moreover, the amino acid composition was similar in bodies of aphids ingesting leaves under both CO₂ treatments, except for two alkaline amino acids, lysine and arginine. This suggests that the nutritional constitution of the phloem sap was important for A. gossypii. Our data suggest that more phloem sap will be ingested by A. gossypii to satisfy its nutritional requirement and balance the break‐even point of amino acid in elevated CO₂. Larger amounts of honeydew produced by A. gossypii under elevated CO₂ will reduce the photosynthesis and result in the occurrence of some Entomophthora spp.     

Improvement of pea biomass and seed productivity by simultaneous increase of phloem and embryo loading with amino acids

The development of sink organs such as fruits and seeds strongly depends on the amount of nitrogen that is moved within the phloem from photosynthetic‐active source leaves to the reproductive sinks. In many plant species nitrogen is transported as amino acids. In pea (Pisum sativum L.), source to sink partitioning of amino acids requires at least two active transport events mediated by plasma membrane‐localized proteins, and these are: (i) amino acid phloem loading; and (ii) import of amino acids into the seed cotyledons via epidermal transfer cells. As each of these transport steps might potentially be limiting to efficient nitrogen delivery to the pea embryo, we manipulated both simultaneously. Additional copies of the pea amino acid permease PsAAP1 were introduced into the pea genome and expression of the transporter was targeted to the sieve element‐companion cell complexes of the leaf phloem and to the epidermis of the seed cotyledons. The transgenic pea plants showed increased phloem loading and embryo loading of amino acids resulting in improved long distance transport of nitrogen, sink development and seed protein accumulation. Analyses of root and leaf tissues further revealed that genetic manipulation positively affected root nitrogen uptake, as well as primary source and sink metabolism. Overall, the results suggest that amino acid phloem loading exerts regulatory control over pea biomass production and seed yield, and that import of amino acids into the cotyledons limits seed protein levels.     

Free Amino Acid Composition of Phloem Sap and Growing Fruit of Lycopersicon esculentum

Pure phloem sap of tomato leaves was collected by stylectomy.Glutamine and glutamate were the predominant free amino acidstranslocated by the phloem stream. In developing fruits glutaminecontent increased significantly, reaching 35% of the total freeamino acids. Comparison in the amino acid composition betweenthe two tissues are discussed. (Received October 6, 1997; Accepted January 27, 1998)     

Removal of Al, Fe and Mn by Pistia stratiotes L. and its stress response

The influence of different chelates applied in the soil primary on Al and secondary on Fe and Mn mobilization and their removal from solution was investigated. The work compared the efficiency of 10 mM tartaric acid and 3 mM EDTA in soil washing process and accumulation potential of Pistia stratiotes in rhizofiltration process. The plant response on the toxic element Al and other elements Fe and Mn was determined through the nitrogen and free amino acids content in plants. The efficiency of chelates decreased in order 10 mM tartaric acid > deionized water > 3 mM EDTA for all studied elements. P. stratiotes was able to remove up to 90% of elements during the 15 days period. Higher content of toxic element Al and potential toxic elements Fe and Mn were observed in the roots than in the leaves with the increased time. The trend of Al accumulation correlated with Fe accumulation (R²=0.89). Toxicity impact of high level of Al was observed by increased free amino acids (AA) level. Proline, histidine, glutamic acid and glycine were the most synthesised free AA in leaves. Total AA content in leaves was significantly higher under chelates addition compared to control.     

The Relationship between Sugar and Amino Acid Export to the Phloem in Young Wheat Plants

The relationship between amino acid and sugar export to thephloem was studied in young wheat plants (Triticum aestivumL. ‘Pro-INTA, Isla Verde’) using the EDTA-phloemcollection technique. Plants grown with a 16 h photoperiod showeda rapid decrease in the concentration of sugars and amino acidsin the phloem exudate from the beginning of the dark period.When plants grown with a 16 h photoperiod were kept in the darkfor longer than 8 h the free amino acid content in leaves andexudate (on a dry weight basis) increased continually throughoutthe 72 h of darkness. During the first 24 h of darkness thesugars in the phloem exudate decreased to 30% of the initialvalue, and returned to the control level when plants were returnedto light. When plants grown under low light intensity for 10d were transferred to high light intensity, they showed an increasein leaf sugar content (dry weight basis) after 3 d but therewere no differences in leaf free amino acid content (dry weightbasis) compared to low-light plants. The sugar concentrationin the phloem exudate was increased by higher light intensities,but there was no difference in the amino acid concentrationof the phloem exudate, and thus the amino acid:sugar ratio inthe phloem decreased in the high-light plants. The present resultssuggest that amino acids can be exported to the phloem independentlyof the export of sugars. Copyright 1999 Annals of Botany Company Sugar exudation, amino acid transport, nitrogen, phloem, transport, wheat, Triticum aestivum L.