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. N-15 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 N-15 was detected, and kinetic parameters were calculated after feeding (NH)-N-15 (4) (+) or (NO)-N-15 (3) (-) to tea plants. The process of N assimilation was studied by monitoring the dynamic N-15 abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with (NH)-N-15 (4) (+) absorbed significantly more N-15 than those supplied with (NO)-N-15 (3) (-) . The kinetics of (NH)-N-15 (4) (+) and (NO)-N-15 (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)). K-M 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 (NH)-N-15 (4) (+) accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with (NO)-N-15 (3) (-) . Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with N-15 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 N-15 labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH (4) (+) .