Haustoria in action: Case studies of nitrogen acquisition by woody xylem-tapping hemiparasites from their hosts

Summary This review discusses studies conducted by the author and his colleagues on mistletoes and root hemiparasites native to Western Australia. Morphological characteristics of haustoria are described and their anatomical features are discussed in relation to uptake, transfer, and metabolism of xylem-borne nitrogenous solutes derived from a host. Experimental approaches used include comparisons of xylem sap composition of parasite and host(s), solute pool analyses and enzymatic properties of haustoria, host xylem feeding of¹⁵N-labelled solutes to follow the fate of label in haustoria and body of the parasite, and studies using species-specific nonprotein amino acids to validate successful uptake from hosts or occasional backflow of xylem-borne solutes to a host. Field studies on promiscuous root hemiparasites assess frequencies of exploitation of different hosts.¹⁵N natural abundance assays of host and parasite dry matter demonstrate marked preference bySantalum acuminatum for N₂-fixing as opposed to nonfixing hosts. The ability ofOlax phyllanthi to continue to exploit deep-rooted hosts ranks of importance when xylem water potentials of other hosts go out of sucking range during periods of water stress. Comparisons of xylem sap composition of parasites feeding on different hosts indicate remarkable versatility by haustoria in uptake and utilization of the different major nitrogenous solutes received from these hosts. Solute pools in parasites partly reflect metabolic transformations accentuated by haustoria while also indicating direct throughput from xylem of a host. The review concludes by showing how empirically based modelling techniques can be used to estimate proportional gains of N by parasites from single hosts and repercussions on host growth which accompany such exploitation.Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday     

The effect of nitrogen supply on growth and water-use efficiency of xylem-tapping mistletoes

Xylem-tapping mistletoes are known to have normally a higher rate of transpiration and lower water-use efficiency than their hosts. The relationships between water relations, nutrients and growth were investigated for Phoradendron juniperinum growing on Juniperus osteosperma (a non-nitrogen-fixing tree) and for Phoradendron californicum growing on Acacia greggii (a nitrogen-fixing tree). Xylem sap nitrogen contents were approximately 3.5 times higher in the nitrogen-fixing host than in the non-nitrogen-fixing host. The results of the present study show that mistletoe growth rates were sevenfold greater on a nitrogen-fixing host. At the same time, however, the differences in water-use efficiency between mistletoes and their hosts, which were observed on the non-nitrogen-fixing host did not exist when mistletoes were grown on hosts with higher nitrogen contents in their xylem sap. Growth rates and the accumulation of N, P, K, and Ca as well as values for carbon-isotope ratios of mistletoe tissues support the hypothesis that the higher transpiration rates of mistletoes represent a nitrogen-gathering mechanism.Abbreviation ¹³C carbon-isotope ratio Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday     

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.     

Water relations of the root hemiparasite Olax phyllanthi (Labill) R.Br. (Olacaceae) and its multiple hosts

Summary Water relations of the root hemiparasite Olax phyllanthi were compared with those of its major species of hosts in natural habitat in coastal heath near Denmark, SW Australia. Leaf water potentials of Olax during winter were 0.4 to 1.4 MPa lower (more negative) than those of all (29) non parasitic host species examined. During the dry summer months (January to March), shallow-rooted hosts developed water potentials up to 3 MPa lower than those of Olax, and were accordingly rated as no longer accessible as a source of water to the hemiparasite. By contrast, deep-rooted hosts, with access to the water table, showed water potentials less negative than Olax, and haustorial contacts retained with these apparently enabled continued extraction of water and nutrients throughout the summer. Three other species of root hemiparasites parasitized by Olax, but not themselves parasitizing Olax, showed leaf water potentials throughout the year very close to, and mostly slightly more negative than those of Olax. Nocturnal measurements of leaf water potential in winter (July and August) in soil at field capacity (water potential –0.006 MPa) showed maintenance of a 0.5–0.8 MPa potential difference between Olax and a range of common host species. By dawn most hosts had equilibrated with the water potential of the soil, whereas both exposed and bagged Olax plants recorded potentials of –0.8 MPa. Daytime rates of transpiration and photosynthesis of Olax were less than those of a range of common hosts, but water use efficiencies were not consistently different between hemiparasite and hosts. This was reflected in almost identical mean values for carbon isotope ratio (¹³C/¹²C) between Olax (mean value –27.0) and thirteen frequently exploited hosts ( value –27.1). The results are discussed in relation to published information on other angiosperm hemiparasites.     

Growth and partitioning of C and fixed N in the shrub legume Acacia littorea in the presence or absence of the root hemiparasite Olax phyllanthi

Nodulated plants of Acacia littorea were pot cultured singlyin minus nitrogen sand culture in the presence or absence ofa transplanted seedling of the root hemiparasite Olax phyllanthiand harvests of cultures made 4 and 8 months after introducingthe parasite. Parasitism decreased host shoot growth while increasingroot growth to a similar extent. Final shoot:root dry weightratio was 2.2 for parasitized versus 4.3 for unparasitized Acacia.Partitioning of fixed N showed 4-fold larger N increments inshoots than roots of unparasitized plants, whereas parasitizedplants lost a small amount of shoot N, made a root gain of Ndouble that of unparasitized plants and lost over half of theirN to Olax. The increment of fixed N in the host:parasite associationwas similar to that of unparasitized Acacia. Data on dry mattergain per unit foliage area and mean CO₂ assimilation rates pershoot of Olax and Acacia (parasitized or unparasitized) werediscussed in relation to an estimated heterotrophic gain ofxylem C from the host equivalent to 40% of the increment ofdry matter C made by the parasite. Growth of Olax was accompaniedby large increases in numbers of haustoria, 9% of which wereattached to root nodules as opposed to roots. Structural andnutritional features of direct parasitism of nodules are described.Models of flow and utilization of C and N in the Acacia:Olaxassociation and unparasitized Acacia are discussed in relationto published data for other host:parasite associations. Key words: Olax phyllanthi, host-parasite relationships, C and N partitioning, Acacia, N₂ fixation     

Haustorium-related Uptake and Metabolism of Host Xylem Solutes by the Root Hemiparasitic ShrubSantalum acuminatum(R. Br.) A. DC. (Santalaceae)

Solute composition of root xylem sap of common native hostsof quandong (Santalum acuminatum) was compared with that ofcorresponding xylem sap and ethanolic extracts of endophytictissues of haustoria of the hemiparasite. Each host transporteda characteristic set of organic nitrogenous solutes, but littleor no nitrate, and the data indicated only limited direct flowof amino compounds between xylem streams of hosts and parasite.Proline predominated in the haustorium and xylem ofSantalum,but was at negligible levels in the xylem of most hosts. Sucrose,fructose, glucose, malate and citrate were at high levels inall saps, and fructose especially prominent inSantalum. Chloride,sulphate and phosphate were the principal inorganic anions ofthe xylem. Based on C:N ratios of xylem and dry matter ofSantalumandassuming a 70% or more dependence on the host for N, it wasestimated thatSantalumwould gain approximately one third ofits C requirement for dry matter production heterotrophicallyfrom the xylem of its hosts. Infiltration of xylem of haustoria-bearingroot segments of a major host (Acacia rostellifera) with a rangeof¹⁵N labelled substrates resulted in 40–80% of the¹⁵Nof endophytes of the attached haustoria being received as proline.Nitrate reductase activity was induced in haustoria followinghost xylem feeding of nitrate. The study concludes that haustoriaofSantalumact as a major site of synthesis and export of prolineand might therefore play an important role in osmotic adjustmentof the parasite and its related acquisition of water from hosts. Root hemiparasite; Santalum acuminatum; ¹⁵N labelled substrates; xylem transport; proline; osmoregulation     

Xylem fluxes of fixed N through nodules of the legume Acacia littorea and haustoria of an associated N-dependent root hemiparasite Olax phyllanthi

Nodulated 1-1.5-year-old plants of Acacia littorea grown inminus nitrogen culture were each partnered with a single seedlingof the root hemiparasite Olax phyllanthi. Partitioning of fixedN between plant organs of the host and parasite was studiedfor the period 4–8 months after introducing the parasite.N fluxes through nodules of Acacia and xylem-tapping haustoriaof Olax were compared using measured xylem flows of fixed Nand anatomical information for the two organs. N₂ fixation duringthe study interval (635 µg N g FW nodules^–1 d^–1)corresponded to a xylem loading flux of 0.20 µg N mm^–2d^–1 across the secretory membranes of the pencycle parenchymaof the nodule vascular strands. A much higher flux of N (4891µg mm^–2 d^–1) exited through xylem at the junctionof nodule and root. The corresponding flux of N from host xylemacross absorptive membranes of the endophyte parenchyma of Olaxhaustorium was 1.15 µg N mm^–1 d^–1, six timesthe loading flux in nodules. The exit flux from haustorium toparasite rootlet was 20.0 pg N mm^–1 d^–1, 200-foldless than that passing through xylem elements of the nodule.Fluxes of individual amino compounds in xylem of nodule andhaustorium were assessed on a molar and N basis. N flux valuesare related to data for transpiration and partitioning of Cand N of the association recorded in a companion paper. Key words: Olax phyllanthi, host-parasite relationships, N flux, Acacia, N₂ fixation     

Xylem Transport and Storage of Amino Acids by S.W. Australian Mistletoes and their Hosts

Amino acid composition of xylem (tracheal) sap and ethanolicextracts of shoots of mistletoes (Amyema spp. and Lysiana casuarinae)and their hosts were compared, using material collected in theirnative habitats. Data indicated that certain host xylem soluteswere transferred directly to the parasite xylem, while otherswere either not absorbed or were metabolized prior to transfer.Certain solutes were major constituents of parasite xylem, butundetected or only in trace amount in the host. Shoot aminoacid pools of parasites differed markedly from those of hosts.The mistletoe, Amyema preissii, exhibited differential storageand transport of arginine when parasitizing three differentspecies, but accumulated proline on only two of these hosts.Host- specific amino acids (djenkolic acid in Acacia saligna,and tyramine in Acacia acuminata) were transported and accumulatedin relatively large amounts by the parasite, but were not detectedin other associations. Proline was the major solute of Amyemalinophyllum parasitizing Casuarina obesa, but arginine predominatedin Lysiana casuarinae on the same host. However, when L. csuarinaeparasitized A. linophyllum, in turn parasitic on C. obesa, theLysiana accumulated equal amounts of proline and arginine andmore asparagine than when directly on the Casuarina. Xylem feedingof ¹⁵N-labelled aspartic acid or ¹³N-(amide labelled) asparagineto cut shoots or whole haustoria-bearing plants of the mistletoeA. preissii resulted in 68–73% of the ¹⁵N of aspartateand 24–30% of that of asparagine appearing in ethanol-solubleshoot amino compounds other than the fed solute. ¹⁵N labellingpatterns of detached shoots were not noticeably different fromthat of whole plants suggesting that the haustorium had relativelylittle effect on processing incoming solutes. Alanine, glutamine,and arginine were principal recipients of ¹⁵N from aspartate,alanine and glutamine in the case of fed asparagine. It is estimatedthat 24% of the carbon requirements for dry matter accumulationin Amyema linophyllm were met by intake of xylem sap solutesfrom its host Casuarina obesa. Key words: Amino acids, xylem transport, mistletoes, host: parasite relations, N metabolism     

Composition of bleeding sap nitrogen from lines of field-grownPhaseolus vulgaris L.

Summary The nitrogenous compounds in the xylem (bleeding) sap of lines of field-grownPhaseolus vulgaris L., known to vary in N₂ fixation and yield, were measured during growth with and without N fertilizer. Forty nine-67% of the total sap N was in the form of nitrate in fertilized plants, with low amounts of the ureides, allantoin and allantoic acid (4–12%). Ureides contributed between 17 and 38% to the total sap N of non-fertilized plants, with nitrate generally comprising less than 40%. Among the nine lines grown without fertilizer there were significant differences in the % of the sap N as ureides and also in the total mol N ml^–1 sap.The ASN/GLN ratio (mol/mol) was greater in sap collected from the two parental lines given fertilizer compared with non-fertilized plants. However the actual ratio varied between the parental lines. Together the two amides generally comprised between 43–62% of the amino-N in sap samples from the two parental lines with or without N fertilizer.For each N treatment (fertilized or non-fertilized) there were no obvious differences in sap composition between the high N₂ fixing lines and the low N₂ fixing lines. However there was generally a positive relationship between the rate of N translocation (total N concn. ml^–1 sap x rate of exudation) and the ranking of the lines on the basis of higher N₂ fixation rates (acetylene reduction), which was to a large extent independent of the source of N available to the plant.     

Pathways of assimilation and transfer of fixed nitrogen in coralloid roots of cycad-Nostoc symbioses

Freshly detached coralloid roots of several cycad species were found to bleed spontaneously from xylem, permitting identification of products of nitrogen transfer from symbiotic organ to host. Structural features relevant to the export of fixed N were described for Macrozamia riedlei (Fisch. ex Gaud.) Gardn. the principal species studied. Citrulline (Cit), glutamine (Gln) and glutamic acid (Glu), the latter usually in a lesser amount, were the principal translocated solutes in Macrozamia (5 spp.), Encephalartos (4 spp.) and Lepidozamia (1 sp.), while Gln and a smaller amount of Glu, but no Cit were present in xylem sap of Bowenia (1 sp.),and Cycas (2 spp.). Time-course studies of ¹⁵N enrichment of the different tissue zones and the xylem sap of ¹⁵N₂-pulse-fed coralloid roots of M. riedlei showed earlier ¹⁵N incorporation into Gln than into Cit, and a subsequent net decline in the ¹⁵N of Gln of the coralloid-root tissues, whereas Cit labeling continued to increase in inner cortex and stele and in the xylem sap. Hydrolysis of the ¹⁵N-labeled Cit and Gln consistently demonstrated much more intense labeling of the respective carbamyl and amide groups than of the other N-atoms. Coralloid roots of M. riedlei pulse-fed ¹⁴CO₂ in darkness showed ¹⁴C labeling of aspartic acid (Asp) and Cit in all tissue zones and of Cit of xylem bleeding sap. Lateral roots and uninfected apogeotropic roots of M. riedlei and M. moorei also incorporated ¹⁴CO₂ into Cit. The ¹⁴C of Cit was restricted to the carbamyl-C. Comparable ¹⁵N₂ and CO₂-feeding studies on corallid roots of Cycas revoluta showed Gln to be the dominant product of N₂ fixation, with Asp and alanine as other major ¹⁴C-labeled amino compounds, but a total absence of Cit in labeled or unlabeled form.Abbreviations Ala alanine - Asp aspartic acid - Cit citrulline - Gln glutamine - Glu glutamic acid - Orn ornithine     

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.     

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     

Gas exchange and water balance of a mistletoe species and its mangrove hosts

Summary The gas exchange and water relations of the hemiparasite Pthirusa maritima and two its mangrove host species, Conocarpus erectus and Coccoloba uvifera, were studied in an intertidal zone of the Venezuelan coast. Carbon uptake and transpiration, leaf osmotic and total water potential, as well as nutrient content in the xylem sap and leaves of mistletoes and hosts were followed through the dry and wet season. In addition, carbon isotope ratios of leaf tissue were measured to further evaluate water use efficiency. Under similar light and humidity conditions, mistletoes had higher transpiration rates, lower leaf water potentials, and lower water use efficiencies than their hosts. Potassium content was much higher in mistletoes than in host leaves, but mineral nutrient content in the xylem sap of mistletoes was relatively low. The resistance of the liquid pathway from the soil to the leaf surface of mistletoes was larger than the total liquid flow resistance of host plants. Differences in the daily cycles of osmotic potential of the xylem sap also indicate the existence of a high resistance pathway along the vascular connection between the parasite pathway along the vascular connection between the parasite and its host. P. maritima mistletoes adjust to the different physiological characteristics of the host species which it parasitizes, thus ensuring an adequate water and carbon balance.     

Evidence for N feedback regulation of N2 fixation in Alnus glutinosa L.

Treatments were applied to vary C and N availability in Alnusglutinosa L. and plant growth, nodule activity (including acetylenereduction) and amino acid composition of the xylem sap weremeasured. Removing the buds, a sink for N, caused a decreasein nodule activity. Flushing root systems daily with 100% O₂destroyed nitrogenase activity and substantially decreased theamount of citrulline in the xylem sap. The amino acid compositionof xylem saps also altered according to the mode of N nutrition.In plants fed , xylem sap composition was similar to N₂-fixing plants, however, when plants were fed, citrulline content increased. The assimilation and subsequent distribution of nitrate wasfollowed in an experiment in which labelled ¹⁵ was added to the base of plant pots. After 12 h7% of root N was from applied ¹⁵ and this increased to 75% at 7 d; substantial enrichment ofN from ¹⁵ also occurred in stems, buds and leaves. After 7 d, 3.5% of nodule N was from¹⁵, consistent with some N being supplied by recycling of shoot N. Xylem saps were alsocollected and after 12 h, glutamate and aspartate were enrichedwith ¹⁵N to 53% and 37% increasing after 7 d to 80% and 49%,respectively. Citrulline content of the xylem sap increasedfrom 3 to 9 µmol cm^–3 following addition of ¹⁵ and at 7 d 80% of the N in the citrullinehad been derived from ¹⁵N. It is hypothesized that the growthand activity of A. glutinosa root nodules is sensitive to theN status of the plant and that the level of citrulline (or otheramino acids) returning to the nodules may feed back to regulatenodule growth and activity. Key words: Alnus glutinosa, citrulline, nitrate, feedback mechanism, N₂-fixation.     

Ecophysiological Aspects of the Woody Root HemiparasiteSantalum acuminatum(R. Br.) A. DC and its Common Hosts in South Western Australia

The water and nitrogen relationships of the xylem-tapping roothemiparasite, quandong (Santalum acuminatum) and its principalhosts were examined at a series of sites in native coastal heathlandsof south west Australia. Assessments based on densities of above-groundbiomass, ground cover and frequencies of haustoria on host rootsindicated that woody N₂fixers (legumes andAllocasuarina) wereprincipal hosts ofSantalum. ¹⁵N values for shoot dry matterof component species suggested these N₂fixers were strongly(70% or more) dependent on atmospheric N and thatSantalumderivedN principally from these species. Structural studies of haustoriashowed the interface with host xylem to be almost entirely comprisedof parenchymatous tissue. No luminal continuities were observedbetween xylem conducting elements of the partners. Formationand functional life of haustoria were closely coordinated withseasonal growth of hosts, with some haustoria surviving summerand overlapping functionally with new ones establishing in thenext autumn. Transpiration and photosynthetic rates of the parasitewere consistently less and water use efficiencies very similarto those of the principal hostAcacia rostellifera. ¹³C valuesof foliage ofSantalumand this host were similar, but large variationsin ¹³C values for above-ground dry matter of parasite and hostsbetween study sites prevented evaluations of water stress orwater-use-efficiency based on carbon isotope discrimination.Specific hydraulic conductivities of roots ofSantalumwere consistentlylower than those ofAcacia, a finding consistent with more conservativewater use by the parasite than the host. Santalum acuminatum; root hemiparasite; ¹⁵N discrimination; water relations; haustorial structure; root conductivity     

Inter-varietal differences in xylem exudate composition and growth under contrasting forms of N supply in cucumber

Variations in the inorganic and organic composition of xylem exudate, growth and N content under contrasting forms of N supply in three cucumber cultivars (Hyclos, Medusa and Victory) were studied in glasshouse conditions. The plants were grown hydroponically with two NO₃ ^-:NH₄ ⁺ ratios (100:0 and 60:40).The xylem sap of Medusa grown with both N sources displayed an increase of organic N and carboxylate concentrations and a decrease of cations, inorganic anions and carbohydrates compared with that of those grown with NO₃ ^- alone, showing a higher growth and N content in tissues and thus better utilization of N supplied as NO₃ ^- and NH₄ ⁺. Mixed N nutrition in Hyclos caused the greatest amounts of NO₃ ^- and NH₄ ⁺ in xylem sap, lower root weight and N levels in the leaves, while its root was unable to generate an adequate supply of organic N compounds. Despite the levels of cations, inorganic and organic anions were reduced by the NH₄ ⁺ supplied to Victory, the ionic balance in the xylem sap, growth and N content remained similar to that of those supplied with NO₃ ^- alone. Finally, the cucumber cultivars studied here, responded differently to the form of N supplied, it may partly be due to their ability of assimilating N in the roots and partly to the form in which the N is translocated to the shoot.     

Distribution and metabolism of xylem-borne ureido and amino compounds in developing soybean shoots

Pulse-chase feeding (30-120 minutes) of ¹⁴C-labeled nitrogenous compounds to cut transpiring shoots was used to investigate the early fate of the major xylem-borne solutes in N₂-fixing soybean (Glycine max) plants at the V₄ growth stage. By comparison with the foliar distribution of [¹⁴C]inulin (a xylem marker), it was determined that the phloem supply of allantoin, allantoic acid, asparagine, glutamine, aspartate, and arginine, respectively, provided about 20, 10, three, two, five, and 20 times the ¹⁴C delivered to the developing trifoliolate in the xylem stream. Recovery of unmetabolized asparagine, aspartate, and arginine in this indicator trifoliolate, and significant declines in the percentage of ¹⁴C from allantoic acid and allantoin recovered in the first trifoliolate, provided some support for the direct xylem-to-phloem transfer of these compounds, but did not preclude the involvement of indirect transfer. Data on stem retention and foliar distribution, expressed as a function of the relative xylem sap composition, indicated that ureides provide the major sources of nitrogen to all plant parts. There was no consistent distinction in distribution patterns between pairs of similar anionic and neutral compounds. The extent of xylem-to-phloem transfer among the ureido or the amino compounds was inversely related to its prominence in xylem sap.     

Influence of sunflecks on the δ 13 C of Adenocaulon bicolor plants occurring in contrasting forest understory microsites

Summary Xylem-tapping mistletoe species growing on Mimosaccae, non-Mimosaceae and hosts performing Crassulacean acid metabolism (CAM) were studied along an aridity gradient in the Namib desert. °¹³C-values of mistletoes became more negative with decreasing nitrogen (N)-concentration in their leaves, while the host plants showed no such relationship. This might suggest that mistletoes regulate their water use efficiency according to the nitrogen supply from the host. However, further inspection of the data indicates that the relations of ¹³C-values with leaf nitrogen in mistletoes may result from carbon input from the host. This is especially true for mistletoes growing on CAM plants which exhibit a very high ¹³C-value, but show no evidence of CAM. It is calculated that about 60% of the carbon in mistletoes growing on C3 and on CAM hosts originated from the host. The hypothesis of Marshall and Ehleringer (1990) that xylem tapping mistletoes are also carbon parasites could explain the change in ¹³C-values with N-supply and the difference in ¹³C-values between mistletoes growing on C3 and CAM hosts.This paper is prepared in memory of J. Visser, who joined the collection and identification of species in 1987, but died in 1990     

Use of labeled nitrogen to monitor transition in nitrogen dependence from storage to current-year uptake in mature walnut trees

Summary Nitrogen (N) derived from both the soil during current-year uptake, and the withintree pool of storage N was distinguished in two groups of Serr walnut trees using labeled fertilizer (¹⁵N-depleted ammonium sulfate) applied in different years. Mass spectrometric analysis of N in xylem sap collected periodically in spring allowed quantification of the relative contributions of N from storage and current season uptake and the transition in N usage from previously assimilated (storage) N to the onset of current season uptake of soil N. N derived from storage accounted for > 50% of the xylem sap N during the staminate and pistillate bloom periods and throughout the period of spur leaf expansion.     

An assessment of soil enrichment by actinorhizal N2 fixation using δ15N values in a chronosequence of deglaciation at Glacier Bay,Alaska

The extent of transfer of fixed N between N₂-fixing and non-N₂-fixing plant species is largely unknown in successional studies. In order to redress this deficiency at a locale intensively studied ecologically, leaf tissue samples were collected from actinorhizal N₂-fixing (Alnus, Shepherdia, and Dryas) and two non-N₂-fixing (Salix) woody species within research plots located along a chronosequence of deglaciated fjord in Glacier Bay National Park, Alaska. The tissue samples were analyzed for ¹⁵N content, and the resulting data analyzed for trends in plant tissue N. Among the non-N₂-fixing Salix species, ¹⁵N values increased from the most recently deglaciated sites to converge with the temporally more-stable values for the symbiotic N₂-fixing species on sites at about 40 years after deglaciation. The lower ¹⁵N values of sequestered N in plant tissues suggested that N derived from N₂-fixing plants accounts for the major portion of N in associated plants up to 40 years after deglaciation. The ¹⁵N isotopic data also suggested that Shepherdia canadensis depends least on soil N, D. drummondii the most, and A. viridis ssp. sinuata somewhere between those two species. The presence of a sere dominated by dense thickets of A. viridis ssp. sinuata at the convergence of ¹⁵N values for the N₂-fixing and non-N₂-fixing species indicated that this species is most responsible for accumulation of fixed N in soil at Glacier Bay. This paper is dedicated to the memory of Steven J. Kohls who died prior to publication of this research.