Across the Northern Hemisphere, during the months of Spring and early Summer, a family of strange plants emerge from the undergrowth.
Some produce tall spires bearing dozens of yellow, orange or purple flowers, others are short and squat, while a few carry only one large bloom on a slender stem. What all of these have in common is that, apart from a few greatly reduced scales on their stems, they have no leaves whatsoever.
The Genus Orobanche
These are the Broomrapes, a genus of 150 parasitic perennials found mainly in Europe, North Africa, North America and Asia. Their common name derives from an old belief that they grew as tubers (rapum) from broom plants (Cytisus). Most have tubular, two-lipped flowers that are frequently covered with glandular hairs, but in some species the blooms flare wide and can be very attractive.
One common European species, the greater broomrape (Orobanche rapum-genistae), specializes in parasitizing brooms and other shrubs. Some, such as the ivy broomrape (Orobanche hederae) can only survive on one host, while others have more cosmopolitan tastes and can parasitize dozens of species.
Broomrapes as Root Parasites
What all broomrapes have in common, is their method of gaining nutrients. Orobanche are holoparasites, which means that they contain no chlorophyll and, being unable to photosynthesize, must take all their food and water from the host plant. Among others, they share this trait with the bizarre and fascinating Cynomorium and the tropical Rafflesia.
All broomrapes are root parasites, attaching themselves beneath the ground to the roots of their hosts, rather than to aerial stems as do some species of mistletoe (Viscum spp and others) and the dodders (Cuscuta spp). They breach the root walls of their hosts with a modified root-like structure called the haustorium. With this, they access a continual supply of water and nutrients from the host´s phloem and xylem, the plant equivalent of a circulatory system.
Evolution of Broomrapes
Why develop a parasitic lifestyle in the first place? Broomrapes probably evolved from a non-parasitic ancestor in the figwort family (Scrophulariaceae). They frequently grow in grasslands and other dense vegetation where competition for nutrients is considerable. In such places it makes evolutionary sense for a plant to dispense with photosynthesis, "plug in" to a ready-made root system and avoid the stressful business of competing for light and food altogether.
The Opportunist Orobanche
Broomrape seeds are tiny, and they are produced in their hundreds of thousands. They need to be in direct contact with a host root in order to germinate, but they can wait 50 years or more before soil disturbance moves them to a suitable location, or the roots of a host come within reach.
Once this happens, germination is triggered by chemical emissions from the host plant. Unlike most seedlings, those of Orobanche are not influenced by gravity and grow sideways to attach to their host. The young plant then produces a swollen subterranean structure from which the flower spikes eventually emerge.
Broomrapes do not always have everything their own way, however. It is believed that some plants can mount a chemical defense to prevent parasitism, but this ability needs to be studied further.
Orobanche as an Agricultural Pest
Some species of Orobanche are under threat from habitat disturbance and a number are protected by regional legislation. Others, especially among the non host-specific species, have travelled far beyond their natural range to become serious agricultural pests.
Because broomrape seed is so small and durable, it has been able to circumnavigate the globe. Aided by human commerce, they gain access to new opportunities by stowing away in ships´ ballast and packaging materials.
Among the worst offenders are Orobanche aegyptiaca, which parasitizes solanums (tomatoes, peppers and aubergines), Orobanche cumana which can devastate sunflower crops, and Orobanche ramosa which attacks solanums and legumes. Although they prefer warmer climates, broomrapes are more cold-tolerant than some related plants and one species has even been found in Finland.
It is in countries such as Australia, however, that alien broomrapes have raised most concern. The branched broomrape, (Orobanche ramosa) is a European species now established in southern Australia, where it is capable of reducing crop yields by more than 70%.
Not only does it parasitize a huge range of commercially valuable crops, but it is difficult to eradicate due to its close association with the host plants and the huge volume of seeds produced. The branched broomrape is subject to a national eradication programe and suspected outbreaks must be reported to the local authorities within three days.
Notwithstanding the significant damage caused by some species, it is hard not to admire these fascinating plants. With their complex ecology and curious appearance, the beguiling broomrapes have the power to captivate wherever they are found.
References
Biodiversity Reference: Orobanche: Broomrapes bioref.lastdragon.org
Department of Agriculture and Food: Government of Western Australia: Broomrape: Branched Broomrape (Orobanche ramose, Orobanche spp. except O. minor) www.agric.wa.gov.au
Daniel L Nickrent, Lytton J Musselman: Introduction to Parasitic Flowering Plants APS net; www.apsnet.org
Peter Ashby - Peter Ashby lives near Granada in southern Spain, is an expert on natural history, and a dedicated traveller, writer and hispanist.
