Genetic adaptation to soil acidification in four grasses

Abstract: Soil acidification has become a source of serious concern in many parts of the world. In southern Sweden, the deposition of acidifying substances, such as nitrogen and sulphur compounds, has decreased the mean pH of forest soils by almost one unit since the 1950s. Apart from H+ ions themselves being harmful to plant roots, the potential toxicity of aluminium (Al) is increased when pH drops below 5. The aim of this thesis was to examine whether increased acidity has caused genetic adaptation to acidic conditions within Elymus caninus, Poa nemoralis, Deschampsia cespitosa and D. flexuosa, four grass species with widely different pH distributions. I have also investigated whether the content of Al in field-collected shoots is related to the Al concentration of the local soil environment, and whether this relationship differs between species with different pH distributions. Finally, I have examined the relationship between the colonisation of arbuscular mycorrhizal (AM) fungi and soil pH in these species. To test for genetic adaptation to acidic conditions, I sampled tussocks (genotypes) of each species from deciduous forests in two regions of southern Sweden, differing in their exposure to acidifying deposition. The tolerance of different genotypes was tested in solution experiments with different pH and Al concentrations. I found little evidence of genetic adaptation to acidic conditions at the regional, populational and micro-site (genotype) level. The association between the Al content in shoots and the Al concentration in the soil, was weak for most species, and showed no consistent relationship with the species? pH distribution in the field. D. flexuosa had the lowest shoot Al content, although this species was growing in soils with relatively high Al levels. D. flexuosa also had the lowest AM colonisation, while E. caninus, which was found on the least acidic soils, had the highest AM colonisation and relatively high shoot Al content. In conclusion, I found that genetic adaptation to acidic soils has not yet occurred in southern Sweden. It is possible that more time is required for acid-stress tolerance to evolve. Adaptation to acidic conditions was most pronounced at the species level: E. caninus, growing in the least acidic soils, was the most sensitive species in the tolerance tests, while D. flexuosa, growing in the most acidic soils, was the least sensitive species. The high tolerance of D. flexuosa may, at least partly, be explained by the existence of an Al exclusion mechanism. However, my results provide no support for a role of AM fungi in acid stress tolerance.