A Critical Assessment of the Role of Abscisic Acid and Calcium Ions in Regulating Stomatal Movements in Commelina communis L.

De Silva, Daniel Lionel Rathnapala (1986) A Critical Assessment of the Role of Abscisic Acid and Calcium Ions in Regulating Stomatal Movements in Commelina communis L. PhD thesis, Lancaster University.

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Abstract

The inhibition of stomatal opening in Commelina communis L. by abscisic acid (ABA) appeared to be dependent on the availability of calcium ions. Synergism between calcium ions and ABA in suppressing stomatal opening was observed, and a factorial experiment revealed a highly significant interaction between calcium and abscisic acid. The effect of calcium appeared to be on the latter stages of the opening process, and might have been the result of an inhibition of potassium accumulation in the guard cells. The ABA-induced stomatal closure was barely detectable when the apoplastic calcium was chelated by EGTA. The divalent cation ionophore, A 23187, reduced stomatal opening presumably because it mimicked the effect of ABA in increasing the permeability of the plasma membrane of the guard cells to calcium. Agents such as La3+, nifedipine and verapamil, which in animal tissues are known to block calcium channels, and thus the free passage of calcium, reduced the ability of stomata to respond to ABA. It is proposed that increased calcium levels in the cytosol of the guard cells could activate the calcium-binding modulator protein, calmodulin, which in turn activates one or more of the enzymes which are responsible for reducing K+ accumulation in the guard cells: compounds which are known to antagonize calmodulin reduced the effects of abscisic acid in the same quantitative way as calcium channel blockers. It is suggested that the action of ABA on guard cells requires a free passage of calcium ions into the cytosol. The calcium ions may then act as 'second messengers' interacting with calmodulin to produce the overt cellular response to ABA. A study on the differential behaviour in opening between adaxial and abaxial epidermes revealed that different endogenous levels of ABA and/or calcium could be responsible for the observed disparity, and that EGTA and La3+ could reduce the normal differences in stomatal opening between the two epidermes. From the results of a preliminary study, it is concluded that electron probe X-ray microanalysis is not a suitable method for the measurement of intracellular calcium ion concentrations. The results are discussed in relation to the concept of hormones in plants, which has recently been attacked by a number of plant physiologists on the basis that there is no convincing evidence that plants have hormones which work in a comparable way to those in animals.