Host recognition in a desert mistletoe: Early stages of development are influenced by substrate and host origin
1. In many root parasites, chemical substances from the host are necessary for the successful initiation of germination and holdfast formation, and are the basis for host recognition and localization within the rhizosphere. In contrast, bird-dispersed mistletoe seeds have no need for the chemical localization of a host, and possible mechanisms of host recognition are poorly understood. Many studies claim that mistletoes germinate readily in the absence of a host and independent of the substrate. 2. In this study, the possible occurrence of substrate dependence and host recognition in mistletoes during the establishment of Plicosepalus acaciae Zucc. seeds on Acacia raddiana Savi was tested experimentally. 3. In a laboratory experiment, no influence of substrate on germination could be found, but a significantly higher proportion of the germinated seeds developed a holdfast on natural (bark) substrate than on artificial (plastic) substrate. Moreover, the origin of the bark had a significant influence on holdfast formation frequency. 4. Host influence was additionally tested in a reciprocal transplant experiment in two field sites with genetically different populations of the same host species. Those mistletoes of local origin germinated significantly more often than their non-local counterparts. 5. These results suggest the existence of interactions between mistletoe seed and host branch on the basis of chemical cues that are active at an early stage of seedling development and are host-specific at the level of populations.