Sink-stimulated photosynthesis and sink-dependent increase in nitrate uptake: Nitrogen and carbon relations of the parasitic association Cuscuta reflexa-Ricinus communis

Ricinus communis L. was grown under limiting N supply in quartz sand culture, fed with 0.2, 1 or 5 mol m(-3) NO3-, or in liquid culture with 0.022, 0.05 or 0.5 mol m(-3) NO3-. Some of the plants were infected with Cuscuta reflexa Roxb, As occurred for the host, dry matter production and growth of C, reflexa were severely depressed with decreasing N supply to the host, When parasitized by C, reflexa, the shoot and root dry weight of Ricinus was diminished at all levels of N nutrition, but the total dry weight of host plus parasite was almost the same as that of uninfected Ricinus. In contrast to the situation in Lupinus albus (Jeschke et al, 1994b), infection by Cuscuta resulted in increased tissue N levels in the host and the N content of the system Ricinus plus C, reflexa was the same or even somewhat larger than that of uninfected plants, This indicated a sink-dependent stimulation of nitrate uptake, As a result of decreased root weights, nitrate uptake g(-1) FW was stimulated by 80, 60 or only 40% at 0.2, 1 or 5 mol m(-3) nitrate supply, Increased nitrate uptake was reflected, particularly at low N supply, in xylem transport; xylem sap nitrate concentrations were substantially elevated, while those of amino acids were decreased in parasitized plants, This indicated an inhibition of nitrate assimilation in roots of parasitized plants under limiting N supply, Besides these effects on N relations, C, reflexa induced a substantial sink-dependent stimulation of net photosynthesis in host leaves and a concomitant increase in stomatal opening and transpiration, This stimulation depended on the relative sink size induced by Cuscuta, on nitrogen nutrition and on leaf age, indicating that delayed senescence of leaves contributes to the overall effects of Cuscuta on its host, The Cuscuta-induced inhibition of nitrate assimilation in the roots and the increase in nitrate uptake suggest that nitrate reduction was shifted towards the leaves in the presence of C, reflexa, The stimulating effects with the strongly inhibitory effects occurring in the tripartite association L. albus-Rhizobium-Cuscuta reflexa.