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 (NH 4 + ) over nitrate (NO 3 − ) as an inorganic nitrogen (N) source. 15N studies were conducted using hydroponically grown tea plants to clarify the characteristics of uptake and assimilation of NH 4 + and NO 3 − by tea roots. The total 15N was detected, and kinetic parameters were calculated after feeding 15NH 4 + or 15NO 3 − to tea plants. The process of N assimilation was studied by monitoring the dynamic 15N abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with 15NH 4 + absorbed significantly more 15N than those supplied with 15NO 3 − . The kinetics of 15NH 4 + and 15NO 3 − 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 V max value for NH 4 + uptake was 54.5 nmol/(g dry wt min), which was higher than that observed for NO 3 − (39.3 nmol/(g dry wt min)). KM estimates were approximately 0.06 mM for NH 4 + and 0.16 mM for NO 3 − , indicating a higher rate of NH 4 + absorption by tea plant roots. Tea plants fed with 15NH 4 + accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with 15NO 3 − . Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with 15N under both conditions. The rate of N assimilation into Thea in the roots of NO 3 − -supplied tea plants was quicker than in NH 4 + -supplied tea plants. NO 3 − 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 NO 3 − as the sole N source. The NH 4 + absorbed by tea plants directly, as well as that produced by NO 3 − reduction, was assimilated through the glutamine synthetase-glutamine oxoglutarate aminotransferase pathway in tea plant roots. The 15N labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH 4 + .