Introductory Crop Physiology (1+1)

MECHANISM OF STOMATAL MOVEMENT

Transpiration (Continued)

The different mechanisms that create osmotic potential in the guard cells and control stomatal movements are,

a. Hydrolysis of starch into sugars in guard cells
b. Synthesis of sugars or organic acids in guard cells
c. The active pumping of K+ ions and, Cl- ions or organic acid counter ions into the guard cells

b. Synthesis of sugars or organic acids in guard cells

• During day light, photosynthesis occurs in guard cells as they contain chloroplast.
• The soluble sugars formed in this process may contribute in increasing the osmotic potential of guard cells and hence resulting in stomatal opening.
• However, very small amounts of soluble sugars (osmotically active) have been extracted from the guard cells which are insufficient to affect water potential.
• As a result of photosynthesis CO2 concentration in guard cells decreases which leads to increased pH.
• There may be some build up of organic acids, chiefly malic acid during this period in guard cells.
• The formation of malic acid would produce proton that could operate in an ATP-driven H+/K+ exchange pump moving protons into the adjacent epidermal cells and K+ ions into guard cells and thus may contribute in increasing the osmotic pressure (decreasing water potential) of the guard cells and leading to stomatal opening.
• The reverse process would occur in darkness.

c. ATP –Driven proton (H+) – K+ exchange pump mechanism in Guard cells

• According to this mechanism, there is accumulation of K+ ions in the guard cells during day light period.
• The protons (H+) are pumped out from the guard cells in to the adjacent epidermal cells and in exchange K+ ions are pumped in to the guard cells from epidermal cells.
• The exchange of H+ and K+ ions is mediated through ATP and thus it is an active process.
• ATP is generated in non-cyclic photo phosphorylation in photosynthesis in the guard cells.
• The ATP required in ion exchange process may also come through respiration.
• The accumulation of K+ ion along with Cl- ions and organic acid is sufficient enough to significantly decrease the water potential of guard cells during day light.
• Consequently, water enters into them from the adjacent epidermal and mesophyll cells thereby increasing their turgor pressure and opening the stomatal pore.
• Reverse situation prevails during dark when stomata are closed.
• There is no accumulation of K+ in guard cells in dark.

ABA Signaling Also Involves Ca2+-Independent Pathways

• Although an ABA-induced increase in cytosolic calcium concentration is a key feature of the current model for ABA-induced guard cell of stomat closure, ABA is able to induce stomatal closure even in guard cells that show no increase in cytosolic calcium (Allan et al. 1994). In other words, ABA seems to be able to act via one or more calcium-independent pathways.
  
• In addition to calcium, ABA can utilize cytosolic pH as a signaling intermediate. As previously discussed, a rise in cytosolic pH can lead to the activation of outward K+channels, and one effect of the abi1 mutation is to render these K+ channels insensitive to pH.
  
• Such redundancy in the signal transduction pathways explains how guard cells are able to integrate a wide range of hormonal and environmental stimuli that affect stomatal aperture, and such redundancy is probably not unique to guard cells.A simplified general model for ABA action in stomatal guard cells is shown in Figure For clarity, only the cell surface receptors are shown.

1. ABA binds to its receptors. 2. ABA-binding induces the formation of reactive oxygen species, which activate plasma membrane Ca2+ channels. 3. ABA increases the levels of cyclic ADP-ribose and IP3, which activate additional calcium channels on the tonoplast. 4. The influx of calcium initiates intracellular calcium oscillations and promotes the further release of calcium from vacuoles. 5. The rise in intracellular calcium blocks K+ in channels. 6. The rise in intracellular calcium promotes the opening if Cl–out (anion) channels on the plasma membrane, causing membrane depolarization. 7. The plasma membrane proton pump is inhibited by the ABA-induced increase in cytostolic calcium and a rise in intracellular pH, further depolarizing the membrane. 8. Membrane depolarization activates K+ out channels. 9. K+ and anions to be released across the plasma membrane are first released from vacuoles into the cytosol.

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3. Simple diffusion of water vapour from intercellular spaces to outer atmosphere through stomata.
The last step in the mechanism of transpiration is the simple diffusion of water vapours from the intercellular spaces to the atmosphere through open stomata. The intercellular spaces are more saturated with moisture on comparison to the outer atmosphere in the vicinity of stomata and the difference in the moisture content causes the diffusion of water vapour.