The role of the mesophyll in stomatal responses to light and CO2

Stomatal responses to light and CO2 were investigated using isolated epidermes of Tradescantia pallida, Vicia faba and Pisum sativum. Stomata in leaves of T. pallida and P. sativum responded to light and CO2, but those from V. faba did not. Stomata in isolated epidermes of all three species could be opened on KCl solutions, but they showed no response to light or CO2. However, when isolated epidermes of T. pallida and P. sativum were placed on an exposed mesophyll from a leaf of the same species or a different species, they regained responsiveness to light and CO2. Stomatal responses in these epidermes were similar to those in leaves in that they responded rapidly and reversibly to changes in light and CO2. Epidermes from V. faba did not respond to light or CO2 when placed on mesophyll from any of the three species. Experiments with single optic fibres suggest that stomata were being regulated via signals from the mesophyll produced in response to light and CO2 rather than being sensitized to light and CO2 by the mesophyll. The data suggest that most of the stomatal response to CO2 and light occurs in response to a signal generated by the mesophyll.