Wheat respiratory O2 consumption falls with night warming alongside greater respiratory CO₂loss and reduced biomass

Warming nights are correlated with declining wheat growth and yield. As a key determinant of plant biomass, respiration consumes O2 as it produces ATP and releases CO₂and is typically reduced under warming to maintain metabolic efficiency. We compared the response of respiratory O2 and CO₂flux to multiple night and day warming treatments in wheat leaves and roots, using one commercial (Mace) and one breeding cultivar grown in controlled environments. We also examined the effect of night warming and a day heatwave on the capacity of the ATP-uncoupled alternative oxidase (AOX) pathway. Under warm nights, plant biomass fell, respiratory CO₂release measured at a common temperature was unchanged (indicating higher rates of CO₂release at prevailing growth temperature), respiratory O2 consumption at a common temperature declined, and AOX pathway capacity increased. The uncoupling of CO₂and O2 exchange and enhanced AOX pathway capacity suggest a reduction in plant energy demand under warm nights (lower O2 consumption), alongside higher rates of CO₂release under prevailing growth temperature (due to a lack of down-regulation of respiratory CO₂release). Less efficient ATP synthesis, teamed with sustained CO₂flux, could thus be driving observed biomass declines under warm nights.