1.1 Plant Parasitism:
PARASITISM is the dependence of one organism on another living organism, to the detriment of the host organism.
SAPROPHITISM is the dependence on a dead and rotting food source.
SYMBIOSIS is the dependence of one organism on another, but to the mutual advantage of both host and dependant organism.
Plant-plant unions can be described as parasitic or as non-parasitic. It is known that the roots of elms trees [Ulmus procera] can fuse together, due to the proximity of the roots. Dutch Elm Disease [Ceratocystis ulmi] 1,2 can be transmitted in this manner. But the plant-plant union itself is clearly not parasitic
A criterion3 proposed for defining a parasitic plant-plant union is the presence of a specialised physiological bridge, haustorium, through which nutrients and water are transported from one plant to another, i.e., from the host plant to the dependant plant. Some species of parasitic plant have become reliant on host plants to the extent of lacking chlorophyll, the green pigment catalysing photosynthesis in all non-parasitic plants. Semi-parasites have retained the photosynthetic ability while still having the need for a support host.
There are two closely related families which contain parasitic species. All genera in the family Orobanchaceae [Broomrapes], and at least twenty-two genera in the family Scrophulariaceae [Figworts] contain parasitic species. The two families are separated primarily on the ground of the total parasitic habit of Orobanchaceae species4. All the parasitic species of both families are root-parasites, as opposed to parasites with a union above ground such as mistletoe on oak trees.
At least one species from these two families could parasitise any crop plant. Striga lutea Lour., [Witchweed] can parasitise the grass family [Gramineae]5 which includes corn, maize, rice, wheat and sorghum; while Orobanche cumana Wallr., is parasitic on sunflower6,7, a crop important for the seed oil used in margarine. In East Sussex, England O. purpurea Jaquin. is known to be parasitic on the minor vegetable fennel [Foeniculum vulgare Miller].
These parasites reproduce in the usual angiospermous manner of seeds. However, these seeds are very small, a diameter of 0.1mm being typical. The minuteness of the seeds leads to ease of transport by air and water; and the production of very large numbers of seeds [ca 100,00] by each flowering plant. The size and number of seeds compound the difficulties of any proposed physical control of the parasites before parasitisation occurs.
Female flies of Smiconyx8 and Phytomyza9 species lay their eggs in the flowers of the parasites, so that the grubs can feed on the developing seeds. But fruit-galls are scarce, stem-galls which do not prevent seeds ripening are more common10. Similarly, the larvae of Ophiomyia strigalis Spencer damage Striga hermonthica by mining into the roots and stems11. Eulocastra argentisparsa Hmps. larvae feed on Striga seed pods, but field populations are always low10. Biological control of the parasitic plant species does not seem viable, since the natural control agents do not exist12.
Disclaimer: I've typed this thesis by hand twice, firstly in 1978 and again 2017. Typographical errors have bound to have crept in.
Copyright © Robert Howe 1978 - 2017. All Rights Reserved.