Increase in night temperature of 1 °C reduces wheat yields by 6-9%. Yield losses may be linked to capacity of physiological processes to acclimate to warm nights. To understand the mechanisms of wheat responses to warm nights we used 12 Australian wheat cultivars released between 1901 and 2012 (over a century of breeding) to assess whether leaf photosynthetic and respiratory processes acclimate to warm nights, and if rates of pollen germination and pollen tube growth were influenced by warm nights. Net CO2 assimilation rate (An) increased by 5.12% under warm nights, driven by higher rates of maximum Rubisco carboxylation and electron transport.

Rates of dark respiratory O2 consumption and CO2 release (Rdark) acclimated to warm nights. However, the type of acclimation differed: Type I for Rdark on O2 basis; and mostly Type II for Rdark on CO2 basis. These results suggest that acclimation was underpinned by changes in substrate availability, membrane fluidity, and mitochondria capacity and density. Warm nights did not alter the ratio of Rdark to An (Rdark/An).

This means representation of carbon fluxes in earth systems models can assume homeostasis of Rdark/An. Warm nights also reduced (P<0.05) pollen germination and pollen tube growth which consequently reduced spikelet fertility and grain weight. We conclude that leaf An and Rdark can acclimate to warm nights, and this is more pronounced in modern cultivars maintaining carbon balance and a more efficient energy economy. Breeding for resilient wheat cultivars should exploit variation in both physiological acclimation capacity and reproductive tolerance to warm nights.