Partitioning of carbon and nitrogen and the nutrition of root and shoot apex in a nodulated legume

Empirically based models depicting exchanges of C, N, and H₂O in phloem and xylem among organs of nodulated white lupin (Lupinus albus cv Ultra) were constructed for the interval 51 to 58 days after sowing. Information was incorporated on the economy of C, N, and H₂O in plant parts, the solute composition of transport fluids collected at selected sites on the plant, and the photosynthetic inputs, transpirational losses, and translocatory activities of different age groups of leaflets and stem + petiole segments of the shoot. Partitioning of C and N showed preferential transfer of N to the shoot apex, which imported 13 milligrams C per milligram N, compared with 54 milligrams C per milligram N for the nodulated root. Leaves translocated assimilates at a C:N weight ratio of 43 to 59, and older leaves serving the roots produced the translocate most rich in N relative to C. The shoot apex was enriched with N, additional to its intake from leaves, by direct uptake of xylem fluid (C:N ratio, 2.4) and receipt of nitrogenous solutes transferred from xylem to upward-moving phloem streams in upper regions of the stem. The models for flow of N and H₂O indicated that xylem streams passing to leaves were substantially less rich in N than the adjacent stream moving through the body of the stem and that a progressive increase in concentration of N occurred within stem xylem elements from base to top of the shoot. This apparently resulted from an abstraction of N from xylem of departing leaf traces, possibly by xylem transfer cells, and a subsequent feedback of this N to xylem streams passing on up the shoot. Upper leaves and shoot apex, therefore, acquired more N from xylem per unit of H₂O transpired than lower parts of the shoot.     

Effects of P deficiency on the uptake, flows and utilization of C, N and H2O within intact plants of Ricinus communis L.

The influence of P deficiency on the uptake, flow and utilizationof C, N and H₂0 by intact NO₃-fed castor bean plants {Ricinuscommunis L.) was studied over a 9 d period in the middle oftheir vegetative growth. The modelling techniques incorporateddata on net increments or losses of C, N and H₂O in plant parts,photosynthetic gains in and respiratory losses of C, molar C:Nratios of solutes in phloem and xylem sap and transpirationallosses of H₂0. Plant growth was inhibited within 3 d of withholdingP supply and dry matter production was less than one-third ofthe controls. Leaf growth was particularly depressed, whileroot growth was much less affected than that of the shoot. Shoot:rootratio of low-P plants was 1.5 compared with 2.6 under P supply.Over the 9 d study period total plant C and N increased by 560and 47 mmol, respectively, in the controls, but by only 113and 6.9 mmol in the low-P treatment. The particularly low incrementof N in P-deficient plants was due principally to decreasedN0₃^- uptake. Flows of C and N during the study period were markedlydifferent between control and P-deficient plants. The partitioningprofile for C in P-deficient plants showed a dramatic inhibitionof net photosynthesis and attendant photoassimilate flow. Proportionaldownward to upward allocation of carbon increased with increasein sink size of the root relative to shoot. This was reflectedin greater relative allocation of C to root dry matter and rootrespiration than in P-sufficient plants, and suppressed cyclingof C from root to shoot via xylem. Nitrogen intake and xylemtransport to the shoot of P-deficient plants were only 15% ofthe control and, as in the case of C, downward allocation ofN predominated over upward phloem translocation. Apart fromthese severe changes, however, the basic patterns of N flowsincluding xylem-to-phloem and xylem-to-xylem transfer of N werenot changed, a feature highlighting the vital nature of thesetransfer processes even under deficiency conditions. The alterationsin flows and partitioning of C, N and H₂O in response to low-Pconditions are discussed in relation to the corresponding effectsof moderate salt stress in Ricinus and the conclusion is reachedthat changes in nutrient flows under P deficiency were morehighly co-ordinated than when plants experience salt stress.Flow profiles under P deficiency which favour root growth andactivity are viewed as a means for increasing the potentialcapability of the plant to acquire P from the nutrient medium. Key words: Ricinus communis L., P deficiency, carbon, nitrogen, water, partitioning, xylem transport, phloem transport     

Contrasting dynamics of water and mineral nutrients in stems shown by stable isotope tracers and cryo‐SIMS

Lateral exchange of water and nutrients between xylem and surrounding tissues helps to de‐couple uptake from utilization in all parts of a plant. We studied the dynamics of these exchanges, using stable isotope tracers for water (H₂¹⁸O), magnesium (²⁶Mg), potassium (⁴¹K) and calcium (⁴⁴Ca) delivered via a cut stem for various periods to the transpiration stream of bean shoots (Phaseolus vulgaris cv. Fardenlosa Shiny). Tracers were subsequently mapped in stem cross‐sections with cryo‐secondary ion mass spectrometry. The water tracer equilibrated within minutes across the entire cross‐section. In contrast, the nutrient tracers showed a very heterogeneous exchange between xylem vessels and the different stem tissues, even after 4 h. Dynamics of nutrients in the tissues revealed a fast and extensive exchange of nutrients in the xylem parenchyma, with, for example, calcium being completely replaced by tracer in less than 5 min. Dilution of potassium tracer during its 30 s transit in xylem sap through the stem showed that potassium concentration was up‐regulated over many hours, to the extent that some of it was probably supplied by phloem recirculation from the shoot.     

Impact of Elevated PCO2 on Mass Flow of Reduced Nitrogen in Trees

To analyze the effects of elevated carbon dioxide concentration （PCO2） on the mass flow of reduced nitro- gen （N） in the phloem and xylem of trees, juvenile beech （Fagus sylvatica L.） and spruce （Picea abies （L.） Karst.） were grown in phytotrons and exposed to ambient and elevated PCO2 （plus 687.5 mg/m^3 CO2） for three growing seasons. Elevated PCO2 significantly decreased the mass flow of N from the shoot to roots of beech by significantly reducing the concentration of soluble amino compounds in the phloem, even if the area of conductive phloem of cross-sectional bark tissue was significantly increased, because of less callus deposition in the sieve elements. In spruce, the downward mass flow of reduced N also tended to be decreased, similar to that in beech. Resembling findings in the phloem, N mass flow from roots to shoot in both tree species was significantly diminished owing to significantly reduced concentrations of amino compounds in the xylem and a lower transpiration rate. Therefore, the mass flow of reduced N between shoots and roots of trees was mainly governed by the concentrations of soluble amino compounds in the phloem and xylem in relation to the loading of reduced N in both long-distance transport pathways.     

Diurnal water balance of the cowpea fruit

The vascular network of the cowpea (Vigna unguiculata [L.] Walp.) fruit exhibits the anatomical potential for reversible xylem flow between seeds, pod, and parent plant. Feeding of cut shoots with the apoplast marker acid fuchsin showed that fruits imported regularly via xylem at night, less frequently in early morning, and only rarely in the afternoon. The dye never entered seeds or inner dorsal pod strands connecting directly to seeds. Root feeding (early morning) of intact plants with ³²PO₄ or ³H₂O rapidly (20 min) labeled pod walls but not seeds, consistent with uptake through xylem. Weak subsequent (4 hours) labeling of seeds suggested slow secondary exchange of label with the phloem stream to the fruit. Vein flap feeding of subtending leaves with [¹⁴C]sucrose, ³H₂O, and ³²PO₄ labeled pod and seed intensely, indicating mass flow in phloem to the fruit. Over 90% of the ¹⁴C and ³H of fruit cryopuncture phloem sap was as sucrose and water, respectively. Specific ³H activities of transpired water collected from fruits and peduncles were assayed over 4 days after feeding ³H₂O to roots, via leaf flaps, or directly to fruits. The data indicated that fruits transpired relatively less xylem-derived (apoplastic) water than did peduncles, that fruit and peduncle relied more heavily on phloem-derived (symplastic) water for transpiration in the day than at night, and that water diffusing back from the fruit was utilized in peduncle transpiration, especially during the day. The data collectively support the hypothesis of a diurnally reversing xylem flow between developing fruit and plant.     

Modeling the transport and utilization of carbon and nitrogen in a nodulated legume

An empirical modeling technique was developed for depicting quantitatively the transport and partitioning of photosynthetically fixed C and symbiotically fixed N during 10-day intervals of a 40-day period in the growth of nodulated plants of white lupin (Lupinus albus L. cv. Ultra). Model construction utilized data for C and N consumption of plant parts and C:N weight ratios of the xylem and phloem fluids serving specific plant organs. Formulas were derived from calculating the net transport of C and N between plant parts in xylem and phloem. The models provided quantitative information on the dependence of growing organs on xylem and phloem for their supply of C and N, the cycling of N through leaflets and of C through nodules, the extent of direct incorporation of fixed N into growing nodules, and the involvement of N from shoot translocate in the nutrition of the nodulated root. Stem plus petioles abstracted considerably more N from xylem than expected from their transpirational activity. Xylem to phloem transfer of recently fixed N in mature stem and petioles was substantiated by the models, being depicted as a device for dispensing N to growing parts of the shoot extra to that attracted transpirationally in xylem or received as translocate from leaflets.     

Diurnal Patterns of Transport and Accumulation of Minerals in Fruiting Plants of Lupinus angustifolius L.

Fluctuations in mineral elements id xylem (tracheal) sap, fruitphloem sap, leaflets and dmloping fruits were studied in a fieldpopulation of Lupinus angustifolius L. by three-hourly samplingover a 39 h period. Elements usually reached maximum contentsor concentrations at or near noon, minimum levels during thenight. Amplitudes of diurnal fluctuations in minerals lay withinthe range ±4–33 per cent of the mean content ofleaflets, and ±17–157 per cent of the mean concentrationsin xylem and phloem sap. Most minerals elements fluctuatcd inphase with daily changes in sugar level of phloem sap and drymatter and carbohydrate fluctuations of leaflets, suggestinga coupling of translocation of photosynthate and minerals fromthe leaflets. Rates of import of minerals by shoots wereestimatedfrom shoot transpiration and mineral concentrations in trachealsap. Average day time rates of import of most elements were12–25 times those at night. Translocation of minerals,nitrogen and carbon to fruits also exhibited diurnal periodicity,average rates of import king three to seven times higher inthe day than at night. A model of transport based on the carbonand water economy of the fruit suggested that P, K, Fe, Zn,Mn and Cu were imported predominantly by phloem. Estimates ofvascular import accounted for 87–104 per cent of the fruit'sactual increment of these elements. Na and Ca were gauged tobe imported mainly by xylem, Mg almost equally by xylem andphloem. However, large discrepancies existed for these threeelements between estimated vascular import and actual intakeby the fruit. Lupinus angustifolius L., mineral transport, accumulation, fruits, xylem sap, phloem sap, transpiration     

Translocation of paclobutrazol and gibberellic acid in Sturt's Desert Pea (Swainsona formosa)

The translocation patterns of paclobutrazol and gibberellic acid were studied by applying these plant growth regulators locally to the main or lateral shoots of Sturt's Desert Pea. Paclobutrazol only reduced the growth of shoots to which it was directly applied indicating that it was readily translocated acropetally within a shoot (via xylem) but not basipetally (via phloem), although some phloem translocation has been reported. Gibberellic acid elongated the main shoot and enhanced apical dominance irrespective of the place of application suggesting it is readily translocated both through xylem and phloem. The translocation patterns did not vary between the main or lateral shoots.     

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.     

Metabolism and translocation of allantoin in ureide-producing grain legumes

Transfer of the nitrogen and carbon of allantoin to amino acids and protein of leaflets, stems and petioles, apices, peduncles, pods, and seeds of detached shoots of nodulated cowpea (Vigna unguiculata L. Walp. cv. Caloona) plants was demonstrated following supply of [2-¹⁴C], [1,3-¹⁵N]allantoin in the transpiration stream. Throughout vegetative and reproductive growth all plant organs showed significant ureolytic activity and readily metabolized [2-¹⁴C]allantoin to ¹⁴CO₂. A metabolic pathway for ureide nitrogen utilization via allantoic acid, urea, and ammonia was indicated. Levels of ureolytic activity in extracts from leaves and roots of nodulated cowpea were consistently maintained at higher levels than in non-nodulated, NO₃⁻ grown plants.

[¹⁴C]Ureides were recovered in extracts of aphids (Aphis craccivora and Macrosiphum euphorbieae) feeding at different sites on cowpea plants supplied with [2-¹⁴C]allantoin through the transpiration stream or to the upper surface of single leaflets. The data indicated that the ureides were effectively transferred from xylem or leaf mesophyll to phloem, and then translocated in phloem to fruits, apices, and roots.

     

Developmental changes in the diurnal water budget of the grape berry exposed to water deficits

The diurnal water budget of developing grape (Vitis vinifera L.) berries was evaluated before and after the onset of fruit ripening (veraison). The diameter of individual berries of potted ‘Zinfandel’ and ‘Cabernet Sauvignon’ grapevines was measured continuously with electronic displacement transducers over 24 h periods under controlled environmental conditions, and leaf water status was determined by the pressure chamber technique. For well-watered vines, daytime contraction was much less during ripening (after veraison) than before ripening. Daytime contraction was reduced by restricting berry or shoot transpiration, with the larger effect being shoot transpiration pre-veraison and berry transpiration post-veraison. The contributions of the pedicel xylem and phloem as well as berry transpiration to the net diurnal water budget of the fruit were estimated by eliminating phloem or phloem and xylem pathways. Berry transpiration was significant and comprised the bulk of water outflow for the berry both before and after veraison. A nearly exclusive role for the xylem in water transport into the berry was evident during pre-veraison development, but the phloem was clearly dominant in the post-veraison water budget. Daytime contraction was very sensitive to plant water status before veraison but was remarkably insensitive to changes in plant water status after veraison. This transition is attributed to an increased phloem inflow and a partial discontinuity in berry xylem during ripening.     

Spontaneous Phloem bleeding from cryopunctured fruits of a ureide-producing legume

The vasculature of the dorsal suture of cowpea (Vigna unguiculata [L.] Walp) fruits bled a sugar-rich exudate when punctured with a fine needle previously cooled in liquid N₂. Bleeding continued for many days at rates equivalent to 10% of the estimated current sugar intake of the fruit. A phloem origin for the exudate was suggested from its high levels (0.4-0.8 millimoles per milliliter) of sugar (98% of this as sucrose) and its high K⁺ content and high ratio of Mg²⁺ to Ca²⁺. Fruit cryopuncture sap became labeled with ¹⁴C following feeding of [¹⁴C]urea to leaves or adjacent walls of the fruit, of ¹⁴CO₂ to the pod gas space, and of [¹⁴C] asparagine or [¹⁴C]allantoin to leaflets or cut shoots through the xylem. Rates of translocation of ¹⁴C-assimilates from a fed leaf to the puncture site on a subtended fruit were 21 to 38 centimeters per hour. Analysis of ¹⁴C distribution in phloem sap suggested that [¹⁴C]allantoin was metabolized to a greater extent in its passage to the fruit than was [¹⁴C] asparagine. Amino acid:ureide:nitrate ratios (nitrogen weight basis) of NO₃-fed, non-nodulated plants were 20:2:78 in root bleeding xylem sap versus 90:10:0.1 for fruit phloem sap, suggesting that the shoot utilized NO₃-nitrogen to synthesize amino acids prior to phloem transfer of nitrogen to the fruit. Feeding of ¹⁵NO₃ to roots substantiated this conclusion. The amino acid:ureide ratio (nitrogen weight basis) of root xylem sap of symbiotic plants was 23:77 versus 89:11 for corresponding fruit phloem sap indicating intense metabolic transfer of ureide-nitrogen to amino acids by vegetative parts of the plant.     

Economy of water, carbon, and nitrogen in the developing cowpea fruit

The nutritional economy of the fruit of cowpea (Vigna unguiculata (L.) Walp cv Vita 3) was assessed quantitatively from intake and utilization of carbon, nitrogen, and water. Fruits failed to make net gains of CO₂ from the atmosphere during daytime, although pod photosynthesis did play a role in the fruit's carbon economy by refixing a proportion of the fruit's respired CO₂. Of every 100 units by weight of carbon entering the fruit, 70.4 were finally incorporated into seeds, 10.3 remained as nonmobilizable material in pod walls, and the remaining 19.3 were lost in fruit respiration. Phloem supplied 97% of the fruit's carbon and 72% of its nitrogen. The xylem contribution of nitrogen occurred mainly in early growth. Ninety-six% of the fruit's nitrogen was incorporated into seeds, approximately 10% of this mobilized from the senescing pod. The mean transpiration ratio of the fruit was very low—8 milliliters water transpired per gram dry matter accumulated. Models of carbon, nitrogen, and water flow were constructed for the two consecutive 11 day periods of fruit development, and indicated a considerably greater entry of water through xylem and phloem than could be accounted for in changes in fruit tissue water and transpiration loss. This discrepancy was greater in the second half of fruit growth and was interpreted as evidence that a significant fraction of the water entering the fruit through phloem cycled back to the parent plant via the xylem.     

Temporal Patterns of Uptake, Flow and Utilization of Nitrate, Reduced Nitrogen and Carbon in a Leaf of Salt-Treated Castor Bean (Ricinus communis L.)

Changes in net photosynthesis, respiration, transpiration andcontents of total C, NO₃-N and reduced N were followed throughoutthe life of leaf 6 of nitrate-dependent plants of castor beanexposed to moderate salinity stress (71 mol m^–3 NaCl).Salt treatment was applied for measuring mineral flows in aparallel study (Jeschke and Pate, 1991b). Concurrent measurementswere made of solute composition and C: N molar ratios and concentrationsof reduced N and collected NO₃-N in phloem sap bleeding fromshallow incisions in the top and at the base of petioles andin xylem exudates from flaps of proximal leaf midribs followingpressurization of the root system. The resulting data were usedto construct empirical models of the respective economies ofC, total N, NO₃ and reduced N for a sequence of defined phasesof leaf life. Water use efficiency increased 3-fold from emergenceto a maximum of 1·5 mmol CO₂ mol^–1 H₂O before decliningto 0·5 mmol CO₂ mol^–1 H₂O at senescence. Xylemmolar ratios of C:N varied from 1·2–2·8,with nitrate always a smaller component than reduced N. Phloemsap C:N increased from 10–40 with leaf expansion and wasthen maintained in the range of 40–50 until falling steeplyto 20 at leaf senescence. Nitrate comprised less than 1% oftotal N in all phloem sap samples. The models of C uptake, flow,and utilization showed a major role of phloem import and thenincreasingly of laminar photosynthesis in providing C for leafgrowth. The carbon budget was thereafter characterized by ratesof phloem export closely matched to net rates of CO₂ fixationby the lamina. Corresponding data for total N depicted an earlymajor role of both xylem and phloem import, but the eventualdominance of xylem import as the N source for leaf growth. Cyclingof N by xylem to phloem exchange commenced before the leaf hadachieved maximum N content, and was the major contributor tophloem export until leaf senescence when mobilized N providedmost exported N. The nitrate economy of the leaf was characterizedby early establishment of tissue pools of the ion in the petioleand to a lesser extent in the lamina, continued high rates ofnitrate reduction in the lamina but negligible assimilationin the petiole, and a release through xylem of previously accumulatedNO₃ from petiole to lamina. Related data for reduced N illustratedthe much greater importance of this form of N than nitrate intransport, storage and cycling of N at all stages of leaf andpetiole life. Xylem to phloem interchanges of reduced N in petiolewere minimal in comparison with cycling through the lamina.The ratio of CO₂ reduction to NO₃ reduction in the lamina wasat first low (57 mol mol^–1) increasing to a peak valueof 294 during mature leaf functioning before declining to 190during the presenescence phase of leaf development. This patternreflected age-related effects on water use efficiency, changesin NO₃ levels in the xylem stream entering the lamina, and therelatively low photosynthetic performances of very young andsenescent laminae. Key words: Ricinus communis, leaf development, phloem transport, xylem transport, carbon, nitrogen, nitrate, reduced nitrogen, nitrate reduction, partitioning     

Xylem and Phloem transport and the functional economy of carbon and nitrogen of a legume leaf

Exchanges of CO₂ and changes in content of C and N were studied over the life of a leaf of Lupinus albus L. These data were combined with measurements of C:N weight ratios of xylem (upper stem tracheal) and phloem (petiole) sap to determine net fluxes of C and N between leaf and plant. Phase 1 of leaf development (first 11 days, leaf to one-third area) showed increasing net import of C and N, with phloem contributing 61% of the imported C and 18% of the N. ¹⁴C feeding studies suggested the potential for simultaneous import and export through phloem over the period 9 to 12 days. Phase 2 (11-20 days, leaf attaining maximum area and net photosynthesis rate) exhibited net import through xylem and increasing export through phloem. Eighty-two% of xylem-delivered N was consumed in leaf growth, the remainder exported in phloem. Phase 3 (20-38 days) showed high but declining rates of photosynthesis, translocation, and net export of N. Phase 4 (38-66 days) exhibited substantial losses of N and declining photosynthesis and translocation of C. C:N ratio of xylem sap remained constant (2.3-2.6) during leaf life; petiole phloem sap C:N ratio varied from 25 to 135 over leaf development. The relationships between net photosynthesis and N import in xylem were: phase 1, 4.8 milligrams C per milligram N; phase 2, 24.7 milligrams C per milligram N; phase 3, 91.9 milligrams C per milligram N; and phase 4, 47.7 milligrams C per milligram N.     

Economy of Carbon and Nitrogen in a Nodulated and Nonnodulated (NO(3)-grown) Legume

Partitioning and utilization of assimilated C and N were compared in nonnodulated, NO₃-fed and nodulated, N₂-fed plants of white lupin (Lupinus albus L.). The NO₃ regime used (5 millimolar NO₃) promoted closely similar rates of growth and N assimilation as in the symbiotic plants. Over 90% of the N absorbed by the NO₃-fed plants was judged to be reduced in roots. Empirically based models of C and N flow demonstrated that patterns of incorporation of C and N into dry matter and exchange of C and N among plant parts were essentially similar in the two forms of nutrition. NO₃-fed and N₂-fed plants transported similar types and proportions of organic solutes in xylem and phloem. Withdrawal of NO₃ supply from NO₃-fed plants led to substantial changes in assimilate partitioning, particularly in increased translocation of N from shoot to root. Nodulated plants showed a lower (57%) conversion of C or net photosynthate to dry matter than did NO₃-fed plants (69%), and their stems were only half as effective as those of NO₃-fed plants in xylem to phloem transfer of N supplied from the root. Below-ground parts of symbiotic plants consumed a larger share (58%) of the plants' net photosynthate than did NO₃-fed roots (50%), thus reflecting a higher CO₂ loss per unit of N assimilated (10.2 milligrams C/milligram N) by the nodulated root than by the root of the NO₃-fed plant (8.1 milligrams C/milligram N). Theoretical considerations indicated that the greater CO₂ output of the nodulated root involved a slightly greater expenditure for N₂ than for NO₃ assimilation, a small extra cost due to growth and maintenance of nodule tissue, and a considerably greater nonassimilatory component of respiration in root tissue of the symbiotic plant than in the root of the NO₃-fed plant.     

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     

Solute fluxes from tobacco to the parasitic angiosperm Orobanche cernua and the influence of infection on host carbon and nitrogen relations

Orobanche species are holoparasites which are very efficient sinks for host-derived solutes. Here, we report the use of direct measurements of xylem sap solute concentrations and water fluxes, together with a modelling procedure to calculate element fluxes within an association between Orobanche cernua and its tobacco host. Infection of tobacco by the parasite markedly influenced carbon acquisition and partitioning; net fixation of carbon was 20% higher in infected tobacco compared with controls. Orobanche cernua caused a 84% increase in net carbon flux moving downward from the tobacco shoot and 73% of this carbon was intercepted by the parasite, almost entirely through the phloem (>99%). Further, the parasite also exerted a large impact on the nitrogen relations of the plant, notably nitrate uptake was stimulated and the amino acid content of xylem sap was lower. The parasite also relied heavily on host phloem for the supply of other resources, with only 5 to 15% of N, and 16% of K, 23% of Na, 63% of Mg and 13% of S being derived from the xylem. Thus, we provide quantitative information on the phloem dependency of the parasite and show that host carbon and nitrogen metabolism is stimulated as a consequence of infection.     

Modelling the flow and partitioning of carbon and nitrogen in the holoparasite Cuscuta reflexa Roxb. and its host Lupinus albus L.: I. Methods for estimating net flows

Nodulated Lupinus albus L. was grown on quartz sand in the greenhouseand supplied with a N-free culture solution. Half the plantswere infected with Cuscuta reflexa Roxb. at 33 DAS. An empiricallybased modelling technique was developed to quantitatively depictuptake, flow and utilization of C and N in the host plant andbetween host and parasite over a 12 d period. The modellingincorporated C: N ratios of solutes in phloem and pressure-inducedxylem sap, net increments of C and N and respiratory lossesof C. For assessing the transfer of solutes from host phloemto Cuscuta it was not possible to use the C: N ratio of phloemsap close to the site of parasite attachment, a procedure whichwould have assumed non-specific withdrawal of phloem-borne solutes,since this would have implied unimpeded mass flow from hostto parasite. The relative intake of C and N by the parasiteby specific withdrawal of nitrogenous and carbonaceous solutesfrom the phloem was obtained independently by assuming thatxylem intake occurred non-specifically. Xylem import was thusobtained (a) from transpiration and tissue water increment ofCuscuta and the concentrations of N and C in xylem sap and (b)from the Ca²⁺ increment of Cuscuta and the ratios Ca: N andCa: C in lupin xylem sap, assuming that Ca²⁺ intake occurredsolely via xylem. By subtracting net xylem import from totaluptake of C and N by Cuscuta the methods resulted in comparableratios of C: N intake from the phloem. The average ratio (53.4)was smaller than the C:N ratio in host phloem (85.6) indicatingspecific withdrawal of solutes with a distinct preference forN. Using this ratio, modelling of flows of C and N was possibleand showed that Cuscuta abstracted C and N mainly from the hostphloem, but xylem supply was nutrient-dependent and amountedto 6.4% of the N but only 0.5% of the C demand. The resultsindicated that Cuscuta exerted a very strong sink and competedefficiently with the root, the major sink of L. albus, by attracting81% of the current photosynthate and more N (223%) than wascurrently fixed. The massive demand of the parasite led to lossesparticularly of N from leaves and the root and apart from causingcarbon losses it appeared to induce a sink-dependent stimulationof photosynthesis. In contrast, nitrogen fixation in the Cuscuta-infectedlupin was inhibited to 37% of the control. Key words: Cuscuta reflexa, Lupinus albus, carbon, nitrogen, phloem, xylem, transport, parasites, modelling     

Phloem Loading and Metabolism of Xylem-Borne Amino Compounds in Fruiting Shoots of a Legume

Cut, fruiting shoots of Lupinus albus L. supplied with ¹⁴C-and ¹⁵N-labelled L-asparagine, L-glutamine, L-aspartic acid,or L-glutamic acid through the transpiration stream readilytransferred the labelled carbon and nitrogen of each compoundto pods and seeds of fruits. A time course of labelling of phloemsap collected from petioles and fruit tips following feedingof labelled asparagine indicated that xylem to phloem exchangein leaflets was an immediate and effective route of transferof the amide to fruits and that this and the loading onto phloemof additional asparagine from unlabelled pools of the amidein stems furnished a major source of the nitrogen for fruitfilling. Xylem to phloem exchange of nitrogen was accomplishedin different ways for each amino acid. The amide nitrogen ofasparagine was transferred mainly in the unmetabolized compound,the nitrogen of aspartate and glutamate largely in a wide rangeof amino acids synthesized in the leaf, and the amide nitrogenof glutamine was transferred in a manner intermediate betweenthese extremes. Glutamine and asparagine were the principalphloem solutes labelled with nitrogen from any of the suppliedcompounds, but the photosynthetically produced amino acids,glutamate, aspartate, serine, alanine, and valine also became¹⁵N-labelled in phloem. The main pathway for glutamine synthesisin vegetative parts of the shoot appeared to be by amidationof glutamate, but asparagine was not considered to be derivedsimilarly from aspartate. Leaflets metabolized glutamine morereadily than asparagine, but in each case the amide nitrogenwas used for synthesis of a variety of amino acids and the carbonwas recovered largely in non-amino compounds.