Carbon isotope discrimination (?) has been found to be either positively or negatively related to grain yield of small grain cereals when grown in contrasting environments. In order to clarify a possible association between grain yield of barley ( Hordeum vulgare L.) and ? of mature kernels, five 6-rowed and five 2-rowed barley cultivars were evaluated in 22 rainfed environments of northern Mediterranean Spain. Analyses of variance suggested that the genotypic ? values were more consistent across environments than the genotypic yields. Genotype×environment (G×E) interaction for grain yield was further explored by fitting an AMMI (additive main effects and multiplicative interaction) model. The first 2 multiplicative axes were found significant. The AMMI2 model provided more accurate estimates of genotypic yields within environments than the conventional unadjusted means across replicates. AMMI2 estimates were used for input into cluster analysis, grouping environments that ranked genotypic yields similarly. Three major groups were obtained, with average yields of 2.42 t/ha (cluster I), 3.06 t/ha (cluster II), and 5.16 t/ha (cluster III). The genotypic ranking for ? did not vary substantially across clusters, but it changed for grain yield. The average genotypic yields in the low-yielding cluster I ranked opposite to those in the high-yielding cluster III, suggesting the existence of a crossover point at an intermediate yield level. The association between grain yield and ? for genotypic means within clusters was variable. In cluster I, yield and ? tended to be negatively related, whereas they were positively related in clusters II and III. Genotypes with lower ?, i.e. with higher transpiration efficiency, performed better in low-yielding environments (mostly those grouped in cluster I). On the contrary, a high genotypic ? was of advantage in medium (cluster II) and high-yielding environments (cluster III). This observation supports the assumption that drought tolerance and high yield potential under non-limiting growing conditions may be antagonistic concepts in barley. Genotypic means for kernel number per m 2 and ? were consistently and positively related within clusters, suggesting that a constitutively high ? may have been driven by a large genotypic reproductive sink. The convenience of using ? as a selection criterion in areas exhibiting a considerable G×E interaction for grain yield is discussed.

Background: Frost tolerance is a key trait with economic and agronomic importance in barley because it is a major component of winter hardiness, and therefore limits the geographical distribution of the crop and the effective transfer of quality traits between spring and winter crop types. Three main frost tolerance QTL (Fr-H1, Fr-H2 and Fr-H3) have been identified from bi-parental genetic mapping but it can be argued that those mapping populations only capture a portion of the genetic diversity of the species. A genetically broad dataset consisting of 184 genotypes, representative of the barley gene pool cultivated in the Mediterranean basin over an extended time period, was genotyped with 1536 SNP markers. Frost tolerance phenotype scores were collected from two trial sites, Foradada (Spain) and Fiorenzuola (Italy) and combined with the genotypic data in genome wide association analyses (GWAS) using Eigenstrat and kinship approaches to account for population structure. Results: GWAS analyses identified twelve and seven positive SNP associations at Foradada and Fiorenzuola, respectively, using Eigenstrat and six and four, respectively, using kinship. Linkage disequilibrium analyses of the significant SNP associations showed they are genetically independent. In the kinship analysis, two of the significant SNP associations were tightly linked to the Fr-H2 and HvBmy loci on chromosomes 5H and 4HL, respectively. The other significant kinship associations were located in genomic regions that have not previously been associated with cold stress. Conclusions: Haplotype analysis revealed that most of the significant SNP loci are fixed in the winter or facultative types, while they are freely segregating within the un-adapted spring barley genepool. Although there is a major interest in detecting new variation to improve frost tolerance of available winter and facultative types, from a GWAS perspective, working within the un-adapted spring germplasm pool is an attractive alternative strategy which would minimize statistical issues, simplify the interpretation of the data and identify phenology independent genetic determinants of frost tolerance.