Taxonomy

One subspecies and three varieties are included in this account. Photograph A shows O. minor var. minor on Tropaeolum majus; B shows O. minor subsp. maritima on Daucus carota; C shows O. minor var. flava on Trifolium repens.

This is a highly variable species which has long been associated with taxonomic and nomenclatural problems. The current taxonomy in Britain and Ireland includes one subspecies, subsp. maritima (Carrot Broomrape), and three varieties, var. minor, var. flava and var. compositarum (Rumsey & Jury, 1991; Rumsey, 2007). O. minor var. compositarum has an incoherent distribution and intergrades with more nominate morphotypes, and may therefore be conspecific with O. minor var. minor.  For all these taxa n=19.

Identification

The key characteristics of the recognised intraspecific taxa are as follows: O. minor var. minor produces lax, purplish spikes of cream-coloured flowers with evenly curved corollas, and bracts equalling or exceeding the flowers. The stigma lobes are distant and pinkish. 

Orobanche minor subsp. maritima can be distinguished from the previous variety by the pronounced yellowish bosses on the lower corolla lip, which has a larger middle lobe than the peripheral lobes. The bracts and calyx lobes of this subspecies are short and rarely exceed the corolla, and the purple stigma lobes are partially united. 

Orobanche minor var. flava produces short, dense spikes which are sub-globose at the apex. The entire plant is yellowish, and devoid of purple pigmentation. 

Orobanche minor var. compositarum is very similar to O. minor var. minor but the smaller, slender corolla tube is sub-erect at anthesis (rather than horizontal) and the filaments are sub-pillose at the base rather than sparsely hairy.

Status

Cheffings & Farrell (2005) classify O. minor s.l. as ‘Least Concern.’ 

Distribution 

Orobanche minor sensu lato is a widespread species, and our commonest broomrape, scattered throughout Britain. It is most frequently encountered in South and Southeast England.

Orobanche minor var. minor is of disputed native status and has probably been introduced in seed mixes throughout its range. This is the most common form of the species.

Orobanche minor subsp. maritima is a scarce subspecies restricted to the south coast of England (Stewart et al., 1994). Populations at the eastward disjunction and west of its range are poorly morphologically defined and may be subject to gene flow with O. minor var. minor. While often confused with pigment-deficient variants of O. minor var. minor, O. minor var. flava sensu stricto is in fact confined to an area of dockland in Newport, Glamorgan, and its continued existence is precarious. Populations previously classified under this variety occurring in the Channel Islands are now considered to be allied more closely with O. minor subsp. maritima (Rumsey, 2007).

Orobanche minor var. compositarum occurs throughout the range of var. minor.

Ecology

Orobanche minor sensu lato has a very broad host range, and occurs on a number of hosts from taxonomically disparate families. Host specific ecotypes may constitute host-defined races in this variable species (Musselman and Parker, 1981) for example plants occurring on the commonly cultivated Senecio greyii (Thorogood et al., unpublished data).

Orobanche minor var. minor is relatively host non-specific but tends to parasitize members of the Fabaceae and Asteraceae most frequently (especially Trifolium pratense). This taxon occurs most commonly in disturbed grassland and dunes.

Orobanche minor subsp. maritima has a distinct ecology, and is largely confined to south-facing sea cliffs on Daucus carota (rarely on Plantago coronopus or Eryngium maritimum).

Orobanche minor var. flava is confined to disturbed gravel and sand banks on Trifolium spp.

Orobanche minor var. compositarum generally occurs on a variety of composites, most commonly on Crepis virens.

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Future work

The genetic structure of populations of O. minor are currently under investigation using molecular markers and DNA sequencing (Thorogood, Rumsey and Hiscock, in prep.). These analyses may resolve the taxonomic and nomenclatural problems long associated with this species, and prompt conservation efforts where the continued existence of intraspecific taxa is precarious.

References

Musselman L.J. & Parker C. 1981. Preliminary host ranges of some strains of economically important broomrapes (Orobanche). Economic Botany, 36, 270-273.    

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

Rumsey, F.J. 2007. A reconsideration of Orobanche maritima Pugsley (Orobanchaceae) and related taxa in southern England and the Channel Islands. Watsonia 26, 473-476.

Acknowledgements

Account and photographs by Chris Thorogood.