Improving winter barley adaptation to freezing and heat stresses in the U.S. Midwest: bottlenecks and opportunities

Continental cropping systems are increasingly exposed to extreme and opposing trends of temperatures over the same growing season. This situation is epitomized by winter barley grown in the Upper Midwest, which is subject to temperatures that can be as low as −30 °C during the winter and over 30 °C during summer. This interaction severely limits the potential of this emerging crop, by threatening the winter survival of the crown which is often exposed to lethal freezing stress and by exposing reproductive organs to high temperature (HT) stress, to which this cool-season grass is highly sensitive. This poses a unique challenge that requires the discovery and capture of well-defined sets of traits enabling adaptation to extreme tail ends of a stressor, with limited trade-offs, while minimizing costs. Here, based on a critical literature review, we propose a framework integrating i) environmental characterization (envirotyping), ii) envirotype-relevant scouting of genetic resources, iii) ecophysiology-informed trait identification and phenotyping and iv) breeding pipelines. We outline propositions and guidelines for implementing these steps and discuss their feasibility, with an emphasis on i) identifying novel genetic resources and eco-physiological traits relevant to this problem, ii) navigating their physiological trade-offs and iii) leveraging this information to develop ad hoc phenotyping methods for deployment in breeding programs. Our review indicates that the eco-physiological and genetic bases for improving tolerance to both stresses in the same organism likely exists based on evidence from crop relatives and extremophile species, with smaller vasculature (freezing tolerance) and transpirational cooling (HT tolerance) being prime examples. Our findings indicate that such traits could be captured with minimal trade-offs, and that is possible to use similar phenotyping concepts (thermal imaging) and infrastructure (cold/heat tents) to screen for these traits and accelerate breeding to enhance adaptation to temperature extremes.