Geoscience Reference
In-Depth Information
Fig. 10.12
Cross section of a leaf, closed and open guard cells, and exchange of CO
2
and H
2
O by
diffusion within a stomatal cavity during photosynthesis
CO
2
entering stomatal cavities, photosynthesis comes to a halt. An increase of turgor
functions in an opposite way to open the stomata. The aperture of each stoma forms
a pathway for diffusive transfer of water vapor. However, the function of the guard
cells including stomatal opening and closing is not a simple process that reacts
merely to cell turgor and relative conditions of plant water stress. Sensors below
each stoma react to concentrations of CO
2
in the air, radiation, temperature, etc. For
example, the presence of toxic gases in the air causes stomatal closure. Potassium
ion concentration and proton transport across the membrane of guard cells play a
dominant role regarding the dynamics of opening and closing of stomatal
apertures.
The primary self-regulating mechanism of transpiration by a plant is realized
through stomata. With a gradual closing of stomata, the resistance against water
vapor fl ux increases and transpiration decreases. However, transpiration does not
