Stomatal Function in Water Movement through PlantsHORT 301- Plant Physiology
September 8, 2008Taiz and Zeiger - Chapters 4 (p. 64-71), 18 (p.449-455) & 23 (p.603-609), Web Topics 4.4 & 18.1 and Web Essays 18.1 &
Transpiration occurs primarily through stomata Transpiration – evaporation of water from leaves and stemsStoma/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 CO2 – stimulus-induced changes in guard cell water potential (ψw) 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 CO2
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 CO2 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
4.14 Electron micrographs of stomata (Part 1)
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
4.14 Electron micrographs of stomata (Part 2)
Guard cell turgor and volume regulate stomatal aperture pore opening, and 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 turgor pressure (ψp) 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
4.16 Radial alignment of the cellulose microfibrils in guard and epidermal cells
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 CO2 uptake
Typically 1 g CO2 (fixed)/500 g H2O 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, CO2 and water deficit (drought)
Light – causes stomatal opening due mainly to blue light
Blue light induces osmotic adjustment by facilitating ion accumulation andbiosynthesis 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 CO2 uptake when light energy is available and reduces water loss at night when photosynthesis is not active
CO2 – higher internal leaf CO2 concentration causes reduction in stomatal aperture, this occurs particularly at night when CO2 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