Uptake and uptake kinetics of nitrate, ammonium and glycine by pakchoi seedlings (Brassica Campestris L. ssp. Chinensis L. Makino)

Abstract

Plants not only absorb inorganic nitrogen (N), but also have the ability to absorb intact amino acids. Studies about pakchoi N uptake (Brassica Campestris L. ssp. Chinensis L. Makino) mainly focused on nitrate (NO3−) uptake and its accumulation in plant edible portion. This study investigated pakchoi seedlings uptake and uptake kinetics of NO3−, ammonium (NH4+) and glycine (Gly) using a mixed equimolar concentrations of the three N forms, and examined passive and active uptake ratios of the three N forms under sterile hydroponics.Our results revealed that pakchoi N uptake was positively related to its substrate N concentration (R=0.89–0.99, p<0.11), while N uptake efficiency showed a negative relation with the N concentration (R=−0.64–−0.77, p<0.36). At the N concentration from 25 to 1500μmolL−1, pakchoi took up significantly more NO3− than NH4+, but no significant difference was detected between uptakes of Gly and NO3− except at 250μmolL−1. At 7500μmolL−1, Gly uptake and uptake rate were significantly more than those for NO3− and NH4+. Regression analysis showed uptake rates for NO3−, NH4+ and Gly were well fitted to the Michaelis–Menten kinetics, and their affinity constant (Km) was in the range of 177–2000μmolL−1 and maximal velocity (Vmax) 18.2–46.8μmolg−1DW h−1. Pakchoi uptake for NO3−, NH4+ and Gly was dominated by active uptake. Gly active uptake at low concentration indicated plants have the ability to uptake amino acids that is relevant to field condition. However, passive uptake should not be overlooked in future studies, especially at high N concentration.