Auxin stimulates Cl- uptake by oat coleoptiles

The effects of auxin on net ion fluxes near parenchyma of oat coleoptiles were studied using the non-invasive MIFE system to measure specific ion fluxes using ion selective microelectrodes. Application of 10 μM 1-naphthaleneacetic acid (NAA) for 3 h caused doubling of coleoptile segment growth, without changing the pH of the unbuffered bathing solution from pH 5.4 during that time. Short term experiments revealed that auxin led to an immediate three-fold increase of chloride influx to 1200 nmol m-2 s-1, maintained for at least 1 h. In the first minutes after auxin application, proton fluxes were small (-25 nmol m-2 s-1, an efflux) and tended to decrease, whereas potassium and calcium fluxes changed little, fluctuating from - 100 to 0 nmol m-2 s-1 and from - 15 to 0 nmol m-2 s-1, respectively. It is suggested that one target of auxin action in plant cells is the plasma membrane chloride transport system mediating increased chloride uptake.