A brief review of the surveys of Thistle Broomrape Orobanche reticulata on Yorkshire Wildlife Trust Nature reserves

Article by Phillip Whelpdale, Yorkshire Wildlife Trust, 1 St Georges Place, York, YO24 1GN

Thistle broomrape (Orobanche reticulata Wallr.) is a rare parasitic plant currently only found in Yorkshire, within the UK. It is classified as ‘endangered' in the British Red Data Book of Vascular Plants and is listed in schedule 8 of the Wildlife and Countryside Act 1981.  It is generally confined to the thin magnesian limestone geology which runs North/South through Yorkshire, with one or two outlier populations in East Yorkshire.

Figure 1: Map based on Foley (1993) and updated with records from NEYEDC and YWT - showing the distribution of Orobanche reticulata in Yorkshire.

Yorkshire Wildlife Trust (YWT) currently manage three sites where O. reticulata is present, and has been monitoring the populations across these sites in recent years. It is widely accepted that there are wide fluctuations in numbers of plants to be found at any given site from year to year and the sudden appearance of flowering spikes after years of apparent absence is considered the norm (Foley, 1993; Knight, 1998; Headley et al 1998).

Since 2013 Wharram Quarry (a small 6.5 Ha disused chalk quarry in Ryedale) has, in comparison to the other broomrape sites managed by YWT, consistently hosted a relatively good population. There is a certain irony to Wharram Quarry sustaining such a good population as it is outside of the native magnesian limestone distribution zone- indeed the plant is believed to have been artificially introduced to Wharram Quarry from a seed source elsewhere in Yorkshire many years ago (Foley pers comm, 2013).

Table 1: Orobanche plant count data post 2013.

(WHQ = Wharram Quarry, HW = Hetchell Wood, RL = Ripon Loop)

YEAR

WHQ

HW

RL

2013

144

-

26

2014

98

10

77

2015

171

117

31

2016

54

21

3

2017

224

7

0

One of the more interesting results from the surveys, particularly at Wharram Quarry, is the morphometric data that is collected alongside the basic plant count. These data have shown that the current maximum height of up to 70cm, as reported in the literature (Rumsey & Jury, 1991) falls short of several plants found at Wharram Quarry over the years (with a height of 81cm recorded on 11th August 2015). Perhaps more interesting is that those plants with exceptional heights are always found in shaded conditions, within dense thickets of vegetation. Being a parasite lacking in chlorophyll it would initially seem strange for it to react to low light levels in such a way. However it is the plant hormone auxin, and not chlorophyll, that is the driver of positive phototropism and this is important for one very reason and that is because auxin is key in how broomrape species are able to parasitise hosts - by them acting as a root, rather than shoot, and therefore also an auxin sink (Bar-Nun et al 2008). It would make sense therefore that larger host plants, like those found in shaded conditions, would contain more auxin. The parasite would be exposed to these increased levels of hormone, thereby potentially causing it to be stimulated to grow towards the light source too. This is an area that requires further study and the relationship between host and parasite height is something we are currently collecting data on.

Finally the usual preferred host species of O. reticulata is Cirsium arvense but at Wharram Quarry the host plant is almost exclusively Cirsium eriophorum. Perhaps this plays some role in the varied fortunes of this plant between the sites that are currently monitored?

In the past there was a great deal of effort going into learning more and actively monitoring this enigmatic species across Yorkshire, with a Broomrape Conservation Workshop being held and reported on (YNU, Naturalist 123, 1998). I’d be interested to hear from anybody with any information which could offer further insight into any progress that has been made into the understanding of the ecology and management of this species since this workshop nearly 20 years ago.

Acknowledgements

I am grateful to the volunteers who have helped with the surveys over the years, in particular Roger Taylor for his enthusiasm and dedication to learning and understanding more about O. reticulata.

References

Bar-Nun N., Sachs T., Mayer A. M. (2008). A role for IAA in the infection of Arabidopsis thaliana by Orobanche aegyptiaca. Ann. Bot. 101 261–265. 10.1093/aob/mcm032

Foley, M. J. Y. (1993). Orobanche reticulata Wallr. populations in Yorkshire (north-east England). Watsonia 19 : 247-257.

Headley, A. D., Abbot, P. P., and Foley, M.J.Y. (1998). Monitoring of Orobanche reticulata Wallr. Populations in Yorkshire

Knight, D.G.E. (1998). The status of Orobanche reticulata Wallr. in the u.k. and the background to the work carried out under the species recovery programme. The Naturalist 123*

Rumsey & Jury (1991). An account of Orobanche L. in Britain and Ireland.Watsonia 18,  257-295.

Yorkshire Naturalists Union (1998). Proceedings of the broomrape conservation workshop. The Naturalist 123*

*The Naturalist vol. 123, issue 1025 includes a number of articles about Broomrape surveillance and conservation in Yorkshire, including the two highlighted above.  To view and download this issue from the Biodiversity Heritage Library, CLICK HERE.

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