Characterization of ammonium and nitrate uptake and assimilation in roots of tea plants
Yang YY., Li XH., Ratcliffe RG., Ruan JY.
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.