Ambient aerosol increases minimum leaf conductance and alters the aperture-flux relationship as stomata respond to vapor pressure deficit (VPD).

Aerosols are important components of the global plant environment, with beneficial and deleterious impacts. The direct effects of aerosol deposition on plant-water relationships remain poorly characterized but potentially important. Vicia faba was grown in ambient urban air and in the same air with aerosol excluded, in a moderately polluted environment using two exposure protocols. Simultaneous measurement of gas exchange and stomatal pore aperture was combined with leaf dehydration kinetics and microscopic evaluation of leaf wetness formation and aerosol deposition patterns. The ambient aerosol was shown to be hygroscopic. Aerosol exposure increased minimum leaf conductance, shown by dehydration kinetics, and nocturnal water vapor flux, shown by dark-adapted gas exchange. Aerosol exposure decreased stomatal apertures at each level of vapor pressure deficit (VPD) and increased stomatal conductance at comparable levels of aperture. Overall, these effects were modest, and largest when stomata were wide open. The uncoupling of conductance (flux-based) from aperture (directly measured microscopically) implies that aerosol-induced water loss is not fully under stomatal control. This reduces drought tolerance and may provide a mechanism by which deposited aerosol plays a direct role in stomatal response to VPD.