An analysis of the physiological basis of commonality between diurnal patterns of NH4+, NO3- and K+ uptake by Phleum pratense and Festuca pratensis

Commonality in diurnal variation in net uptake of NH4+, NO3- 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 NH4+ (mean min:max uptake=0.24) than NO3- (min:max=0.41) or K+ (min:max=0.34). There were significant differences between NH4+ and the other two ions (NO3- 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 NH4+ over NO3- increased sharply during the first half of the light period; in relative terms NH4+ uptake accelerated twice as fast as NO3- uptake. Fescue always absorbed more NH4+ than NO3-, but timothy showed a preference for NO3- 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.