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, NO₃⁻, H₂PO₄⁻ and K⁺, as well as moderate increases in Cl⁻ and SO₄²⁻. Mg²⁺ and Ca²⁺ 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⁺, NO₃⁻, H₂PO₄⁻ and amino acids, and decreased concentrations of Ca²⁺ and Mg²⁺. Estimates of NO₃⁻ 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.