Wheat leaf dark respiration acclimates more strongly at night than in the day when responding to nocturnal warming

Rising night temperatures pose a significant threat to wheat productivity, yet the physiological basis of wheat adaptation to nocturnal warming remains poorly understood. We evaluated leaf photosynthetic and respiratory traits in response to warm nights under temperature-controlled environments in 10 Australian wheat cultivars released between 1901 and 2012. When exposed to warmer nights, rates of leaf net CO2 assimilation measured at 25 °C (Anet25) remained stable across cultivar release date despite declines in photosynthetic capacity (Vcmax and J1500) in newer cultivars. In most cultivars leaf respiratory CO2 release in the dark (Rdark) exhibited divergent thermal responses: warm nights suppressed temperature-normalised night Rdark (Rdark_night) but stimulated or maintained Rdark in the daytime (Rdark_day). The results suggest that a century of yield-focused breeding might have inadvertently maintained Anet25 under warmer nights in modern cultivars. This most likely reflects the selection of genotypes with more efficient photosynthetic capacity (i.e. greater return per protein investment) under warm nights. It is also likely that modern cultivars exhibit reduced respiratory demand for maintenance of processes such as Rubisco protein turnover and synthesis. Our findings highlight trait-based targets for enhancing energy efficiency and climate resilience in wheat and opportunities to improve the parameterisation of Rdark in Earth system models.