Background and Aims This work has been conducted to assist theoretical modelling of the different stages of the blue light (BL)-induced phototropic signalling pathway and ion transport activity across plant membranes. Ion fluxes (Ca2+, H+, K+ and Cl-) in etiolated oat coleoptiles have been measured continuously before and during unilateral BL exposure.Methods Changes in ion fluxes at the illuminated (light) and shadowed (dark) sides of etiolated oat coleoptiles (Avena sativa) were studied using a non-invasive ion-selective microelectrode technique (MIFE). The bending response was also measured continuously, and correlations between the changes in various ion fluxes and bending response have been investigated. For each ion the difference (A) between the magnitudes of flux at the light and dark sides of the coleoptile was calculated.Key results Plants that demonstrated a phototropic bending response also demonstrated Call influx into the light side approximately 20 min after the start of BL exposure. This is regarded as part of the perception and transduction stages of the BL-induced signal cascade. The first 10 min of bending were associated with substantial influx of H+, K+ and Cl- into the light (concave) side of the coleoptiles.Conclusions The data suggest that Ca2+ participates in the signalling stage of the BL-induced phototropism, whereas the phototropic bending response is linked to changes in the transport of H+, K+ and Cl-. (C) 2004 Annals of Botany Company.