Diversity in Nordic spring wheat cultivars (1901-93)

Nordic plant breeders have selected superior genotypes and released new spr ing wheat cultivars throughout this century. However, the extent of phenoty pic diversity that exists in this improved germplasm has not been accuratel y investigated. This study phenotypically assessed 75 selected cultivars re leased to farmers in Denmark, Finland, Norway and Sweden within the last 10 decades. Grouping of cultivars was not related to their geographical origi n or decade of release. The respective within-cluster variances were always larger than the among-cluster variances. The average phenotypic diversity for Nordic spring wheat germplasm was 0.22, which was not surprising becaus e some newer cultivars were derived from older cultivars. There was a signi ficant influence of geographical origin on days to heading, straw, grain an d biomass weight, and kernel number. Likewise, the country x decade of rele ase interaction was significant for all characteristics except plant height . These findings suggest that irrespective of location, Nordic breeders hav e reduced plant height in recently released cultivars at a rate of - 0.5 cm year(-1), thereby reducing lodging in this germplasm. There was no signifi cant change in biomass, straw and kernel weight between old and new cultiva rs. Nonetheless, relative genetic gains in this germplasm during this centu ry were significant for agronomic characteristics, such as days to heading (8%), plant height (36%), grain yield (20%), harvest index (19%), and numbe r of kernels per unit area (18%). On average, the absolute genetic gain for grain yield was about 18.5 kg ha(-1) year(-1). Negative changes in days to heading (at a rate of - 0.06 year(-1)) and plant height, and positive chan ges in harvest index (0.06% year(-1)), and kernels m(-2) (45 year(-1)) were associated with gains observed in grain yield, i.e. an early flowering pla nt with short straw, but many fertile tillers, had high grain yield. This s ustained genetic gain could result from the accumulation of favourable alle les and intergenomic interactions between homologous loci during systematic plant breeding for grain yield.