Since water spontaneously ionizes, protons cannot be removed from the medium: their free concentration in cells must be regulated through actively controlling W-related transport across membranes, by active and passive buffering, and by setting a certain pH within the metabolic network. Whereas these are the basic tools that provide effective H+ homeostasis, cellular compartmentation serves as an intermediate store into which protons can be shifted temporarily and from which protons can be regained when required. on the other hand, intracellular compartments can also serve as a final proton sink. pH regulation is not confined to intracellular spaces, but also comprises the apoplast. Whereas the pH of the cytosol is kept slightly alkaline at 7.2 to 7.5, with an average buffer capacity of 20 to 80 mM H+ per pH unit, the apoplastic pH may vary among tissues but is always acidic, with values between pH 5 and 6 and with a buffer capacity in the lower millimolar range per pH unit. pH can be a signal and/or a messenger, a distinction not always clearly made. Here, "signal" should be understood as information about an ongoing or preceding process, whereas "messenger" would be the carrying of certain information that will lead to a change of state. As such, pH would signal light intensity changes, drought, lack of oxygen and the presence of symbiotic partners or microbial attackers. On the other hand, pH would be a messenger in situations where pH changes are preconditions for certain processes, e.g., the gravity response or for activation of certain transporters in stomatal movements, and possibly for growth. The function of pH as a cellular messenger raises the question of whether pH should be understood as a "second messenger" in the way this is done for Ca2+. In an effort to give a comprehensive answer to this problem, the different roles of Ca2+ and H* in cellular signalling are discussed and a number of Ca2+/pH interactions are presented.
