A critical review on methods to measure apoplastic pH in plants

Various attempts have been made to estimate or measure apoplastic pH over the last few decades. These approaches include pH indicators in agar or beads, measurement of the pH of apoplastic fluid, weak acid influx, ion-selective electrodes and optical probes. Each of these has its own applications and limitations, and has contributed to the understanding of the processes taking place in the apoplast in relation to pH changes. However, convincing methods allowing us to probe this cell compartment are still lacking and invite exploration. The distinction between apoplast components is also discussed to clarify the indiscriminate use of the term 'apoplastic pH'.