Modelling the flow and partitioning of carbon and nitrogen in the holoparasite Cuscuta reflexa Roxb. and its host Lupinus albus L.

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Abstract
Nodulated Lupinus albus L. was grown on quartz sand in the greenhouse and supplied with a N-free culture solution. Half the plants were infected with Cuscuta reflexa Roxb. at 33 DAS. An empirically based modelling technique was developed to quantitatively depict uptake, flow and utilization of C and N in the host plant and between host and parasite over a 12 d period. The modelling incorporated C: N ratios of solutes in phloem and pressure-induced xylem sap, net increments of C and N and respiratory losses of C. For assessing the transfer of solutes from host phloem to Cuscuta it was not possible to use the C: N ratio of phloem sap close to the site of parasite attachment, a procedure which would have assumed non-specific withdrawal of phloem-borne solutes, since this would have implied unimpeded mass flow from host to parasite. The relative intake of C and N by the parasite by specific withdrawal of nitrogenous and carbonaceous solutes from the phloem was obtained independently by assuming that xylem intake occurred non-specifically. Xylem import was thus obtained (a) from transpiration and tissue water increment of Cuscuta and the concentrations of N and C in xylem sap and (b) from the Ca2+ increment of Cuscuta and the ratios Ca: N and Ca: C in lupin xylem sap, assuming that Ca2+ intake occurred solely via xylem. By subtracting net xylem import from total uptake of C and N by Cuscuta the methods resulted in comparable ratios of C: N intake from the phloem. The average ratio (53.4) was smaller than the C:N ratio in host phloem (85.6) indicating specific withdrawal of solutes with a distinct preference for N. Using this ratio, modelling of flows of C and N was possible and showed that Cuscuta abstracted C and N mainly from the host phloem, but xylem supply was nutrient-dependent and amounted to 6.4% of the N but only 0.5% of the C demand. The results indicated that Cuscuta exerted a very strong sink and competed efficiently with the root, the major sink of L. albus, by attracting 81% of the current photosynthate and more N (223%) than was currently fixed. The massive demand of the parasite led to losses particularly of N from leaves and the root and apart from causing carbon losses it appeared to induce a sink-dependent stimulation of photosynthesis. In contrast, nitrogen fixation in the Cuscuta-infected lupin was inhibited to 37% of the control.

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