Genetic gain in yield and associated changes in phenotype, trait plasticity and competitive ability of South Australian wheat varieties released between 1958 and 2007

To quantify the genetic gain in yield and associated changes in phenotype, we compared 13 South Australian wheat varieties released between 1958 and 2007. Crops were grown in three environments with a range of yield between 4.1 and 6.1 t/ha. Yield increased linearly with year of cultivar release at a rate of 25 +/- 3.4 kg/ha per year. Yield improvement was associated with a linear increase in harvest index over the whole period 1958-2007 and increased shoot biomass for varieties released after the early 1980s. A non-linear model with an inflection point at 1982 +/- 1.6 emphasised two phases in the time trend of grain size: it decreased between 1957 and 1982 and increased afterwards. The plasticity of grain size increased 2-fold after 1982. Grain number increased until the early 1980s and stabilised afterwards. Grain number was associated with crop growth rate between stem elongation and anthesis, and grain size was associated with crop growth rate per grain. Crop growth rate between stem elongation and anthesis increased after the early 1980s in parallel with increased radiation-use efficiency and independently of changes in capture of radiation. Candidate traits to explain the improvement in radiation-use efficiency include increased stomatal conductance and greener leaves. The concentration of water-soluble carbohydrates in shoots at anthesis increased with year of cultivar release at 0.12 +/- 0.018% per year. In two out of three environments, yield response to competition declined with year of cultivar release, which was consistent with the communal plant ideotype.