Use of an extracellular, ion-selective, vibrating microelectrode system for the quantification of K+, H+, and Ca2+ fluxes in maize roots and maize suspension cells

An ion-selective vibrating-microelectrode system, which was originally used to measure extracellular Ca²⁺ gradients generated by Ca²⁺ currents, was used to study K⁺, H⁺ and Ca²⁺ transport in intact maize (Zea mays L.) roots and individual maize suspension cells. Comparisons were made between the vibrating ion-selective microelectrode, and a technique using stationary ion-selective microelectrodes to measure ionic gradients in the unstirred layer at the surface of plant roots. The vibrating-microelectrode system was shown to be a major improvement over stationary ion-selective microelectrodes, in terms of sensitivity and temporal resolution. With the vibrating ion microelectrode, it was easy to monitor K⁺ influxes into maize roots in a background K⁺ concentration of 10 mM or more, while stationary K⁺ electrodes were limited to measurements in a background K⁺ concentration of 0.3 mM or less. Also, with this system it was possible to conduct a detailed study of root Ca²⁺ transport, which was previously not possible because of the small fluxes involved. For example, we were able to investigate the effect of the excision of maize roots on Ca²⁺ influx. When an intact maize root was excised from the seedling at a position 3 cm from the site of measurement of Ca²⁺ transport, a rapid fourfold stimulation of Ca²⁺ influx was observed followed by dramatic oscillations in Ca²⁺ flux, oscillating between Ca²⁺ influx and efflux. These results clearly demonstrate that wound or perturbation responses of plant organs involve transient alterations in Ca²⁺ transport, which had previously been inferred by demonstrations of touch-induced changes in cytoplasmic calcium. The sensitivity of this system allows for the measurement of ion fluxes in individual plant cells. Using vibrating K⁺ and H⁺electrodes, it was possible to measure H⁺efflux and both K⁺ influx and efflux in individual maize suspension cells under different conditions. The availability of this technique will greatly improve our ability to study ion transport at the cellular level, in intact plant tissues and organs, and in specialized cells, such as root hairs or guard cells.