Effects of cadmium stress on pakchoi (Brassica chinensis L.) growth and uptake of inorganic and organic nitrogenous compounds

Exposure of cultivated plants to cadmium (Cd) can affect their growth and safety as food sources. Nitrogen (N) is important in regulating both plant growth and Cd uptake, but little research has been conducted studying the effects of Cd stress on the uptake of different forms of nitrogen. In this study, we measured the relative rates of uptake of nitrate, ammonium, and glycine by pakchoi (Brassica chinensis L.) plants under Cd stress, using substrate-specific 15N-labelling in a sterilized environment. Cd stress significantly increased the proportional contribution of nitrate in comparison with controls, while decreasing the contribution of ammonium in shoots. Overall glycine uptake decreased under Cd stress, but the proportional contribution of glycine significantly increased in roots while decreasing in shoots. The mechanism of Cd stress on glycine uptake and metabolism was detected using 15N-labelling and 15N-gas chromatography mass spectrometry (GC-MS) analysis. In shoots, Cd stress inhibited the active uptake of 15N-glycine, while enhancing the passive uptake of 15N-glycine significantly. Additionally, the short-term total uptake of glycine in shoots and roots was slightly affected by Cd stress. In comparison to controls, 15N-labelled glycine was found to be significantly higher and serine lower in Cd-stressed plants, indicating that Cd stress inhibited the conversion of glycine to serine. We posit that Cd stress affects the overall nitrogen uptake in pakchoi plants, but that an inhibition in the metabolism of glycine to serine, rather than root uptake, is the limiting step for glycine contribution.