Plant 296

Orobanche species (Orobanchaceae)


Orobanche, known as the broomrapes, is a large genus of parasitic, herbaceous plants that are mainly found in the Northern Hemisphere. A few species, like common broomrape (Orobanche minor) are native to the United Kingdom but the majority are found in Europe (particularly the Mediterranean Basin), North Africa, western and Central Asia and North America.

Broomrapes are leafless but have alternate scales on their stems that are oblong to lanceolate in shape. Stems produce heads of colourful flowers that may occupy half the length of the stem. The flowers eventually produce capsular fruits, each of which contains many thousands of minute, dust-like seeds that may remain viable in the soil for decades.

To germinate, broomrape seeds require a chemical stimulus from the roots of a so-called host plant from which the plant steals its food. This occurs when the seeds are in close proximity to the host's roots. For a broomrape seedling to survive, such connections must happen within a few days of germination, since broomrapes are completely dependent on their host plants for water and organic and inorganic nutrients.

The stems of Orobanche can vary in colour from yellow to brown or purple but lack any green colour; they are achlorophyllous which means that they do not have any chlorophyll and therefore cannot photosynthesise. To obtain nutrients, broomrapes parasitise the roots of different plant species through haustoria, which are structures that penetrate the tissues of the host and absorbs nutrients and water from it. Some species, such as common broomrape, can parasitise a wide variety of different plant species from many families, but others are far more specific, such as ivy broomrape (Orobanche hederae) which parasitises ivy, and related species, almost exclusively.

Besides occurring in natural habitats, some broomrapes, like Orobanche crenata and Phelipanche ramosa, can be pests to fields of crops. They have become a serious economic threat to tomatoes, potatoes, aubergine, tobacco, cabbage, bell pepper, sunflowers, celery and beans through root parasitism and can cause complete crop failure. Their parasitic nature makes broomrapes particularly difficult to eradicate and the long-term viability of their seed means susceptible crops cannot be grown on infested land for years after an infection. The economically-devastating witchweeds (Striga) of Africa are also root parasites in the family Orobanchaceae.

Researchers in the Oxford Botanic Garden have conducted research into how broomrapes shifting between different host plants are in the process of forming new species.

Further reading

Parker, C 2008. Invasive Species Compendium: Orobanche ramosa (branched broomrape).

Silverside, A 2008. Orobanche: broomrapes.

Emma McFarline