Differences in the influx of glutamine and nitrate into Viscum album from the xylem sap of its hosts.

The flux of inorganic and organic nitrogen into the mistletoe Viscum album L. from the xylem sap of a deciduous (Populus x euamericana) and a coniferous host (Abies alba Mill.) was analyzed. For this purpose, a perfusion system was developed in which the xylem sap of the host was replaced by an artificial perfusion solution. With this system flux rates into the mistletoe were determined in feeding experiments either with the organic nitrogen source [1,2-13C2]glutamine at high and the inorganic nitrogen source 15NO3- at low concentration or vice versa. Glutamine influx was already saturated at the low concentration in the xylem sap and was--different from nitrate--not enhanced, when a 250-fold higher concentration was applied. Nitrate influx matched glutamine influx only at high inorganic/organic nitrogen ratios in the perfusion solution. This result indicates a preferential influx of glutamine over nitrate from the host xylem into the mistletoe at the concentrations found in the xylem sap of trees. Surprisingly, a high percentage of both N sources were accumulated in the mistletoe stem, indicating excessive N nutrition of the mistletoe leaves. Since 13C isotope signature was significantly reduced in the outflowing perfusion solution, either an upload of organic compounds from the phloem into the xylem, or an efflux of organic compounds from haustorium of mistletoe into the xylem has to be assumed. 15N isotope signatures enriched in the outflowing perfusion solution support the idea of a nitrate uptake system at the host xylem-haustorium interface, which favors the light N isotope of nitrate.     

Nitrogen distribution in young Norway spruce (Picea abies) trees as affected by pedospheric nitrogen supply

In numerous locations in Europe spruce trees are exposed to high loads of nitrogen. The present study was performed to characterize the distribution of nitrogen compounds under these conditions. For this purpose Norway spruce ( Picea abies [L.] Karst.) trees were cultivated under close-to-natural conditions of a forest understory in soil from an apparently nitrogen-limited field site in the Black Forest either with, or without supplementation of nitrogen as ammonium nitrate. After 11 and 20 months, growth, total nitrogen contents of the biomass, and total soluble non-proteinogenic nitrogen compounds (TSNN, i.e. nitrate, ammonium, soluble proteinogenic and non-proteinogenic amino compounds) in needles, xylem sap and phloem exudate were analysed. After 20 months of growth, N-fertilization had slightly enhanced the biomass of current-, but not of 1-year-old shoots. At both harvests, total N-content of 1-year-old needles was increased by N-fertilization, whereas current-year needles were not significantly affected. By contrast, TSNN was elevated by N-fertilization in both current-year and 1-year-old needles. The increase in TSNN was mainly attributed to an accumulation of arginine. Xylem sap analysis showed that the increase in TSNN of the needles was a consequence of enhanced nitrogen assimilation of the roots rather than the shoot. Since also TSNN in phloem exudates was enhanced, it appears that N-fertilization elevates the cycling pool of amino compounds in young Norway spruce trees. However, this pool seems to be subject to metabolic interconversion, since mainly glutamine and aspartate are transported in the xylem from the roots to the shoot, but arginine accumulated in the needles and the phloem.     

Seasonal and spatial variation of reduced sulphur compounds in mistletoes (Viscum album) and the xylem sap of its hosts (Populus × euramericana and Abies alba)

In the present field study with adult trees inhabited by Viscum album , the question was addressed as to whether European mistletoes are able to remove reduced sulphur from the xylem sap of its hosts. For this purpose the reduced sulphur composition and content of the xylem sap of Viscum album and the corresponding hosts Populus  ×  euramericana and Abies alba were analysed. The xylem sap of Viscum was enriched in reduced sulphur compared to the hosts but still reflected the higher reduced sulphur content of Populus compared to Abies . Despite similar xylem sap composition of the hosts with glutathione as the dominating thiol, Viscum on Populus contained predominantly cysteine, Viscum on Abies predominantly glutathione in its xylem sap. These findings suggest selective and different removal of reduced sulphur from these hosts. Still the amount of reduced sulphur removed was too small to result in changes of the concentration of thiols in the xylem sap of the hosts that are statistically significant, probably due to the high variability encountered under field conditions. Despite the differences in the reduced sulphur composition and contents of the xylem sap between Viscum on Populus and Viscum on Abies , total thiol content as well as thiol composition of Viscum leaves on the two hosts were similar throughout the seasons. The seasonal pattern in the thiol composition and contents of Viscum leaves showed high levels in spring and autumn and low levels in summer. The significance of these seasonal changes is discussed.     

Detection and characterization of nitrogen circulation through the sieve tubes and xylem vessels of rice plants

Nitrogen movement through the xylem vessels and sieve tubes in rice plants was studied using xylem and phloem sap analysis in combination with stable and radioactive nitrogen isotope techniques.More than 90% of nitrogen was translocated in the sieve tubes of rice plants as amino acids. When 15N (99.6 atom%) was applied as a nitrate to the root, 15N first appeared in phloem sap of the leaf sheath within 10 minutes and increased to 37 atom% excess 5 hours after the experiment had started. In long-term experiments, 63% of nitrogen in the phloem sap of the leaf sheath and 15% in that of the uppermost internode came from nitrogen absorbed within the last 24 hours and 50 hours, respectively.To obtain information about the more rapid circulation of nitrogen in the plant, radioactive 13N was used as a tracer. A positron-emitting tracer imaging system was used to show that 13N was transferred to the leaf sheath within 8 minutes of its application to the roots. Analysis of the xylem sap of the leaf sheath showed that when the nitrate was applied to the roots, most of the nitrogen in the xylem was transported as a nitrate.These data showed that phloem and xylem sap analysis together with the stable and radioactive nitrogen techniques provide a good method for the detection of nitrogen cycles in plants.     

Regulation of sulfur nutrition in wild-type and transgenic poplar over-expressing gamma-glutamylcysteine synthetase in the cytosol as affected by atmospheric H2S

This study with poplar (Populus tremula x Populus alba) cuttings was aimed to test the hypothesis that sulfate uptake is regulated by demand-driven control and that this regulation is mediated by phloem-transported glutathione as a shoot-to-root signal. Therefore, sulfur nutrition was investigated at (a) enhanced sulfate demand in transgenic poplar over-expressing gamma-glutamylcysteine (gamma-EC) synthetase in the cytosol and (b) reduced sulfate demand during short-term exposure to H2S. H(2)S taken up by the leaves increased cysteine, gamma-EC, and glutathione concentrations in leaves, xylem sap, phloem exudate, and roots, both in wild-type and transgenic poplar. The observed reduced xylem loading of sulfate after H2S exposure of wild-type poplar could well be explained by a higher glutathione concentration in the phloem. In transgenic poplar increased concentrations of glutathione and gamma-EC were found not only in leaves, xylem sap, and roots but also in phloem exudate irrespective of H(2)S exposure. Despite enhanced phloem allocation of glutathione and its accumulation in the roots, sulfate uptake was strongly enhanced. This finding is contradictory to the hypothesis that glutathione allocated in the phloem reduces sulfate uptake and its transport to the shoot. Correlation analysis provided circumstantial evidence that the sulfate to glutathione ratio in the phloem may control sulfate uptake and loading into the xylem, both when the sulfate demand of the shoot is increased and when it is reduced.     

Nitrogen and Cation Nutrition of Three Ecologically Different Plant Species

Apple rootstocks M.7 were given a nitrogen application either in the spring or in the preceding autumn. At the time of the spring application some rootstocks were ringed. During the 50-day experimental period from bud-break, shoot growth and the amount of nitrogen incorporated into the new shoots were slightly reduced in the spring-treated trees and strongly reduced in the ringed trees of both treatments. Roots of unringed autumn-fertilized trees showed higher levels of total and amino nitrogen than those of similar trees in the spring treatment; to a lesser degree, the reverse held for xylem sap from the stem. Ringing increased the amino-nitrogen level in the roots, which suggests a reduced translocation rate. The nitrogen treatments led to marked differences in the percentage composition of the amino-nitrogen fraction of roots and xylem sap. The distribution of amino acids and amides in the roots and that in xylem sap of the same trees was divergent, but arginine and asparagine often were the most important constituents. Aspartic acid was rather abundant in xylem sap. Ringing did not affect the composition of the amino-nitrogen fraction in the roots quantitatively but increased the proportion of arginine in the sap. The possible relationship between the composition of xylem sap and soluble nitrogen in the roots is discussed. It is argued that especially in spring-fertilized trees appreciable amounts of nitrogen must be translocated via the phloem in addition to the transport in the xylem.     

Glutamine Transfer from Xylem to Phloem and Translocation to Developing Leaves of Populus deltoides

The distribution of ¹⁴C from xylem-borne [¹⁴C]glutamine, the major nitrogen compound moving in xylem sap of cottonwood (Populus deltoides Bartr. ex Marsh), was followed in rapidly growing shoots with a combination of autoradiographic, microautoradiographic, and radioassay techniques. Autoradiography and ¹⁴C analyses of tissues showed that xylem-borne glutamine did not move with the transpiration stream into mature leaves. Instead, most of it was transferred from xylem to phloem in the upper stem and then translocated to young developing tissues. Microautoradiography showed that metaxylem parenchyma, secondary xylem parenchyma, and rays were the major areas of uptake from xylem vessels in the stem. Accumulation in phloem (high ¹⁴C concentrations in sieve tubes) took place in internodes subtending recently mature leaves. Little ¹⁴C from xylem-borne glutamine was found in phloem of mature leaves, which indicates restricted retransport of glutamine that did enter the leaf. In the primary tissues of the upper stem, most ¹⁴C was found in the phloem. Cottonwood stems have an efficient uptake and transfer system that enhances glutamine movement to developing tissues of the upper stem.     

Regulation of Sulfur Nutrition in Wild-Type and Transgenic Poplar Over-Expressing γ-Glutamylcysteine Synthetase in the Cytosol as Affected by Atmospheric H2S

This study with poplar (Populus tremula × Populus alba) cuttings was aimed to test the hypothesis that sulfate uptake is regulated by demand-driven control and that this regulation is mediated by phloem-transported glutathione as a shoot-to-root signal. Therefore, sulfur nutrition was investigated at (a) enhanced sulfate demand in transgenic poplar over-expressing γ-glutamylcysteine (γ-EC) synthetase in the cytosol and (b) reduced sulfate demand during short-term exposure to H₂S. H₂S taken up by the leaves increased cysteine, γ-EC, and glutathione concentrations in leaves, xylem sap, phloem exudate, and roots, both in wild-type and transgenic poplar. The observed reduced xylem loading of sulfate after H₂S exposure of wild-type poplar could well be explained by a higher glutathione concentration in the phloem. In transgenic poplar increased concentrations of glutathione and γ-EC were found not only in leaves, xylem sap, and roots but also in phloem exudate irrespective of H₂S exposure. Despite enhanced phloem allocation of glutathione and its accumulation in the roots, sulfate uptake was strongly enhanced. This finding is contradictory to the hypothesis that glutathione allocated in the phloem reduces sulfate uptake and its transport to the shoot. Correlation analysis provided circumstantial evidence that the sulfate to glutathione ratio in the phloem may control sulfate uptake and loading into the xylem, both when the sulfate demand of the shoot is increased and when it is reduced.     

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.     

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

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

Spatial and temporal variations in phloem sap composition of plantation-grown Eucalyptus globulus

Spontaneous bleeding of sugar-rich sap from cambial-deep incisions in the bark of trunks was demonstrated for Eucalyptus globulus and other eucalypts across a range of localities and seasonal conditions in south-west Australia. High levels of sucrose and raffinose (up to 31% w/v total sugars) were present in the exudates, and upward and downward gradients in exudate sugar concentrations were recorded between samples obtained at different heights up trunks of E. globulus. The data indicated a phloem origin for the exudates, with source:sink pressure gradients driving translocation. Concentration ratios of sugars to amino acids were consistently lower in exudate from upper (distal) than basal regions of trunks, suggesting preferential partitioning of nitrogen upwards towards the trunk apex. A comparison of phloem and xylem sap composition from one plantation over a season showed nitrate in xylem but not phloem and substantial amounts of sodium, and high concentrations of chloride and sulphate relative to phosphate in xylem and phloem. Phloem sap sampled across a range of 29 contrasting plantations of E. globulus at peak stress (autumn) showed great inter-site variability in concentrations of amino acids, sulphur, sodium and certain trace elements and in C:N and Na:K ratios of sap. Carbon isotope ratios (δ¹³C) were strongly correlated with sugar concentrations of the sap samples from these and other plantations. Use of sap compositional attributes of phloem and δ¹³C values of translocated carbon is suggested for assessing the current nutritional condition and water status of E. globulus plantings. Received: 9 April 1998 / Accepted: 20 August 1998     

施用磷对王桉 (Eucalyptus regnans F. Muell.)幼苗生长及其氨基酸组

王桉 (Eucalyptus regnans F. Muell.) 是澳大利亚桉树中最重要的商业用材和人工造林树种之一.研究王桉的施肥与其体内氨基酸的积累和转化及与食叶虫害之间的相关性具有重要的经济和生态意义.在温室内利用2种不同来源的土壤对王桉幼苗进行了不同磷施用量(100 kg hm~(-2)和 200 kg hm~(-2))处理.结果显示,不同土壤和不同磷施用量对苗木生长影响显著,但均未显著影响苗木各部分的氮和磷含量水平.苗木木质部渗出液中的氨基酸含量以谷氨酰胺为主,并与苗木生长和磷施用量呈反相关.不同土壤和磷施用量对苗木组织中游离氨基酸组分和含量的影响不显著,但游离氨基酸的组分和相对水平随叶龄变化明显,尤其是精氨酸在嫩叶氨基酸总量中只占2%～3%,但在老叶中占到20%多;精氨酸在老叶中的积累极有可能是某些蛋白质降解而精氨酸即时合成所致,因为精氨酸一般不在韧皮部转运.谷氨酰胺在树液中含量最高并与苗木生长呈反相关或许可以作为预测桉树发生食叶昆虫危害的一个有用指标.     

Mineral uptake and transport in xylem and phloem of the proteaceous tree,Banksia prionotes

Xylem sap of sinker (tap) root, cluster feeding roots, lateral roots and from an age series of main stem extensions of 6-year trees of Banksia prionotes was collected and analyzed for principal organic and inorganic solutes. During the phase of root uptake activity in winter and spring, cluster roots were principal xylem donors of malate, phosphate, chloride, sodium, potassium and amino acid N whereas other parts of the root served as major sources to the shoot of other cations, nitrate and sulphate. Sinker root xylem sap was at all times less concentrated in solutes than that of lateral roots into which cluster roots were voiding exported solutes. Phosphate was abstracted from xylem by stem tissue during winter and it and a range of other solutes released back to xylem immediately prior to extension growth of the shoot in summer. Phloem sap collected from mid regions of stems was unusually low in potassium and phosphate relative to chloride and sulphate in comparison with phloem sap of other species, and its low potassium: sodium ratio relative to xylem indicated poor discrimination against sodium during phloem loading. Data are discussed in relation to the asynchronous seasonal cycles of nutrient uptake and shoot growth.     

Nodule growth and activity may be regulated by a feedback mechanism involving phloem nitrogen*

We present a mechanism of regulation of growth and activity of legume root nodules which is consistent with published experimental observations. The concentration of reduced nitrogen compounds, probably amino acids, flowing into the nodules from the phloem, is sensed by the nodules; growth and activity of the nodules is adjusted accordingly. In many legumes this response may involve changes in the oxygen diffusion resistance of the nodule cortex. A straightforward feedback mechanism in which nodule activity is lowered when reduced N in the phloem is high and increased when it is low is envisaged. Almost all import into nodules is via the phloem sap originating in the lower leaves. As a plant develops, these mature leaves no longer utilize nitrogen delivered in the xylem and so export it in the phloem. In plants with an adequate nitrogen supply (from nodules or combined nitrogen in soil), a high concentration of nitrogen containing compounds in the phloem from the lower leaves may inhibit nodule growth as well as activity. This suggestion is an alternative to the hypotheses of carbohydrate deprivation or nitrate inhibition which are commonly used to explain the effects of combined nitrogen on nodule growth and activity.     

A phloem-sap feeder mixes phloem and xylem sap to regulate osmotic potential

Phloem-sap feeders (Hemiptera) occasionally consume the dilute sap of xylem, a behaviour that has previously been associated with replenishing water balance following dehydration. However, a recent study reported that non-dehydrated aphids ingested xylem sap. Here, we tested the hypothesis that the consumption of xylem sap, which has a low osmolality, is a general response to osmotic stresses other than dehydration. Alate aphids were subjected to different treatments and subsequently transferred onto a plant, where electrical penetration graph (EPG) was used to estimate durations of passive phloem sap consumption and active sucking of xylem sap. The proportion of time aphids fed on xylem sap (i.e., time spent feeding on xylem sap/total time spent feeding on phloem plus xylem sap) was used as a proxy of the solute concentration of the uptake. The proportion of time alate aphids fed on xylem sap increased: (1) with the time spent imbibing an artificial diet containing a solution of sucrose, which is highly concentrated in phloem sap and is mainly responsible for the high osmotic potential of phloem sap; (2) with the osmotic potential of the artificial diet, when osmotic potential excess was not related to sucrose concentration; and (3) when aphids were deprived of primary symbionts, a condition previously shown to lead to a higher haemolymph osmotic potential. All our results converge to support the hypothesis that xylem sap consumption contributes to the regulation of the osmotic potential in phloem-sap feeders.     

Seasonal courses of key parameters of nitrogen,carbon and water balance in European beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) grown on four different study sites along a European North–South climate gradient during the 2003 drought

During the growing season of the exceptionally dry and warm year 2003, we assessed seasonal changes in nitrogen, carbon and water balance related parameters of mature naturally grown European beech (Fagus sylvatica L.) along a North–South transect in Europe that included a beech forest stand in central Germany, two in southern Germany and one in southern France. Indicators for N balance assessed at all four sites were foliar N contents and total soluble non-protein nitrogen compounds (TSNN) in xylem sap, leaves and phloem exudates; C and water balance related parameters determined were foliar C contents, δ¹³C and δ¹⁸O signatures. Tissue sampling was performed in May, July and September. The N related parameters displayed seasonal courses with highest concentrations during N remobilization in May. Decreased total foliar N contents as well as higher C/N ratios in the stands in central Germany and southern France compared to the other study sites point to an impaired N nutrition status due to lower soil N contents and precipitation perception. TSNN concentrations in leaves and phloem exudates of all study sites were in ranges previously reported, but xylem sap content of amino compounds in July was lower at all study sites when compared to literature data (c. 1 μmol N mL⁻¹). In September, TSNN concentrations increased again at the two study sites in southern Germany after a rain event, whereas they remained constant at sites in central Germany and southern France which hardly perceived precipitation during that time. Thus, TSNN concentrations in the xylem sap might be indicative for water balance related N supply in the beech trees. TSNN profiles at all study sites, however, did not indicate drought stress. Foliar δ¹³C, but not foliar C and δ¹⁸O followed a seasonal trend at all study sites with highest values in May. Differences in foliar δ¹³C and δ¹⁸O did not reflect climatic differences between the sites, and are attributed to differences in altitude, photosynthesis and δ¹⁸O signatures of the water sources. Except of low TSNN concentrations in the xylem sap, no physiological indications of drought stress were detected in the trees analysed. We suppose that the other parameters assessed might not have been sensitive to the drought events because of efficient regulation mechanisms that provide a suitable physiological setting even under conditions of prolonged water limitation. The uniform performance of the trees from southern France and central Germany under comparably dry climate conditions denotes that the metabolic plasticity of mature beech from the different sites studied might be similar.     

Physiological performance of beech (Fagus sylvatica L.) at its southeastern distribution limit in Europe: seasonal changes in nitrogen, carbon and water balance

To assess the physiological performance of drought-sensitive European beech ( Fagus sylvatica L.) under the dry Mediterranean climate prevailing at its southeastern distribution limit in Europe, we analyzed seasonal changes in carbon, nitrogen and water balance of naturally grown adult trees. We determined the foliar C and N contents, delta13C and delta18O signatures, total soluble non-protein nitrogen compounds (TSNN) in xylem, leaves, and phloem, as well as leaf water potential and photosynthetic quantum yield in northern Greece during 2003. Tissue sampling was performed in May, July, and September, while field measurements were conducted regularly. Climatic conditions for the 2003 growing season fall within the typical range of the studied area. The N- and C-related parameters displayed distinct seasonal courses. TSNN was highest in May in all tissues, and asparagine (Asn) was then the most abundant compound. Thereafter, TSNN decreased significantly in all tissues and both its concentration and composition remained constant in July and September. In both months, glutamate (Glu) prevailed in leaves, gamma-aminobutyric acid (GABA) in phloem exudates from twigs and trunks, and arginine (Arg) in the xylem sap, where loading with amino acids was rather low during that period, amounting to only 0.8 micromol N ml-1 in September. Highest total foliar N and C contents were detected in May, and the elevated abundance of nutrients as well as an increased foliar delta13C signature at the beginning of the growing season is attributed to remobilization processes. The signatures of delta18O, quantum yield and leaf water potentials varied only slightly throughout the growing season. Although summer precipitation at the study site was considerably lower compared to what is usual for typical central European beech forests, no intensive drought responses of the physiological apparatus were detected in the studied beech trees. This suggests efficient internal regulation mechanisms, constantly ensuring a favourable physiological status under the relatively dry Mediterranean climate.     

Root-shoot interactions in mineral nutrition

In this paper four classes of co-operative root-shoot interations are addressed. (I) Nitrogen concentrations in the xylem sap originating from the root and in the phloem sap as exported from source leaves are much lower than those required for growth by apices and developing organs. Enrichment of xylem sap N is achieved by xylem to xylem (X-X) transfer, by which reduced N, but not nitrate, is abstracted from the xylem of leaf traces and loaded into xylem vessels serving the shoot apex. Nitrogen enrichment of phloem sap from source leaves is enacted by transfer of reduced N from xylem to phloem (X-P transfer). Quantitative data for the extent of the contribution of X-X and X-P transfer to the nutrition of young organs of Ricinus communis L. and for their change with time are presented. (II) Shoot and root cooperate in nitrate reduction and assimilation. The partitioning of this process between shoot and root is shifted towards the root under conditions of nitrate- and K-deficiency and under salt stress, while P deficiency shifts nitrate reduction almost totally to the shoot. All four changes in partitioning can be attributed to the need for cation-anion balance during xylem transport and the change in electrical charge occurring with nitrate reduction. (III) Even maintenance of the specificity of ion uptake by the root may – in addition to its need for energy – require a shoot-root interaction. This is shown to be needed in the case of the maintenance of K/Na selectivity under the highly adverse condition of salt stress and absence of K supply from the soil. (IV) Hormonal root to shoot interactions are required in the whole plant for sensing mineral imbalances in the soil. This is shown and addressed for conditions of salt stress and of P deficiency, both of which lead to a strong ABA signalling from root to shoot but result in different patterns of response in the shoot.     

Modelling of the Partitioning, Assimilation and Storage of Nitrate within Root and Shoot Organs of Castor Bean (Ricinus communis L.)

An experimentally-based modelling technique was developed todescribe quantitatively the uptake, flow, storage and utilizationof NO₃-N over a 9 d period in mid-vegetative growth of sandcultured castor bean (Ricinus communis L.) fed 12 mol m^–3nitrate and exposed to a mean salinity stress of 128 mol m^–3NaCl. Model construction used information on increments or lossesof NO₃-N or total reduced N in plant parts over the study periodand concentration data for NO₃-N and reduced (amino acid) Nin phloem sap and pressure-induced xylem exudates obtained fromstem, petiole and leaf lamina tissue at various levels up ashoot. The resulting models indicated that the bulk (87%) of incomingnitrate was reduced, 51% of this in the root, the remainderprincipally in the laminae of leaves. The shoot was 60% autotrophicfor N through its own nitrate assimilation, but was oversuppliedwith surplus reduced N generated by the root and fed to theshoot through the xylem. The equivalent of over half (53%) ofthis N returned to the root as phloem translocate and, mostly,then cycled back to the shoot via xylem. Nitrate comprised almosthalf of the N of most xylem samples, but less than 1% of phloemsap N. Laminae of leaves of different age varied greatly inN balance. The fully grown lower three leaves generated a surplusof reduced N by nitrate assimilation and this, accompanied byreduced N cycling by xylem to phloem exchange, was exportedfrom the leaf. Leaf 4 was gauged to be just self-sufficientin terms of nitrate reduction, while also cycling reduced N.The three upper leaves (5–7) met their N balance to varyingextents by xylem import, phloem import (leaves 6 and 7 only)and assimilation of nitrate. Petioles and stem tissue generallyshowed low reductase activities, but obtained most of theirN by abstraction from xylem and phloem streams. The models predictedthat nodal tissue of lower parts of the stem abstracted reducedN from the departing leaf traces and transferred this, but notnitrate, to xylem streams passing further up the shoot. As aresult, xylem sap was predicted to become more concentratedin N as it passed up the shoot, and to decrease the ratio ofNO₃-N to reduced N from 0·45 to 0·21 from thebase to the top of the shoot. These changes were reflected inthe measured N values for pressure-induced xylem exudates fromdifferent sites on the shoot. Transfer cells, observed in thexylem of leaf traces exiting from nodal tissue, were suggestedto be involved in the abstraction process. Key words: Ricinus communis, nitrogen, nitrate, nitrate reduction, partitioning, phloem, xylem, flow models     

Consequences of elevated CO2, augmented nitrogen-deposition and soil type on the soluble nitrogen and sulphur in the phloem of beech (Fagus sylvatica) and spruce (Picea abies) in a competitive situation

Mixed spruce-beech plantations grown in large open-top chambers (OTC) were used to study consequences of elevated CO2, nitrogen-deposition and soil type on plant internal nitrogen and sulphur cycling of juvenile beech (Fagus sylvatica L.) and spruce (Picea abies Karst.) in a competitive situation. Processes of re-cycling as a consequence of protein turnover during leaf senescence in autumn were of further interest. For this purpose, phloem sap was collected in September 1998 and analysed for the composition and concentrations of organic and inorganic nitrogen and sulphur compounds. The phloem exudate of spruce showed higher total soluble non-protein nitrogen (TSNN) concentration on calcareous soil than on acidic soil, independent of the treatment. N-fertilization increased the N-concentration of phloem exudate significantly on both soil types, mainly by an increase of Arg and Gln concentrations. Elevated CO2 slightly increased TSNN on calcareous, but not on acidic soil. The combination of elevated CO2 and augmented N-deposition induced a further increase of TSNN on calcareous soil, but caused a lower N-effect on TSNN on acidic soil. Arg, the main TSNN component in phloem exudate, mediated this effect. Since Arg is considered to be a major nitrogen storage compound, it is concluded that in autumn elevated CO2 and augmented N-deposition, influence storage of N rather than N-supply of spruce. An effect of elevated CO2 and augmented N-deposition on GSH and sulphate concentrations in phloem exudate of spruce was not observed on acidic soil. On calcareous soil augmented N-deposition enhanced, elevated CO2 decreased phloem exudate GSH contents. In combination, elevated CO2 compensated the positive effect of N-deposition. The effects of elevated CO2 and augmented N-deposition on phloem sap N- and S-contents described above were not observed for beech trees. Apparently, elevated CO2 and augmented N-deposition did not affect plants internal S and N cycling of beech grown in spruce-beech plantations.