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Populus nigra L. is a pioneer tree species of riparian ecosystems that is threatened with extinction because of the loss of its natural habitat. To evaluate the existing genetic diversity of P. nigra within ex-situ collections, we analyzed 675 P. nigra L. accessions from nine European gene banks with three amplified fragment length polymorphism (AFLP) and five microsatellite [or simple sequence repeat (SSR)] primer combinations, and 11 isozyme systems. With isozyme analysis, hybrids could be detected, and only 3% were found in the gene bank collection. AFLP and SSR analyses revealed effectively that 26% of the accessions were duplicated and that the level of clonal duplication varied from 0% in the French gene bank collection up to 78% in the Belgian gene bank collection. SSR analysis was preferred because AFLP was technically more demanding and more prone to scoring errors. To assess the genetic diversity, we grouped material from the gene banks according to topography of the location from which the accessions were originally collected (river system or regions separated by mountains). Genetic diversity was expressed in terms of the following parameters: percentage of polymorphic loci, observed and effective number of alleles, and Nei's expected heterozygosity or gene diversity (for AFLP). Genetic diversity varied from region to region and depended, to some extent, on the marker system used. The most unique alleles were identified in the Danube region (Austria), the Rhône region (France), Italy, the Rijn region (The Netherlands), and the Ebro region (Spain). In general, the diversity was largest in the material collected from the regions in Southern Europe. Dendrograms and principal component analysis resulted in a clustering according to topography. Material from the same river systems, but from different countries, clustered together. The genetic differentiation among the regions (F(st)/G(st)) was moderate.

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A micrografting technique for use on shoots derived by shoot‐tip culture is described. Autografts of Prunus domestica cv. Hauszwetsche as well as heterografts of several sour cherry cultivars (Prunus cerasus L. cvs Schattenmorelle, Weiroot 158, Köröser) were established. Successful graft formation in vitro was confirmed by translation of 86Rb+ from the stock root into growing scion tissues. A mechanically strong graft union was formed during the course of a 3‐week subculture of micrografts in a liquid medium without the addition of growth regulators. In the case of graft rejection, 86Rb+ was mainly attracted to new developing shoots from lateral meristems of the stock plant. Histological examination of the graft union revealed callus formation, cytodifferentiation and xylogenesis leading to the formation of vascular connections. Stem elongation after micrografting was related to vigour of the stock and scion genotypes. Early stem elongation could be used as a criterion for preselection of growth vigour in graft combinations. Micrografts were transferred to soil and grown in the greenhouse.

Eleven laboratories have collaborated to study chloroplast DNA (cpDNA) variation in black poplar (Populus nigra L.) across Europe in order to improve our understanding of the location of glacial refugia and the subsequent postglacial routes of recolonisation. A common analysis based on the restricted fragments produced by five primer pairs was used to determine the cpDNA haplotype of 637 samples obtained from genebank collections established in nine European countries. Haplotype 2 was particularly common and was found in 46% of the non-hybrid samples. A total of 81 non-hybrid chloroplast variants were www.elsevier.com/locate/foreco Forest Ecology and Management 219 (2005) 293-312 DOIs of related articlesdetected. Three haplotypes (from four trees believed to originate from Eastern Europe) clustered together and were very different from the rest of the samples. The remaining samples were divided into two groups, one of which had a largely eastern distribution and samples from the other group were mostly located in the west. This, along with the fact that Spain in the southwest and Austria and Italy in the southeast had high diversity, suggest that there were ice age refugia of black poplar in both southwestern (Spain) and southeastern Europe (Italy and/or Balkan). Results also indicate that the Pyrenees formed a significant barrier, since only 7 of the 45 haplotypes in Spain exist elsewhere in Europe. #