Prof. Samih Tamimi Biology751 Gas Exchange Gateway Prof. Samih Tamimi Biology751

1. Definition Transpiration is the evaporation of water from the aerial parts of plants. Of all the water plant absorbs, over 95-99% is transpired to the air as water vapor.

4. From where water is transpired? Aerial parts of whole young plant Lenticels (lenticular transpiration) 0.1% Cutin (cuticular transpiration) 3%~10% Stomatum (stomatal transpiration) ~ 90%

Prof. Samih Tamimi Biology751 What is most likely leaving through the stomata of the leaf picture here? Water (H2O) What is this process called? Stomatal Transpiration Prof. Samih Tamimi Biology751 4

Why should symplasmic resistances be variable? Because water molecules cross cell membranes through a Nobel prize winning molecule : AQUAPORINS

Prof. Samih Tamimi Biology751 Stomatal transpiration Cuticle Prevents water loss Mesophyll Site of photosynthesis Stomata Guard cells Openings allow gases and water to move in and out of leaf Open and close the stomata Prof. Samih Tamimi Biology751

Understanding water pathways in leaves… Mesopyll cell wall stomatal chambers

Stomata regulate gas exchange: CO2 in, O2 and water out H2O H2O

Water loss Water loss is regulated by : Difference in water vapor concentration The pathway resistance

Transpiration rate (a flux) is a function of the water vapor diffusion gradient (the driving force) and stomatal aperture (the conductance of the pathway). Flux = driving force x conductance PP04100.jpg

Prof. Samih Tamimi Biology751 Water Pathway through the Leaf \figures\ch04\pp04100.jpg Prof. Samih Tamimi Biology751

The driving force of transpiration is the “vapor pressure gradient The driving force of transpiration is the “vapor pressure gradient.” This is the difference in vapor pressure between the internal spaces in the leaf and the atmosphere around the leaf Diffusional resistance comprises leaf resistance and boundary layer resistance

5. Characteristics of guard cells

Guard cell properties and their relationship with stomatal control Thickness of CW varies in the ventral and dorsal part of the guard cells. Contains chloroplast and can perform light reaction. (not dark reaction for the lack of key enzymes) Structurally isolated from epidermal cells for the lack of plasmodesmata (water and ions transmit only through cellular pathway, thus helps to build up water gradient) Little volume, little amount of water absorption or loss controls stomtal aperture.

Mechanical Features Required for Proper Stomatal Function 1) the cell walls of guard cells can allow large elastic deformations(30% volume increase). 2) thickening of the ventral wall surrounding the pore compared tothe dorsal wall (away from the pore) favors a bending mode of the cell. 3) radial re-inforcement of the cell wall of the guard cell by cellulose microfibrils favors elongation of the cell. . Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751

The mechanism of guard cell movement

Ion and organic molecules Guard cell ys Decrease (ys = -RTCs) yw decrease Water moves into the guard cell yp increase Stoma open

The guard cells control the opening and closing of the stomata Guard cells flaccid Guard cells turgid Thin outer wall Thick inner wall Stoma closed Stoma open

Regulating Stomatal Opening:-the potassium ion pump hypothesis Guard cells flaccid K+ K+ ions have the same concentration in guard cells and epidermal cells K+ K+ K+ K+ Light activates K+ pumps which actively transport K+ from the epidermal cells into the guard cells K+ K+ K+ K+ K+ K+ K+ Stoma closed

Regulating Stomatal Opening:-the potassium ion pump hypothesis H2O Increased concentration of K+ in guard cells K+ K+ Lowers the  in the guard cells K+ K+ K+ K+ K+ K+ K+ Water moves in by osmosis, down  gradient K+ K+ K+

Increased concentration of K+ in guard cells Guard cells turgid Increased concentration of K+ in guard cells H2O H2O K+ K+ Lowers the  in the guard cells K+ H2O H2O K+ K+ K+ K+ K+ K+ H2O H2O K+ Water moves in by osmosis, down  gradient K+ K+ Stoma open

The guard cells control the opening and closing of the stomata Guard cells flaccid Guard cells turgid Thin outer wall Thick inner wall Stoma closed Stoma open

Regulating Stomatal Opening:-the potassium ion pump hypothesis Guard cells flaccid K+ K+ ions have the same concentration in guard cells and epidermal cells K+ K+ K+ K+ Light activates K+ pumps which actively transport K+ from the epidermal cells into the guard cells K+ K+ K+ K+ K+ K+ K+ Stoma closed

Regulating Stomatal Opening:-the potassium ion pump hypothesis H2O Increased concentration of K+ in guard cells K+ K+ Lowers the  in the guard cells K+ K+ K+ K+ K+ K+ K+ Water moves in by osmosis, down  gradient K+ K+ K+

Increased concentration of K+ in guard cells Guard cells turgid Increased concentration of K+ in guard cells H2O H2O K+ K+ Lowers the  in the guard cells K+ H2O H2O K+ K+ K+ K+ K+ K+ H2O H2O K+ Water moves in by osmosis, down  gradient K+ K+ Stoma open

(1). Light Stomata of most plant open in the day and close at night, while CAM plants are just the opposite. Stomata opening are sensitive to red light and blue light, and blue light is more effective, it stimulates opening by a blue-light receptor: zeaxanthin.

24h Cycle of Stomatal Opening and Closing 12.00 Why is this cycle an advantage to most plants? 09.00 15.00 18.00 06.00 21.00 3.00 24.00

Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751 But blue light activates Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751

Guard cell protoplasts Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751

Activation of H+ pump by blue light Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751 Role of the proton-pumping ATPase in the regulation of stomatal movement PP5e-Fig-18-15-0.jpg Prof. Samih Tamimi Biology751

Phosphorylation of threonin (Thr950) on C terminal is induced by blue radiation. For activation of H+-ATPase is necessary also binding of protein 14-3-3.

Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751 Summary Prof. Samih Tamimi Biology751

Ion Transport—stomatal opening Proton extrusion makes membrane potential more negative Water influx Potassium uptake. Membrane hyperpolarization activate the Kin channel Inside cell Membrane Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751 Stomatal swelling E. Water influx increases pressure, but water is incompressible, so guard-cell volume increases. The increase results from stretching of the dorsal wall. Prof. Samih Tamimi Biology751

Prof. Samih Tamimi Biology751 Stomatal closing Membrane Inside cell Potassium efflux. Mechanism: Membrane depolarization activates the Kout channel Prof. Samih Tamimi Biology751

Air boundary layer A thin film of still air on the surface of leaf and its resistance to water vapor diffusion is proportional to its thickness. The thickness of the boundary layer is determined primarily by wind speed.

When air surrounding the leaf is still, increases in stomatal aperture have little effect on transpiration rate. The thickness of the boundary layer is the primary deterrent to water vapor loss from the leaf. When wind velocity is high, the stomatal resistance has the largest amount of control over water loss.

Boundary layer resistance Thickness of the layer is determined by wind speed. Still air – layer may be so thick that water is effectively stopped from leaving the leaf Windy conditions – moving air reduces the thickness of the boundary layer at the leaf surface The size and shape of leaves influence air flow – but the stomata itself play the most critical role leaf transpiration Prof. Samih Tamimi Biology751

Osmotic potential decrease Water potential decrease ADP + Pi Osmotic potential decrease Water potential decrease Water enter into vacuole Increase turgor Stomata open

Mechanism for stomatal close Uptake of Ca+2 into the cytosol Depolarize the membranes Anion channel opened and Cl- and malate released from the vacuole. K+ channel opened and K+ released from vacuole and subsequently into subsidiary cells.

Osmotic potential increase Water potential increase ADP + Pi Ca+2 Ca+2 Osmotic potential increase Water potential increase Water comes out from vacuole decrease turgor Stomata close Ca+2

6. Mechanism of stomatal opening ----K+ absorption theory H+-ATPase in PM is light activated Its function is out-pumping H+ HCO3-+PEP Mal PM Mal－ 　＋H+ light Inward rectifier K+ channel is voltage dependent, PM hyperpolarization activates the channel and carry K+ inward H＋ H＋ V K＋ K＋ H＋ H＋ Cl- is transported through Cl- /H+ symport or Cl-/OH-antiport Cl- Cl-

7. Factors influencing stomatal aperture Light Temp. CO2 Water content Plant hormone

Main channels of ion transport in guard cells plasmalemma , tonoplast K+in, K+out, Ca2+in from Schroeder et al. Annu. Rev. Plant Physiol. Plant mol. Biol. 2001.

Blue radiation receptor can be zeaxanthin in guard cell chloroplasts Blue radiation receptor can be zeaxanthin in guard cell chloroplasts. After its excitation, the signal is tranferred into cytoplasm where serin/threonin kinase is activated. This kinase activates ATPase and stimulate transport of H+  K+in. Zeaxanthin is formed by deepoxidation of violaxanthin (xanthophyl cycle). Zeaxanthin formation is dependent on electron transport rate and consumption of ATP and NADPH during CO2 fixation. Thus concentration of CO2 can affect response of stomata to irradiance (Zeiger, E. Trends Plant Sci. 5: 183, 2000)

Guard-Cell Function.