Warming trends involve two agronomically relevant aspects: a gradual increase in long-term mean temperature with the primary effect of shifting phenological patterns, and an increasing incidence of heat waves. Depending on timing, intensity and duration, heat can reduce crop growth and disrupt reproduction. Agronomic and breeding adaptations to elevated temperature have been listed but there is an overall lack of frameworks for systematic analysis. This paper provides agronomic and physiological background for the quantitative assessment of spatial patterns of the thermal regimes for wheat, barley, canola, field pea and chickpea. First, we revise the notion that Australian agriculture is ‘European’ and ill-adapted to the local environments. By showing that Australian agriculture in the southern and western regions is rather Levantine, we advance a more accurate and relevant framework to the thermal regimes of winter crops. Second, we outline the direct and indirect effects of temperature on crop traits and highlight the limitations of different approaches to investigate crop responses to temperature. This is important to make explicit the assumptions of studies dealing with crop responses to temperature; for example, indirect effects of temperature on crops mediated by effects on weeds, pathogens or herbivores could be important. Third, we compare the cardinal temperatures (including base, optimal, and critical thresholds) of our target crops. Cardinal temperatures respond to both natural and agronomic selection and are relevant for crop adaptation. Fourth, we develop a conceptual framework to assess thermal effects on crop yield and adaptive practices and traits, based on the notions of yield being a primary function of seed number, the species-specific critical window for the determination of seed number, and two complementary perspectives involving the photothermal quotient and crop growth rate in the critical window. The framework accounts for both aspects of warming: non-stressful elevated temperature and heat stress. Testable propositions are advanced that inform future research on crop adaptation to elevated temperature.
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Vol. 66 • No. 11