Stomatal Responses to Light and CO2

We have shown that stomata in isolated epidermes show very small responses to light and CO2.  Yet, when an isolated epidermis is placed back in contact with exposed mesophyll of a leaf of the same species or a different species, the stomata respond normally to light and CO2.  Water or silicone oil between the epidermis and the mesophyll closes the stomata immediately.  We suggest that there is a signal from the mesophyll to the guard cells that mediates most of the stomatal response to light and CO2.