Characterization of ammonium and nitrate uptake and assimilation in roots of tea plants

It has been pointed out that tea (Camellia sinensis (L.) O. Kuntze) prefers ammonium (NH4+) over nitrate (NO3-) as an inorganic nitrogen (N) source.15N studies were conducted using hydroponically grown tea plants to clarify the characteristics of uptake and assimilation of NH4+and NO3-by tea roots. The total15N was detected, and kinetic parameters were calculated after feeding15NH4+or15NO3-to tea plants. The process of N assimilation was studied by monitoring the dynamic15N abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with15NH4+absorbed significantly more15N than those supplied with15NO3-. The kinetics of15NH4+and15NO3-influx into tea plants followed a classic biphasic pattern, demonstrating the action of a high affinity transport system (HATS) and a low affinity transport system (LATS). The Vmaxvalue for NH4+uptake was 54.5 nmol/(g dry wt min), which was higher than that observed for NO3-(39.3 nmol/(g dry wt min)). KMestimates were approximately 0.06 mM for NH4+and 0.16 mM for NO3-, indicating a higher rate of NH4+absorption by tea plant roots. Tea plants fed with15NH4+accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with15NO3-. Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with15N under both conditions. The rate of N assimilation into Thea in the roots of NO3--supplied tea plants was quicker than in NH4+-supplied tea plants. NO3-uptake by roots, rather than reduction or transport within the plant, seems to be the main factor limiting the growth of tea plants supplied with NO3-as the sole N source. The NH4+absorbed by tea plants directly, as well as that produced by NO3-reduction, was assimilated through the glutamine synthetase-glutamine oxoglutarate aminotransferase pathway in tea plant roots. The15N labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH4+. © 2013 Pleiades Publishing, Ltd.