Two decades of harnessing standing genetic variation for physiological traits to improve drought tolerance in maize (Zea mays L.).

We review approaches to maize breeding for improved drought tolerance during flowering and grain filling in central and western US corn belt and place our findings in context of results from public breeding. Here we show that after two decades of dedicated breeding efforts, the rate of crop improvement under drought increased from 6.2 to 7.5 g m -2 y -1 closing the genetic gain gap with respect to the 8.6 g m -2 y -1 observed under water sufficient conditions. The improvement relative to the long-term genetic gain was possible by harnessing favorable alleles for physiological traits available in the reference population of genotypes. Experimentation in managed stress environments that maximized the genetic correlation with target environments was key for breeders to identify and select for these alleles. We also show that the embedding physiological understanding within genomic selection methods via crop growth models can hasten genetic gain under drought. We estimate a prediction accuracy differential (Δr) above current prediction approaches of ~ 30% (Δr = 0.11, r = 0.38), which increase with increasing complexity of the trait x environment system as estimated by Shannon Information. We propose this framework to inform breeding strategies for drought stress across geographies and crops.