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

Nitrogen (N) supply, including NO3--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 NO3--N uptake and root morphology have received little attention. In this study, we evaluated the effects of exogenous glycine (Gly) on root growth and NO3--N uptake in pak choi (Brassica campestris ssp. Chinensis L.). Addition of Gly to NO3--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 NO3--N in hydroponic culture, Gly significantly reduced 15NO3--N uptake but significantly increased the number of root tips per unit root length, root activity and 15NO3--N uptake rate per unit root length. In addition, 15N-Gly was taken up into the plants. In contrast to absorbed NO3--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 AgNO3) partly reversed Gly-induced inhibition of primary root elongation on agar plates and increased the NO3--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.

氮素（N）供应，包括硝酸态氮（NO3--N）与氨基酸形式的有机氮，均可调控植物的形态特征。氨基酸可为植物提供营养，但外源氨基酸对硝酸态氮吸收及根系形态的影响却鲜有关注。本研究以小白菜（Brassica campestris ssp. Chinensis L.）为材料，探究了外源甘氨酸（Gly）对其根系生长与硝酸态氮吸收的影响。在硝酸态氮琼脂培养基或水培液中添加甘氨酸，会显著降低小白菜幼苗的根长；但甘氨酸对根系形态的上述效应，并非由其贡献的氮素供给比例所致。水培条件下，当小白菜幼苗暴露于甘氨酸与硝酸态氮的混合营养液时，甘氨酸可显著抑制15N标记的硝酸态氮（15NO3--N）吸收，却显著提升了单位根长的根尖数、根系活力以及单位根长的硝酸态氮吸收速率。此外，植株可吸收15N标记的甘氨酸（15N-Gly）。与多数被转运至地上部的吸收态硝酸态氮不同，吸收的甘氨酸更多滞留于根系中。外源甘氨酸可增强根系的1-氨基环丙烷-1-羧酸合酶（ACS）与氧化酶（ACO）活性，并促进乙烯生成。乙烯拮抗剂氨基乙氧基乙烯甘氨酸（0.5 μM AVG）与硝酸银（10 μM AgNO3）可部分逆转甘氨酸在琼脂平板上诱导的主根伸长抑制，并在水培条件下提高硝酸态氮吸收速率，表明外源甘氨酸至少部分通过增强根系乙烯生成来发挥上述效应。本研究结果显示，甘氨酸可显著影响植物根系形态与氮素吸收，为有机氮源诱导的特异性响应提供了新的科学依据。