Stomatal behavior controls the balance between carbon fixation and water loss and therefore plays a central role in crop performance. Faster stomatal responses to a fluctuating environment can improve intrinsic water use efficiency and reduce the risk of transient water deficits. The speed of stomatal responses depends on guard cell size, and recent work has highlighted that plants have rapid stomatal closure presenting higher water use efficiency, which could be explained by plants grown at high nitrogen conditions contributing to higher stomatal density and smaller guard cell size.

In this study, we used wheat with diploid, tetraploid and hexaploid ploidy levels to investigate how stomatal responses to light fluctuations at the different stages of wheat growth under elevated CO2 and nitrogen fertilizer conditions. We measured the variation of guard cell size and density, showing that flag leaves with smaller guard cell size and higher density in diploid wheat increased the rapidity of stomatal responses than hexaploidy wheat.

Meanwhile, elevated CO2 and nitrogen fertilizer also improved the faster stomatal opening and closing and therefore enhanced the water use efficiency in wheat. However, the effects of elevated CO2 and nitrogen fertilizer on stomatal kinetics were diminished during the ripening stage compared to the flowering stage.

Overall, the findings indicate that stomatal rapidity in wheat is dependent on guard cell size, elevated CO2 and nitrogen fertilizer conditions, and suggest that diversity in the speed of stomatal changes could be a potential breeding target for optimizing the physiological responses of wheat to dynamic field conditions.