A possible evaporation site in the guard cell wall and the influence of leaf structure on the humidity response by stomata of woody plants.

Gas exchange measurements made with single leaves from Wych elm (Ulmus glabra Huds.), Lombardy poplar (Populus nigra 'Italica' L.) and common oak (Quercus robur L.) seedlings grown at high irradiance showed significant interspecific differences in the stomatal response to variation in atmospheric humidity. Elm and poplar seedlings showed low conductances at high vapour pressure differences (VPD), while the stomatal conductance of oak was little influenced by an increase in VPD between the leaf and the surrounding air. Mild water stress (leaf water potentials of approximately 0.4 MPa) reduced the sensitivity of stomata of elm to humidity and caused almost complete stomatal closure in poplar. Oak seedlings grown at low irradiance showed enhanced stomatal sensitivity to changes in VPD and comparatively high water use efficiencies.Significant reductions in stomatal conductance and transpiration of elm leaves at high VPD suggest that some water loss must occur directly from the external surfaces of the guard cell complex. At high VPD, this loss may maintain the guard cells in a flaccid condition and therefore the stomatal pore will remain closed, even though the plant may be relatively turgid. Differential staining of guard cell walls in light microscope sections of elm and oak leaves suggested an area of the wall that may be permeable to water and transmission electron micrographs revealed a corresponding cuticle-free area on the inner wall of the guard cells of elm leaves. S.E.M. and T.E.M. pictures and light micrographs form the basis of a hypothesis to explain the modifying influence of mild water stress and leaf structure on the response of elm and oak stomata to humidity.