Uptake and transport characteristics of soluble organic and inorganic nitrogen by tea plant

Objectives Whether tea plants in subtropics could directly take up soluble molecular organic nitrogen from soil or not does not have agreeable conclusions. Studying the transport charecteristics of soluble organic and inorganic nitrogen in tea plants will help the deep understanding of the problem.

Methods The isotope tracing method was used in a pot experiment using tea seedlings (Huangjingui) as tested material. The isotope tracers were 2-¹³C-¹⁵N-glycine, (¹⁵NH₄)₂SO₄and ¹⁵N-KNO₃, which were injected into soil at the bottom of tea seedlings. Aboundance of ¹³C and ¹⁵N in shoots and roots were measured by isotope ratio mass spectrometer with a Eurovector Elemental Analyser.

Results The ratios of ¹³C to ¹⁵N excess in roots and whole seedlings of Huangjingui were much close to 1:1, the theroretical ratio of ¹³C to ¹⁵N aboundance, at 2 h and 6 h after the 2-¹³C-¹⁵N-glycine application in soil. The ¹³C excess was not detected in Huangjingui shoots at 2 h and 6 h,while it reached 0.284 μmol/(g, DW) in shoots at 72 h after the 2-¹³C-¹⁵N-glycine application. The ¹⁵N excesses of roots, shoots and whole seedlings of Huangjingui at 2 h, 6 h and 72 h after the (¹⁵NH₄)₂SO₄application were significantly higher than those after the ¹⁵N-KNO₃and 2-¹³C-¹⁵N-glycine application in soil. The shoot/root ratio in ¹⁵N excess at 6 h after the (¹⁵NH₄)₂SO₄application was 34.7% and 65.0% higher than those after the ¹⁵N-KNO₃and 2-¹³C-¹⁵N-glycine application, while the ratio at 72 h after the (¹⁵NH₄)₂SO₄application was 88.6% and 133.0% higher than those after the ¹⁵N-KNO₃and 2-¹³C-¹⁵N-glycine application, respectively, and the differences of the ratios between the (¹⁵NH₄)₂SO₄and ¹⁵N-KNO₃or 2-¹³C-¹⁵N-glycine treatments reached the 1% level.

Conclusions Tea plants of Huangjingui could directly take up intact glycine molecule from the soil, but preferred to uptake ammonium-N and nitrate-N. The glycine molecule taken up by Huangjingui could be transported from roots to shoots. The transport ability of different forms of nitrogen in Huangjingui plants showed following order:ammonium-N>nitrate-N>glycine-N. These results provided further evidence that uptake of soluble organic N by plants is a widespread adaptation strategy in terrestrial ecosystems.