MODELING THE FLOW AND PARTITIONING OF CARBON AND NITROGEN IN THE HOLOPARASITE CUSCUTA-REFLEXA ROXB AND ITS HOST LUPINUS-ALBUS L .1. METHODSFOR ESTIMATING NET FLOWS

Nodulated Lupinus albus L. was grown on quartz sand in the greenhouse and supplied with a N-free culture solution. Half the plants were infe cted with Cuscuta reflexa Roxb. at 33 DAS. An empirically based modell ing 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 an d 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 phl oem sap close to the site of parasite attachment, a procedure which wo uld have assumed non-specific withdrawal of phloem-borne solutes, sinc e this would have implied unimpeded mass flow from host to parasite. T he relative ive intake of C and N by the parasite by specific withdraw al of nitrogenous and carbonaceous solutes from the phloem was obtaine d independently by assuming that xylem intake occurred non-specificall y. Xylem import was thus obtained (a) from transpiration and tissue wa ter increment of Cuscuta and the concentrations of N and C in xylem sa p and (b) from the Ca2+ increment of Cuscuta and the ratios Ca:N and C a: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 Coscuta 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 d istinct preference for N. Using this ratio, modelling of flows of C an d N was possible and showed that Cuscuta abstracted C and N mainly fro m the host phloem, but xylem supply was nutrient-dependent and amounte d to 6.4% of the N but only 0.5% of the C demand. The results indicate d that Cuscuta exerted a very strong sink and competed efficiently wit h the root, the major sink of L. albus, by attracting 81% of the curre nt photosynthate and more N (223%) than was currently fixed. The massi ve demand of the parasite led to losses particularly of N from leaves and the root and apart from causing carbon losses it appeared to induc e a sink-dependent stimulation of photosynthesis. In contrast, nitroge n fixation in the Cuscuta-infected lupin was inhibited to 37% of the c ontrol.