Determining the fluxes of Tl+ and K+ at the root surface of wheat and canola using Tl(I) and K ion-selective microelectrodes

The objectives of this study were to develop and evaluate a Tl+ ion-selective microelectrode (ISME) and to determine the basis for observed differences in Tl accumulation by durum wheat (Triticum turgidum L. var ‘Kyle’) and spring canola (Brassica napus L. cv ‘Hyola 401’). Seedlings were grown hydroponically and fluxes of K+ and Tl+ were measured at the root surface in solutions containing 5 μM Tl+ or 3 mM K+. After testing two different Tl(I) ionophores, a functional Tl+ ISME was developed from calix[4]arene tetra-n-propyl ether which had a detection limit of 2.5 µM and a slope of 56.6 mV/dec. Measurements of Tl+ flux indicate that Tl+ efflux occurred within 300–500 µm of the root tip, and influx farther from the root tip. Compared with canola, wheat had a slightly larger region of efflux and a smaller region of maximal influx, resulting in flux per root branch that was 2.3 to 4 times greater in canola than in wheat. The magnitude and pattern of K+ fluxes by the two species were more similar. Our results indicate that observed differences in Tl accumulation by wheat and canola are due both to differences in the magnitude of Tl flux per root branch of these species, and to differences in root morphology resulting in more root tips in canola than in wheat roots.