Domestication has reduced leaf water use efficiency associated with the anatomy of abaxial stomata in cotton

Cotton domestication has resulted in relatively more and larger abaxial stomata, and this is associated with higher maximum stomatal conductance and lower water use efficiency. Crop domestication for increasing growth rates and yields appears to have altered the features of adaxial and abaxial stomata, but its effect on leaf water use efficiency (WUE) have not been experimentally verified. In this study, we characterized stomatal anatomy and carbon isotope discrimination (delta C-13) in 32 wild and 36 domesticated genotypes of cotton grown under agricultural field conditions. The results showed that domesticated genotypes possessed lower WUE, as indicated by low or more negative delta C-13 compared with wild genotypes. Higher theoretical maximum stomatal conductance (g(smax)) after domestication was accounted for by more stomata rather than significantly enlarged stomata. Specifically, abaxial stomatal density was higher whilst there was no change in the adaxial density. The size of both adaxial and abaxial stomata was greater due to larger guard cells but without there being any increase in pore size. However, there was a negative relationship between delta C-13 and stomatal size across wild and domesticated genotypes, especially on the abaxial leaf surface, because bigger stomata resulted in a lower maximum stomatal response rate to fluctuating canopy light, resulting in increased water loss. Overall, our results indicate that cotton domestication has resulted in substantial variation in stomatal anatomy, and that WUE and drought tolerance can potentially be improved in future breeding by decreasing the size of abaxial stomata to produce a faster stomatal response and hence a reduction in unnecessary water loss.

Automated summary

Cotton domestication has increased the number and size of abaxial stomata, resulting in higher maximum stomatal conductance and lower water use efficiency. This study found that domesticated cotton genotypes have lower water use efficiency compared to wild genotypes. The increase in stomatal conductance after domestication is due to an increase in stomata, rather than enlarged stomata. However, there is a negative relationship between stomatal size and carbon isotope discrimination, especially on the abaxial leaf surface, indicating that bigger stomata lead to increased water loss. This study suggests that future breeding efforts could improve water use efficiency and drought tolerance by decreasing the size of abaxial stomata to increase stomatal response and reduce unnecessary water loss.