Phenotypic plasticity of tillering modulates winter wheat agronomic optimum plant density: A quantitative framework for interpretation of G × E × M interactions

Context Agronomic optimum plant density (AOPD) is the minimum number of plants per unit area that maximizes grain yield and varies with genotype, environment, and management interactions. Predominantly, studies on cultivar-specific AOPD treat cultivars as fixed effects, limiting the findings to the pool of cultivars studied. Objective To explore genotype-specific yield response to plant density from the perspective of phenotypic plasticity of tillering, i.e., the ability of a genotype to produce tillers in response to the environment. Method A complete factorial experiment evaluated 24 commercial wheat genotypes seeded at two plant densities (100 and 300 seeds m −2 ) in 12 environments across Kansas, the largest winter wheat-producing state in the United States. Percentile-plasticity regressions evaluated whether plasticity was a positive, neutral, or negative trait for grain yield and its components, and a Bayesian hierarchical model estimated grain yield response to density with varying phenotypic plasticity of tillering. Results Tillering plasticity at the crop level was associated positively with tiller number, head number, biomass, harvest index, and grain yield in environments where these traits had high expression; but mostly neutral in environments of low trait expression. Higher tillering plasticity genotypes had lower AOPD in 90% of cases, ranging from null to ~17 plants m −2 unit plasticity −1 . Larger AOPD reductions were associated with weather conditions favorable for tiller formation and survival (e.g., greater spring precipitation, greater winter temperatures). Conclusions Phenotypic plasticity of tillering was an adaptive trait in high tillering environments, with no tradeoff, as more plastic cultivars reduced wheat AOPD in environments conducive to tiller formation and survival compared to more stable cultivars. Implications This quantitative framework can be expanded to other crops and regions to explore genotype-specific responses to plant density.