Stomatal behavior in response to drought has been the focus of intensive research, but less attention has been paid to stomatal density. In this study, 5-week-old maize seedlings were exposed to different soil water contents. Stomatal density and size as well as leaf gas exchange were investigated after 2-, 4- and 6-weeks of treatment, which corresponded to the jointing, trumpeting, and filling stages of maize development. Results showed that new stomata were generated continually during leaf growth. Reduced soil water content significantly stimulated stomatal generation, resulting in a significant increase in stomatal density but a decrease in stomatal size and aperture. Independent of soil water conditions, stomatal density and length in the trumpeting and filling stages were greater than in the jointing stage. Irrespective of growth stage, severe water deficit significantly reduced stomatal conductance (Gs), decreasing the leaf transpiration rate (Tr) and net photosynthetic rate (Pn). Stomatal density was significantly negatively correlated with both Pn and Tr but more strongly with Tr, so the leaf instantaneous water use efficiency (WUEi) correlated positively with stomatal density. In conclusion, drought led to a significant increase in stomatal density and a reduction in stomatal size and aperture, resulting in decreased Pn and Tr. Because the negative correlation of stomatal density to Tr was stronger than that to Pn, leaf WUEi tended to increase.
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