An analysis of the physiological basis of commonality between diurnal patterns of$\mathrm{N}{\mathrm{H}}_{4}^{+}$,$\mathrm{N}{\mathrm{O}}_{3}^{-}$and K+uptake by Phleum pratense and Festuca pratensis

Commonality in diurnal variation in net uptake of $\mathrm{N}{\mathrm{H}}_{4}^{+}$, $\mathrm{N}{\mathrm{O}}_{3}^{-}$ and K+ by the grasses Phleum pratense L. cv. Bodin (timothy), and Festuca pratensis Huds. cv. Salten (fescue) was evaluated in flowing solution culture under a semi-natural light regime. Hourly uptake rates from constant 20 mmol m-3 concentrations of each ion were measured concurrently over 7 d, without physical disturbance. The light period was 11 h, natural light supplemented by constant artificial illumination, with a step-transition to a 13 h dark period. Uptake of all three ions showed a broadly similar pattern of diurnal variation, rates increasing during the light period and decreasing during the darkness to a minimum within ± 2 h of the end of the dark period. The amplitude was greatest during high-irradiance days, and both proportionately and absolutely greater for $\mathrm{N}{\mathrm{H}}_{4}^{+}$ (mean min:max uptake = 0.24) than $\mathrm{N}{\mathrm{O}}_{3}^{-}$ (min:max = 0.41) or K+ (min:max = 0.34). There were significant differences between $\mathrm{N}{\mathrm{H}}_{4}^{+}$ and the other two ions ($\mathrm{N}{\mathrm{O}}_{3}^{-}$ and K+) whose behaviour coincided in almost all respects, in timing of maximum and minimum rates, and acceleration in uptake during the light period. Preferential uptake of $\mathrm{N}{\mathrm{H}}_{4}^{+}$ over $\mathrm{N}{\mathrm{O}}_{3}^{-}$ increased sharply during the first half of the light period; in relative terms $\mathrm{N}{\mathrm{H}}_{4}^{+}$ uptake accelerated twice as fast as $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake. Fescue always absorbed more $\mathrm{N}{\mathrm{H}}_{4}^{+}$ than $\mathrm{N}{\mathrm{O}}_{3}^{-}$, but timothy showed a preference for $\mathrm{N}{\mathrm{O}}_{3}^{-}$ during part of the dark period. The results are interpreted in terms of the hypothesis that diurnally fluctuating 'sink-strength' for nutrients is the primary determinant of nutrient transport rates, although uptake may become temporarily uncoupled from nutrient demand during periods of physiological stress or perturbation, such as towards the end of the dark period, during which uptake rates are determined by factors (e.g. carbohydrate supply) other than current nutrient demand.