Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, UK

Background and objectives
Environmental factors are of prime importance in determining the level of a parasite's activity and thus the infection pressure it imposes on its host's population. Environmental factors in different parts of a host's range could vary from those which are so unfavourable to the parasite that the parasite's infection pressure is zero, to those which are extremely favourable and where the infection pressure may be so high that only totally resistant host genotypes can survive. Within these extremes the parasite is likely to be active to varying degrees and be able to establish infections on its host. These infections will impose selection pressures on the host population favouring those genotypes that are best able to cope with that infection. This contribution reviews the extent to which favoured genotypes are likely to be those with partial resistance to and/or tolerance of the parasite [1] (a tolerant genotype is defined as one which suffers less reduction in reproductive output in response to the same or greater levels of infection than other genotypes of the same species). Studies will be presented that indicate that tolerance could be an important component of the survival strategy of some host plants in some environments.

Results and conclusions
Our studies on the role of tolerance in the survival strategies of host plants developed from studies on relationships between the native annual weed plant Senecio vulgaris (groundsel) and its powdery mildew parasite Erysiphe fischeh. Many plants in groundsel populations regularly support heavy levels of infection yet the host plant remains common and the frequency of susceptible plants in the population does not appear to change. Thus, tolerance of infection, rather than resistance, may play the greater role in the survival strategy which enables groundsel to cope with mildew infection. Growth cabinet studies supported this conclusion by showing that the growth and development of groundsel plants are much less affected by high levels of mildew infection than are those of some cultivated crops suffering the same or much lower levels of infection [2]. The finding that groundsel suffers less from mildew infection than some crops such as cereals also suggested that, during domestication, crops could have lost some of the tolerance that their wild ancestors may have possessed. This latter possibility has been tested by comparing the effects of Erysiphe graminis infections on cultivated and wild oat (Avena sativa and A. fatua, respectively) [3,4] and cultivated and wild barley(Hordeum vulgare and H. spontaneum, repectively). The comparison involved growth analysis to determine the effects of different levels of infection on the growth and development of the whole plant and measurements with oxygen electrodes to determine the effects of different levels of infection on photosynthesis and respiration in detached leaves. Lines of the wild species were generally less affected by heavy levels of infection than were cultivars of the related cultivated species. However, differences between some cultivars and their wild relatives were quite small and bigger differences were found between some cultivars. Thus, while the comparisons indicate that the wild relatives were more tolerant of mildew infection than any of the cultivated cereals they also revealed significant differences in tolerance between cultivars, with some cultivars possessing quite high levels.

1. Clarke DD, 1996. In: Morris CE, Nicot PC, Nguyen-The C, eds. Aerial Plant Surface Microbiology. New York: Plenum Press, pp.91-101.
2. Harry IB, Clarke DD, 1992. Physiological and Molecular Plant Pathology 40, 211-224.
3. Sabri N, Clarke DD, 1997. Physiological and Molecular Plant Pathology 49, 405-421.
4. Sabri N, Clarke DD, 1997. Physiological and Molecular Plant Pathology. 50, 321-335.