The Structure and Orientation of Guard Cells in Plants Showing Stomatal Responses to Changing Vapour Pressure Difference

Transmission electron micrographs revealed that a substantialpart of the guard cell wall of both Quercus robur L. and Populusnigra ‘italica’ L. was either free of cuticle orcovered with a greatly reduced cuticular layer. In Quercus thestructure of the guard cell was such that the area of limitedcuticular development would be exposed to the evaporating powerof the atmosphere even when the stomata were closed. Lanthanumstaining confirmed that this area might be an important siteof evaporation. A similar evaporation site was identified inthe guard cell wall of Pinus sylvestris L. Light micrographsrevealed that this area could also be exposed on the outsideof the leaf when the stomata were closed. It appears that guardcell orientation with respect to the epidermal plane dependsupon epidermal turgor. Changes in orientation of the guard cellcoupled with the exact location of the cuticle-free area inthe guard cell wall may explain the nature of the stomatal responseof individual species to changing VPD and the effect of othervariables, e.g. water deficit, on this response. Quercus robur L, oak, Populus nigra L, poplar, stomata, guard cells, cuticle, evaporation, vapour pressure difference