Studies of the Uptake of Nitrate in Barley: II. Energetics

Q10 values for ^{13}\text{NO}{}_{3}^{-}$ influx were determined in 'uninduced' (NO3 --starved) and 'induced' (NO3 --pretreated) roots of barley (Hordeum vulgare L.) plants at various concentrations of external NO3 - ($[\text{NO}_{3}{}^{-}]_{0}$). At 0.02 mole per cubic meter $[\text{NO}_{3}{}^{-}]_{0}$, Q10 values for influx were from 3 to 4 between 5 and 10°C. As $[\text{NO}_{3}{}^{-}]_{0}$ increased Q10 values decreased, reaching values of 1.2 and 2.0, respectively, at 20 moles per cubic meter in uninduced and induced plants. The metabolic dependence of ^{13}\text{NO}{}_{3}^{-}$ influx at low and high $[\text{NO}_{3}{}^{-}]_{0}$ (0.1 and 20.0 moles per cubic meter, respectively) in uninduced and induced plants was probed by the use of various inhibitors. These experiments confirmed the findings of the Q10 studies, demonstrating that at low $[\text{NO}_{3}{}^{-}]_{0}$ ^{13}\text{NO}{}_{3}_{-}$ influx was extremely sensitive to metabolic inhibition. By contrast, at high $[\text{NO}_{3}{}^{-}]_{0}$, influx was relatively insensitive to the presence of inhibitors.