Midday water use efficiency in sorghum is linked to faster stomatal closure rate, lower stomatal aperture and higher stomatal density

Most studies assume midday gas exchange measurements capture the leaf's daytime performance. However, stomatal conductance (g(s)) and photosynthesis (A(n)) fluctuate diurnally due to endogenous and environmental rhythms, which can affect intrinsic water use efficiency (iWUE). Six Sorghum lines with contrasting stomatal anatomical traits were grown in environmentally controlled conditions, and leaf gas exchange was measured three times a day. Stomatal anatomy and kinetic responses to light transients were also measured. The highest A(n) and g(s) and the lowest iWUE were observed at midday for most lines. Diurnally averaged iWUE correlated positively with morning and midday iWUE and negatively with the time taken for stomata to close after transition to low light intensity (k(close)). There was significant variation among sorghum lines for k(close), and smaller k(close) correlated with lower g(s) and higher stomatal density (SD) across the lines. In turn, g(s) was negatively correlated with SD and regulated by the operational stomatal aperture regardless of stomatal size. Altogether, our data suggest a common physiology to improve iWUE in sorghum related to the control of water loss without impacting photosynthesis relying on higher SD, lower stomatal aperture and faster stomatal closing in response to low light intensity.

Automated summary

Most studies assume midday gas exchange measurements can reflect the leaf's daytime performance, but the fluctuation of stomatal conductance and photosynthesis due to diurnal rhythms can affect water use efficiency. This study on six Sorghum lines found that the highest photosynthesis and stomatal conductance and lowest water use efficiency were observed at midday. Water use efficiency correlated positively with morning and midday values and negatively with the time taken for stomata to close after transition to low light intensity.