| Abstract: | A recently developed empirically based modelling technique wasused to quantify uptake, flow and utilization of C and N inLupinus albus L., uninfected and parasitized by Cuscuta reflexaRoxb. plants over a 12 d period during flowering and early fruitsetting of the host. The modelling combined data on molar C:Nratios in host phloem and pressure-induced xylem sap, net incrementsof C and N in host and parasite plant parts and respiratorylosses of C. The modelling of the solute transfer between hostand Cuscuta was achieved by assuming non-specific intake fromthe xylem. The models predicted that Cuscuta derived 99.5% ofits carbon and 93.6% of its nitrogen demand from the host phloem.The overriding sink strength of the parasite diverted most ofthe basipetally translocated host assimilates and massivelycompeted with the host root and inhibited fruit setting. Carbonincorporation in Cuscuta consumed 56%, respiration 24% and secretionby extrafloral nectaries 1.8% of the current host photosynthate.Root respiration was inhibited by 59% and carbon was mobilizedfrom host root and leaves. Competition by the parasite for Nwas even more severe and Cuscuta incorporated nitrogen equalling223% of current fixation, but N2 fixation of the host was severelyrestricted to 37%. Withdrawal of N from host phloem led to severelosses of N from leaves and the root and marked decreases inN concentration. It required massive xylem-to-phloem transferof N, because the xylem as the major supply route for N wasnot exploited substantially by Cuscuta. The results are discussedin relation to likely causes for parasite-induced pathogeniceffects, suggesting that Cuscuta affected the host adverselyby depriving it mainly of its nitrogen, but that causal to incipientnitrogen deficiency and restricted N2 fixation was the superiorsink potential of Cuscuta, which prevented adequate supply ofassimilates to the nodulated root. The dominating sink potentialof Cuscuta is compared with the similarly strong sink competitionexerted by fruits at the stage of seed filling in annual plants. Key words: Cuscuta reflexa, Lupinus albus, parasitism, carbon, nitrogen, phloem, xylem, transport |
