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

Q₁₀ values for ¹³NO₃⁻ influx were determined in `uninduced' (NO<sub>3</sub><sup>−</sup>-starved) and `induced' (NO₃⁻-pretreated) roots of barley (Hordeum vulgare L.) plants at various concentrations of external NO₃⁻ ([NO₃⁻]₀). At 0.02 mole per cubic meter [NO₃⁻]₀, Q₁₀ values for influx were from 3 to 4 between 5 and 10°C. As [NO₃⁻]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 ¹³NO₃⁻ influx at low and high [NO₃⁻]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 [NO₃⁻]₀¹³NO₃⁻ influx was extremely sensitive to metabolic inhibition. By contrast, at high [NO₃⁻]₀, influx was relatively insensitive to the presence of inhibitors.