2.7 Measurement of total N content and15N-NO3 absorption
Total N content was measured as described by Makino et al. (1997). NO3 quantitation in plant tissue was measured with high performance liquid chromatography (Agilent Technologies, USA) (Patterson et al. , 2010).15N-NO3 uptake activity was determined on an isotope-ratio mass spectrometry system (Thermo Scientific, USA) and15N-NO3 influx rate was calculated based on the amount of 15N taken up per unit weight of the sample per unit time (Zhang et al. , 2021).