Comparative studies of plant growth and distribution in relation to waterlogging

summaryPopulations of Geum rivale L. and G. urbanum L. were collected from a range of habitats; G. rivale usually occurred in wetter and less‐shaded habitats than G. urbanum. Populations differed significantly in a number of respects; most differences between G. rivale populations were related to the sizes of leaves produced and to plant stature, whereas most of the variation in G. urbanum was related to differences in growth rates. The two species responded very differently to waterlogging, but there was only slight intraspecific variation in waterlogging tolerance in either species. Progeny of hybrid individuals were raised as families; their response to waterlogging was similar to that of one or other of the parent species. In G. rivale and other relatively tolerant families waterlogging generally caused a slight increase in shoot: root dry weight ratio, a small decrease in root dry weight, and had little effect on specific leaf area. Waterlogging of G. urbanum and less‐tolerant families caused a large increase in shoot: root ratio, a large decrease in root dry weight, and a decrease in specific leaf area. Tolerant families always had pronounced axillary bud proliferation, usually accompanied by the development of a rhizome‐like caudex. Less‐tolerant families and G. urbanum lacked a caudex, and had reduced axillary bud proliferation. Many of these traits were found to be highly heritable in hybrid families. The ratio of drained: flooded values was used as a measure of response to waterlogging, and showed highly heritable differences between hybrid families for root dry weight, shoot: root ratio and specific leaf area, indicating that these measures of waterlogging tolerance are genetically determined. Families whose maternal parents were morphologically similar to G. rivale were always relatively tolerant of waterlogging, but some families that were similar to G. urbanum were also tolerant. The results emphasize the effect of waterlogging on plant root formation; while there were also important effects of waterlogging on shoot growth and development, the primary effect appeared to be on the root system. These results are discussed in relation to theories of hybrid ecology, and the need for further comparative studies of hybrid ecology is emphasized.