Nitrate Uptake into Barley (Hordeum vulgare) Plants

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Abstract
Evidence is presented that chlorate is an extremely good analog for nitrate during nitrate uptake by intact barley (H. vulgare cv. Fergus) roots. The depletion of ClO3- or NO3- from uptake media over 2-6 h by seedlings was 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 h, 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 min and 20 h, respectively. Data pooled from a number of different experiments were used to calculate kinetic constants (Km and Vmax) for 36ClO3- influx into barley roots at different external ClO2-/NO3- ratios, using short (10 min) 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 (0.05 mM). At higher concentrations, in the range of > 1 mM, similar interactions between these ions were evident.