15.2.2 Solute relations of guard cells

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Stomata open due to an increase in guard cell volume driven by an influx of water which in turn results from a decrease in guard cell water potential. Solute accumulation, especially of potassium chloride and malate, is principally responsible.

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Table 15.2 Changes in ionic concentrations, pH and membrane potential for epidermal cells, subsidiary cells and guard cells. O = open stomata; C = closed stomata

One key event in stomatal opening is activation of an electrogenic H+-ATPase in the plasma membrane of guard cells (see Section 4.1). Upon activation, protons are extruded across the plasma membrane. Consequently the electrical potential across the membrane increases, with the inside becoming more negative, thereby causing entry of potassium ions (K+ influx; Figure 4.11). Potassium-specific stains and microelectrodes, as well as X-ray microprobe analysis of epidermal peels, have shown that during opening, guard cell potassium content increases, while the potassium content of subsidiary cells declines. Subsidiary cells thus act as sites of storage of potassium (and chloride). Changes in the pH of guard cells and subsidiary cell also occur in accordance with the operation of a H+-pumping ATPase (Table 15.2).

K+ influx occurs through ion channels in the plasma membrane. There are several classes of such channels, including a voltage gated influx channel and voltage gated efflux channel (Section 4.1.3). Aluminium ions, which inhibit stomatal opening but not closure, only inhibit the influx channel. In contrast, barium inhibits the activity of both. Upon hyperpolarisation of the plasma membrane due to the proton pump, K+ influx occurs through the K+in channel.

K+ influx, associated with stomatal opening, is balanced by either anion influx and/or anion synthesis in guard cells, to balance the positive charge moving into the guard cell (other-wise the voltage across the membrane would be dissipated as the stoichiometry of H+/K+ exchange is not 1:1). Guard cells employ two means to accomplish charge balance. First, anion transporters in the plasma membrane allow both chloride and malate influx. This could offset some of the positive charge accumulating inside guard cells. A partial offset is also accomplished by protons that are pumped out of the guard cell by the H+ pump.

Second, malic acid (malate) is also synthesised within the guard cell, from starch, using phosphoenolpyruvate carboxylase (PEP carboxylase). This malate helps balance the positive charge accumulating due to K+ influx. Malate, accumulated in the vacuole, is also osmotically active and contributes to a drop in guard cell water potential, thereby drawing water into the guard cells. Turgor pressure rises, volume increases and stomata open. In addition, malic acid may also be a source of protons required by the H+-pumping ATPase. Although most species accumulate malate during stomatal opening, some, such as onion (Allium cepa), use chloride as the principal counter-anion to potassium.

Starch levels increase during stomatal closure, and decline during stomatal opening. This is consistent with a role for starch as a source of carbon skeletons for PEP synthesis and hence malate synthesis. During stomatal opening, when malate is accumulated, starch levels decline as the carbon is used in the synthesis of PEP.

 

 

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