Nutrient mediation of sink strength in the Orobanche minor – Red clover association

Holoparasites are non-photosynthetic plants which derive all their growth requirements from their host plant and are thought to act as a very strong sink for host resources. Here, we grew red clover plants in split-root boxes to explore the effect of nutrient supply to Orobanche minor parasitized or unparasitized host roots. Where nutrients were supplied to parasitized roots, parasite growth was strongly promoted at the expense of the host. Conversely, host growth did not differ significantly from unparasitized controls where nutrients were supplied to unparasitized roots. While ¹⁵N labelling suggested both strong parasitic ammonium abstraction and reduced nitrate uptake in parasitized roots, the total N content of systems where nutrients were fed to parasitized roots was approximately 26 % higher than control plants, suggesting that changes in host and parasite growth rates were due to changes in sink strength, rather than nutrient uptake. Parasitism and nutrient supply had strong effects on leaf carbohydrate metabolism but did not affect photosynthetic rates or leaf N concentration. In the second experiment, we investigated the importance of light level on the host – parasite relationship, concluding that parasitism had a diminished effect on host growth under low light conditions. Total system mass was unaffected by the apparent sink strength of the parasite. Our results suggest a dynamic relationship between host shoot and parasite sink strengths, mediated by changes in nutrient status.