Nitrate Uptake into Barley (Hordeum vulgare) Plants 1: A New Approach Using ³⁶ClO₃⁻ as an Analog for NO₃⁻

Evidence is presented that chlorate is an extremely good analog for nitrate during nitrate uptake by intact barley (Hordeum vulgare cv. Fergus) roots. The depletion of ClO₃⁻ or NO₃⁻ from uptake media over 2 to 6 hours by seedlings was found to be dependent on combined NO₃⁻ plus ClO₃⁻ concentrations, and total anion uptake was equivalent at different NO₃⁻/ClO₃⁻ ratios. After loading barley seedlings with ³⁶ClO₃⁻ for 6 hours, kinetic parameters were derived from the analysis of efflux of [³⁶Cl] chlorate into unlabeled solution. On the basis of this analysis, the half times for exchange for the cytoplasmic and vacuolar phases were 17 minutes and 20 hours, respectively.

Data pooled from a number of different experiments were used to calculate kinetic constants (K_m and V_max) for ³⁶ClO₃⁻ influx into barley roots at different external ClO₃⁻/NO₃⁻ ratios, using short (10 minutes) influx times. There appeared to be no discrimination by the root cells between ClO₃⁻ and NO₃⁻. Lineweaver-Burk analysis of the interaction between nitrate and chlorate were characteristic of competitive inhibition at low nitrate concentrations (0-0.5 mm). At higher concentrations, in the range of >1 mm, similar interactions between these ions were evident.