Domestication reduces leaf water use efficiency associated with the abaxial stomatal anatomy in cotton.

Crop domestication appears to alter the adaxial and abaxial stomatal features for increasing growth rate and yield. However, its effect on leaf water use efficiency (WUE) has not been experimentally verified under domestication. In this study, we characterized stomatal anatomy and carbon isotope (δ  13C) in 32 wild and 36 domesticated genotypes of cotton grown under agricultural field. The results showed that domesticated genotypes possessed lower WUE as indicated by low or more negative δ  13C compared with wild genotypes. Higher maximum theoretical stomatal conductance (gsmax) was underpinned by more stomata rather than significantly enlarged stomata after domestication. Specifically, abaxial stomatal density was higher but no change in adaxial stomatal density following domestication, while both adaxial and abaxial stomatal size were greater due to larger guard cell without the contribution to stomatal pore size. However, there was a negative relationship between δ  13C and SS, especially the abaxial SS across wild and domesticated genotypes, because enlarged stomata resulted in a lower stomatal response rate to fluctuating canopy light, consequently excessive water loss. In conclusion, cotton domestication has triggered substantial variation in stomatal anatomy. WUE and drought tolerance in cotton can be improved by decreasing abaxial stomatal size for faster stomatal response and reducing unnecessary water loss in future cotton breeding.