Simultaneous collection of xylem sap from Rhinanthus minor and the hosts Hordeum and Trifolium: hydraulic properties, xylem sap composition and effects of attachment

Abstract

The facultative hemiparasite Rhinanthus minor was grown alone, or with either Hordeum vulgare or Trifolium alpestre as a host. All plants were fed 5 mM nitrate. In the parasites, successful attachment led to dramatic increases in growth (particularly with barley as host) and in tissue concentrations of total N, NO $ − , H # PO % − and K + , as well as moderate increases in Cl − and SO % #−. Mg# + and Ca# + concentrations either remained unchanged, increased or decreased, depending on the host. Xylem sap was collected from leaf veins (barley) or the stem (Rhinanthus and clover), by applying pneumatic pressure to the rooted soil to raise the soil water potential so that in vivo xylem sap exuded from small incisions once the applied pressure balanced the tension exerted by transpiration. High balancing pressures were needed for unattached Rhinanthus, whereas simultaneous collection of xylem sap from attached Rhinanthus and its host, at a much lower balancing pressure, was possible only when the parasite was enclosed in polyethylene film and high transpiration abolished. From plots of xylem sap volume flow versus applied pressure, the hydraulic conductivity L p and the overall hydraulic resistance r were obtained. The value of r was high in unattached Rhinanthus and dramatically decreased by attachment to a host, highlighting the improved access to water gained by Rhinanthus when parasitizing a host. For the parasites, attachment resulted in greatly increased concentrations and solute flow rates in xylem of K + , NO $ − , H # PO % − and amino acids, and decreased concentrations of Ca# + and Mg# + . Estimates of NO $ − reduction in the parasite showed that 99 % was reduced in unattached plants, 85 % in those attached to clover and 52 % in those attached to barley. In the parasitized hosts, ion concentrations in xylem sap were somewhat increased relative to unparasitized controls. Conversely, xylem sap amino-acid concentrations in infected hosts were decreased. Glutamine (Gln) was the principal amino acid in xylem sap of unattached Rhinanthus, but after attachment to both hosts asparagine (Asn) predominated. In clover, Asn was the major transport amino acid, suggesting massive transfer of Asn from host to parasite. In unparasitized barley, however, Gln was the major xylem sap amino acid, but unexpectedly, in this species Asn was induced as the principal amino acid when parasitized. The results are discussed in terms of mutual host-parasite interactions.

Transport and partitioning of phosphorus in wheat as affected by P withdrawal during flag-leaf expansion

“…Pressure was applied to both the sand substrate and the roots of the treated plants. After slowly applying pressure, xylem sap started to exude from the midribs after reaching a balancing pressure (Seel and Jeschke, 1999), and sap was collected at 100 kPa above this pressure. The first exudate was discarded to avoid contamination from cut cells.…”

“…For xylem sap collection, the plants were grown in 2.1 L special pots (Seel and Jeschke, 1999), but treated in the same way as the plants for harvests. To collect xylem sap, the pot was placed in a pressure container for the application of pressure to the root system (Figure 1).…”

Effects of light availability on attachedRhinanthus minor (L.), an angiospermatic root hemiparasite

“…Thus, R. minor might respond to alterations in light quality and/or quantity by growing taller and adopting a "shadeavoidance" strategy. It is also possible that, because R. minor extracts from its host some of the resources needed for growth (Seel and Jeschke, 1999), it might be able to tolerate a reduction in irradiance availability through increased dependence on host-derived materials, thereby compensating for the resources limited by shade. This latter scenario would demonstrate a "shade-tolerance" strategy.…”

“…In general, the variation in the amount of 15N present in the parasite should indicate the flux of solutes derived from the host. Much of the N transferred from host to parasite is in the form of amino acids (Seel and Jeschke, 1999). Nitrogen nutrition for xylem-tapping parasites (including R. minor) involves a carbon flux to the parasites equal to approximately three times the nitrogen flux (Raven, 1983).…”

Overexpression of stress-related genes in Cuscuta campestris in response to host defense reactions

“…To this genus belong the holoparasitic flowering plants that must take up nutrients, water, and other macromolecules from the host plants with the help of specialized absorptive organs (Seel and Jeschke, 1999;Haupt et al, 2001;Birschwilks et al, 2006Birschwilks et al, , 2007.…”