The Physiological Mechanism Of Nitrate-Nitrogen Uptake And Metabolism Of Pakchoi As Affected By Glycine

Growth and development of green leafy vegetables are mainly dependent on the supply of nitrogen?N?.N is found in a variety of inorganic and organic forms in soil.Nitrate?NO₃^-?-N has been traditionally viewed as the main form of inorganic N taken up by green leafy vegetables.In addition,organic N in the form of amino acids can also be an important source of plant N.Furthermore,in organic cropping systems that rely on recycling and decomposition of organic N sources,amino acids may represent an important plant-available N pool.More attention has often been focused on the effects of inorganic N or single organic N source on the growth and development of green leafy vegetables.However,the plant amino acid-N nutrition and the effects of organic N in the form of amino acids on growth,NO₃^--N uptake and metabolism are still not clear when amino acid-N and NO₃^--N are supplied together.In this study,we first selected the suited ones to N environment under organic management system from 16 pakchoi?Brassica campestris ssp.Chinensis L.?cultivars according to their yields and N related indexes.Secondly,a sterile hydroponic or an agar plate system were used to investigate the effects of exogenous glycine?Gly?on pakchoi growth,root morphology,NO₃^--N uptake,N metabolism,N related enzyme activity and ethylene production under the synergetic application of Gly and NO₃^--N.The main results were as follows:1 Cultivar differences in nitrogen uptake by pakchoi under organic and conventional management systemsYields under organic management system?ORG?were significantly lower than those under conventional management system?CON?,whereas the NUpE in organic pakchoi were significantly higher than those in conventional pakchoi.ORG produced pakchoi with lower nitrate,higher free amino acid and soluble protein contents.Moreover,all the traits were significantly influenced by the cultivar.In addition,significant management system×cultivar interactions for NUtE in fall and for soluble protein in winter were observed.Among cultivars,the yield was closely correlated with the NUpE under ORG and CON,except under ORG in fall.Cultivar ranking analysis using Spearman's rank correlation coefficient?R_S?showed a rank correlation between cultivar rankings for yield and NUpE in winter,while no rank correlation was observed in fall.The rankings of cultivars showed major changes between ORG and CON in fall.The hierarchical classification analysis of yield and NUpE showed that there were three patterns of cultivars under ORG and CON.‘Huawang'was defined as the cultivar with higher yield,NUpE and used in subsequent experiments.2 The effects of Gly on growth and root morphology of pakchoi treated with mixed N sources Addition of the increasing concentrations of Gly-N into the nutrient solution or agar plate with NO₃^--N significantly inhibited the root growth and elongation of pakchoi?‘Huawang'?.Under hydroponic conditions,exogenous Glysignificantly decreased the primary root length,total root length,root surface area,root volume,but significantly increased the root average and root/shoot ratio compared with seedlings treated with the single source of NO₃^--N.Under agar plate conditions,exogenous Gly significantly decreased the primary root length and lateral root length in a concentration-dependent manner compared with NO₃^--N as a single N source.However,the presence of low to medium concentrations of Gly??5 mM Gly?significantly increased the lateral root number compared to seedlings cultured in the absence of Gly.In comparison,higher concentration of NO₃^--N?>1 mM?significantly decreased the primary root length and lateral root number,while low concentration of NO₃^--N?<1 mM?significantly increased the lateral root number.Furthermore,the exposure to the concentration less than 0.2 mM NO₃^--N significantly increased the lateral root length.Compared with equimolar concentrations of NO₃^--N,Gly-N reduced the the elongated primary root length.These results suggest the effects of Gly on root length were not due to a change in the concentration of Gly-N available to the roots.3 Nitrate-N uptake by pakchoi in response to GlyWhen pakchoi seedlings were exposed to mixtures of Gly and NO₃^--N in hydroponic culture,Gly significantly decreased ¹⁵NO₃^--N excess,¹⁵NO₃^--N uptake and uptake rate,but significantly increased the number of root tips per unit root length,root activity and ¹⁵NO₃^--N uptake rate per unit root length.Moreover,¹⁵N-Gly was taken up into the seedlings.¹⁵N-Gly excess in the roots was significantly higher than that in the shoots.In comparison,there was no significant difference in ¹⁵NO₃^--N excess between roots and shoots.The fraction of ¹⁵N-Gly derived from N source accounted for 28.2-35.7%of shoot N,and 53.2-61.7%of root N.In contrast to absorbed NO₃^--N,which was mostly transported to the shoots,a larger proportion of absorbed Gly was retained in the roots.Additionally,exogenous Gly significantly increased the N content in the roots,C content in the seedlings,but significantly decreased the N content in the shoots and accumulation of C and N in the seedlings.4 The nitrate accumulation and nitrogen metabolism of pakchoi in response to exogenous GlyWhen pakchoi seedlings were exposed to mixtures of Gly and NO₃^--N in hydroponic culture,Gly significantly decreased the nitrate content,but significantly increased the free ammonium,free amino acid and soluble protein contents in the seedlings of pakchoi in a concentration-dependent manner.In addition,Gly?2.5 mM?significantly increased the amino acid,soluble protein contents,root glutamine synthetase?GS?,glutamate synthetase?GOGAT?,glutamate oxaloacetate transaminase?GOT?and glutamate-pyruvate transaminase?GPT?activity;but Gly did not significantly influenced the nitrate reductase?NR?and nitrite reductase?NiR?activity;the equimolar concentrations of NO₃^--N?2.5 mM?significantly increased nitrate content,root NR and GS activity in the shoots,decreased soluble sugar content in the roots.5 Effects of exogenous Gly on ethylene production in pakchoi roots and the role of ethylene in root length and nitrate-N uptake in response to exogenous GlyFor the pakchoi seedlings cultured in a mixture of NO₃^--N and Gly,Gly significantly increased root 1-aminocyclopropane-1-carboxylic acid synthase?ACS?,oxidase?ACO?activities and ethylene production.Moreover,ethylene biosynthesis precursor?ACC?deepened the inhibition of primary root length in response to NO₃^--N+Gly.However,application of optimal concentrations ethylene antagonists?AVG,CoCl₂ or AgNO₃?markedly reversed the inhibition of primary root elongation induced by Gly;0.5?M AVG and 10?M AgNO₃ had the most significant effect after1 and 2 d of treatment.Additionally,in NO₃^--N+Gly-treated plants,the ¹⁵NO₃^--N uptake rate was significantly increased,while the ¹⁵N-Gly uptake rate was significantly decreased by 0.5?M AVG or 10?M AgNO₃.In summary,organic N in the form of Gly can influence the root growth,N uptake and metabolism of pakchoi in the presence of NO₃^--N.The results suggested that Gly inhibited the root elongation,and the inhibitory effect was attributed to the specific N forms,rather than the total N concentration.When treated with mixed N sources,pakchoi seedlings took up in the form of nitrate-N and Gly.Furthermore,NO₃^--N uptake and nitrate content was reduced,whereas the free amino acid,soluble protein content was increased by application of Gly.The low accumulation of nitrate was related to the inhibition of root length and NO₃^--N uptake rather than promotion of NO₃^--N reduction in the tissues in response to exogenous Gly.Meanwhile,the absorbed Gly can contribute to plant N nutrition and could be transformed into other amino acids through deamination and transamination in the plants,which further promoted the accumulation of amino acids and increased ethylene production in the roots.The inhibition of root growth and reduction in NO₃^--N uptake induced by Gly was probably mediated by the increase of ethylene levels.The findings presented in this study help to enrich the theory of plant organic nitrogen nutrition that has been generated using inorganic N or single organic N source test systems,and give some advices on improvement of quality in organic farming.