Reciprocal parasitization in Rhinanthus serotinus: A model system of physiological integration in clonal plants

Physiological integration among shoots of a clone occurs in a wide range of species and is often interpreted to be adaptive and especially beneficial in heterogeneous environments. Translocation of resources is, however, not restricted to clonal plants, but can also occur between shoots of genetically different individuals. We analysed ''non-clonal'' integration between individuals of the facultative root hemiparasite Rhinanthus serotinus that reciprocally parasitized each other. The parasites were grown pairwise in pots without hosts. In half of the pots the parasites were separated below ground by a plastic division. In the other half of the pots the plants developed connections through haustoria. In a first experiment the separation treatment was combined with different levels of soil nutrients. The mean performance of the two plants in a pot was not influenced by separation, but increased considerably with increasing soil nutrient levels. The size variation between plants in a pot was much higher if plants were connected below ground. Because the environment within pots was homogeneous, this must be interpreted as asymmetric competition for belowground resources. However, the level of these resources did not significantly affect the size variation and therefore the strength of competition. In a second experiment a heterogeneous environment was simulated by applying fertilizer to the leaves of one parasite per pot. Under these conditions size variation between plants within a pot was significantly lower if the parasites were connected below ground than if they were not. This indicates that resource translocation similar to that observed in clonal plants can occur between genetically different hemiparasites. Since non-clonal systems can at least partly mimic phenomena found in clonal systems, adaptive explanations for clonal integration have to be applied with caution.