Transfer cells and flange cells in sinkers of the mistletoePhoradendron macrophyllum (Viscaceae), and their novel combination

Summary The unusual thick-walled cells in contact with host and parasite vessels, first noted by Calvin 1967 in sinkers (structures composed of tracheary elements and parenchyma that originate from parasite bark strands that grow centripetally to the host vascular cambium and become embedded by successive development of xylem) of the mistletoePhoradendron macrophyllum (Englem.) Cockerell, have been investigated by modern methods of microscopy. The wall is thickest in cells abutting large-diameter host vessels, less so against smaller host vessels and those abutting sinker vessels. Transmission electron microscopy reveals the wall to be complex, consisting of a basement primary wall, upon which two developments of secondary-wall material occur. These are represented by lignified thickenings, in the form of flanges, and a labyrinth of wall ingrowths characteristic of a transfer cell. The wall ingrowths occur mostly in the primary-wall regions between the flanges, but when in contact with a large host vessel the ingrowths also differentiate on top of the flanges. Cells with such a transfer cell labyrinth have not been previously reported in the endophytic system of a mistletoe. The cells are confined to the xylary portion of the primary haustorium and sinkers. InP. macrophyllum, however, the cells differ from ordinary transfer cells in that they have differentiated as part of a flange parenchyma cell. This arrangement represents a novel anatomical situation. The name flange-walled transfer cell is used for these cells. The xylem of primary haustorium and sinkers also contain numerous ordinary flange cells. In both flange-walled transfer cells and ordinary flange cells the flanges are lignified and form a reticulate pattern of thickenings, separated by rounded areas of primary pit fields. The extent of development of the flange wall can vary in different parts of a sinker. At the host interface, the existence of a flange-walled transfer cell in direct contact with a vessel reflects a site associated with high loading into the parasite. Similarly, a labyrinth against a sinker vessel indicates a site of unloading from surrounding sinker tissue into the vessel for subsequent longdistance transport within the parasite.Dedicated to the memory of Dr. Katherine Esau (1898–1997)     

Ultrastructure of the haustorial interface and apoplastic continuum between host and the root hemiparasiteOlax phyllanthi (Labill.) R. Br. (Olacaceae)

Summary Structural features of haustorial interface parenchyma of the root hemiparasiteOlax phyllanthi are described. Walls contacting host xylem are thickened non-uniformly with polysaccharides, not lignin, and show only a thin protective wall layer when abutting pits in walls of host xylem vessels or tracheids. Lateral walls of interface parenchyma exhibit an expanded middle layer of open fibrillar appearance, sometimes with, but mostly lacking adjoining layers of dense wall material. Free ribosomes and rough endoplasmic reticulum are prominent and occasional wall ingrowths present. Experiments involving transpirational feeding of the apoplast tracers lanthanum nitrate or uranyl acetate to host roots cut below haustorial connections, indicate effective apoplastic transfer from host to parasite root via the haustorium. Deposits of the tracers suggest a major pathway for water flow through host xylem pits, across the thin protective wall layer, and thence into the haustorium via the electronopaque regions of the terminal and lateral walls of the contact parenchyma. Graniferous tracheary elements and walls of parenchyma cells of the body of the haustorium appear to participate in tracer flow as do walls of cortical cells, stele parenchyma and xylem conducting elements of the parasite root, suggesting that both vascular and non-vascular routes are involved in extracytoplasmic transfer of xylem sap from host to parasite. The Casparian strip of the endodermis and the suberin lamella of the exodermis of theOlax root act as barriers to flow within the system.     

Organic substances in xylem sap delivered to above-ground organs by the roots

Squash (Cucurbita maxima) xylem sap, an apoplastic fluid, contains t-zeatin riboside, glutamine, methylglycine, myo-inositol, fructose, oligosaccharides of arabinogalactan, glucan, galacturonan, and pectins (rhamnogalacturonan-I and rhamnogalacturonan-II), as well as various proteins, including arabinogalactan and pathogen-related proteins. These substances are mainly produced in stele (xylem) parenchyma and the pericycle in the root-hair zone where ion transporter genes are expressed. Glycine-rich protein genes (CRGRPs) cloned by antiserum raised against whole xylem sap of cucumber (Cucumis sativus) were abundantly expressed in the parenchyma cells surrounding xylem vessels in the root-hair zone. CRGRP proteins accumulated and immobilized in the lignified walls of metaxylem vessels and perivascular fibers in shoots, suggesting a systemic delivery mechanism of wall materials via xylem sap. A major 30-kDa protein (XSP30) found in cucumber xylem sap was homologous to the B chains of a lectin (ricin) and bound to a nonfucosylated core N-acetylglucosamine dimer of N-linked glycoproteins abundant in leaf parenchyma cells. XSP30 gene expression, abundant in root xylem parenchyma and pericycle, and the level of XSP30 protein fluctuated diurnally under the control of a circadian clock, and the amplitude was up-regulated by gibberellic acid produced in young leaves, suggesting a long-distance control system between organs.     

Xylem parenchyma cells deliver the H<Subscript>2</Subscript>O<Subscript>2</Subscript> necessary for lignification in differentiating xylem vessels

Lignification in Zinnia elegans L. stems is characterized by a burst in the production of H₂O₂, the apparent fate of which is to be used by xylem peroxidases for the polymerization of p-hydroxycinnamyl alcohols into lignins. A search for the sites of H₂O₂ production in the differentiating xylem of Z. elegans stems by the simultaneous use of optical (bright field, polarized light and epi-polarization) and electron-microscope tools revealed that H₂O₂ is produced on the outer-face of the plasma membrane of both differentiating (living) thin-walled xylem cells and particular (non-lignifying) xylem parenchyma cells. From the production sites it diffuses to the differentiating (secondary cell wall-forming) and differentiated lignifying xylem vessels. H₂O₂ diffusion occurs mainly through the continuous cell wall space. Both the experimental data and the theoretical calculations suggest that H₂O₂ diffusion from the sites of production might not limit the rate of xylem cell wall lignification. It can be concluded that H₂O₂ is produced at the plasma membrane in differentiating (living) thin-walled xylem cells and xylem parenchyma cells associated to xylem vessels, and that it diffuses to adjacent secondary lignifying xylem vessels. The results strongly indicate that non-lignifying xylem parenchyma cells are the source of the H₂O₂ necessary for the polymerization of cinnamyl alcohols in the secondary cell wall of lignifying xylem vessels.     

Thermal dissipation probe measurements of sap flow in the xylem of trees documenting dynamic relations to variable transpiration given by instantaneous weather changes and the activities of a mistletoe xylem parasite

The thermal dissipation probe was described in the early 1930s for the demonstration of a volume and mass flow of sap in the conductive elements of the xylem in trees. It was subsequently developed further and is now widely used in physiological ecology including measurements in the field. Thermal dissipation demonstrates the occurrence of sap flow and allows determination of its velocity. Here we report simultaneous continuous measurements of sap flow using the thermal dissipation technique and of transpiration by infrared gas analysis for diurnal and annual cycles in a deciduous and an evergreen oak tree, Quercus robur L. and Quercus turneri Willd., respectively, in a deciduous and an evergreen conifer, Larix decidua Mill. and Pinus griffithii McClell., respectively, and the host/mistletoe consortium of the deciduous linden Tilia mandschurica Rupr. & Max. and the evergreen Viscum album L. We show (1) that in diurnal cycles sap flow closely follows dynamic changes of the rate of transpiration elicited by daily fluctuations of weather parameters (sunshine, cloudiness, air temperature and humidity), (2) that in annual cycles sap flow reflects autumnal yellowing and shedding of leaves of the deciduous trees. We report for the first time comparative measurements of sap flow towards mistletoe shoots and host branches in a parasite/host consortium. This demonstrates (3) that mistletoes maintain considerably larger sap flow rates in their xylem conduits than the adjacent host branches dragging the transpiration stream of their host towards their own shoots. We also show (4) that even after the deciduous host has shed its leaves and itself does not transpire any more the evergreen mistletoe towards its shoots can maintain the persistence of a continuous sap flow via the stem and branches of the host as long as frost does not prevent that. The work presented underlines the contention that transpiration is the driving force for sap flow with continuous files of water in the xylem. It shows for the first time that mistletoes direct the flow of water through host roots and stems towards its own shoots by not only performing stronger transpiration as it is known from the literature but also by maintaining larger sap flow rates in the xylem conduits of its stems.     

Water relations in silver birch during springtime: How is sap pressurised?

• Positive sap pressures are produced in the xylem of birch trees in boreal conditions during the time between the thawing of the soil and bud break. During this period, xylem embolisms accumulated during wintertime are refilled with water. The mechanism for xylem sap pressurization and its environmental drivers are not well known.
• We measured xylem sap flow, xylem sap pressure, xylem sap osmotic concentration, xylem and whole stem diameter changes, and stem and root non‐structural carbohydrate concentrations, along with meteorological conditions at two sites in Finland during and after the sap pressurisation period.
• The diurnal dynamics of xylem sap pressure and sap flow during the sap pressurisation period varied, but were more often opposite to the diurnal pattern after bud burst, i.e. sap pressure increased and sap flow rate mostly decreased when temperature increased. Net conversion of soluble sugars to starch in the stem and roots occurred during the sap pressurisation period. Xylem sap osmotic pressure was small in comparison to total sap pressure, and it did not follow changes in environmental conditions or tree water relations.
• Based on these findings, we suggest that xylem sap pressurisation and embolism refilling occur gradually over a few weeks through water transfer from parenchyma cells to xylem vessels during daytime, and then the parenchyma are refilled mostly during nighttime by water uptake from soil. Possible drivers for water transfer from parenchyma cells to vessels are discussed. Also the functioning of thermal dissipation probes in conditions of changing stem water content is discussed.

     

The haustorium of the root parasite Triphysaria (Scrophulariaceae), with special reference to xylem bridge ultrastructure

Haustoria of Triphysaria pusilla and T. versicolor subsp. faucibarbata from a natural habitat were analyzed by light and electron microscopy. Secretory trichomes (root hairs) participate in securing the haustorium to the surface of the host root. The keel-shaped intrusive part of the secondary haustorium penetrates to the depth of the vascular tissue of the host. Some of the epidermal interface cells differentiate into xylem elements. A significant number of haustoria do not differentiate further, but in most haustoria one to five of the epidermal xylem elements terminate a similar number of xylem strands. The strands mostly consist of vessel members and they connect host xylem or occasionally host parenchyma to the plate xylem adjacent to the stele of the parasite root. Each strand of this xylem bridge is accompanied by highly protoplasmic parenchyma cells with supposed transfer cell function. Increased surface area of the plasmalemma occurs in these cells as it does in interface parenchyma cells. Graniferous tracheary elements are restricted to the haustorium and occur most frequently in the plate xylem. The plate xylem is also accompanied by highly protoplasmic parenchyma cells. Hyphae of mycorrhizal fungi of the host root occasionally penetrate into the distal part of the xylem bridge. We combine structural observations and physiological facts into a hypothesis for translocation of water and nutrients between host and parasite. Some evolutionary aspects related to endogeny/exogeny of haustoria are discussed, and it is argued that the Triphysaria haustorium represents a greatly advanced and/or reduced condition within Scrophulariaceae.     

Anomalous secondary vascular tissue inHelminthostachys zeylanica (Ophioglossaceae)

The vascular anatomy ofHelminthostachys zeylanica was examined with special reference to anomalous secondary tissue. Primary xylem development gradually takes place centrifugally. In branched rhizomes with destroyed apices, the vascular cylinder apical to the insertion of branch traces is generally composed of primary xylem, accessory xylem, inner parenchyma of radially arranged cells, outer parenchyma of irregularly arranged cells, and partly crushed phloem, listed in order going outwards. The accessory xylem as well as the inner parenchyma ofHelminthostachys zeylanica is probably secondarily produced, partly to contribute to the branch traces, in a position corresponding to that of secondary vascular tissue developed from a normal cambium inBotrychium sensu lato. It is suggested that although a cambium is lacking inHelminthostachys zeylanica, the secondary vascular tissues are comparable between the genera. The phylogenetic implication of this tissue is discussed.     

Structure and Development of the Endophyte in the Parasitic Angiosperm <Emphasis Type="Italic">Cuscuta japonica</Emphasis>

The endophyte, that is, the haustorial part within the tissues of the host plant Impatiens balsamina, of the parasitic angiosperm Cuscuta japonica was studied with light and electron microscopy. The endophyte consisted mainly of vacuolated parenchymatous axial cells and elongate, superficial (epidermal) cells. Then the elongate, epidermal cells separated from each other and transformed into filamentous cells, called searching hyphae. The hyphae grew independently either intercellularly or intracellularly in the host parenchyma. The apical end of the hyphal cells was characterized by conspicuous, large nuclei with enlarged nucleoli and very dense cytoplasm with abundant organelles, suggesting that the hyphal cells penetrating host tissue were metabolically very active. Numerous osmiophilic particles and chloroplasts were noted in the hyphae. The osmiophilic particles were assumed to be associated with elongation of the growing hyphe. Plasmodemata connections between the searching hyphal cells of the parasite and the host parenchyma cells were not detected. Hyphal cells that reached the host xylem differentiated into water-conducting xylic hyphae by thickening of the secondary walls. A xylem bridge connecting the parasite and the host was confirmed from serial sections. Some hyphal cells that reached the host phloem differentiated into nutrient-conducting phloic hyphae. Phloic hyphae had a thin layer of peripheral cytoplasm with typical features of sieve-tube members in autotrophic angiosperms, i.e., parallel arrays of smooth endoplasmic reticulum, mitochondria, and plastids with starch granules. Interspecific open connections via the sieve pores of the host sieve elements and plasmodesmata of the parasite phloic hyphae were very rarely observed, indicating that the symplastic translocation of assimilate to the parasite from the host occurred.     

An anatomical study of the haustoria of Rhinanthus minor attached to roots of different hosts

Roots of a range of potential hosts responded differently when Rhinanthus minor attempted to form haustoria. Roots of Fabaceae show the weakest reaction as apart from slight lignification, no reaction was observed at the interface between the endophyte and the cortical tissue of the host root. Grass roots react with strong lignification of all cells within the stele with the exception of a small number of phloem cells whilst the endodermis fully enters the tertiary stage. In the case of Phleum bertolonii the cortical cells also become lignified. The lignification is even observed in the host root tissue in a distance of about 1 mm from the haustorium (both apically and basipetally). In the case of Leucanthemum vulgare, strong suberisation can be observed in the cell walls of the interface between endophyte (tip of the sucker) and host. Plantago lanceolata exhibits the strongest reactions against the haustorial tissues. Cells of the interface between the endophyte and the host cortex are completely destroyed, as well as a few cell layers outside the central xylem cylinder, even in some distance from the haustorium. Thus, host xylem is completely isolated from the haustorium in this case. Extraction of sap from xylem vessels is likely to be drastically impaired in such a situation.     

Green tissue within the haustorium of the dwarf mistletoe Korthalsella (Viscaceae). An ultrastructural comparison between chloroplasts of sucker and aerial stem tissues

Summary Korthalsella (Viscaceae) is a dwarf mistletoe attached to its host branch by a single haustorium. Plants are leafless with flattened or cylindrical stems that function in photosynthesis. When a fresh haustorium is cut the sucker within the host appears bright green. Transmission electron microscopy reveals that this greening is due to chloroplasts, but that their organization differs from those of the aerial stem. The three representatives of Korthalsella endemic to New Zealand were the main species investigated. In the stem, chloroplasts have short stacks of cylindrical grana interconnected by stroma thylakoids typical of normal chloroplasts. Sucker chloroplasts have a more variable organization, with most containing extensive granal stacks and poorly differentiated stroma thylakoids. These granal thylakoids exhibit extensive partitions formed by appression of adjacent membranes. Some sucker plastids also approach etioplasts in having a prominent prolamellar body from which radiate thylakoids with short partitions. Sucker chloroplasts usually contain a few large starch grains, plastoglobuli, and sometimes also a stroma centre. The extensive granal thylakoids in sucker chloroplasts of Korthalsella resemble that found in certain shade plants and tissue grown under low light conditions. Sucker chloroplasts probably have a low level of photosynthesis. This activity might provide a local source of osmotically active material used to assist transport between host and parasite.     

The apoplastic continuum,nutrient absorption and plasmatubules in the dwarf mistletoeKorthalsella lindsayi (Viscaceae)

Summary The transfer of nutrients between host and parasite in mistletoes has generally been considered to occur via the xylem to xylem contacts at the host-parasite interface in the haustorial organ of attachment. A few workers, however, have recently begun to question this assumption and have suggested an alternative pathway of transport involving the intervening parenchyma cells which are often abundant in the parasite at the interface. But no morphological experimental evidence has yet been forthcoming in support of an apoplastic continuum across this interface between parasite and host.Our observations on the dwarf mistletoeKorthalsella lindsayi first indicate an absence of plasmodesmata at the interface, with the conclusion that symplastic transport between the two plants is not involved. However, application of apoplastic markers, such as Calcofluor white and lanthanum and uranyl ions, to the stem of the host results in the transfer of these tracers across the interface and into the tissues of the parasite. This demonstrates the existence of an apoplastic continuum between the two plants, and a pathway that is probably used in the normal transfer of water and other nutrients from host to parasite.From the apoplastic continuum provided by the walls of the haustorial parenchyma tissue, nutrients are transferred to the symplast for eventual distribution to other parts of the plant. Evidence for the active uptake of substances from the apoplast by the protoplasts of the parenchyma cells is shown by the convoluted appearance of the plasmalemma and its differentiation often into plasmatubules.     

Freezing behavior of xylem ray parenchyma cells in softwood species with differences in the organization of cell walls

Summary By cryo-scanning electron microscopy we examined the effects of the organization of the cell walls of xylem ray parenchyma cells on freezing behavior, namely, the capacity for supercooling and extracellular freezing, in various softwood species. Distinct differences in organization of the cell wall were associated with differences in freezing behavior. Xylem ray parenchyma cells with thin, unlignified primary walls in the entire region (all cells inSciadopitys verticillata and immature cells inPinus densiflora) or in most of the region (mature cells inP. densiflora and all cells inP. pariflora var.pentaphylla) responded to freezing conditions by extracellular freezing, whereas xylem ray parenchyma cells with thick, lignified primary walls (all cells inCrytomeria japonica) or secondary walls (all cells inLarix leptolepis) in most regions responded to freezing by supercooling. The freezing behavior of xylem ray parenchyma cells inL. leptolepis changed seasonally from supercooling in summer to extracellular freezing in winter, even though no detectable changes in the organization of cell walls were apparent. These results in the examined softwood species indicate that freezing behavior of xylem ray parenchyma cells changes in parallel not only with clear differences in the organization of cell walls but also with subtle sub-electron-microscopic differences, probably, in the structure of the cell wall.     

Integrated nitrogen,carbon, and water relations of a xylem-tapping mistletoe following nitrogen fertilization of the host

Xylem-tapping mistletoes transpire large volumes of water (E) while conducting photosynthesis (A) at low rates, thus maintaining low instantaneous wateruse efficiency (A/E). These gas-exchange characteristics have been interpreted as a means of facilitating assimilation of nitrogen dissolved at low concentration in host xylem water; however, low A/E also results in substantial heterotrophic carbon gain. In this study, host trees (Juniperus osteosperma) were fertilized and gas exchange of mistletoe (Phoradendron juniperinum) and host were monitored to determine whether mistletoe A/E would approach that of the host if mistletoes were supplied with abundant nitrogen. Fertilization significantly increased foliar N concentrations (N), net assimilation rates, and A/E in both mistletoe and host. However, at any given N concentration, mistletoes maintained lower A and lower A/E than their hosts. On the other hand, when instantaneous water-use efficiency and A/N were calculated to include heterotrophic assimilation of carbon dissolved in the xylem sap of the host, both water-use efficiency and A/N converged on host values. A simple model of Phoradendron carbon and nitrogen budgets was constructed to analyze the relative benefits of nitrogen- and carbonparasitism. The model assumes constant E and includes feedbacks of tissue nitrogen concentration on photosyn-thesis. These results, combined with our earlier observation that net assimilation rates of mistletoes and their hosts are approximately matched (Marshall et al. 1994), support part of the nitrogen-parasitism hypothesis: that high rates of transpiration benefit the mistletoe primarily through nitrogen gain. However, the low ratio of A/E is interpreted not as a means of acquiring nitrogen, but as an inevitable consequence of an imbalance in C and N assimilation.This research was supported by the National Science Foundation (grants BSR-8706772 and 8847942).     

Transpiration, CO2 assimilation, WUE, and stomatal aperture in leaves of Viscum album (L.): Effect of abscisic acid (ABA) in the xylem sap of its host (Populus x euamericana).

Leaves of the mistletoe Viscum album (L.) show a high rate of transpiration, even when the host is under severe drought stress. The hypothesis that a strong control of ABA influx from the xylem sap of the host into the mistletoe prevents stomatal closure in mistletoe leaves was tested under the following conditions: sections of poplar twigs carrying a mistletoe were perfused with artificial xylem sap that contained different ABA concentrations and both transpiration and ABA levels were analysed in mistletoe leaves. Despite variation by a factor of 10(4), the ABA content of the host xylem did not affect ABA levels, leaf transpiration, CO(2) assimilation, WUE, or the degree of stomatal aperture in mistletoe leaves. These observations support the hypothesis of a strong control of ABA influx from the host of the xylem into the mistletoe, although degradation of ABA before it enters the mistletoe leaves cannot be excluded. This mechanism may ensure a water and nutritional status favourable for the mistletoe, even if the water status of the host is impaired. Despite the lack of short-term sensitivity of ABA levels in mistletoe leaves to even strong changes of ABA levels in the xylem sap of the host, ABA levels in mistletoe leaves were relatively high compared to ABA levels in the leaves of several tree species including poplar. Since significant transpiration of the mistletoe leaves was observed despite high ABA levels, a diminished sensitivity of the stomata of mistletoe leaves to ABA has to be concluded. The stomatal density of adaxial Viscum leaves of 89+/-23 stomata per mm is lower than those reported in a study performed at the end of the 19th century.     

RELATION OF DWARFMISTLETOE (ARCEUTHOBIUM) TO THE XYLEM TISSUE OF CONIFERS. I. ANATOMY OF PARASITE SINKERS AND THEIR CONNECTION WITH HOST XYLEM

Srivastava , L. M., and K. Esau . (U. California, Davis.) Relation of dwarf mistletoe (Arceuthobium) to the xylem tissue of conifers. I. Anatomy of parasite sinkers and their connection with host xylem. Amer. Jour. Bot. 48(2): 159–167. Illus. 1961.—The anatomy of the sinkers of Arceuthobium infecting 7 species of conifers was studied by the use of serial cross, radial, and tangential sections of the host wood. The sinkers were found to be composed of parenchyma cells only, or of parenchyma cells and tracheary elements, including vessel elements. In all species tracheary cells of the sinkers had direct contacts with the host tracheids of axial and radial systems. Typically the sinkers were associated with rays of the host wood. In some species, the centripetal ends of sinkers were wedged in radially among the axial tracheids of the host, but centrifugally such sinkers were usually found associated with rays. In the region of the host cambium the sinker contained parenchyma cells meristematic in appearance and, in 6 out of 7 species, also mature tracheary elements. The oldest of these elements became stretched and ruptured, a circumstance indicating that growth occurred in the part of the sinker embedded in the host cambium. This growth appeared to be coordinated with that of the host cambium, so that the sinker became embedded in the host xylem and phloem. Radial centripetal penetration of sinkers among differentiating axial tracheids of the host possibly occurred to a limited extent.     

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

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

Effects of parasitism by a mistletoe on the structure and functioning of branches of its host

Infestation of Acacia acuminata by the xylem-tapping mistletoe Amyema preissii invariably results in inhibition of growth, defoliation and eventual death of host branch parts distal to the mistletoe. Branch sectional areas proximal (P) and distal (D) to mistletoes are used to classify stages of parasitism, with P:D area ratios of 5–6 invariably associated with distal branch senescence. As monopolization of the branch proceeds, mistletoe leaf area increases in parallel with declining host foliage area, and the specific hydraulic conductivity of distal host wood declines sharply relative to that of proximal wood, mineral composition and concentrations of nitrogenous solutes in xylem sap are at no stage appreciably different from those of proximal wood. After the demise of the distal branch parts, the transectional area of the host branch stump increases linearly with increasing mistletoe leaf area, the branch area supporting a unit of mistletoe leaf area always being about 3 times greater than that supporting a unit of host foliage area on unparasitized branches. This differential, compounded with high transpiration rates and selective uptake of host xylem solutes by the haustorium, fosters substantial mineral enrichment of the mistletoe relative to its host. The study provides a background for future investigation of possible cellular mechanisms continuously driving structural and functional changes in favour of the mistletoe.     

Seasonal and spatial variation of carbohydrates in mistletoes (Viscum album) and the xylem sap of its hosts (Populus x euamericana and Abies alba)

In the present field study we analysed the seasonal pattern of carbohydrate composition and contents in the xylem sap of Viscum album and the xylem sap of a deciduous ( Populus × euramericana ) and a coniferous ( Abies alba ) host tree species. The results were compared with the soluble carbohydrate composition and contents of mistletoe tissues. On both hosts significant amounts of glucose, fructose, and sucrose were found in the xylem sap of Viscum throughout the seasons. The general seasonal pattern of sugar contents, i.e. high concentrations in spring and lower concentrations in other seasons on Populus , and intermediate concentrations throughout the year on Abies , largely reflected the xylem sap carbohydrate composition and contents of the respective host. These observations provide indirect evidence for carbohydrate flux from the xylem sap of the host into the mistletoe. However, in both hosts xylem sap seems to be deviated into the mistletoe without specific control of carbohydrate flux. Differences observed between the seasonal pattern of xylem sap carbohydrate concentrations in Viscum on Populus and Abies may originate from the different time of leaf development of these species. A clear-cut seasonal pattern of soluble carbohydrates was not observed in the leaves of Viscum on both hosts. Still soluble carbohydrates seem to be reallocated from the senescing to the newly developed leaves of Viscum indicating that the seasonal requirement of carbohydrate for growth and development can only completely be met by carbohydrate acquisition from the host and their own photosynthesis.     

Morphology and Anatomy of the Haustorium of the Root Hemiparasite Olax phyllanthi (Olacaceae), with Special Reference to the Haustorial Interface

Gross morphology and internal structure of haustoria of Olaxphyllanthi are described in parasitism with a range of hosts,including roots of woody and herbaceous dicotyledons and certainmonocotyledons, and occasional instances of autoparasitism andhaustorial formation on monocotyledon rhizomes. Successful penetrationto xylem occurs on virtually all hosts across broad diameters,ages and anatomies of host root, but anatomical impedimentsto haustorial establishment and penetration are recorded forcertain host taxa. Each haustorium is a comparatively simpleand ephemeral structure. Its developing sucker (endophytic regionof the haustorium) spreads laterally around the surface of thehost xylem, yet never completely encircles the host stele. Damageto hosts is minimal and secondary thickening (of hosts) continueson the side of a host root opposite to a haustorium. The haustorialsucker lacks phloem and its interface with host xylem is comprisedalmost entirely (more than 98.7%) of parenchyma. The few terminatingtracheids at an interface lie in very close proximity to oroccasionally directly against exposed xylem vessels, but lumento lumen continuity between tracheary elements of the partnersis not achieved. Three dimensional reconstructions based onserial transverse sectioning indicate that well defined filesof tracheids connect back from an interface to the core of graniferoustracheary elements in the external body of the haustorium, andthence to the xylem of the parent parasite root. The findingsare discussed in relation to existing studies on haustorialanatomy. Root parasite, Olacaceae, haustorial anatomy, host specificity