Stomatal Function in Water Movement through Plants HORT 301- Plant Physiology September 8, 2008 Taiz and Zeiger - Chapters 4 (p ), 18 (p ) & 23 (p ), Web Topics 4.4 & 18.1 and Web Essays 18.1 & 18.3 Transpiration occurs primarily through stomata Transpiration – evaporation of water from leaves and stems Stoma/stomate (singular) – microscopic pores in the epidermis of leaves and stems Guard cells regulate stomatal opening and closing, and transpiration – specialized cells in the epidermis that border the stomate and regulate opening and closing of the pore Guard cell turgor and volume regulate stomatal aperture, conductance of water and CO 2 – stimulus-induced changes in guard cell water potential (ψ w ) regulate guard cell function, balances need for photosynthesis with water use

Transpiration occurs primarily through stomata – pores are up to 20 μm (diameter) in the leaf epidermis, primarily on the bottom surface or abaxial side About 95% of plant water loss occurs through stomata Cuticle – multilayered structure of complex lipid molecules on the surface of leaf and stem epidermal cells that forms a barrier to water and CO 2 exchange Stomatal complex – pore surrounded by a pair of guard cells that control the aperture size

Sub-stomatal cavity – cavity subjacent to the pore composed of leaf mesophyll cell and other intercellular spaces Water vapor diffuses from and CO 2 diffuses into this cavity

Guard cell anatomy – two guard cells in the epidermis form the aperture of the stomate Guard cell types – two classes, dumbbell shaped (most grasses) and kidney shaped Dumbbell-shaped guard cells – each cell is bulbous at the end with heavily thickened walls along the pore Change in guard cell volume affects pore opening and closing, enlargement increases pore aperture and volume reduction decreases the aperture

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

There are numerous guard cells in a leaf, about 25% of epidermal cells

Guard cell turgor and volume regulate stomatal aperture pore opening, and conductance of water and CO 2 Pore opening – stimulus induced osmotic adjustment, uptake of ions (primarily) and synthesis of organic solutes Solute/osmotic potential (ψ s ) becomes more negative resulting in a more negative intracellular water potential (ψ w ) ψ w gradient (between apoplast and symplast) facilitates turgor pressure (ψ p ) and water uptake resulting in cell volume increase (cell size doubles)

Kidney-shaped cells expand with more flexibility where the cell walls are thinner causing a curvature that increases pore size Dumbbell-shaped cells expand at the ends, which increases pore size along the thickened cell walls Pore closing - turgor and volume reduction causes stomatal closure

Raven, Evert & Eichhorn 2005 Biology of Plants Turgor and volume change, and cellulose microfribril arrangement control stomatal opening and closing

Stomatal pore resistance changes caused by opening and closing are critical for balancing transpiration with CO 2 uptake Typically 1 g CO 2 (fixed)/500 g H 2 O transpired, water use efficiency 0.002% Why is transpiration necessary: Facilitates cooling Necessary for uptake of water and nutrients/essential organic molecules

Stomatal opening and closing (pore resistance) are regulated by different stimuli, including light, diurnal rhythm, CO 2 and water deficit (drought) Light – causes stomatal opening due mainly to blue light Blue light induces osmotic adjustment by facilitating ion accumulation and biosynthesis of organic solutes, more negative cellular osmotic potential (  s ) leading to increased guard cell turgor pressure (  p) and volume

Circadian rhythm - day (open)/night (closed) diurnal cycling is a temporal control mechanism Facilitates CO 2 uptake when light energy is available and reduces water loss at night when photosynthesis is not active CO 2 – higher internal leaf CO 2 concentration causes reduction in stomatal aperture, this occurs particularly at night when CO 2 is not fixed by photosynthesis

Water deficit (drought) – stomatal closure ABA is the signal that accumulates in response to water deficit and initiates guard processes that result in ion leakage, turgor loss and stomatal closure Maize plants