Simultaneous collection of xylem sap from Rhinanthus minor and the hosts Hordeum and Trifolium: hydraulic properties, xylem sap composition and effects of attachment
We. Seel et Wd. Jeschke, Simultaneous collection of xylem sap from Rhinanthus minor and the hosts Hordeum and Trifolium: hydraulic properties, xylem sap composition and effects of attachment, NEW PHYTOL, 143(2), 1999, pp. 281-298
The facultative hemiparasite Rhinanthus minor was grown alone, or with eith
er Hordeum vulgare or Trifolium alpestre as a host. All plants were fed 5 m
M nitrate. In the parasites, successful attachment led to dramatic increase
s 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 pressur
e to the rooted soil to raise the soil water potential so that in vivo xyle
m sap exuded from small incisions once the applied pressure balanced the te
nsion exerted by transpiration. High balancing pressures were needed for un
attached Rhinanthus, whereas simultaneous collection of xylem sap from atta
ched Rhinanthus and its host, at a much lower balancing pressure, was possi
ble only when the parasite was enclosed in polyethylene film and high trans
piration abolished. From plots of xylem sap volume flow versus applied pres
sure, the hydraulic conductivity L-p and the overall hydraulic resistance r
were obtained. The value of r was high in unattached Rhinanthus and dramat
ically 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 rat
es in xylem of K+, NO3-, H2PO4- and amino acids, and decreased concentratio
ns of Ca2+ and Mg2+. Estimates of NO3- reduction in the parasite showed tha
t 99% was reduced in unattached plants, 85% in those attached to clover and
52% in those attached to barley. In the parasitized hosts, ion concentrati
ons in xylem sap were somewhat increased relative to unparasitized controls
. Conversely, xylem sap amino-acid concentrations in infected hosts were de
creased. Glutamine (Gln) was the principal amino acid in xylem sap of unatt
ached Rhinanthus, but after attachment to both hosts asparagine (Asn) predo
minated. In clover, Asn was the major transport amino acid, suggesting mass
ive transfer of Asn from host to parasite. In unparasitized barley, however
, Gin 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.