Amino acid content in xylem sap of regrowing alfalfa (Medicago sativa L.): Relations with N uptake,N2 fixation and N remobilization

During vegetative regrowth of Medicago sativa L., soil N, symbiotically fixed N₂ and N reserves meet the nitrogen requirements for shoot regrowth. Experiments with nodulated or non-nodulated plants were carried out to investigate the changes in N flows originating from the different N sources and in xylem transport of amino acids during regrowth. Exogenous N uptake, N₂ fixation and endogenous N remobilization were estimated by ¹⁵N labelling and amino acids in xylem sap were analysed. Removal of shoots resulted in great declines of exogenous N flows derived either from N₂ or from NH₄NO₃ during the first week of regrowth, thereafter recovery increased linearly. Mineral N uptake as well as N₂ fixation occurred mainly between the 10th and 18th day after removal of shoots while exogenous N assimilation in intact plants remained at a steady level. Nitrogen remobilization rates in defoliated plants increased by at least three to five-fold, especially during the first 10 days following shoot removal. Compared to control plants, contents of amino acids in xylem sap, during the first 10 days of regrowth, were reduced by about 72% and 82% in NH₄NO₃ grown and in N₂ fixing plants, respectively. Asparagine was the main amino acid transported in xylem sap of both treated plants. Its relative contents during this period significantly decreased from 75% to 59% and from 67% to 36% respectively in non-nodulated plants and in nodulated ones. This decline was accompanied by compensatory increase in the relative contents of aspartate and glutamine.     

4-methyleneglutamine in peanut plants: dynamics of formation, levels, and turnover in relation to other free amino acids

Neither 4-methyleneglutamine nor 4-methyleneglutamic acid were found in free or bound form in ungerminated peanut seeds (Arachis hypogaea L.). Both, however, were formed soon after germination; whereas, 4-methyleneglutamic acid appeared slightly before 4-methyleneglutamine, the former remained at a low concentration while the level of 4-methyleneglutamine rose rapidly between 2 and 10 days of germination and declined slowly thereafter. Free proline and glutamine followed a pattern similar to 4-methyleneglutamine; on the other hand, asparagine increased for at least 20 days but other free amino acids remained at relatively low, constant levels. In mature peanut plants, 4-methyleneglutamine occurred in all parts except developing pods, was virtually the only free amino acid in xylem sap, and constituted about 70% of the total soluble nitrogen of sap. In contrast, 4-methyleneglutamic acid was found only in leaves and stems in highly variable amounts.     

Major nitrogen compounds transported in xylem vessels from roots to top in Citrus trees

Four-year-old citrus trees ( Citrus unshiu Marcovitch) were fed via the roots with (¹⁵NH₄)2sO₄ or K¹⁵NO₃ as a nitrogen source. Nitrogenous compounds and their isotopic abundances in fine roots and xylem sap from trunks were assayed in order to obtain information on the species of nitrogen released by the root system into the ascending xyiem stream.
Arginine, asparagine, nitrate and proline in xylem sap accounted for 48, 21, 13 and 10%, respectively, of the total nitrogenous constituents tested in the sap. However, in the trees fed with labelled ammonium the main nitrogenous compound labelled with ¹⁵N in the xylem sap was asparagine and glutamine, which accounted for 79% and 18%, respectively, of total labelled nitrogen. In the xylem sap of trees fed with labelled nitrate, nitrate accounted for 94% of total labelled nitrogen. Nitrate and asparagine followed by glutamine showed the highest ratios of isotopic abundance in xylem sap as compared to fine roots. Proline and arginine had much lower ratios. These results indicate that nitrate, asparagine and glutamine are the main nitrogenous compounds released by the roots to the xylem stream, whereas arginine and proline are released into the xylern vessels by the trunk tissues. Furthermore, nitrate and asparagine are probably in steady movement upward in the trunk xylem, whereas glutamine is more easily taken up by the trunk tissues than nitrate and asparagine.     

Characterization of nitrogen relationships between Sorghum bicolor and the root-hemiparasitic angiosperm Striga hermonthica (Del.) Benth. using K15 NO3 as isotopic tracer

The role of the host in the nitrogen nutrition of Striga hermonthica (Del.) Benth. (Scrophulariaceae) parasitic on Sorghum bicolor cv. SH4 Arval has been investigated using (15)N-nitrate as the tracer. It is shown that, when nitrate is absorbed only by the roots of the host plant, a rapid transfer of nitrogen to the parasite can be detected. The xylem sap of S. hermonthica contained approximately equal amounts of nitrate and amino acids, mostly glutamine and asparagine. Infection altered the free amino acid profile of the host tissues, leading notably to a large increase in asparagine and a decrease in glutamine. The haustoria of S. hermonthica, although rich in nitrate, showed a low concentration of free amino acids, particularly lacking in asparagine and glutamine. The roots of S. hermonthica, in contrast, were rich in both asparagine and glutamine while, in the shoots, asparagine constituted 80% of the total FAA pool. Asparagine was also found to be the primary (15)N-enriched amino acid in the shoots of S. hermonthica while, interestingly, it was glutamate that was most strongly enriched in the roots. It is concluded that nitrogen nutrition in S. hermonthica is based on a supply of both nitrate and amino acids from the host. This implies a non-specific transfer in the transpiration stream. Nitrate reduction probably occurs mainly in the leaves of the parasite. Assimilation also occurs in S. hermonthica and excess nitrogen is stored as the non-toxic nitrogen-rich compound, asparagine. This specific trait of nitrogen metabolism of the parasite is discussed in relation to the effect of nitrogen fertilization on reducing infestation.     

Short-term changes in xylem N compounds in Lolium perenne following defoliation

Previous studies have indicated that an increased asparagine to glutamine ratio (Asn : Gln) occurs in the xylem fluid of Lolium perenne 24 h after defoliation. However, the absolute changes in Asn and Gln leading to the increased Asn : Gln ratio are unknown. The present study tested the hypotheses that: (1) defoliation-induced changes in xylem amino acid composition occur in L perenne within the first 24 h following defoliation, irrespective of phasing with respect to the diurnal light/dark cycle; and (2) the increase in Asn : Gln ratio in the xylem fluid of L perenne following defoliation is due to an increase in Asn content. Plants of L perenne L. 'Aurora' were grown in flowing solution culture for 40 d. Plants were then either left intact, defoliated at the end of the light period or defoliated at the end of the dark period. 15N-labelled NO3- was supplied following defoliation to discriminate between the recovery of N absorbed prior to, and following, defoliation. Xylem samples were collected over the subsequent 24 h period with amino acids speciated by GC-MS. There was support for the first hypothesis: increased Asn : Gln ratios occurred within the first 24 h, irrespective of the phasing of defoliation with respect to light/dark cycles. The second hypothesis was not supported: the concentration of all amino acids in the xylem exudate declined after defoliation, and the increased Asn : Gln ratio was accounted for by a disproportionately large reduction in Gln levels. Low concentrations of amino acids in the xylem of defoliated plants precluded accurate discrimination of their nitrogen content into pre- and post-defoliation sources.     

Xylem to phloem transfer of solutes in fruiting shoots of legumes,studied by a phloem bleeding technique

Summary Comparisons were made of the levels of various solutes in xylem (tracheal) sap and fruit tip phloem sap of Lupinus albus (L.) and Spartium junceum (L.). Sucrose was present at high concentration (up to 220 mg ml^-1) in phloem but was absent from xylem whereas nitrate was detected in xylem (up to 0.14 mg ml^-1) but not in phloem. Total amino acids reached 0.5–2.5 mg ml^-1 (in xylem) versus 16–40 mg ml^-1 in phloem. Phloem: xylem concentration ratios for mineral nutrients (K, Na, Mg, Ca, Fe, Zn, Mn, Cu) spanned the range 0.7 to 20, the ratios generally reflecting an element's phloem mobility and its availability to the xylem from the roots.The accessibility of nitrate to xylem and phloem was studied in Lupinus. Increasing the nitrate supply to roots from 100 to 1000 mg NO₃–Nl^-1 increased nitrate spill over into xylem, but nitrate always failed to appear in phloem. However, phloem loading of small amounts of nitrate was induced by feeding 750 or 1000 mg NO₃–Nl^-1 directly to cut shoots via the transpiration stream. Transfer of reduced nitrogen to phloem was demonstrated by feeding ¹⁵NO₃ to shoots and recovering ¹⁵N-enriched amides and amino acids in phloem sap. Increased nitrate supply to roots led to increased amino acid levels in xylem and phloem but did not alter markedly the balance between individual amino acids.The fate of xylem-fed ¹⁴C-labelled asparagine, glutamine and aspartic acid and of photosynthetically fed ¹⁴CO₂ was studied in Spartium, with reference to phloem transport to seeds. Substantial fractions of the ¹⁴C of all sources appeared in non-amino compounds. [¹⁴C]asparagine passed largely in unchanged form to the phloem whereas the ¹⁴C from aspartic acid or glutamine appeared in phloem attached to other amino acids (e.g. asparagine and glutamic acid). Serine, asparagine and glutamine were the main amino compounds labelled in phloem sap after feeding ¹⁴CO₂. The wide distribution of ¹⁴C amongst free and bound amino acids of seeds suggested that extensive metabolism of phloem-borne solutes occurred in the fruits.     

Nickel speciation in the xylem sap of the hyperaccumulator Alyssum serpyllifolium ssp. lusitanicum growing on serpentine soils of northeast Portugal

Nickel speciation was studied in the xylem sap of Alyssum serpyllifolium ssp. lusitanicum, a Ni-hyperaccumulator endemic to the serpentine soils of northeast Portugal. The xylem sap was collected from plants growing in its native habitat and characterized in terms of carboxylic and amino acids content. The speciation of nickel was studied in model and real solutions of xylem sap by voltammetric titrations using Square Wave Voltammetry (SWV). The results showed that Ni transport in the xylem sap occurs mainly as a free hydrated cation (about 70%) and complexed with carboxylic acids, mainly citric acid (18%). Altogether, oxalic acid, malic acid, malonic acid and aspartic acid complexed less than 13% of total Ni. A negligible amount bounded to the amino acids, like glutamic acid and glutamine (<1%). Histidine did not play a role in Ni translocation in the xylem sap of A. serpyllifolium under field conditions. Amino acids are one of the main forms of N transport in the xylem sap, and under field conditions, N is usually a limited nutrient. We hypothesize that the translocation of Ni in the xylem sap as a free ion or chelated with carboxylic acids is ‘cheaper’ in terms of N resources.     

Copper uptake and translocation in chicory (Cichorium intybus L. cv Grasslands Puna) and tomato (Lycopersicon esculentum Mill. cv Rondy) plants grown in NFT system. II. The role of nicotianamine and histidine in xylem sap copper transport

The effect of rooting media Cu concentration (0.05–20 mg Cu L^-1) on amino acid concentrations and copper speciation in the xylem sap of chicory and tomato plants was measured using 6 week old plants grown in a nutrient film technique system (NFT). Irrespective of the Cu concentration in the nutrient solutions, more than 99.68% and 99.74% of total Cu in tomato and chicory xylem sap was in a bound form. When exposed to high Cu concentrations in the rooting media, amino acid concentrations in the sap increased. Relative to other amino acids, the concentrations of glutamine (Gln), histidine (His), asparagine (Asn), valine (Val), nicotianamine (NA) and proline (Pro) in tomato xylem saps, and His, γ-aminobutyric acid (Gaba), glutamic acid (Glu), leucine (Leu), NA and phenylalanine (Phe) in chicory xylem saps showed the greatest increases. The data indicate that induced synthesis of some free amino acids as a specific and proportional response to Cu treatment. For a single complexation amino acid, the solution Cu²⁺concentration vs pH titration curve for NA at 0.06–0.07 mM was most similar, closely followed by His at 0.5–0.6 mM, to the solution Cu²⁺concentration behaviour in both tomato and chicory xylem sap. It is concluded that increased Cu concentrations in the rooting media induced selective synthesis of certain amino acid which include NA, His, Asn and Gln which have high stability constants with Cu. NA and His have the highest binding constants for Cu and the concentrations of NA and His in chicory and tomato xylem saps can account for all the bound Cu carried in the sap. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photoautotrophic growth of soybean cells in suspension culture IV. Free amino acid pools and the effect of nitrogen on chlorophyll levels

A photoautotrophic soybean suspension culture was used to study free amino acid pools during a subculture cycle. Free amino acid analysis showed that the intracellular concentrations of asparagine, serine, glutamine, and alanine reached peaks of 200, 10, 9 and 7 mM, respectively, at specific times in the 14-day subculture cycle. Asparagine and serine levels peaked at day 14 but glutamine level rose quickly after subculture, peaking at day three and then declined gradually. Roughly similar patterns were found in the conditioned culture medium although the levels were 1000-fold lower than those found in cells. Photoautotrophic (SB-P) and photomixotrophic (SB-M) cultures were quantitatively similar with regard to free asparagine and serine but not glutamine or free ammonia. Heterotrophic (SB-H) cells had 81–85% less free asparagine on day seven than did SB-M or SB-P cells. Hence, similar to the phloem sap of a soybean plant, asparagine, glutamine, alanine and serine were the predominant amino acids in photoautotrophic soybean cell cultures. Varying the amount of total nitrogen in culture medium for two subcultures at 10, 25, 50, and 100% Of normal levels showed that growth was inhibited only at the 10 and 25% levels but that growth on medium containing 50% of the normal nitrogen was as good as that on 100% nitrogen. Moreover, cellular chlorophyll content correlated exceptionally well with initial nitrogen content of the medium. Thus, the photosynthesis of SB-P cells was not limited by chlorophyll content. SB-P cells grown for two subcultures on 10% nitrogen contained very low free amino acid levels and only 1% of the free ammonia levels found in cells growing on a full nitrogen complement.Abbreviations SB-P photoautotrophic soybean cells (no sucrose, high CO₂, high light) - SB-M photomixotrophic soybean cells (1% w/v sucrose, high light) - SB-H heterotrophic soybean cells (3% sucrose, dark)     

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.     

Effects of the coordination mechanism between roots and leaves induced by root-breaking and exogenous cytokinin spraying on the grazing tolerance of ryegrass

The grazing tolerance mechanism of ryegrass was investigated by examining the effects of roots on leaves under frequent defoliation. The study consisted of four treatments: (1) with root breaking and cytokinin spraying, (2) root breaking without cytokinin spraying, (3) cytokinin spraying with no root breaking, and (4) no root breaking and no cytokinin spraying. Results showed that root breaking or frequent defoliation inhibited the ryegrass regrowth, which resulted in low biomass of the newly grown leaves and roots, as well as low soluble carbohydrate content and xylem sap quantity in the roots. Spraying with exogenous cytokinin promoted the increase in newly grown leaf biomass, but decreased root biomass, root soluble carbohydrate content, and root xylem sap quantity. Determination of gibberellic acid, indole-3-acetic acid, abscisic acid, and zeatin riboside (ZR) in roots, newly grown leaves, and stubbles showed that cytokinin is a key factor in ryegrass regrowth under frequent defoliation. Root breaking and frequent defoliation both decreased the ZR content in roots and in newly grown leaves, whereas spraying with exogenous cytokinin increased the ZR content in roots and in newly grown leaves. Therefore, cytokinin enhances the above ground productivity at the cost of root growth under frequent defoliation.     

Cycling of amino compounds in symbiotic lupin

The composition of amino acids was determined in the xylem andphloem sap of symbiotic lupins grown under a variety of treatmentsdesigned to alter the rate of nitrogen fixation. Asparaginewas the major amino acid in both xylem and phloem with glutamine,glutamate and aspartate also major components. GABA had a highconcentration in the xylem while valine was a major componentin the phloem. Exposure to combined nitrogen in the form ofeither ammonium or nitrate caused a reduction in specific nitrogenaseactivity and was associated with subsequent changes in bothof the translocated saps. Inhibiting nitrogen fixation by exposingnodules to oxygen produced a lower amide to amine ratio in thexylem sap (1.3:1) compared with control and nitrate ratios (2.6:1)and ammonium ratios (7.1:1). Similar ratios for amide aminewere also observed in the phloem sap. Labelling studies using¹⁵N₂ to follow nitrogen fixation, ammonium assimilation andamino acid transport have shown rapid accumulation of labelinto glutamine with subsequent enrichment in glutamate, aspartate,alanine, and GABA. Asparagine was found in high concentrationsin nodules and became slowly enriched. Labelled nitrogen fixedand assimilated in nodules was detected 40 min later in stemxylem extracts, largely as the amides glutamine and asparagine.These experiments provide evidence that large amounts of nitrogenouscompounds are cycled through the root nodules of symbiotic plants(contributing approximately 50% of xylem N) and that differencesin the composition of the phloem sap may influence nodule growthand activity. Key words: Nitrogen fixation, nitrogen translocation, isotope labelling, legumes, GC-MS     

Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch)

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine. Pulse-feeding nodulated roots with ¹⁵N₂ gas showed asparagine to be the principal nitrogen product exported from N₂-fixing nodules. Maintaining root systems in an N₂-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N₂ fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of ¹⁵N-nitrate, and a ¹⁵N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N₂ or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N₂. Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N₂ and nitrate are being used in peanuts.     

Gradients in the Nitrogenous Constituents of the Sap extracted from Apple Shoots of Different Ages

A comparison was made of the nitrogenous constituents in thesap extracted under vacuum from apple shoots of different ages,as regards changes both with season and in response to fertilizernitrate applied in summer or autumn. Before blossoming the N concentration of the sap changed markedlywith age of shoot, with the values doubling in the samples fromthe proximal half of the 3-year-old wood to the distal 2-year-oldsection, followed by a significant decrease in the 1-year-oldshoot. After blossoming the gradients in sap concentration wereless pronounced but usually the lowest values were found inthe youngest part of the shoot. Fourteen days after a soil application of nitrate in July therewas a marked increase in the concentration of asparagine inthe sap, but only in the 3-year-old section of the shoot. Sevendays later the xylem sap from all parts of the shoot containedincreased levels of asparagine, aspartic acid, and glutamine. No changes in the xylem sap of shoots in response to fertilizerapplied in October were observed until the following April.Then increased amounts of asparagine and glutamine were foundin all sections, with the greatest increase being seen in theyoungest part. It is suggested that this was due to acceleratedmobilization of N reserves into the xylem sap in response togrowth regulators originating in the roots rather than to movementof recently absorbed nutrients.     

Root xylem transport of amino acids in the root hemiparasitic shrub Olax phyllanthi (Labill) R.Br. (Olacaceaea) and its multiple hosts

The amino acid compositions of the root xylem saps of Olax phyllanthi and a range of its common hosts were examined in native coastal heath in Western Australia and in pot cultures of Olax reliant on single hosts. When hosts specializing in the xylem transport of one major solute (asparagine, glutamine, histidine, arginine or proline) were exploited, the endophytic tissue of haustoria and the xylem sap of Olax showed much lower proportions of this than of other solutes, suggesting pronounced metabolic transformation prior to xylem loading by the parasite. However, the xylem sap of Olax did partly reflect the compositions of its hosts; for example, djenkolic acid and pipecolic acid were present when Olax was parasitic on species of Acacia, and levels of citrulline and aspartic acid were higher than normal when it exploited hosts transporting large amounts of these compounds. Back-flow of S-ethenyl cysteine, a novel amino acid specific to Olax, was observed to another root hemiparasite (Exocarpos sparteus) in native habitat and to certain non-parasitic hosts in water-stressed pot cultures. Haustoria exhibited high levels of glutamine synthetase but showed appreciable in vivo nitrate reductase activity only when on hosts with high xylem levels of nitrate.     

d-Glycerate Transport by the Pea Chloroplast Glycolate Carrier : Studies on [1-C]d-Glycerate Uptake and d-Glycerate Dependent O(2) Evolution

Rapid direct conversion of exogenously supplied [¹⁴C]aspartate to [¹⁴C] asparagine and to tricarboxylic cycle acids was observed in alfalfa (Medicago sativa L.) nodules. Aspartate aminotransferase activity readily converted carbon from exogenously applied [¹⁴C]aspartate into the tricarboxylic acid cycle with subsequent conversion to the organic acids malate, succinate, and fumarate. Aminooxyacetate, an inhibitor of aminotransferase activity, reduced the flow of carbon from [¹⁴C]aspartate into tricarboxylic cycle acids and decreased ¹⁴CO₂ evolution by 99%. Concurrently, maximum conversion of aspartate to asparagine was observed in aminooxyacetate treated nodules (30 nanomoles asparagine per gram fresh weight per hour. Metabolism of [¹⁴C]aspartate and distribution of nodulefixed ¹⁴CO₂ suggest that two pools of aspartate occur in alfalfa nodules: (a) one involved in asparagine biosynthesis, and (b) another supplying a malate/aspartate shuttle. Conversion of [¹⁴C]aspartate to [¹⁴C]asparagine was not inhibited by methionine sulfoximine, a glutamine synthetase inhibitor, or azaserine, a glutmate synthetase, inhibitor. The data did not indicate that asparagine biosynthesis in alfalfa nodules has an absolute requirement for glutamine. Radioactivity in the xylem sap, derived from nodule ¹⁴CO₂ fixation, was markedly decreased by treating nodulated roots with aminooxyacetate, methionine sulfoximine, and azaserine. Inhibitors decreased the [¹⁴C]aspartate and [¹⁴]asparagine content of xylem sap by greater than 80% and reduced the total amino nitrogen content of xylem sap (including nonradiolabeled amino acids) by 50 to 80%. Asparagine biosynthesis in alfalfa nodules and transport in xylem sap are dependent upon continued aminotransferase activity and an uninterrupted assimilation of ammonia via the glutamine synthetase/glutamate synthase pathway. Continued assimilation of ammonia apparently appears crucial to continued root nodule CO₂ fixation in alfalfa.     

Induction of xylem sap methylglycine by a drought and rewatering treatment and its inhibitory effects on the growth and development of plant organs

In higher plants, the xylem vessels functionally connect the roots with the above-ground organs. The xylem sap transports various organic compounds, such as proteins and amino acids. We examined drought and rewatering-inducible changes in the amino acid composition of root xylem sap collected from Cucurbita maxima roots. The major free amino acids in C . maxima root xylem sap were methylglycine (MeGly; sarcosine) and glutamine (Gln), but MeGly was not detected in the xylem sap of cucumber. MeGly is an intermediate compound in the metabolism of trimethylglycine (TMG; betaine), but its physiological effects in plants are unknown. Drought and rewatering treatment resulted in an increase in the concentration of MeGly in root xylem sap to 2.5 m M . After flowering, the MeGly concentration in the xylem sap dropped significantly, whereas the concentration of Gln decreased only after fruit ripening. One milli molar MeGly inhibited the formation of adventitious roots and their elongation in C . maxima , but glycine, dimethylglycine, or TMG had no effect. Similar effects and the inhibition of stem elongation were observed in shoot cuttings of cucumber and Phaseolus angularis . These observations seem to imply a possible involvement of xylem sap MeGly in the physiological responses of C . maxima plants to drought stress.     

Seasonal variation of nitrogenous compounds in the xylem sap of Citrus

Samples of tracheal sap of Citrus sinensis (L.) Osbeck cv. Washington Navel were taken from field trees throughout the year and the nitrogen composition of the sap was determined. The nitrogenous fraction of the sap was composed mainly of free amino acids (92–97% of total nitrogen) and nitrates throughout the year. Proline was the most abundant amino acid during almost the entire cycle, and its concentration was especially high during the autumn and winter period. Nevertheless, a significant part (40–60%) of the total organic nitrogen was transported as arginine. Total nitrogen as well as amino acids and nitrates were maximal at spring flush. At spring flush and summer flush there was also a diversification of α-amino nitrogen among different amino acids. During the spring flush, nitrates, asparagine and γ-aminobutyric acid in the xylem sap seemed to have a radicular origin, whereas glutamic acid and arginine were released from the surrounding parenchyma. The results suggest a metabolic transformation in the wood parenchyma of nitrogenous compounds coming from the roots (including reduction of nitrates) and a turnover of different nitrogen metabolites between the xylem and surrounding cells.     

Nicotianamine and histidine/proline are, respectively, the most important copper chelators in xylem sap of Brassica carinata under conditions of copper deficiency and excess

The effect of two different copper conditions (deficiency andexcess) on the amino acid composition in B. carinata xylem sapwas analysed. When the Cu in the nutrient solution was increasedfrom 0.12 to 2.5 or 5 µM, the concentrations of histidine,threonine, glutamine, proline, methionine, and glycine weremuch increased in the xylem sap. When Cu was made deficientin the nutrient solution by decreasing its concentration from0.12 µM to 0 µM, nicotianamine, glutamine, and threoninewere significantly increased in the xylem sap. Aqueous solutionscontaining different Cu–amino acid complexes (simulatedsaps) responded in a specific way to the changes in pH, providinga signature that was used to evaluate, by comparison with thereal xylem sap, the importance of each amino acid in the xylemtransport of Cu. For a single amino acid, the free solutionCu²⁺ concentration versus pH titration curves for histidineand proline were the most similar to that for xylem under Cuexcess. Under Cu deficiency, this Cu concentration versus pHtitration curve appeared to be very similar to that for nicotianamine.It is concluded that increased Cu concentrations induced theselective synthesis of certain amino acids in the sap, of whichhistidine and proline are the most important. Under Cu deficiency,the concentration of nicotianamine was induced the most. Thefact that nicotianamine is induced under Cu starvation and notunder Cu excess, is in contrast to similar studies indicatingspecies-specific reactions. However, the induction of nicotianamineunder Cu starvation is in line with recent molecular data ofthe role of nicotianamine in intracellular Cu delivery. Key words: Brassica carinata, copper, histidine, nicotianamine, proline, xylem sap Received 30 September 2008; Revised 16 October 2008 Accepted 20 October 2008     

Synthesis, Storage, and Utilization of Amino Compounds in White Lupin (Lupinus albus L.)

Changes in total N and in free amino compounds were followed during growth of nodulated white lupin. Leaflets contained the greatest fraction of plant N but had lower proportions (1 to 4%) of their N in soluble amino form than stem + petioles (10 to 27%) and reproductive parts (15 to 33%). Mobilization of free amino compounds from plant parts to fruits contributed at most only 7% of the total N intake of fruits, compared with 50% in mobilization of other forms of N and 43% from fixation during fruiting. Asparagine was usually the most abundant free amino compound in plant parts, followed by glutamine and alanine. Valine, glycine, isoleucine, aspartic acid and γ-aminobutyric acid comprised the bulk of the remaining soluble amino N. Composition of tissue pools of amino-N closely resembled that of xylem and phloem exudates. Data on N flow and utilization were combined with information on composition of transport fluids to quantify syntheses, exchanges, and consumptions of asparagine, glutamine, aspartic acid, and valine by organs of the 51- to 58-day plant. These amino compounds carried 56, 29, 5, and 2%, respectively, of the N exported from nodules and contributed in roughly commensurate proportions to transport exchanges and N increments of plant parts. There were, however, more than expected involvements of glutamine and valine in mobilization of N from lower leaves, of asparagine in xylem to phloem transfer, and of aspartic acid in cycling of N through the root, and there was a less than expected participation of aspartic acid in xylem to phloem transfer and in phloem translocation to the shoot apex. The significance of these differences is discussed.