Fig. 1.5.4. Regulation of stomatal closure by ABA. A Immediately after application of ABA to isolated epidermal strips the concentration of free Ca2+ in the guard cells increases. Only 5 min later the stomates begin to close (after Taiz and Zeiger 2000). B Simplified scheme of stoma closure caused by ABA. ABA affects the ion relations in the guard cells in many ways, but mainly by increasing the intracellular pH (left part of figure) and by directly and indirectly increasing the intracellular concentration of free calcium (right part of figure). Many ion channels are regulated by Ca2+ or metabolites (gated channels), and others open or close in response to the membrane potential and are thus dependent on the pH gradient across the plasma membrane. Some reactions are controlled by both pH and the concentration of free Ca2+ (e.g. proton export pumps of the plasma membrane). Overall, there is a gross net export of K+ from the cell, followed by export of anions. Ion export leads to the shift of cellular water to the neighbouring cells, resulting in turgor loss of the guard cells and finally to closure of the stoma. (After Assman and Shimazaki 1999)
—Plasm a membrane
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