There are lots of publications regarding the inducing effect of calcium in stomatal closure. In the past there was this believe that increase in cytosolic calcium concentration induced stomatal closure, later it was reported that oscillations in calcium concentrations causes stomatal closure. calcium ion has some osmotic effects, since for the opening of the stomata, concentration of some ions including K+, Cl- and in a lesser extent Na+ and NO3- increase in guard cells cytosol. Now my question is: Although calcium concentration in cytosol decreases during stomatal opening, but if we presume increase in calcium concentration, it can have osmotic effect to induce influx of water towards guard cells?
Sasan Aliniaeifard,
Stomatal Closing (Loss of guard cell turgor) is mainly mediated by ABA dependent and independent pathways. During this process ABA will induce guard cell cytosolic calcium calcium by two ways form the vacuolar membrane efflux and cell membrane influx. Increased calcium will induce Slow activating sustained S - anion channel and Rapid transient R- anion channel and membrane will get depolarize.
But Stomatal Opening (turgor gaining/swelling) is mainly mediated by H+ -ATP ase Ion transporters (efflux H+ ) during this time membrane will get hyper polarized and K+ will influx.
Now If you increase Calcium concentration in cytosol It may not effect on influx of water. However there are some unknown mechanisms still exists in co regulation between water channels (AQPs) and Ion channels as well.
Rajesh Vinnakota
There is no direct osmotic role of calcium! It is because Ca2+ cytoplasmic concentration must be kept at very low level due to its signalling role. Calcium ions however mediate transport of K+ and Cl–, the most abundant ions in plant cells. Therefore changes in K+ and Cl– concentrations have the significant osmotic effect.
(Hope I understood your question properly)
I can only suggest a reading of Enid MacRobbie review about the mechanisma of stomatal closure. Recently even low humidity appears to trigger an ABA mediated responseand K+ release. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692354/pdf/9800209.pdf