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 ten Australian wheat cultivars released between 1901 and 2012 to warm nights under temperature-controlled environments. When exposed to warmer nights, rates of leaf net CO ₂ assimilation measured at 25 °C ( A _net ²⁵ ) remained stable across cultivar release date despite declines in photosynthetic capacity ( V _cmax and J ₁₅₀₀ ) in newer cultivars. In most cultivars leaf respiratory CO ₂ release in the dark ( R _dark ) exhibited divergent thermal responses: warm nights suppressed temperature-normalised night R _dark ( R _night ) but stimulated or maintained R _dark in the daytime ( R _day ). The results suggest that century-long, yield-focused selection may have inadvertently maintained A _net ²⁵ under warmer nights in modern cultivars through selection for more night-temperature sensitive but efficient photosynthetic capacity (i.e. greater return per protein investment) and overall 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 parameterization of R _dark to warm nights in crop and Earth system models.