Exogenous glycine inhibits root elongation and reduces nitrate-N uptake in pak choi (Brassica campestris ssp. Chinensis L.)

Nitrogen (N) supply, including NO(3)(-)-N and organic N in the form of amino acids can influence the morphological attributes of plants. For example, amino acids contribute to plant nutrition; however, the effects of exogenous amino acids on NO(3)(-)-N uptake and root morphology have received little attention. In this study, we evaluated the effects of exogenous glycine (Gly) on root growth and NO(3)(-)-N uptake in pak choi (Brassica campestris ssp. Chinensis L.). Addition of Gly to NO(3)(-)-N agar medium or hydroponic solution significantly decreased pak choi seedling root length; these effects of Gly on root morphology were not attributed to the proportion of N supply derived from Gly. When pak choi seedlings were exposed to mixtures of Gly and NO(3)(-)-N in hydroponic culture, Gly significantly reduced (15)NO(3)(-)-N uptake but significantly increased the number of root tips per unit root length, root activity and (15)NO(3)(-)-N uptake rate per unit root length. In addition, (15)N-Gly was taken up into the plants. In contrast to absorbed NO(3)(-)-N, which was mostly transported to the shoots, a larger proportion of absorbed Gly was retained in the roots. Exogenous Gly enhanced root 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and oxidase (ACO) activities and ethylene production. The ethylene antagonists aminoethoxyvinylglycine (0.5 μM AVG) and silver nitrate (10 μM AgNO(3)) partly reversed Gly-induced inhibition of primary root elongation on agar plates and increased the NO(3)(-)-N uptake rate under hydroponic conditions, indicating exogenous Gly exerts these effects at least partly by enhancing ethylene production in roots. These findings suggest Gly substantially affects root morphology and N uptake and provide new information on the specific responses elicited by organic N sources.