The Effect of Reduced Hydraulic Conductance on Stomatal Conductance and Xylem Cavitation

The vulnerability of xylem conduits to cavitation theoreticallydetermines the maximum flow rate of water through plants, andhence maximum transpiration (E), stomatal conductance (g_s),and leaf area (A₁. Field-grown Betula occidentalis with a favourablewater supply exhibit midday xylem pressures (     

Water Transport during Acid-Induced Growth of Excised Hypocotyl Sections of Vigna unguiculata under Xylem Perfusion

Elongation growth of abraded hypocotyl sections of Vigna unguiculataunder xylem perfusion was markedly promoted a few minutes afterthe application of an acid aerosol generated from a solutionof HCl. At the beginning of the acid-induced growth, intracellularpressure (Pⁱ) began to decrease and the membrane potential betweenthe symplast and the xylem apoplast (V^px) began to depolarize.Subsequently, Pⁱ and V^px remained at a reduced level and a depolarizedlevel, respectively, while the promotion of elongation growthcontinued for more than 4 hours. The electrogenic componentof the xylem membrane potential (V^px_act) gradually increasedto about twice that before acid treatment. There was a closecorrelation between the enhanced growth and the decrease inintracellular pressure within 30 min after application of acidbut little correltion after 60 min. By contrast, there was littlecorrelation between the promotion of growth and the activityof the xylem pump after 30 min while a close correlation wasobserved after 60 min. It is inferred that the acid-induced activation of water uptakeconsists of two major processes, in series, that are drivenby different forces: the rapid uptake of water for more than30 min, driven by hydrostatic force generated by loosening ofcell walls; and a long-lasting enhancement of water uptake forat least 4 h, which is driven by osmotic force that is generatedby the canal system within the xylem. (Received October 17, 1994; Accepted January 23, 1995)     

Modulation of eIF5A1 expression alters xylem abundance in Arabidopsis thaliana

Eukaryotic translation initiation factor 5A (eIF5A) is thoughtto facilitate protein synthesis by participating in the nuclearexport of specific mRNAs. In Arabidopsis, there are three isoformsof eIF5A. One of them, AteIF5A1, has been shown to be expressedin vascular tissue, specifically developing vessel members,using GUS as a reporter. In order to determine whether AteIF5A1plays a role in xylem formation, its full-length cDNA was constitutivelyover-expressed in transgenic Arabidopsis plants. Microscopicanalysis revealed that the cross-sectional area of the xylemin the main inflorescence stems of transgenic plants was 1.9-foldhigher than those of corresponding inflorescence stems of wild-typeplants. In wild-type stems, the primary xylem typically comprisedsix cell layers and was 105 µm thick, but increased to9–11 cell layers, 140–155 µm thick, in transgenicstems. Similarly, the secondary xylem increased from six celllayers, 70 µm thick, in control stems to 9 cell layers,95–105 µm thick, in transgenic stems. Moreover,constitutive down-regulation of AteIF5A1 using antisense technologyresulted in the major suppression of xylem formation comparedwith control plants, and the antisense transgenic plants werealso stunted. These data collectively indicate that eIF5A1 playsa role in xylogenesis. Key words: Arabidopsis thaliana, eukaryotic translation initiation factor 5A, inflorescence stem, xylem Received 5 November 2007; Revised 26 December 2007 Accepted 10 January 2008     

Mechanisms of Inhibition of Water Movement in Anaerobically Treated Roots of Zea mays L.

Changes in water flux (Jv) across detopped, 7-d-old, maize rootswere characterized during the initial 24 h of being made anoxicby exposure to an anaerobic nutrient solution. Suction (50 kPa)was applied to the xylem and samples of the xylem sap were collectedat intervals and the osmolality and ionic content were measured. Values of Jv through anoxic roots fell below those of aerobiccontrols 1 h after the equilibrium oxygen partial pressure inthe bathing medium dropped below 20 kPa (air = 20.6 kPa). Thereduction in Jv was due primarily to a nullification of thediurnal rhythm in hydraulic conductivity (Lp) that was measuredin aerobic roots. However, about one-quarter of the reductionin Jv could be accounted for by a smaller osmotic componentof the driving force () on water movement. The significance of changes in Jv in anoxic roots is discussedin terms of the reliability of estimates of Lp, the reflectioncoefficient () and . Key words: Anaerobiosis, hydraulic conductivity, osmotic potential, water     

Seasonal Water Acquisition and Redistribution in the Australian Woody Phreatophyte, Banksia prionotes

Sap flows in the xylem of plant roots in response to gradientsin water potential, either between soil and atmosphere (transpiration)or soil layers of different moisture content (termed hydraulicredistribution). The latter has the potential to influence waterbudgets and species interactions, but we lack information forall but a few plant communities. We combined heat pulse measurementsof sap flow with dye and isotope tracing techniques to gaugethe movement of xylem sap within, and exudation from, rootsof Banksia prionotes (Lindley). We demonstrated ‘ hydrauliclift’ during the dry season and provide some evidencethat extremely dry soils limit hydraulic lift. In addition wereport difficulties posed by spiralled xylem tissue in rootsfor the application of heat pulse techniques. Copyright 2000Annals of Botany Company Banksia prionotes, sap flow, hydraulic lift, heat ratio method, deuterium, stable isotopes, root architecture.     

On the Conductivity of the Xylem in Trees

The xylem of three tree species, the ring porous Fraxinus excelsiorL., and the diffuse porous Acer pseudoplatanus L., and Salixatrocinerea Brot., has been examined to determine how closelyflow through it obeys the Poiseuille equation. The equationwas most closly obeyed in the relationship between flow-rateand pressure difference between the ends of the xylem cylinder,though there were differences between the ring and diffuse poroustypes. The relationship of flow-rate and specific conductivityto the radius of the xylem cylinder was complex.Fraxinus showeda rise with increase of radius up to a given size (0.54cm),followed by a steep decline. This decline was shown to be causedby a progressive increase in the number of vessels which wereunable to conduct water. Small increases in the viscosity ofthe xylem liquid produced a disproportionately large fall inthe rate of flow, and the specific conductivity of the xylemdecreased. With viscosities greater than 2.5 centipoise, thespecific conductivity remained constant. An effect of alternatelyraising and lowering the hydrostatic pressure of the xylem wateron the specific conductivity of the wood is described. These results are discussed in relation to the structure ofthe xylem, and particularly with regard to the possibility thatthe conduction channels in the wood consist of a system of microcapillariessurrounding the vessel cavities.     

An anatomical assessment of branch abscission and branch-base hydraulic architecture in the endangered Wollemia nobilis

Background and Aims: The branch-base xylem structure of the endangered Wollemia nobiliswas anatomically investigated. Wollemia nobilis is probablythe only extant tree species that produces only first-orderbranches and where all branches are cleanly abscised. An investigationwas carried out to see if these unusual features might influencebranch-base xylem structure and water supply to the foliage. Methods: The xylem was sectioned at various distances along the branchbases of 6-year-old saplings. Huber values and relative theoreticalhydraulic conductivities were calculated for various regionsof the branch base. Key Results: The most proximal branch base featured a pronounced xylem constriction.The constriction had only 14–31 % (average 21 %)of the cross-sectional area and 20–42 % (average28 %) of the theoretical hydraulic conductivity of themore distal branch xylem. Wollemia nobilis had extremely lowHuber values for a conifer. Conclusions: The branch-base xylem constriction would appear to facilitatebranch abscission, while the associated Huber values show thatW. nobilis supplies a relatively large leaf area through a relativelysmall diameter ‘pipe’. It is tempting to suggestthat the pronounced decline of W. nobilis in the Tertiary isrelated to its unusual branch-base structure but physiologicalstudies of whole plant conductance are still needed.     

Modelling of quinolizidine alkaloid net flows in Lupinus albus and between L. albus and the parasite Cuscuta reflexa: new insights into the site of quinolizidine alkaloid synthesis

The present work reveals new and completely different conclusionsabout the alkaloid economy of symbiotically fed Lupinus albusand L. albus parasitized by Cuscuta reflexa in the study periodof 43–55 d after sowing of lupin. Net flows of alkaloidswithin lupin and between host and parasite were calculated usingthe molar ratio of alkaloid nitrogen: total nitrogen combinedwith known net flows of nitrogen in the transport fluids andanalysing alkaloid accumulation in plant organs by HRGC. Incontrast to previous studies, quinolizidine alkaloids were predictedto be synthesized mainly in the root of L. albus and to be predominantlytransported via xylem to the apical plant shoot organs. Parasitismby C. reflexa for 12 d induced a decline of alkaloid contentin the host L. albus up to 53% compared to control plants andalkaloid synthesis was halved—apparently due to a shortageof the precursor lysine. In spite of an additional decreasein nitrogen levels at the second harvest, the host-parasitesystem showed a1.3-fold higher alkaloid content than the controlplants, 63% of the total alkaloids being attracted by Cuscuta.This indicates (a) restriction of catabolic processes withininfected lupins, (b) a massive shift of nitrogen metabolismin the direction of alkaloids and (c) an enormous sink potentialof Cuscuta for nitrogenous compounds. Although xylem was foundto be the main translocation system for alkaloids, the modellingof alkaloid flows predicts Cuscuta to derive only 4.5% of itstotal alkaloid supply from the xylem and 95.5% from the phloem.By analogy with nitrogen flows, this finding requires xylemphloemtransfers which were assumed to occur within the stem axis oflupin. A similar proportion regarding the contribution of xylemand phloem to the supply of Cuscuta was obtained for the netflows of two selected alkaloids, lupanine and 13     

Fibres and Fibre-sclereids in Wild-typeArabidopsis thaliana

Examination of the fibre system in roots and shoots ofArabidopsisthaliana(L.) Heynh. revealed three types of fibres, distinguishedaccording to their site of differentiation: (1) short fibre-sclereidsin the secondary phloem of roots and shoots at the rosette level;(2) long fibres in the secondary xylem of the main root; and(3) very long fibres in the xylem of the inflorescence stems.These three types are in addition to the small number of primaryphloem fibres that are formed even in smallA. thalianaindividuals.These findings provide a basis for the use ofArabidopsis thalianaasa model system to study the processes leading to fibre differentiationin dicotyledons. Arabidopsis thaliana; differentiation; fibre-sclereids; inflorescences; phloem fibres; xylem     

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     

DoesAcetobacter diazotrophicusLive and Move in the Xylem of Sugarcane Stems? Anatomical and Physiological Data

We have previously shown that the nitrogen-fixing endophyteof sugarcane,Acetobacter diazotrophicuslives in the sugar solutionin the intercellular-space apoplast of the stem cortex. Variousauthors have claimed that it inhabits the xylem apoplast. Thispossibility has been investigated in the clone Ja 60-5 and shownto be most unlikely for the following reasons: (1) an adequatecarbon source is lacking in the xylem sap, and cannot be suppliedfrom the intercellular-space apoplast of the cortex. Diffusionof solutes into and out of the vascular bundles is preventedby complete lignification and suberization of the bundle sheathcell walls except at the nodes. (2) Longitudinal movement ofparticles as large as bacteria is severely limited at the nodes.Vessel end walls were found in 90% of vessels at each node,and only 1% of open vessels extended through two nodes. Noneextended as far as three nodes. In addition to vessel end walls,vessel continuity at nodes was interrupted by living cells.Dye solution in the transpiration stream in metaxylem vesselsdid not pass through these living cells, but accumulated incrystals (sump formation) in the vessels below the node. Onlyin some protoxylem vessels and cavities did dye solution movethrough many nodes. It is likely that selection of sugarcaneclones such as Ja 60-5 for resistance to bacterial wilt diseaseshave selected for clones that have limited vessel continuity.(3) When culturedA. diazotrophicuswas introduced into the transpirationstream, the xylem parenchyma reacted by secreting a bright redpolymer which killed the bacteria and blocked the movement ofwater. We conclude that the xylem flow-apoplast of this cloneof sugarcane is an unsuitable habitat forA. diazotrophicusandthat additional habitats to those of the intercellular-spaceapoplast should be sought elsewhere. Acetobacter diazotrophicus; endophytic bacteria; nitrogen fixation; sugarcane; vessel end walls; xylem apoplast; xylem bacteria; xylem segmentation     

Vascular Development and Sap Flow in Apple Pedicels

Xylem and phloem tissues of the pedicel of apple fruit increasein cross-sectional area throughout development. The increasein phloem is similar in the two cultivars examined (Cox's OrangePippin and Royal Gala) and reflects a steadily increasing phloemsap flow to the fruit. The increase in xylem tissue is due toa proliferation of non-conducting, structural, components sinceclose examination reveals no increase in the number of vesselelements from just after flowering onwards. The greater number,and the larger diameter, of the vessels in Cox's explains theinitially higher xylem conductance found in this cultivar. In vitro measurements of xylem exudation reveal a decline duringthe growing season in the xylem conductance of both cultivarsand an increasing proportion of fruit (particularly in Cox's)in which the xylem comes to be totally non-conducting. Thisobservation is in line with previously reported measurementsof xylem sap flow in vivo. The straightforward techniques used in this study offer a feasiblealternative to more arduous methods of assessing xylem and phloemsap flows and their balance during growth.Copyright 1994, 1999Academic Press Apple, xylem, phloem, vascular development, sap flow, Malus domestica Borkh     

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     

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     

The Effect of Hydrosatic Pressure Gradients on the Movement of Sodium and Calcium across the Root Cortex

The hydrostatic pressure gradient across the root cortex inducedby transpiration was simulated by raising the pressure on themedium surrounding the roots of detopped tomato (Lycopersicumesculentum) and castoroil plants (Ricinus communis).Applyinga pressure of 2 atm. resulted in a doubling of the sodium fluxfrom medium to xylern (table I). This flux depended on a maintainedsuply of Na in the medium (fig.1) indicating that there wasa small but readily available storage and exchange capacityfor Na in the root tissues. In the presence of metabolic inhaibitorsthe Na flux was considerablyu reduced and equalled the fluxof mannitol (talble II) believed to be a passive mass flow.The Na flux under a pressure gradient appeared therefore tobe metabollism-facilitated. Metabolic inhibition after the rootshad been allowed to take up Na led to a large flux of sodiuminto the xylem (Fig.2). This was considered to come from a largeunavailable store in the roots. Metabolism-facilitated processes also appeared to predominatein the flux of calcium to the xylem of Ricinus, buit not inthe flux through tomato roots. The location of the centripetal metabolism-facilitated transferand the available and unavailable stores is discussed in relationto the effects of pressure gradients.     

Influence of Glutathione (GSH) on Sulphate Influx, Xylem Loading and Exudation in Excised Tobacco Roots

In short-term experiments sulphate influx of excised tobaccoroots {Nicotiana tabacum L. var. 'Samsun') followed monophasicMichaelis-Menten kinetics with an approximate K_m of 12 ±4 µM and v_max of 657 ± 211 nmol g^–1 FW h^–1.An inhibition of sulphate influx, xylem loading and exudationof more than 70% was achieved with 01 mM GSH within 1 h. Cysteinewas two orders of magnitude more effective as an inhibitor thanGSH. An inhibition of more than 75% was already obtained with1.0µM cysteine. It may, therefore, be assumed that GSHis decomposed to yield cysteine concentrations that may inhibitsulphate influx, xylem loading and exudation. When BSO, a specificinhibitor of the initial step of GSH synthesis, was added, cysteine-mediatedinhibition on sulphate influx, xylem loading and exudation wasstrongly diminished. Apparently, GSH synthesis is required toobtain inhibition of these processes by cysteine. The physiologicalmechanisms that may cause the inhibition of sulphate influx,xylem loading and exudation by glutathione are discussed. Key words: Sulphate transport, Nicotiana, Solanaceae, glutathione, cysteine, buthionine sulphoximine     

Observations on the Exchange of Salt Between the Xylem and Neighbouring Cells in Zea mays Primary Roots

Comparison of two preparations of Zea mays primary roots hasshown that the salt concentration in the xylem fluid is alteredas it passes through a region of the root where no absorptionof salts or water from the external bathing medium can occur.Salts are secreted to the xylem by such a region, presumablyfrom the vacuoles of the cortical cells. It has also been demonstratedthat longitudinal water movement through the root cortex isnegligibly small during root-pressure exudation.     

Characterization of Cd translocation and identification of the Cd form in xylem sap of the Cd-hyperaccumulator Arabidopsis halleri

Arabidopsis halleri is a Cd hyperaccumulator; however, the mechanismsinvolved in the root to shoot translocation of Cd are not wellunderstood. In this study, we characterized Cd transfer fromthe root medium to xylem in this species. Arabidopsis halleriaccumulated 1,500 mg kg^–1 Cd in the shoot without growthinhibition. A time-course experiment showed that the releaseof Cd into the xylem was very rapid; by 2 h exposure to Cd,Cd concentration in the xylem sap was 5-fold higher than thatin the external solution. The concentration of Cd in the xylemsap increased linearly with increasing Cd concentration in theexternal solution. Cd transfer to the xylem was completely inhibitedby the metabolic inhibitor carbonyl cyanide 3-chlorophenylhydrazone(CCCP). Cd concentration in the xylem sap was decreased by increasingthe concentration of external Zn, but enhanced by Fe deficiencytreatment. Analysis with ¹¹³Cd-nuclear magnetic resonance (NMR)showed that the chemical shift of ¹¹³Cd in the xylem sap wasthe same as that of Cd(NO₃)₂. Metal speciation with Geochem-PCalso showed that Cd occurred mainly in the free ionic form inthe xylem sap. These results suggest that Cd transfer from theroot medium to the xylem in A. halleri is an energy-dependentprocess that is partly shared with Zn and/or Fe transport. Furthermore,Cd is translocated from roots to shoots in inorganic forms.     

Diurnal change in the relationship between stomatal conductance and abscisic acid in the xylem sap of field-grown peach trees

To evaluate whether abscisic acid (ABA) in the xylem sap playsan important role in controlling stomatal aperture of field-grownPrunus persica trees under drought conditions, stomatal conductance(g) and xylem ABA concentrations were monitored both in irrigatedand non-irrigated trees, on two consecutive summer days (threetimes a day). Stomata1 conductance of non-irrigated trees hada morning maximum and declined afterwards. The changes in gduring the day, rather than resulting from variations in theconcentrations of ABA in the xylem sap or the delivery rateof this compound to the leaves, were associated with changesin the relationship between g and xylem ABA. The stomata ofwater-stressed trees opened during the first hours of the day,despite the occurrence of a high concentration of ABA in thexylem sap. However, stomatal responsiveness to ABA in the xylemwas enhanced throughout the day. As a result, a tight inverserelationship between g and the logarithm of xylem ABA concentrationwas found both at midday and in the afternoon. A similar relationshipbetween g and ABA was found when exogenous ABA was fed to leavesdetached from well-watered trees. These results indicate thatABA derived from the xylem may account for the differences ing observed between field-grown peach trees growing with differentsoil water availabilities. Several possible explanations forthe apparent low stomatal sensitivity to xylem ABA in the morning,are discussed, such as high leaf water potential, low temperatureand high cytokinin activity. Key words: Prunus persica L., stomata, xylem ABA, water deficits, root-to-shoot communication     

Naturally Occurring Regenerative Differentiation of Xylem Around Adventitious Roots in Luffa cylindrica Seedlings

Regeneration of xylem induced by adventitious root formationin the hypocotyl of Luffa cylindrica Roem. seedlings is described.This naturally occurring form of xylem regeneration involvesthe formation of a bypass of regenerated tracheary elementsaround a root without external severance of the vascular strands.The regeneration of xylem around an adventitious root is polarand is very similar in its developmental pattern to the well-knownxylem regeneration induced by wounding vascular strands. Adventitious root formation, Luffa cylindrica Roem, regenerated tracheary elements, vascular differentiation, xylem regeneration