ROLE OF CALCIUM AND OTHER IONS IN DIRECTING ROOT HAIR TIP GROWTH IN LIMNOBIUM-STOLONIFERUM .1. INHIBITION OF TIP GROWTH BY ALUMINUM

The magnitude and spatial localization of Ca2(+), K+ and H+ fluxes in growing and non-growing Limnobium stoloniferum root hairs was determin ed using non-invasive, ion-selective vibrating microelectrodes. Both t he spatial pattern and magnitude of the ionic flux was dependent on th e particular ion in question. Both H+ and Ca2+ influx was localized al most exclusively to the tips of growing root hairs, suggesting that th ese 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 indicated that the magnitude of the Ca2+ flux ent ering the root hair tip did not determine growth rate; however, the pr esence of Ca2+ on the external face of the membrane was implicit for r oot hair integrity. Aluminum proved to be a potent inhibitor of root h air growth. At an exogenous Al concentration of 20 mu M a complete blo ckage 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 mu M), Ca2+ fluxes were unaffected wh ile inhibition of growth was still observed along with a distinct swel ling of the root hair tip. The swelling at the root hair tips was iden tical in appearance to that seen in the presence of microtubule inhibi tors, suggesting that Al could influence a number of different sites a t the plasma-membrane surface and within the cell. The possible role(s ) of Ca2+ and H+ fluxes in directing tip growth are discussed.