Effects of Calcium and Abscisic Acid on Volume Changes of Guard Cell Protoplasts ofCommelina

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Abstract
Calcium ions contracted guard cell protoplasts (GCP) of Commelina communis L., being particularly effective within the concentration range of 0 to 0.2 mol m−3. Abscisic acid (ABA) in the presence of EGTA, which chelates free Ca2+ in the medium, contracted GCP to a similar extent to Ca2+ alone or Ca2+ and ABA together. Similarly, ABA in the absence of free Ca2+ (i.e. an ABA/EGTA treatment) inhibited K+-induced swelling of contracted GCP, as did Ca2+ alone or ABA and Ca2+ together. Lanthanum, a Ca2+ channel blocker, prevented the contraction of GCP by Ca2+ but had no effect if ABA was also present with Ca2+. The inhibition of swelling of GCP by Ca2+ was also prevented by the presence of lanthanum or verapamil (another Ca2+ channel blocker). These results indicate that Ca2+ and ABA can act independently of each other in contracting swollen GCP and in preventing K+-induced swelling of contracted GCP of C. communis. If swelling and contraction of GCP are equivalent to stomatal opening and closure, respectively, the results do not support the hypothesis that ABA opens Ca2+ channels in the plasma membrane of guard cells allowing Ca2+ to enter the cells and, as a second messenger, to set in motion closing processes.