1. Stomatal Function in Water Movement through Plants
HORT 301- Plant Physiology
October 1, 2007
Taiz and Zeiger - Chapter 4 (p ), Web Topic 4.4
Transpiration occurs primarily through stomata – stoma/stomate (singular), cuticle/waxes over remainder of leaf (and stem) restrict direct water loss
Guard cells regulate transpiration through stomata – specialized cells in the epidermis that open and close the stomatal pore
Guard cell turgor and volume regulates stomatal conductance of water and CO2 – stimulus-induced changes in guard cell water potential (ψw) regulate guard cell function, balances need for photosynthesis with water use

2. Transpiration through stomata (~95%) – pores (0 to 20 μm diameter) in the leaf epidermis (primarily on the bottom surface, abaxial side)
Stomatal complex – pore surrounded by a pair of guard cells that control the aperture size
Pore opens to a sub-stomatal cavity (leaf mesophyll intercellular space), diffusion of CO2 from and water into the atmosphere

3. Guard cell anatomy – two principal guard cell types, dumbbell shaped and kidney shaped
Dumbbell-shaped guard cells (most grasses) – bulbous at the ends, heavily thickened walls are prominent along the pore, cell turgor and enlargement cause guard cell swelling and increase the pore size

4. Kidney shaped guard cells - dicots and non-grass monocots (most plants), elliptical contour with the pore in the center

5. There are numerous guard cells in a leaf

6. Cell wall alignment and differential thickness affect pore opening and closing
Kidney-shaped cells – thinner cell walls adjacent to and opposite of pore

7. Guard cell turgor and volume regulates pore opening (stomatal conductance of water and CO2)
Pore opening – stimulus induced osmotic adjustment, uptake of ions (primarily) and synthesis of organic solutes
Solute/osmotic potential (ψs) becomes more negative resulting in a more negative intracellular water potential (ψw)
ψw gradient (between apoplast and symplast) facilitates water uptake resulting in cell volume increase (cell size doubles) and positive hydrostatic pressure/pressure potential/turgor pressure (ψp)

8. Kidney-shaped cells expand with more flexibility where the cell walls are thinner causing a curvature that increase pore size
Dumbbell-shaped cells expand the ends which increases pore size along the thickened cell walls
Turgor and volume reduction reverses the processes, i.e. stomatal closure

9. Stomatal resistance (pore size reduction) - essential for balancing transpiration (water loss) with CO2 uptake, typically 1 g CO2 (fixed)/500 g H2O transpired (0.002%)
Transpiration facilitates cooling of plants
Stomatal aperture opening and closing (pore resistance) - controlled by light (blue light), CO2 concentration and water availability (via abscisic acid, ABA), Ca2+
Day (open)/night (closed) diurnal cycling - temporal control mechanism that facilitates CO2 uptake when light energy is available and reduces water loss at night when photosynthesis is not active