#47 Thistle Broomrape by Seebra Young
Meet Seebra Young, Ecological Data Officer at North and East Yorkshire Ecological Data Centre!
Seebra works at North and East Yorkshire Ecological Data Centre as Assistant Ecological Data Officer. Whilst working at NEYEDC Seebra has tried her hand at many of the centre’s activities, although is progressing onto a more Local Wildlife Sites based role. As an Ecological Data Officer, Seebra’s role involves preparing data to add to our database, writing articles, and organising community outreach activities. Seebra works with qGIS and R to organise, standardise and analyse large datasets of species records and habitat data, and is particularly interested in taking advantage of new technology and open-source software to streamline and advance processes within the centre. Before moving to Yorkshire Seebra taught gardening and ecology to children in East London and worked as a commercial gardener. This and her studies in Natural Sciences have spurred an interest in botany, and she is a member of the Botanical Society of Britain and Ireland, and Bradford Botany Group. Working with botanical surveyor Kay McDowell, Seebra recently completed surveys of sections of the River Wharfe and River Ure looking for rare protected species Thistle Broomrape.
Seebra can be contacted at seebra.young@neyedc.org.uk, and would be very interested to hear from anyone who has any ideas with respect to some of the questions she has about this species!
Seebra’s chosen species is Thistle Broomrape (Orobanche reticulata). O. reticulata is a totally parasitic species, an obligate parasite, with no chlorophyl and total reliance on its host. Although most found hosting on Creeping Thistle (Cirsium arvense), Woolly Thistle (Cirsium eriophorum) is also a well-known host, and this broomrape is thought to host on any of the Cirsium and Carduus species. The inability to survive without its host, despite a cost to the plant, affords it a few luxuries; without the need for leaves, which are reduced to small bracts on the stem, energy can be directed towards developing the reproductive organs and producing large quantities of seeds.
Figure 1. The old flowering head and sturdy stem of Thistle Broomrape, poking out of scrub in September 2023.
There is some debate about the impact of scrub and high vegetation on the growth of the plant. As the plant doesn’t photosynthesise it doesn’t require sunlight, however, it does still need its flowers to be exposed to pollinators. Thus, although it is sometimes written that the plant prefers open, sunny locations, this may in fact be a reporter bias (the plant is easier to spot in these conditions). On the other hand, thick scrub is also likely to reduce chances of successful seed germination. Although O. reticulata grows in a rather mundane environment (usually dominated by Cirsium arvense and other generalist species), it tends to have a beneficial impact on its surrounding. By reducing the number of competitively dominant species, a wider pool of species are allowed to grow.
A fascination to botanists of Yorkshire since it was first discovered in the 1900s, this plant is an iconic resident, also known as Yorkshire Broomrape. Appearing in Yorkshire, but also in other parts of Europe (Turkey), the plant is fairly rare. However, there is some deliberation over the extent of the rarity of O. reticulata as it is possible the variety found in the UK, subsp. pallidiflora, is a different species entirely. If this is accepted, the plant is in fact extremely rare, and found nowhere else in world other than Yorkshire.
Although never a common plant in recorded history, O. reticulata is thought to have suffered drastically with the invention of herbicides and the agricultural boom following the second World War. A simple search on Google Scholar yields many results primarily focused on the control of Orobanche sp. within the agricultural sector. These control measures target other Orobanche sp. such as Orobanche fabacea, which parasitizes on a broad range of crop species causing a significant decrease in productivity. Efforts to control these Orobanches are likely to have affected Reticulata in the process.
As is observed in other Orobanche species, abundances are unpredictable and vary considerably from year to year. One factor contributing to this is the sensitive germination conditions required; Orobanche germination is stimulated by the release of phytohormones from the roots of the host species, for this chemical reaction to happen seeds must be within a few mm of the root. A basic connective structure, the haustorium, then grows connecting the parasite to its host and allowing the pass of water, carbon, and nutrients. This chance meeting is a requirement for the plant to germinate; to combat this the plant produces vast quantities of long-lived seeds, which remain in the seed bank for some time awaiting the rare germination conditions.
Given the specific conditions required for germination, it makes sense that the plant would have a preference for disturbed soil. The plant is generally found growing in flood plains, recent disturbance increasing the likelihood of exposed thistle roots and thus chances for seed germination. The re-routing of the A1M in Yorkshire is thought to have provided new habitat for O. reticulata by disturbance of soil, and populations can be observed here. Seeds are also thought to be dispersed by water. The distribution of O. reticulata in Yorkshire, sticks closely to the River Wharfe and Ure, and new observations tend to occur downstream rather than upstream which further confirms the hypothesis of water dispersal.
Figure 2. Scientific illustration of Thistle Broomrape, which can be found on the front cover of West Yorkshire Plant Atlas, Lavin and Wilmore 1994.
There are several features of O. reticulata that made it interesting to Seebra. First, the plant has a strong preference for Magnesium Limestone habitat and is almost exclusively found on the Magnesium Limestone belt. This is a little unusual as you would expect it to have a similar niche as its host, Cirsium arvense, which is most ubiquitous. A similar pattern can be observed in Thyme Broomrape, “(Orobanche alba) is mainly confined to the large-scale, base-rich habitats of the Burren limestones and the Basaltic Plateau of the north-east (of Ireland). Yet its predominant host, Thymus polytrichus (Wild Thyme), is of widespread occurrence and indifferent to soil pH,” as described in Broomrapes of Britain and Ireland, 2021. This could perhaps indicate that the basic environment is an essential component to the biochemical processes involved with O. Reticulata, although there is little information available on this topic. On the other hand, an exception is found in Wharram Quarry, where a well-established population of the plant can be found despite no Magnesium Limestone. It is possible this population has been introduced artificially, but nonetheless the presence of O. Reticulata here causes further complications: why is this population able to persist without the usual conditions? Is Magnesium Limestone in fact not a necessity for O. Reticulata to grow?
Figure 3. Orobanchol pictured above, is the specific Strigolactone thought to be involved in Orobanche sp. germination.
The other interesting feature of this plant revolves around the Phytohormones which induce germination. A class of Phytohormones, called Strigolactones (SLs), are thought to induce seed germination in Orobanche sp., as well as other parasitic species in the Striga and Pelipanchi genera. This group of compounds is also involved in stimulating the hyphal branching of arbuscular mycorrhizal fungi, and in other internal signalling within the plant. The compound Orobanchol has been identified as the specific SL detected by Orobanche species, which is crucially defined by its stereochemistry. Interestingly, recent studies in grapevines have illustrated that SL production increases in plants under calcified stress (The effects of strigolactones on some biochemical traits in calcified media on grapevine, 2022). If a similar mechanism applies to Cirsium sp., the Magnesium Limestone environment could increase the production of SLs. The presence of these SLs has been proven to be a requirement for Orobanche sp. germination, so an increased concentration of these compounds may increase germination rate. This hypothesis could explain the preference of O. reticulata for Magnesium Limestone environments. However, as all Orobanche. species are thought to depend on SLs for germination, you would expect to see a preference for basic environments in all other species in the group. This is observed in some other species, such as O. alba mentioned above, but not in all which suggests the mechanism is much more complicated than this simple hypothesis. There are many more questions posed by this species, including the relationship between mycorrhizal fungi and Orobanche parasitism. Given the detection of SLs by both Orobanche species and mycorrhizal fungi, perhaps the interactions between these three parties can shed further light on the complex relationships at play…
Seebra would welcome any further discussion of her ideas and notes that they are tentative, based on scouring of the internet!
Recording and monitoring
Whilst conducting the survey mentioned above, only one species of the plant was found. Although disappointing this doesn’t necessarily mean the plant is in danger. There are a number of hypothesis’ as to why only a few plants were observed: anecdotally the plant is popular with slugs thus following 2023’s very wet summer it is possible many of the specimens had been eaten by slugs; a very cold spell in April 2023 may also have affected germination; it is also likely this has just been a bad year for the plant and low records this year form part of its usual fluctuations (this was also confirmed anecdotally by landowners); and finally limitations of the survey likely impacted results (including time of year, survey boundaries, and inaccessibility). Whilst conducting this survey it was very reassuring to meet local landowners/managers who were extremely knowledgeable and aware of O. reticulata on their land and keeping a close eye on it themselves. For example, at High Batts Nature Reserve SSSI, Ripon, staff always look out for the plant and have plans to manually ‘disturb’ land to create the ideal habitat for O. reticulata to grow.
If you’d like to see O. reticulata some strongholds of the species are Quarry Moor SSSI and Hook Moor, Leeds, where it can often be found in large populations at the right time of year. The best time of the year to find O. reticulata in its prime is early-mid summer where you may be able to see the exotic purple and yellow flowers. However, the sturdy brown spikes can also be seen poking out above vegetation for many months following this. Searching for the plant later into winter, may also yield good results as other scrub and vegetation will have died down and O. reticulata spikes may be easier to spot. However, looking later in the year does pose the risk that spikes may have been mowed, trampled, ploughed, washed away during flooding or eaten by slugs!
Any observations of this plant should be reported to the Environment Agency, the Botanical Society of Britain and Ireland or your Local Environmental Record Centre! As a Schedule 8 protected plant, do not disturb the plant and only share the location with trusted individuals.
Further information and acknowledgements
NEYEDC would like to thank Seebra for her time and expertise helping to create this blog. If you’d be interested in contributing to the project, please contact Lucy at lucy.baldwin@neyedc.co.uk.