Short-term pH regulation in plants

Cellular pH regulation consists of two features: (i) Long-term pH homeostasis, which ensures that all H⁺ or OH^? produced in excess is ultimately removed from the cell and which requires metabolic energy; (ii) short-term reactions of the cell(s) to sudden shifts in intracellular pH, in order to prevent acute disturbances of metabolism. Recent progress in measuring and understanding of mainly short-term cellular regulation is summarized, including cellular responses to pH loads that arise from different sources such as external pH, weak acids/bases, protonophores, metabolic inhibitors, H⁺/cotransport, light and phytohormones. Whereas the plasma membrane H⁺ pump and metabolic adjustments may serve both long- and short-term pH control, physico-chemical buffering and the translocation of H⁺ from and to cellular compartments render only time-limited capacity for the neutralization of pH loads and seem exhausted within minutes. In spite of the widespread opinion that, because of tight regulation, intracellular pH does not vary with time, there is good evidence for long-lasting pH changes in plant cells, i.e. after hormonal stimulation, light/dark changes or carboxylation during crassulacean acid metabolism (CAM). This emphasizes that cytoplasmic pH, besides being well regulated, is essential not only for the regulation of membrane transport but also as a cellular messenger.