lastdragon.org
BIODIVERSITY REFERENCE
 
   Orobanche L.   
 
Broomrapes
 
(page 1)


Kingdom: Plantae
Phylum: Tracheophyta – vascular plants
Subphylum: Magnoliophytina – flowering plants
Class: Magnoliopsida – dicotyledons
Order: Scrophulariales
Family: Orobanchaceae


British distribution (genus): Nine native species, mostly southern, several very local and rare.
World distribution (genus): About 150 species, in temperate and subtropical parts of the world: Europe (especially the Mediterranean), N.Africa, Western and Central Asia, N.America, with some introduced as weeds outside their native ranges.


Orobanche minor var. minor
Orobanche minor var. minor, parasitic on Medicago lupulina, Hertfordshire, June 2002


Biology and Ecology
Orobanche is a genus of wholly parasitic plants, attached to the roots of their hosts. They lack chlorophyll and hence any green colouration, and their leaves are vestigial. Above-ground stems are produced only for the purpose of flowering and setting seed; in perennial species the plant may persist below ground, unseen for a number of years.

Most species are highly host-specific, sometimes restricted to a single host species or genus. Others are capable of parasitising a number of unrelated plants, but usually still show strong regional preferences. The seeds germinate when in contact with host roots, triggered by chemical recognition. The fine root of the broomrape grows into the host root, reaching and entering the vascular tissue. An underground tuber develops, from which, eventually, the flowering stems may develop.

Broomrapes are thermophilic (warmth-loving) and often highly demanding in their habitat prefences. Frequently they require dry, open, often nutrient-poor grasslands but they are vulnerable to agricultural 'improvement', scrub development or other types of habitat loss. Some also appear to be sensitive to minor climatic changes and may vanish from sites that still appear suitable. Many are extremely local throughout their entire geographical ranges and are now endangered, requiring conservation measures and legislative protection.

A review of European species (excluding those which are exclusively Mediterranean) is provided by Kreutz (1995) – illustrated by numerous, stunningly superb, colour photographs.
 

Broomrapes as weeds
In contrast from requiring urgent conservation, a very few Orobanche species are destructive arable weeds, especially in semi-arid parts of the world. Host plants are usually severely affected by the parasite, making poor growth and often not flowering. Crop yields are substantially reduced, causing major problems in areas where agriculture is already marginal or subject to drought. With a single plant being capable of producing hundreds of thousands of seeds, heavy infestations can quickly develop, which may destroy entire crops in subsequent years. Their underground existence and intimate attachment to their hosts makes their control extremely difficult, beyond hand-removal of flowering stems to prevent further seed production.

Important weed species are O. crenata (a number of different hosts but causing especially severe losses to fields of peas, beans or other legumes), O. minor (numerous hosts but especially on clover (Trifolium) crops), O. ramosa (especially on tobacco, tomatoes, maize and hemp), O. aegyptiaca (on tomatoes and other Solanaceae) and O. cumana (more or less restricted to sunflowers (Helianthus annuus) but causing major economic losses where this is an important crop).

In Britain, Orobanche species cannot be regarded as a significant agricultural problem. O. minor has been a problem on clover in the past but is now rarely seen in crops, though it is widespread in England on a number of wild hosts. O. ramosa was once frequent on hemp (Cannabis sativa), when this was cultivated in East Anglia, but it is now considered extinct here (Rumsey & Jury, 1991). O. crenata has been sporadically established, especially in a small area of S.Essex, but it is more a candidate for conservation than for control. O. aegyptiaca occurred transiently in a tomato nursery in Sussex in 1952; O. cumana has never occurred here (nor has O. cernua, in which species O. cumana is included by some authorities).
 

Identification: British species
Identification of several species requires attention to critical detail and many past records of the rarer species are suspect or known to be errors. A detailed account of British species, with full descriptions, a key and excellent line illustrations, was provided by Rumsey & Jury (1991). A useful recent account is provided in Sell & Murrell (2009).

 
Orobanche minor Sm. (Lesser Broomrape)
subsp. minor

Orobanche minor var. minor

 
This variable species occurs on numerous hosts of many different plant familiies, though most often on the dandelion family, Asteraceae (formerly 'Compositae') and on the pea family, Fabaceae (formerly 'Leguminosae'). It is much the most common British broomrape, widespread and locally frequent in south-eastern England, but becoming much rarer westwards and northwards, and with only transient occurrences on disturbed sites in Scotland.

The photograph is of var. minor, the commonest variant in Britain, though it may well be an ancient introduction from Central Europe. It grows on a wide range of hosts, including, though now rarely, clover crops. It favours roadside banks and other ruderal habitats, where it is often transient or sporadic.

 
Photograph: Hertfordshire, June 2002, growing in some abundance on a new roadbank, on Black Medick (Medicago lupulina) and other legumes.


Orobanche minor, yellow variant

Rare, yellow variants of O. minor occur, the purple pigment being missing. Shown here is a form of var. minor, photographed on a road verge in Hertfordshire, 2003. Another yellow variant, differing in calyx characters, has ocurred in the Channel Islands and in dockland at Newport in South Wales and was referred to the supposedly separate var. flava by Rumsey & Jury, though the correct classification and application of names to these yellow variants seems less than certain, and the value of their recognition is arguable.

More recently, Rumsey (2007) has come to the conclusion that at least the yellow Channel Islands plant (which may well be extinct), and perhaps also the Newport plant, are better regarded as a variant of O. minor subsp. maritima, whereas that shown here surely remains just a colour variant of O. minor subsp. minor.

These photographs show the sticky ('glandular') hairs that cover the inflorescences of these plants. It seems most probable that the function of these hairs is to discourage ants and other non-flying insects that might steal the nectar. Flying insects are able to alight on the landing-platform formed by the lower lip of the corolla, such insects being more likely to transfer pollen between different plants, effecting cross-fertilisation. The page on the ant, Myrmica ruginodis, shows ants bypassing the defences of one of the plants in this colony.

Kreutz (1995) has photographs of yellow variants of many of the European speces; they are simply variants with defective pigment formation and should not be assigned undue significance.


Orobanche minor, yellow variant, close-up of flowers


Another distinctive and probably native colour variant, differing also slightly in corolla shape, occurs on dunes in east Kent, especially on Sea Holly (Eryngium maritimum). It has been claimed as the Mediterranean O. amethystea, but Rumsey & Jury (1991) give reasons why it does not match that species. They consider that it falls within the range of variation of O. minor subsp. (as var.) minor. Similar plants are frequent there too on Restharrow (Ononis repens) and are scattered on other hosts, though looking less distinctive and grading into more typical O. minor, as Rumsey & Jury themselves point out. The hosts themselves may be influencing phenotype.

Orobanche minor, amethyst variant
The amethyst variant of Orobanche minor var. minor on Sea Holly (Eryngium maritimum), East Kent, June 2002.

O. minor is a substantially self-fertilising species and so it would be expected to form genetically distinct local populations. Races showing physiological adaptation to particular hosts are known to occur (Musselmann &, Parker, cited from Rumsey & Jury, op. cit.). The Kentish plant on Sea Holly seems best regarded as one such biological entity within the species, conspicuous and attractive though it is. True O. amethystea is reported in Britain as an introduced weed in Cambridge Botanic Garden (Sell & Murrell, 2009).


 

Orobanche minor Sm. subsp. maritima (Pugsl.) Rumsey (= O. maritima Pugsl.) (Carrot Broomrape)

Orobanche minor subsp. maritima

 
This has been regarded as a species in its own right, or as a native, coastal variant of O. minor, but seems best regarded as a subspecies. Surprisingly it has been formally accorded this status only recently. In Britain it occurs in scattered sites along the coast of south and south-west England and in the Channel Isles. It has been considered rare, but it is very easily overlooked and often sporadic in its occurrence at any particular site. Rumsey (in Stewart et al., 1994) provides an account of its status (updated in Rumsey (2007)).

It occurs on slopes on sea-cliffs and on dry, coastal banks, and almost exclusively parasitises the southern coastal subspecies of Wild Carrot (Daucus carota subsp. gummifer), which has much the same geographic range (but which extends up the Welsh coast). The occurrence of subsp. maritima outside Britain is uncertain, though it is likely to be on the coasts of France and Spain.

 
Photograph: on cliff slope, Guernsey, Channel Isles, 1970.


 


GO TO NEXT PAGE
(other British species)


© A.J. Silverside
Page first hosted at www-biol.paisley.ac.uk/bioref/, December 2000, December 2008; restructured, updated and transferred to lastdragon.org, February 2010.
This remains a teaching page for the University of the West of Scotland (UWS), Paisley Campus
For text layout and clarity it is best viewed with Internet Explorer
 
Return to main Index
Conditions of Use
Lastdragon.org home page