The Uptake Of Amino Acids And The Effects Of PH And Cd Stress On Plants Absorption

Plants uptake soluble organic nitrogen(ON)to partially fulfil their growth and development demands.Small ON compounds,including amino acids and oligopeptides,are abundant in soils,with many studies demonstrating that both non-mycorrhizal and mycorrhizal plant species can use such compounds.The use of dual(13C or 14C,15N)labelled amino acids injected into the soil has clearly demonstrated the ability of plants to exploit this potential ON source in the field,providing evidence that plants can circumvent the need to rely on the microbial production of inorganic N.Molecular mechanisms have also been identified that plants can use amino acids present in the soil,showing that ON may be a potentially important N source for plant N nutrition in some situations.However,the effects of long-term fertilization and other environmental factors on the uptake of amino acids by plants and soil microorganisms have not been well documented.The wheat-rice cropping system is one of the main cropping systems for cereal food production in East and South Asia,However,the effect of long-term fertilization of inorganic N and organic manure-inorganic fertilizers on the relative uptake of amino acids by wheat and soil microorganisms has not been documented.Therefore,we conducted this in-situ ON uptake test to study the effect of fertilizer regime on the competition of ON between plant and microorganisms,and to determine which key groups of microorganisms are active in amino acids uptake under various fertilizer regimes.Further,we cultivated pak choi in a sterile environment,in order to prevent the sources of N being transformed by microorganisms.The objectives of further study were to determine whether amino acids are an important source of N for pak choi growth in a sterile environment,and the mechanism by which pH and Cd stress affects the uptake and metabolism of glycine.1.Fertilizer regime affects plant growth and soil microbial community composition,however,its impact on organic nitrogen(N)uptake by plants remains poorly understood.To address this,we undertook an in-situ,short-term uptake experiment based on 13C,15N labelling,and 13C-PLFA analysis at two long-term(6 year)fertilizer trial sites(Jintan and Changshu).Each site had five treatments:a control without fertilizer,NPK fertilizer,50%NPK fertilizer + 6 t/ha pig manure,100%NPK fertilizer + cereal straw,and 50%NPK fertilizer + 6 t/ha pig manure and cereal straw.Overall,we found that 6-21%and 6-11%of the added 13C-15N-glycine was taken up intact by wheat,while 18-35%and 8-20%were captured by soil microorganisms in Jintan and Changshu locations,respectively.These results indicate that wheat has an appreciable capacity to utilize organic N,even in fertile agricultural soils.Organic N uptake by wheat correlated positively with ammonium and nitrate soil contents,indicating that inorganic N may enhance organic N capture by increasing plant biomass.The 13C:15N ratio in the microbial biomass showed that 32-71%and 13-71%of the 15N was absorbed through a direct uptake route in Jintan and Changshu soils.Chemical fertilizer reduced microbial activity and increased the proportion of intact glycine uptake by wheat.Gram-positive bacteria accounted for 18-23%,and 13-15%of the total 13C labelled PLFA in Jintan and Changshu,respectively,while Gram-negative bacteria accounted for 43-48%and 66-72%indicating that they are the dominant competitors with plants for soil nutrients.Total 15N uptake by wheat and microorganisms was highest in the 50%NPK fertilizer + pig manure and cereal straw treatment at both sites,indicating that it represents the best fertilizer practice for sustainable food production,as it not only reduced chemical fertilizer application,improved wheat growth and microbial biomass,but also increased the utilization of soil organic N.2.Although plants can utilize amino acids as a N source,it is unclear how pH affects the uptake of amino acids.We investigated the relative uptake of glycine,nitrate,and ammonium by pak choi at different pH levels using specific substrate 15N-labelling in a sterile environment.Further,the mechanisms underlying the influence of pH on glycine uptake and metabolism were examined by 15N-labelling short-term uptake and 15N-gas chromatographyrlass spectrometry analysis.The optimal pH for glycine uptake was 7.0,for both single and mixed nitrogen sources,whereas that for pak choi growth(mixed N sources)was 6.2.Glycine served as an important N source in the sterile environment,accounting for 19.0%-33.1%of N in the mixed N sources,and its contribution peaked at pH 7.0.Little 15N-labelled glycine was detected in shoots,indicating that the majority of the glycine absorbed by the roots was metabolized to other amino acids.The limiting step for N contribution by glycine was root uptake at lower pH levels,particularly active uptake,and root metabolism of glycine to serine at pH 8.0.These results indicate that pak choi can efficiently utilize glycine,and that pH has a substantial effect on glycine uptake and metabolism.3.Organic nitrogen can play an important role in plant growth,and soil pH changed greatly due to the over-use of chemical fertilizers,but the effects of soil pH on the competitive uptake of amino acids by plants and rhizosphere microorganisms are lack of detailed research.To study the effects of soil pH on the uptake of amino acids by maize and soil microorganisms,two soils from Hangzhou and Tieling were selected,and the soil pH was changed by the electrokinesis.Then,the 15N-labeled glycine was injected to the centrifuge tube with a short-term uptake of 4 h.Soil pH had a significant effect on the shoots and roots biomass,and the optimal pH for maize shoots growth was 6.48 for Hangzhou red soil,while it was 7.65 for Tieling brown soil.For Hangzhou soil,the 15N abundance of maize shoots under pH=6.48 was significantly higher than other treatments,and the uptake amount of 15N-glycine was much higher.However,the 15N abundance of maize shoots and roots under pH=7.65 Tieding soil was significantly lower than it under pH=5.78,but the uptake alount of 15N-glycine under pH=7.65 was much higher.The microbial biomass C was much higher under pH=6.48 Hangzhou soil,while it was much lower in pH=7.65 Tieling soil.According to the results of root uptake,root to shoot transportation,and the competition with microorganisms,we suggested that although facing the fierce competition with microorganisms,the maize grown in pH=6.48 Hangzhou soil increased the uptake of glycine by increasing its root uptake and root to shoot transportation.While under pH=7.65 Tieling soil,the activity of microorganisms was decreased,which decreased the competition with maize for glycine,and increased the uptake of glycine by maize.4.Long-term application of organic fertilizers not only increases soil fertility but also increases the risk of soil heavy metal pollution.Exposure of cultivated plants to cadmium(Cd)can affect their growth and safety as food sources.Nitrogen 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.