SINK-STIMULATED PHOTOSYNTHESIS, INCREASED TRANSPIRATION AND INCREASEDDEMAND-DEPENDENT STIMULATION OF NITRATE UPTAKE - NITROGEN AND CARBON RELATIONS IN THE PARASITIC ASSOCIATION CUSCUTA-REFLEXA COLEUS-BLUMEI

When parasitizing Coleus blumei Benth,, grown in quartz sand culture a nd fed with 0.2, 1 or 5 mM nitrate, the biomass production of Cuscuta reflexa Roxb, was inhibited to a similar extent as that of the host su pplied limiting concentrations of nitrate, In the presence of Cuscuta the growth and dry matter increase of the host plant was severely inhi bited, However, dry matter production of host plus parasite was only s lightly less than or at 0.2 mM nitrate almost the same as that of unin fected Coleus plants, Under all conditions of nitrate nutrition, paras itism by Cuscuta led to a substantial increase in photosynthesis in ho st leaves under light-saturating conditions and in transpiration. Part icularly with 0.2 mM and mM NO3- parasitism by Cuscuta reflexa also le d to an increase in the nitrogen concentration in almost all host tiss ues and to a substantial increase in the chlorophyll concentration of leaves. The total nitrogen content in the system, host plus parasite, was as large with 0.2 and 1 mM NO3- or only somewhat smaller with 5 mM NO3- than that of the unparasitized host, Since the root weight of in fected plants was considerably decreased, it followed that nitrate upt ake by roots of parasitized plants was substantially increased compare d to the uninfected Coleus, Increased photosynthesis and accelerated n itrate uptake are discussed in relation to an increased sink for assim ilates and to higher demand for nitrogen in the parasitized plants and to opposing effects of Cuscuta on symbiotically nitrogen-fed hosts.