RECIPROCAL PARASITIZATION IN RHINANTHUS-SEROTINUS - A MODEL SYSTEM OFPHYSIOLOGICAL INTEGRATION IN CLONAL PLANTS

Physiological integration among shoots of a clone occurs in a wide ran ge of species and is often interpreted to be adaptive and especially b eneficial 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 hemiparasit e Rhinanthus serotinus that reciprocally parasitized each other. The p arasites were grown pairwise in pots without hosts. In half of the pot s the parasites were separated below ground by a plastic division. In the other half of the pots the plants developed connections through ha ustoria. In a first experiment the separation treatment was combined w ith different levels of soil nutrients. The mean performance of the tw o plants in a pot was not influenced by separation, but increased cons iderably with increasing soil nutrient levels. The size variation betw een plants in a pot was much higher if plants were connected below gro und. Because the environment within pots was homogeneous, this must be interpreted as asymmetric competition for belowground resources. Howe ver, the level of these resources did not significantly affect the siz e variation and therefore the strength of competition. In a second exp eriment a heterogeneous environment was simulated by applying fertiliz er to the leaves of one parasite per pot. Under these conditions size variation between plants within a pot was significantly lower if the p arasites were connected below ground than if they were not. This indic ates that resource translocation similar to that observed in clonal pl ants can occur between genetically different hemiparasites. Since non- clonal systems can at least partly mimic phenomena found in clonal sys tems, adaptive explanations for clonal integration have to be applied with caution.