Which hosts do root hemiparasitic plants chose from a plant community?

Submitted by editor on 9 October 2018.Get the paper!

As a part of my PhD I wanted to test the quality of 13 different host species for the hemiparasite Rhinanthus alectorolophus, and I planned to include also one host mixture, because this might be the more natural host environment for the parasite. But which mixture should I chose? Even with only one plant per species, 13 species can be combined to four-species mixtures in 715 different combinations. After many hours of planning I ended up with 15 different mixtures, designed to represent different numbers of legume species (which are usually good hosts for Rhinanthus) and different numbers of host functional groups (legumes, grasses and forbs). The mixtures became the most interesting part of the experiment.

Natural population of Rhinanthus alectorolophus (light green inflorescences) growing with many potential hosts, including Trifolium pratense, Lotus corniculatus, Taraxacum officiniale

We know from community studies that Rhinanthus species grow better in more diverse than in species-poor grasslands, and that dominant grasses are often suppressed while non-leguminous forbs benefit. But does this mean that Rhinanthus prefers grasses, or are they simply less tolerant to parasite attack? Potting experiments can help, as plant species that support large hemiparasites can be regarded as good host species. And there are usually huge differences in the quality of different species as hosts for Rhinanthus. But can these results be transferred to the natural systems? Intermediate-scale experiments that document the effect of a parasite on every host species for mixtures of more than two different host species are still missing.

The question is appealing: which host species is used by a hemiparasite growing in a diverse community? To answer this question, very different methods have been applied in previous studies, including counting of root connections between parasites and hosts (“haustoria”) and DNA fingerprinting. We thus know that parasites can connect to several hosts simultaneously, but it has not been possible to quantify the importance of individual hosts for parasites. Our approach used plant biomass as a measure of fitness. When a parasite grows, it withdraws resources from the host xylem and thus suppresses the growth of a host. By comparing the effects of the parasite on the growth of the host species alone and in mixtures of four different hosts, we were able to estimate whether a particular host was preferred over the other hosts or not.

The hemiparasite Rhinanthus alectorolophus (yellow flowers) grown with a combination of four hosts – before (left) and after separation of the above-ground biomasses (right). But which of the plants contributed most to parasite biomass?

Our results suggest that the generalist Rhinanthus alectorolophus indeed prefers some plants over others, but that this preference is not a simple function of the quality of species in single-species pot experiments. Rather, what is a good or poor host for a parasitic plant may depend on the presence of other species, as hosts suppressed strongly when grown alone with the parasite were sometimes hardly suppressed in mixtures when more attractive hosts were present, and vice versa. In particular, the parasites appeared to prefer a mixed diet, as legumes became more attractive when grasses where also present. In line with this, parasites grew best in diverse mixtures containing both grasses and legumes. These findings leave us with even more respect for the complex interactions all around us.

The authors through Tobias Sandner

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