Sink-stimulated photosynthesis, increased transpiration and increased demand-dependent stimulation of nitrate uptake: Nitrogen and carbon relations in the parasitic association Cuscuta reflexa-Coleus blumei

Abstract

When parasitizing Coleus blumei Benth., grown in quartz sand culture and 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 supplied limiting concentrations of nitrate. In the presence of Cuscuta the growth and dry matter increase of the host plant was severely inhibited. However, dry matter production of host plus parasite was only slightly less than or at 0.2 mM nitrate almost the same as that of uninfected Coleus plants. Under all conditions of nitrate nutrition, parasitism by Cuscuta led to a substantial increase in photosynthesis in host leaves under light-saturating conditions and in transpiration. Particularly with 0.2 mM and mM NO3/- parasitism by Cuscuta reflexa also led to an increase in the nitrogen concentration in almost all host tissues 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 infected plants was considerably decreased, it followed that nitrate uptake by roots of parasitized plants was substantially increased compared to the uninfected Coleus. Increased photosynthesis and accelerated nitrate uptake are discussed in relation to an increased sink for assimilates and to higher demand for nitrogen in the parasitized plants and to opposing effects of Cuscuta on symbiotically nitrogen-fed hosts.