The Yellow Rattle

The yellow rattle (Rhinanthus minor) is a pretty yellow flower that is widespread across much of the northern hemisphere. It parasitizes a wide range of host species in meadows. To form a parasitic relationship with another plant species, the yellow rattle will produce haustoria, which are growths in the host roots. These haustoria puncture the host and draw water and nutrients into the yellow rattle. The yellow rattle is indiscriminate about which plants it parasitizes and has even been shown to attack a sheet of Perspex. However, it parasitizes different species to different degrees. Some species of forbs show resistance to parasitism by a yellow rattle, whereas others such as grasses are more susceptible [3].

Image Credit: Derek Harper (CC BY-SA 2.0)

The differential effect of yellow rattle on different plants is what allows there to be high floral diversity in infected meadows. This is because grasses, which normally have a competitive advantage over herbs and flowers, are most susceptible to infection. This infection stunts the growth of the grass species and allows other species to thrive [4]. The mechanism behind the herb resistance is called the hypersensitive response, which creates a physical barrier against infection by depositing lignin in the root cells. This effectively blocks the parasite from creating the haustoria connection between itself and the host [4].

Other factors also influence the ecological makeup of these meadow communities. For example, the leaf litter dropped by parasitic plants is often very nutrient-rich and can indirectly benefit other species in the area [5].

Despite being a parasite, the overall effect of the plant on meadow ecosystems is positive, as it encourages a greater diversity of wildflowers [1]. Around 7.5 million acres of wildflower meadows have been lost since the 1930s [2] and the yellow rattle provides an easy and cost-effective solution to halting this decline.

Image Credit: Kleine Klapper (Deutschlands Flora in Abbildungen) (Public Domain)

In the UK, less than 1% of wild meadow habitat remain [2]. The increased diversity promoted by this unassuming yellow flower is vital for sustaining and re-establishing meadows. The yellow rattle is a UK native and its seeds are cheap and easy to spread [6]. Meadows are a concentrated hotspot for insect biodiversity too and can contain up to 40 species per square metre [2]. Many fields in the UK are sprayed with insecticide to prevent pests from harming crop yields. These floral wilderness areas have the potential to provide refuge for pollinators, capture and store carbon in soils and mitigate against flooding [2].

References

[1] Gibson, C.C., Watkinson, A.R. The role of the hemiparasitic annual Rhinanthus minor in determining grassland community structure. Oecologia 89, 62–68 (1992). https://doi-org.sheffield.idm.oclc.org/10.1007/BF00319016
[2] http://www.bbc.co.uk/earth/story/20150702-why-meadows-are-worth-saving
[3] Cameron DD, Seel WE (2007) Functional anatomy of haustoria formed by Rhinanthus minor : linking evidence from histology and isotope tracing. New Phytologist 174: 412-419.
[4] DUNCAN D. CAMERON, ALISON M. COATS, WENDY E. SEEL, Differential Resistance among Host and Non-host Species Underlies the Variable Success of the Hemi-parasitic Plant Rhinanthus minor, Annals of Botany, Volume 98, Issue 6, December 2006, Pages 1289–1299, https://doi.org/10.1093/aob/mcl218
[5] Fisher JP, Phoenix GK, Childs DZ, et al. Parasitic plant litter input: a novel indirect mechanism influencing plant community structure. New Phytol. 2013;198(1):222-231. doi:10.1111/nph.12144
[6] https://www.theguardian.com/lifeandstyle/gardening-blog/2012/sep/27/yellow-rattle-meadow-grassland