Mallaburn, Peter S.; (1992) The physiological anatomy of the haustorium of Striga hermonthica (Del.) Benth. (Scrophulariaceae). Doctoral thesis (Ph.D.), University College London.

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Abstract

The parasitic angiosperms are a diverse group of plants that obtain a proportion of the resources needed for growth, development and reproduction by parasitizing other flowering plants. This is achieved by a haustorium, a structure thought to be derived from the root system which penetrates host tissues to form a structural conduit for the withdrawal of nutrients from the host xylem and phloem. The abstraction of resources can reduce the yield of the host, and one genera of parasitic angiosperms, Striga, can cause considerable economic damage by parasitising crops such as sorghum, millet and cowpea. The most important member of this genus is Striga hermonthica. which predominates in Africa and Asia, and can completely eliminate crop yield in subsistence agriculture. The literature on parasitic flowering plants is reviewed with particular emphasis on the role of the haustorium in Striga nutrition and host resistance. The development of the haustorium and the establishment of a nutritional flux from the host are identified as areas of particular uncertainty in the understanding of the parasitic habit. A range of anatomical and anatomically-related techniques are applied to the problem. The early development and structure of the haustorium in relation to host resistance is investigated using light microscope histology and transmission electron microscopy. The flow of host-derived solutes through the haustorium is assessed using vital dyes and fluorescent tracers, and the physiological anatomy and ionic distribution of the haustorium is analysed by enzyme cytochemistry and scanning electron microscopy. These studies show that early in the development of the parasite, the hyaline tissue within the haustorium takes up, processes and stores metabolites entering the parasite by sequestering the input within cells that display a high metabolic activity. Metabolites are then passed on to the parasitic shoot. This activity is sensitive to alterations in the nutrient status of the host and changes as the parasite develops. The results are discussed in the context of a model of haustorial function in S. hermonthica. It is proposed that the haustorium acts not just as a conduit for host resources, but also as a symplast bridge between the internal environments of the two species. In this respect, the haustorium appears to regulate the input to the parasite and possible to modify certain nutrients. The concept of the haustorium as a sink organ is introduced and developed in the light of the impact of the parasitism on host resource allocation and crop yield.

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