Variation and Phenotypic Plasticity of Heading Date and Yield Across Multiple U.S. Great Plains Environments.

In wheat, heading is a phenological stage that describes the emergence of the inflorescence and indicates a shift from vegetative to reproductive development. Variation in heading date reflects genotypic ‘earliness’ and is critical for adaptation to specific environments. Poorly timed floral development can expose sensitive reproductive tissues to cold, heat, or water stress and reduce grain yield and/or quality. Heading date is highly variable across environments, and is affected by major genes (vernalization and photoperiod) as well as quantitative earliness per se (eps) genes. Most eps genes are detected by QTL mapping and are very environment-specific. Since heading date is so variable, it is useful to consider the range of phenotypic responses of each genotype. 

Phenotypic plasticity is described as the ability of a single genotype to produce a variable phenotype under different environmental conditions (Nicotra, 2010). The extent of plasticity, as well as whether it is beneficial or detrimental varies among traits and when evaluated on different germplasm or environments (Bradshaw, 1965). Fine-tuning and optimizing reproductive development in wheat is complex, and involves many quantitative genes that are not well characterized or necessarily stable across environments. A more complete understanding of phenotypic plasticity of heading date could allow deeper understanding of crop adaptation.

We assessed variation and phenotypic plasticity of heading date and yield within a panel of 299 hard winter wheat varieties at ten environments across the U.S. Great Plains in 2012 and 2013. There was significant variation for both traits across environments, and there was a significant negative correlation between heading date and yield. Heading date plasticity was driven by earliest heading date, and had a significant positive correlation with yield. Thus, heading date plasticity was a favorable trait in our germplasm and environments. Yield plasticity was driven by minimum yield, and also had a significant positive correlation with yield.  There was also a significant positive relationship between heading date plasticity and yield plasticity, which suggests both plasticity traits could be targeted for improvement at the same time. Future work might involve investigating the genetic basis of trait plasticity through GWAS, and identifying plasticity QTL that co-localize with, or are independent of the trait per se.