Solute fluxes from tobacco to the parasitic angiosperm Orobanche cernua and the influence of infection on host carbon and nitrogen relations

Orobanche species are holoparasites which are very efficient sinks for host -derived solutes. Here, we report the use of direct measurements of xylem s ap solute concentrations and water fluxes, together with a modelling proced ure to calculate element fluxes within an association between Orobanche cer nua and its tobacco host. Infection of tobacco by the parasite markedly inf luenced carbon acquisition and partitioning; net fixation of carbon was 20% higher in infected tobacco compared,vith controls. Orobanche cernua caused a 84% increase in net carbon flux moving downward from the tobacco shoot a nd 73% of this carbon was intercepted by the parasite, almost entirely thro ugh the phloem (>99%). Further, the parasite also exerted a large impact on the nitrogen relations of the plant, notably nitrate uptake was stimulated and the amino acid content of xylem sap was lower. The parasite also relie d heavily on host phloem for the supply of other resources, with only 5 to 15% of N, and 16% of K, 23% of Na, 63% of Mg and 13% of S being derived fro m the xylem, Thus, we provide quantitative information on the phloem depend ency of the parasite and show that host carbon and nitrogen metabolism is s timulated as a consequence of infection.