Abstract
Stomatal pore area is heterogeneous across leaf surfaces. This has been considered as “patchy stomatal conductance,” and may have substantial implications for photosynthetic efficiency. Aerosols have always been important elements of plant environments, but their effects on stomatal control of plant water relations, and stomatal heterogeneity specifically, have not been considered. Here we evaluate the spatial coordination of pore area in the glabrous and homobaric leaves of Vicia faba grown under two aerosol treatments and measured at four levels of VPD. We construct a large dataset (n > 88,000 discrete comparisons) of paired pore areas and distances between the pores. Plants were grown in ambient urban air and in filtered air (FA) to determine the effect of ambient aerosol on stomatal properties. Pore area exhibited spatial organization, as well as considerable variability among closely co-located pores. The difference between pore areas was positively correlated with the distance between the pores, in both aerosol treatments and at all VPDs. However, aerosol deposition reduced both the magnitude of variability between pores and the rate at which this variability increased with pore separation distance. These data support previous conclusions that deposition of hygroscopic aerosol may create a thin aqueous film across the leaf surface that connects neighboring stomata to each other and to the leaf interior. Aerosol impacts on stomatal heterogeneity and gas exchange are not adequately considered in current assessments of stomatal control.
Highlights
Heterogeneous stomatal opening across the surface of individual leaves may result in stomatal “patchiness,” random variation, or spatially coherent trends in pore area across the surface
We evaluate the distribution of pore areas, the local and larger scale heterogeneity of stomatal opening, and the effect of four levels of VPD and two levels of ambient aerosol on these characteristics
Plants were randomly assigned either to a greenhouse ventilated with ambient air (AA) or an adjacent greenhouse ventilated with filtered air (FA)
Summary
Heterogeneous stomatal opening across the surface of individual leaves may result in stomatal “patchiness,” random variation, or spatially coherent trends in pore area across the surface. Under conditions of high boundary layer conductance, stomatal heterogeneity may be detrimental to gas exchange efficiency, but under conditions of low wind or large leaves, which reduce boundary layer conductance, and under conditions of low overall stomatal conductance, heterogeneity may improve photosynthetic efficiency (Buckley et al, 1999). This reflects the effects of evaporative cooling of the leaf on fluxes and gradients of both water and CO2, and the transport efficiency of those pores that remain widely open. The significance and ubiquity of stomatal heterogeneity and its relationship with environmental conditions (Cheeseman, 1991; Gunasekera and Berkowitz, 1992; Cardon et al, 1994) as well as the role of aerosol deposition (Burkhardt and Grantz, 2017) in generating such heterogeneity have not been adequately considered
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