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

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Abstract
Transmission electron micrographs revealed that a substantial part of the guard cell wall of both Quercus robur L. and Populus nigra ‘italica’ L. was either free of cuticle or covered with a greatly reduced cuticular layer. In Quercus the structure of the guard cell was such that the area of limited cuticular development would be exposed to the evaporating power of the atmosphere even when the stomata were closed. Lanthanum staining confirmed that this area might be an important site of evaporation. A similar evaporation site was identified in the guard cell wall of Pinus sylvestris L. Light micrographs revealed that this area could also be exposed on the outside of the leaf when the stomata were closed. It appears that guard cell orientation with respect to the epidermal plane depends upon epidermal turgor. Changes in orientation of the guard cell coupled with the exact location of the cuticle-free area in the guard cell wall may explain the nature of the stomatal response of individual species to changing VPD and the effect of other variables, e.g. water deficit, on this response.