Eight wild Hordeum species: H. bogdanii, H. intercedens, H. jubatum, H. lechleri, H. marinum, H. murinum, H. patagonicum, and H. secalinum, and cultivated barley (H. vulgare) were grown in nutrient solution containing 0.2 (control), 150, 300, or 450 mol m(-3) NaCl. In saline conditions, the wild Hordeum species (except H. murinum) had better Na+ and Cl- 'exclusion', and maintained higher leaf K+, compared with H. vulgare. For example, at 150 mol m(-3) NaCl, the K+:Na+ in the youngest, fully expanded leaf blades of the wild Hordeum species was, on average, 5.2 compared with 0.8 in H. vulgare. In H. marinum grown in 300 mol m(-3) NaCl, K+ contributed 35% to leaf psi(pi), whereas Na+ and Cl- accounted for only 6% and 10%, respectively. By comparison, in H. vulgare grown at 300 mol m(-3) NaCl, K+ accounted for 19% and Na+ and Cl- made up 21% and 25% of leaf psi(pi), respectively. At 300 mol m(-3) NaCl, glycinebetaine and proline together contributed almost 15% to psi(pi) in the expanding leaf blades of H. marinum, compared with 8% in H. vulgare. Decreased tissue water content under saline conditions made a substantial contribution to declines in leaf psi(pi) in the wild Hordeum species, but not in H. vulgare. A number of the wild Hordeum species were markedly more salt tolerant than H. vulgare. H. marinum and H. intercedens, as examples, had relative growth rates 30% higher than H. vulgare in 450 mol m(-3) NaCl. Hordeum vulgare also suffered up to 6-fold more dead leaf material (as a proportion of shoot dry mass) than the wild Hordeum species. Thus, several salt-tolerant wild Hordeum species were identified, and these showed an exceptional capacity to 'exclude' Na+ and Cl- from their shoots.

Growth, root aerenchyma, and profiles of radial O2 loss (ROL) along adventitious roots were evaluated in 35 'wild' Hordeum accessions and cultivated barley (H. vulgare L. ssp. vulgare) when grown in stagnant nutrient solution (deoxygenated and containing 0.1% agar). When grown in stagnant solution, accessions from wetland and 'intermediate' habitats were superior, compared with accessions from non-wetland habitats, in maintaining relative growth rate, tillering, and adventitious root mass. Constitutive aerenchyma formation in adventitious roots was ≥�10% in 22 accessions (cf. H. vulgare at 2%). When grown in stagnant solution, aerenchyma was ≥� 20% in the adventitious roots of 14 accessions (cf. H. vulgare at 12%). Variation among the accessions in the volume of aerenchyma formed when grown in aerated or stagnant solution was not determined by the waterlogging regime of the species' natural habitat. However, the genus Hordeum comprises four genomes and when grown in stagnant solution accessions with the X genome formed, on average, 22% aerenchyma in adventitious roots (50 mm behind apex), whereas those with the H genome averaged 19%, and those with the Y or I genomes averaged 16 and 15%, respectively. Sixteen accessions formed a barrier to ROL in the basal region of adventitious roots when grown in stagnant solution. The formation of a barrier to radial O2 loss was predominant in accessions from wet habitats, and absent in accessions from non-wetland habitats. In addition, this trait was only present in accessions with the X or H genomes. The combination of aerenchyma and a barrier to ROL enhances the longitudinal diffusion of O2 within roots towards the apex. The possibility of a link between having a barrier to ROL and the X or H genomes in Hordeum species might, in future studies, enable a genetic analysis of this important trait.

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The meiotic pairing behaviour of 39 new interspecific combinations between diploid Hordeum species are reported. On the basis of this data, four "basic genomes" are probably present in the genus. Hordeum bulbosum and H. vulgare have the same genome (I); H. marinum ("X") and H. murinum ("Y") each have one distinct genome. All other diploid taxa have either the same or a somewhat modified form of genome H. In this latter group of diploids, the South American taxa together with H. pusillum and H. intercedens in North America constitue a homogeneous group with respect to genomic structure, which differs somewhat from that found in the other Asiatic and North American species. Hordeum roshevitzii from Central Asia is unique, showing high affinity to both the Asiatic and to the American taxa. Evidence suggesting genetic regulation of chromosome pairing (both pairing promoting and pairing reducing) was obtained from a number of the diploid hybrids.Key words: Hordeum, interspecific hybrids, meiosis, diploids.

The phylogenetic relationship between four basic genomes designated H, I, Xa, and Xu in the genus Hordeum was studied using a nuclear DNA sequence. The sequence, cMWG699, is single copy in the H. vulgare genome, and tightly linked to the vrs1 locus which controls two- and six-rowed spikes. DNA fragments homologous to cMWG699 were amplified from diploid Hordeum species and the nucleotide sequences were determined. A phylogeny based on both base substitutions and an insertion-deletion event showed that the H- and Xa-genome groups are positioned in one monophyletic group indicating that the Xa-genome taxa should be included in the H-genome group. The large H-genome group is highly homogeneous. The I and Xu genomes are distinctly separated from H and Xa, and form sister groups. Another phylogeny pattern based on data excluding the insertion-deletion gave a result that the Xa genome forms a sister group to the H-genome group. The difference between the H and Xa genomes was affected only by a single base insertion-deletion event, thus the H and Xa genomes are likely to be closely related. The I and Xu genomes were again distinctly separated from the H and Xa genomes.