Night-time warming in the field reduces nocturnal stomatal conductance and grain yield but does not alter daytime physiological responses

Global nocturnal temperatures are rising more rapidly than daytime temperatures and have a large effect on crop productivity. In particular, stomatal conductance at night (gsn) is surpris-ingly poorly understood and has not been investigated despite constituting a significant pro-portion of overall canopy water loss. Here, we present the results of 3 yr of field data using 12 spring Triticum aestivum geno-types which were grown in NW Mexico and subjected to an artificial increase in night-time temperatures of 2 degrees C. Under nocturnal heating, grain yields decreased (1.9% per 1 degrees C) without significant changes in daytime leaf-level physiological responses. Under warmer nights, there were significant dif\ferences in the magnitude and decrease in gsn, values of which were between 9 and 33% of daytime rates while respiration appeared to acclimate to higher temperatures. Decreases in grain yield were genotype-specific; genotypes categorised as heat tolerant demonstrated some of the greatest declines in yield in response to warmer nights. We conclude the essential components of nocturnal heat tolerance in wheat are uncoupled from resilience to daytime temperatures, raising fundamental questions for physiological breeding. Furthermore, this study discusses key physiological traits such as pollen viability, root depth and irrigation type may also play a role in genotype-specific nocturnal heat toler-ance.