Stomatal Function in Water Movement through Plants HORT 301- Plant Physiology October 1, 2007 Taiz and Zeiger - Chapter 4 (p. 64-71), Web Topic 4.4 paul.m.hasegawa.1@purdue.edu 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

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

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

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

There are numerous guard cells in a leaf

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

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)

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

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