Everybody who has seen the BBC Television documentary series, "The Private Life of Plants", will remember the time lapse sequence of the parasitic plant dodder invading a colony of stinging nettles.
With its circling red stems reaching out to embrace fresh victims, the behaviour of dodder seems almost like that of an animal, rather than a plant. Everything about it is designed for conquest. It can grow and spread at a phenomenal rate, claiming new territory by means of its tough seeds, spreading like wildfire through crops at considerable economic cost.
A few species are such dire pests that in some countries, all Cuscuta species have been blacklisted as noxious weeds. What is the truth behind this highly specialized group of plants, and why are they so successful?
Cuscuta In Context
There are some 200 species of dodder and they are found throughout the temperate and tropical regions of the world in a huge range of habitats from cool and tropical forests, to grasslands and montane environments.
Dodders were formerly considered to be a distinct family, the Cuscutaceae. Despite some disagreement among researchers, recent genetic studies suggest the genus is part of the family Convolvulaceae, that also contains the bindweeds and morning glories.
Devilish Dodder
Dodders are supremely adapted for a life of plunder and pillage. Reduced in form to scrambling and twining threads, they appear to be completely leafless, although closer inspection reveals tiny scale leaves pressed close to the stems.
These leaves, however, are superfluous to requirement because most dodders contain no chlorophyll and cannot manufacture their own food via photosynthesis. Such plants are entirely dependent on their hosts for water, carbohydrates and other nutrients and are called holoparasites.
Among other holoparasites are the Broomrapes, Orobanche, and the bizarre Mediterranean Cynomorium. These, however, are root parasites that emerge above ground only to flower. Dodder, on the other hand, is a stem parasite that performs its acts of botanical rape in full view.
From Seed To Strangler
Cuscuta species generally produce round clusters of tiny white or pink flowers. The resulting seeds are amazingly tough, enduring for at least 20 years under natural conditions, though there are records of them germinating after 60 years. Some appear in the spring following production but others remain dormant, possibly as a survival strategy in case of a poor growing season.
The first few days of life for a dodder seedling are critical. Unlike Orobanche, dodder seeds do not need chemical stimulation from a host plant to grow. Indeed, they are capable of germinating and reaching the surface from a depth of several centimetres.
A dodder seedling produces only a rudimentary root that anchors it temporarily to the ground. It has no cotyledons or seed leaves, only a questing filamentary shoot that depends entirely on the food reserves stored in the seed for its first five to ten days of life. If it does not find a host within this time, the infant dodder is doomed.
As it grows, the seedling circumnutates, or swings around anti-clockwise at a rate of around one revolution an hour as it hunt for its first host. However, recent research has demonstrated that the process is not as random as it seems, for the seedlings are able to detect the volatile chemicals released by the host plant and grow towards it. Furthermore, this process is truly selective as the seedlings will ignore a non-host species in favour of their chosen victim.
If the seedling finds a host, it immediately alters its behaviour. Rapidly coiling around the plant, it sends out multiple haustoria that breach the host´s stem and tap into its supply of water and nutrients. The root withers, and the dodder is now entirely disconnected from the soil.
The haustorium is a modified root-like structure, evolved by many parasitic plants especially for this purpose, and dodder can produce so many that its stems appear swollen and distorted wherever they encounter those of the host.
Dodder plants do not literally strangle their victims, but in the case of crop species they can debilitate them enough to seriously affect yields. Some dodders have a cosmopolitan taste in hosts and can easily bridge gaps to parasitize a whole range of different species. Sometimes, whole trees can be covered with a tangled mass of yellow, orange or purplish threads - a fearsome sight indeed!
Dodder In Danger
Dodders are generally annual plants, but in mild regions growth can continue year round. In addition, the haustoria sometimes survive the winter within the tissues of the host, regrowing once Spring arrives.
Not all dodders are pests. In fact, only around 15 species are a threat to agriculture. Many kinds are specific to only one host and a half of all species are in need of protection. In Britain, the common dodder, Cuscuta epithymum, has been declining since the 1930s due to loss of lowland heath and the ploughing of chalk grasslands.
This is clearly a genus of plants that deserves greater understanding and, in the case of threatened species, appropriate measures to ensure their survival.
References
Convolvulaceae Unlimited: Cuscuta
Floridata: Cuscuta spp
Purdue University: Purdue Plant and Pest Diagnostic Laboratory: Dodder
The University of Waikato: Can plants hunt?
Peter Ashby - Peter Ashby lives near Granada in southern Spain, is an expert on natural history, and a dedicated traveller, writer and hispanist.
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