Flora (1986) 178: 233-242 VEB Gustav Fischer Verlag J ena Quantitative Aspects of Hybridization in Mixed Populations of Rumex obtusifolius L. and R. crispus L. (Polygonaceae) ALBRECHT ZIBURSKI, JOACHIM W. KADEREIT and PETER LEINS Institut fiir Systematische Botanik und Pflanzengeographie, Heidelberg, BRD Summary The analysis of hybrid frequency in mixed natural populations of R. obtusifolius and R. crispus on the level of the seed and the adult population and the analysis of Fl fertility and the F2 genera- tion leads to the following conclusions: 1. The variation in hybrid frequency found in the comparison of the three different sites near Heidelberg examined, between the individual seed parents and between the two parent species from the three sites cannot be correlated with the overall composition of the adult population and its spatial structure in most cases. The extent of hybridization rather depends on the circumstances found by the individual plant during its flowering period (e.g. climatic conditions, flowering period of neighbouring individuals). Differences in the hybridization behaviour of R. obtusifolius and R. crispus can partly be explained with their differing flowering period. 2. Changes in the proportional representation of R. obtusifolius, R. crispus and R. X pratensis from the seed to the adult population are suggested to depend on the different degree of seed dormancy in the three taxa and the hybrid vigour of R. X pratensis. 3. Introgressive hybridization between R. obtusifolius and R. crispus can take place, but is not likely to be a very frequent event. In the populations examined, the dominant direction of gene flow will be from R. crispus into R. obtusifolius. 4. Hybridization between R. obtusifolius and R. crispus has a detrimental effect on both parent species through the loss of progeny in form of hybrid seeds, competition between the parent species and the hybrid in the adult population and probably through the loss of seed dormancy in at least R. obtusifolius resulting from gene flow into this species. Introduction Although there exists a wide knowledge on the circumstances and evolutionary significance of hybridization in higher plants (e.g. STACE 1975), little information is available on quantitative aspects of hybridization in natural populations. The present account wants to present and discuss quantitative evidence on hybridization in mixed populations of Rumex obtusifolius L. and R. crispus L. on three different levels: 1. The percentage of hybrids found in the seed population, 2. The percentage of hybrids found in the adult population and 3. The fertility of F 1 hybrids and the formation of an F 2 generation. R. obtusifolius and R. crispus were chosen for their availability and the frequent occurrence of their hybrid, R. Xpratensis MERT. et KOCH. Exhaustive accounts of the morphology and ecology of the two species have been presented by RECHINGER (1958), CAVERS & HARPER (1964) and LOUSLEY & KENT (1981). Material and Methods Three sites near Heidelberg were chosen for the examination of their Rumex population: 1. Meadow near Maisbach (Site M), topographische Kartel: 25,000, 6618 Heidelberg-Siid, grid