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     

Heterotrophic gain of carbon from hosts by the xylem-tapping root hemiparasite Olax phyllanthi (Olacaceae)

Heterotrophic gains of carbon from various host species by the root hemiparasitic shrub Olax phyllanthi (Labill) R.Br. were assessed using techniques based on carbon isotope discrimination (¹³C) on C3 and C4 hosts and C:N ratios of xylem sap and dry matter of host and parasite. Heterotrophic benefits (H) to Olax based on ¹³C values were 30% and 19% from two nonnative C4 hosts (Portulaca oleracea and Amaranthus caudatus respectively) compared with 13% and 15% from these hosts when computed on the basis of C:N ratios of host xylem sap and C and N increments of Olax dry matter. Nitrate was the source of N available to pot cultures of the above species and estimates based on C:N ratios assumed that all N accumulated by Olax had come from nitrate absorbed by the host. Equivalent estimates of H for Olax, grown in nitrogen-free pot culture with the native N₂-fixing host Acacia littorea as its sole source of N, indicated 63% and 51% dependence on host carbon when assessed in terms of xylem sap composition of host parasite respectively. Comparisons of xylem sap solutes of Olax and a range of partner hosts indicated marked selectivity in haustorial uptake and transfer of nitrate, amino compounds, organic acids and sugars. Possible implications of variations between hosts in absolute levels of C and inorganic and organic forms of N in xylem are discussed in relation to evidence of much better growth performance of Olax on Acacia littorea and other N₂-fixing legumes than on non-fixers.     

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.     

Haustorial Development and Growth Benefit to Seedlings of the Root Hemiparasitic Tree Nuytsia floribunda(Labill.) R.Br. in Association with Various Hosts

Dry matter gains and haustorial production of pot-cultured seedlingsof Nuytsia floribunda were assessed after a 12 month periodof association singly with each of a range of potential woodyhost species. One species,Adenanthos cygnorum , of similar sizeto most parasitized hosts, served as measure of response ofNuytsia in a non-benefiting situation. Rated on this basis,all 23 parasitized hosts elicited greater mean dry weights ofNuytsia than when on Adenanthos, and seven of these instanceswere highly significant. Numbers and weights of penetratingand presumably functional haustoria formed on a host were broadlycorrelated with growth benefit to Nuytsia, but there were notableinstances of unusually poor or great benefit from a host relativeto the complement of haustoria involved. Experiments in whichhaustoria-bearing associations of Nuytsia partnered with nodulatedAcacia hosts (Acacia acuminata and A. cyclops) were fed¹⁵N₂showedsignificant transfer of¹⁵N to the parasite, but failed to determinewhether the label had been acquired through haustoria or directlyby Nuytsia roots following turnover of nodule and root residuesof the host in the rooting medium. A parallel study using theunusual non-protein amino acid, djenkolic acid, as a markerof benefit from the djenkolic acid-containing host A. cyclops,showed appearance and progressive build-up of the compound infoliage of Nuytsia over a 6 month period after partnering thespecies in pot culture. Presence of the compound at final harvestin xylem sap of both partners but not in soil solution of thecultures strongly indicated xylem transfer via haustoria asthe principal avenue for N benefit to the parasite. Resultsare discussed in relation to a recent evaluation of haustorialstructure and functioning of N. floribunda. Copyright 2000 Annalsof Botany Company Root hemiparasite, Nuytsia, Loranthaceae, growth benefit, haustorial production, nitrogen transfer from hosts     

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.     

Water Relations of the Hemiparasite Rhinanthus serotinus before and after Attachment

Growth of the hemiparasite Rhinanthus serotinus (Schönh.) Oborny was greatly stimulated after attachment of the parasite to the roots of the host plant, Hordeum vulgare L. Before attachment the hydrostatic pressure in the xylem, determined by the pressure bomb technique, was found to be lower in Rhinanthus than in the host. It increased after the formation of haustoria between host and parasite. Apparently, the water transport to Rhinanthus was facilitated. The hydrostatic pressure remained lower than that of the host, accounting for the flow of water and solutes in the direction of the parasite and indicating that there exists a resistance to water transport in the haustoria. Water and solutes were absorbed by the cells, which increased in size. The turgor pressure of the parasite rose steeply, but the osmotic potential was hardly affected.     

Heterotrophic carbon gain by the root hemiparasites, Rhinanthus minor and Euphrasia rostkoviana (Orobanchaceae)

Hemiparasitic plants gain virtually all mineral nutrients and water from their host plant whilst organic carbon is provided, at least in part, by their own photosynthetic activity, although their rates of assimilation are substantially lower than that found in non-parasitic plants. Hence, hemiparasites must gain at least some of their organic carbon heterotrophically from the host plant. Despite this, heterotrophic carbon gain by root hemiparasites has been investigated only for a few genera. We investigated heterotrophic carbon gain by two root hemiparasites, Rhinanthus minor L. and Euphrasia rostkoviana Hayne (Orobanchaceae), using natural abundance stable isotope (δ¹³C) profiles of both parasites attached to C₃ (wheat) and C₄ (maize) hosts coupled to a linear two-source isotope-mixing model to estimate the percentage of carbon in the parasite that was derived from the host. Both R. minor and E. rostkoviana attached to maize hosts were significantly more enriched in ¹³C than those attached to wheat hosts with R. minor becoming more enriched in ¹³C than E. rostkoviana. The natural abundance ¹³C profiles of both parasites were not significantly different from their wheat hosts, but were less enriched in ¹³C than maize hosts. Using a linear two-source isotope-mixing model, we estimated that R. minor and E. rostkoviana adult plants derive c. 50 and 25% of their carbon from their hosts, respectively. In light of these results, we hypothesise that repeatedly observed negative effect of competition for light on hemiparasites acts predominantly in early ontogenetic stages when parasites grow unattached or the abstraction of host nutrients is less effective.     

Xylem Transfer of Organic Solutes inSantalum albumL. (Indian Sandalwood) in Association with Legume and Non-legume Hosts

Indian sandalwood (Santalum album), a commercially importantroot hemi-parasitic angiosperm, was partnered singly in potculture with one of three nitrogen-fixing legumes or a eucalypthost. Xylem (tracheal) sap of stems of host and parasite andethanolic extracts of endophytic tissue of haustoria of theparasite were analysed for amino acids, organic acids and sugarsto determine which sets of solutes were available to and obtainedby the parasite from different hosts. There were high concentrationsof asparagine, followed by glutamate, aspartate and     

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.     

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     

Haustorial Structure and Functioning of the Root Hemiparastic Tree Nuytsia floribunda(Labill.) R.Br. and Water Relationships with its Hosts

Observations on the origin and mature structure of the haustoriumof the Western Australian Christmas tree (Nuytsia floribunda)corroborate and extend the findings of earlier workers. We showthat the previously described sclerenchymatous ‘horn’or ‘prong’ formed within the haustorium acts asa sickle-like cutting device which transversely severs the hostroot and then becomes lodged in haustorial collar tissue directlyopposite to that where it originated. The cutting process isdeduced to be rapid and the gland-like fluid filled structurein the haustorium is suggested to generate a hydrostatic forcedriving the device through the host root. The haustorial parenchymacells at the tight junction between the endophytic part of thehaustorium and the cut face of the host root develop balloon-likeoutgrowths which intrude into the lumina of severed xylem vesselsof the host. Experiments feeding 0.05% (w/v) basic fuchsin tofreshly cut ends of host root segments distal to terminally-attachedmature haustoria demonstrate an apoplastic pathway from hostxylem elements fractured at the interface into haustorial parenchyma,and thence through vascular tissue to the haustorium into thetranspiring plant of Nuytsia. Application of labelled water(D₂O) to uncut basal roots of potted plants ofAcacia acuminataparasitized by Nuytsia results in labelling of leafy shootsof parasite and host, indicative of haustorial uptake of waterby Nuytsia from host root xylem in the intact association. Measurementsof xylem water potentials of pot-cultured seedling Nuytsia associatedwith a range of hosts, or of mature trees of Nuytsia and partnerwoody hosts in the native habitat, demonstrate consistentlymore negative potentials in the parasite than host, suggestingthat the parasite may regularly obtain xylem water through itshaustorial apparatus. Copyright 2000 Annals of Botany Company Root hemiparasite, Nuytsia floribunda, Loranthaceae, haustorial structure, host–parasite water relations     

Autotrophy and heterotrophy in root herniparasites

More than 3000 species of flowering plants are at least partially parasitic, acquiring water and solutes from the host via haustoria. More than one third of all parasitic angiosperms - the root hemiparasites - possess green leaves and root systems. In these species there are potentially two opportunities for the capture of water and solutes: an autotrophic or abiotic supply from the external environment, and a heterotrophic or host-derived supply via the haustoria. Most root hemiparasites occur in the Scrophulariaceae, a family also containing autotrophic and holoparasitic plants. Between these two extremes, the root hemiparasites provide an ideal opportunity to investigate the balance between the autotrophic and heterotrophic modes of nutrition in parasitic plants. The tropical hemiparasites within this family are important weeds of cereals and legumes, causing considerable crop losses, and thus fuelling research into the nutritional dependency of these plants on their hosts. These studies have led to some exciting new ideas, particularly with respect to the carbon relations of these plants.     

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.     

Interactions between hemiparasitic plants and their hosts: The importance of organic carbon transfer

Hemiparasitic plants display a unique strategy of resource acquisition combining parasitism of other species and own photosynthetic activity. Despite the active photoassimilation and green habit, they acquire substantial amount of carbon from their hosts. The organic carbon transfer has a crucial influence on the nature of the interaction between hemiparasites and their hosts which can oscillate between parasitism and competition for light. In this minireview, we summarize methodical approaches and results of various studies dealing with carbon budget of hemiparasites and the ecological implications of carbon heterotrophy in hemiparasites.Key words: haustorium, heterotrophy, parasitic plant, mistletoe, Rhinanthus, Striga, δ¹³CHemiparasitic plants withdraw resources from the vascular system of their hosts through a specialized transfer organ called haustorium.¹ Hemiparasites attack the host''s xylem, in contrast to the holoparasites that infect both phloem and xylem, and as a consequence, hemiparasitic plants have access to water and mineral nutrients but little carbon.¹ Due to their reduced or non-existing root networks, hemiparasitic plants acquire virtually all mineral nutrients and water from the host while organic carbon is provided, at least in part, by their own photosynthetic activity.^2,3 This is in contrast to holoparasitic plants which rely on the host for the supply of both organic and inorganic nutrients. The location of the attachment to the host and the degree of host dependency represent the most important characters defining the three basic functional types within hemiparasitic plants. Root hemiparasites attack host roots but their above-ground appearance is usually not substantially different from that of a non-parasitic plant. This group can be further divided in two—facultative and obligate hemiparasites consisting of plants that are able (at least sometimes) or unable to complete their life cycle without an attachment to the host respectively. Stem hemiparasites are attached to the host stem (usually trunk or branches) and are all obligate parasites, unable to survive without a host.Hemiparasitic plants have an ambiguous relationship with their hosts which, on the one hand, represent exclusive sources of inorganic nutrients but on the other hand, the co-occurrence of these host plants in the hemiparasite vicinity imposes competition for light. The nature and intensity of this competitive relationship varies across different groups and species of hemiparasites. The ability of hemiparasites to acquire organic carbon (largely in the form of xylem-mobile organic and amino acids) is certainly the key factor affecting this interaction since hemiparasites that are capable of efficient organic carbon abstraction should be minimally affected by shading from their host. The fact that hemiparasites can exhibit substantial carbon heterotrophy is now supported by a large number of studies, although a traditional point of view on hemiparasites that highlights the importance of inorganic resources (mainly nitrogen) acquisition is still prevailing. Therefore, we decided to summarize available information on hemiparasite heterotrophy, outline techniques for assessing the proportion of heterotrophy and estimating the overall carbon budget, and discuss possible implications of this phenomenon on hemiparasite ecology.     

Is palatability of a root-hemiparasitic plant influenced by its host species?

Palatability of parasitic plants may be influenced by their host species, because the parasites take up nutrients and secondary compounds from the hosts. If parasitic plants acquired the full spectrum of secondary compounds from their host, one would expect a correlation between host and parasite palatability. We examined the palatability of leaves of the root-hemiparasite Melampyrum arvense grown with different host plants and the palatability of these host plants for two generalist herbivores, the caterpillar of Spodoptera littoralis and the slug Arion lusitanicus. We used 19 species of host plants from 11 families that are known to contain a wide spectrum of anti-herbivore compounds. Growth of M. arvense was strongly influenced by the host species. The palatability of the individual host species for the two herbivores differed strongly. Both A. lusitanicus and S. littoralis discriminated also between hemiparasites grown with different host plants. There was no correlation between the palatability of a host species and that of the parasites grown on that host, i.e., hemiparasites grown on palatable host species were not more palatable than those grown on unpalatable hosts. We suggest an interacting pattern of specific effects of chemical anti-herbivore defences and indirect effects of the hosts on herbivores through effects on growth and tissue quality of the parasites.     

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     

Does legume nitrogen fixation underpin host quality for the hemiparasitic plant Rhinanthus minor?

The high quality of leguminous hosts for the parasitic plantRhinanthus minor (in terms of growth and fecundity), comparedwith forbs (non-leguminous dicots) has long been assumed tobe a function of the legume's ability to fix atmospheric nitrogen(N) from the air and the potential for direct transfer of compatibleamino compounds to the parasite. Using associations betweenRhinanthus minor and Vicia faba (Fabaceae) that receive N eitherexclusively via symbiotic associations with rhizobia supplyingorganic N fixed from N₂ or exclusively through the supply ofinorganic nitrate to the substrate, the underlying reasons forthe quality of legumes as hosts for this parasite are unravelled.It is shown that sole dependence of the host, V. faba, on Nfixation results in lower growth of the attached parasite thanwhen the host is grown in a substrate supplied exclusively withinorganic N. In contrast, the host plants themselves achieveda similar biomass irrespective of their N source. The physiologicalbasis for this is investigated in terms of N and abscisic acid(ABA) partitioning, haustorial penetration, and xylem sap aminoacid profiles. It is concluded that legume N fixation does notunderpin the quality of legumes as hosts for Rhinanthus butrather the well-developed haustorium formed by the parasite,coupled with the lack of defensive response of the host tissuesto the invading haustorium and the presence of sufficient nitrogenouscompounds in the xylem sap accessible to the parasite haustoria,would appear to be the primary factors influencing host qualityof the legumes. Key words: ABA, haustorium, legume, nitrogen fixation, nodules, parasitic plant Received 14 November 2007; Revised 7 January 2008 Accepted 8 January 2008     

Multiple choice: hemiparasite performance in multi‐species mixtures

Hemiparasitic plants have green leaves, but extract water and solutes from neighbouring plants. It is still poorly understood how different host plants in communities contribute to parasite performance, as species that are good hosts in single‐host experiments may not necessarily be preferred hosts in mixtures. We grew the root hemiparasite Rhinanthus alectorolophus (Orobanchaceae) together with each of 13 host species (experiment 1) and with 15 different four‐species mixtures of these hosts (experiment 2) that differed in the number of legumes and of host functional groups. Parasites profited from mixtures including more legumes and from mixtures including different host functional groups. Some host species and mixtures were very tolerant of parasitism and supported large parasites without being strongly suppressed in their own growth, but the suppression of a species in the single‐host experiment did not explain the suppression of a species in a host mixture. We thus calculated for each host species an index of the difference in suppression between the two experiments which may be related to host use in a mixture. Host quality (mean parasite biomass with a host species) in the single‐host experiment could explain 64% of the variation in parasite biomass with a host mixture when it was weighted by the proportion of the host species in the mixture without the parasite and by the suppression difference index. Our results suggest that plant species which are the best hosts in single‐host experiments are not always those used most strongly by a parasite growing with a mixture. Together with the finding that hemiparasites benefit from a mixed diet based on hosts from different functional groups this suggests that parasites prefer certain host species to obtain a mixed diet.     

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.     

Uptake of EDTA-complexed Pb, Cd and Fe by solution- and sand-cultured Brassica juncea

Direct plant uptake of metals bound to chelating agents has important implications for metal uptake and the free-ion activity model. Uptake of hydrophilic solutes such as metal–EDTA complexes is believed to occur via bypass apoplastic flow, but many questions remain about the relative importance and selectivity of this pathway. In this study, Brassica juncea (Indian mustard) plants grown in solution- and sand-culture conditions were exposed to metal–EDTA complexes and to PTS, a hydrophilic fluorescent dye previously used as a tracer of apoplastic flow. The results suggest that there are two general phases of solute uptake. Under normal conditions, xylem sap solute concentrations are relatively low (i.e., <0.5% of concentration in solution) and there is a high degree of selectivity among different solutes, while under conditions of stress, xylem sap concentrations are significantly higher (i.e., >3% of concentration in solution) and the selectivity among solutes is less. In healthy plants, xylem sap metal–EDTA concentrations were generally an order of magnitude higher than those of PTS and differences among complexes were observed, with CdEDTA²⁻ exhibiting slightly higher xylem sap concentrations than PbEDTA²⁻ or FeEDTA⁻. Metal–EDTA complexes were found to dominate xylem sap metal speciation and the fraction of metal in xylem sap present as metal–EDTA was greater for non-nutrient metals (Pb, Cd) than for the nutrient metal Fe. Despite differences in root morphology between plants grown under solution- and sand-culture conditions, uptake of solutes was similar under both sets of growth conditions.