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

• Published in: Russian journal of plant physiology 2013, Vol.60 (1), p.91-99
• Main Authors: Yang, Y. Y, Li, X. H, Ratcliffe, R. G, Ruan, J. Y
• Format: Article
• Language: English
• Subjects: absorption; ammonium nitrate; Biomedical and Life Sciences; Camellia sinensis; free amino acids; glutamine; hydroponics; Life Sciences; monitoring; nitric oxide; nitrogen; Plant Physiology; Plant Sciences; Research Papers; roots; tea; theanine
• ISSN: 1021-4437; 1608-3407
• DOI: 10.1134/S1021443712060180

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 ⁺ .