Role of calcium and other ions in directing root hair tip growth

The magnitude and spatial localization of Ca2+, K+ and H+ fluxes in growing and non-growing Limnobium stoloniferum root hairs was determined using non-invasive, ion-selective vibrating microelectrodes. Both the spatial pattern and magnitude of the ionic flux was dependent on the particular ion in question. Both H+ and Ca2+ influx was localized almost exclusively to the tips of growing root hairs, suggesting that these fluxes may be involved in directing growth. Influx of K+ showed no distinct localization and uptake appeared uniform along the length of the root hair. Competitive inhibition of Ca2+ influx using a range of Mg2+ concentrations in dicated that the magnitude of the Ca2 + flux entering the root hair tip did not determine growth rate; however, the presence of Ca2+ on the external face of the membrane was implicit for root hair integrity. Aluminum proved to be a potent inhibitor of root hair growth. At an exogenous Al concentration of 20 |iM a complete blockage of Ca2 + influx into root hair tips was observed, suggesting that Al blockage of Ca2 + influx could be involved in Al toxicity. However, at a lower Al concentration (2 (iM), Ca2 + fluxes were unaffected while inhibition of growth was still ob served along with a distinct swelling of the root hair tip. The swelling at the root hair tips was identical in appear ance to that seen in the presence of microtubule inhibitors, suggesting that Al could influence a number of different sites at the plasma-membrane surface and within the cell. The possible role(s) of Ca2+ and H+ fluxes in directing tip growth are discussed.