Qingxu Ma

Light intensity affects the uptake and metabolism of glycine by pakchoi (Brassica chinensis L.)

The uptake of glycine by pakchoi (Brassica chinensis L.), when supplied as single N-source or in a mixture of glycine and inorganic N, was studied at different light intensities under sterile conditions. At the optimal intensity (414 μmol m−2 s−1) for plant growth, glycine, nitrate, and ammonium contributed 29.4%, 39.5%, and 31.1% shoot N, respectively, and light intensity altered the preferential absorption of N sources. The lower 15N-nitrate in root but higher in shoot and the higher 15N-glycine in root but lower in shoot suggested that most 15N-nitrate uptake by root transported to shoot rapidly, with the shoot being important for nitrate assimilation, and the N contribution of glycine was limited by post-uptake metabolism. The amount of glycine that was taken up by the plant was likely limited by root uptake at low light intensities and by the metabolism of ammonium produced by glycine at high light intensities. These results indicate that pakchoi has the ability to uptake a large quantity of glycine, but that uptake is strongly regulated by light intensity, with metabolism in the root inhibiting its N contribution.

Elevational Variation in Soil Amino Acid and Inorganic Nitrogen Concentrations in Taibai Mountain, China

Amino acids are important sources of soil organic nitrogen (N), which is essential for plant nutrition, but detailed information about which amino acids predominant and whether amino acid composition varies with elevation is lacking. In this study, we hypothesized that the concentrations of amino acids in soil would increase and their composition would vary along the elevational gradient of Taibai Mountain, as plant-derived organic matter accumulated and N mineralization and microbial immobilization of amino acids slowed with reduced soil temperature. Results showed that the concentrations of soil extractable total N, extractable organic N and amino acids significantly increased with elevation due to the accumulation of soil organic matter and the greater N content. Soil extractable organic N concentration was significantly greater than that of the extractable inorganic N (NO3−-N + NH4+-N). On average, soil adsorbed amino acid concentration was approximately 5-fold greater than that of the free amino acids, which indicates that adsorbed amino acids extracted with the strong salt solution likely represent a potential source for the replenishment of free amino acids. We found no appreciable evidence to suggest that amino acids with simple molecular structure were dominant at low elevations, whereas amino acids with high molecular weight and complex aromatic structure dominated the high elevations. Across the elevational gradient, the amino acid pool was dominated by alanine, aspartic acid, glycine, glutamic acid, histidine, serine and threonine. These seven amino acids accounted for approximately 68.9% of the total hydrolyzable amino acid pool. The proportions of isoleucine, tyrosine and methionine varied with elevation, while soil major amino acid composition (including alanine, arginine, aspartic acid, glycine, histidine, leucine, phenylalanine, serine, threonine and valine) did not vary appreciably with elevation (p>0.10). The compositional similarity of many amino acids across the elevational gradient suggests that soil amino acids likely originate from a common source or through similar biochemical processes.

Hexavalent chromium stress enhances the uptake of nitrate but reduces the uptake of ammonium and glycine in pak choi (Brassica chinensis L.)

Chromium (Cr) pollution affects plant growth and biochemical processes, so, the relative uptake of glycine, nitrate, and ammonium by pak choi (Brassica chinensis) seedlings in treatments with 0mgL-1 and 10mgL-1 Cr (VI) were detected by substrate-specific 15N-labelling in a sterile environment. The short-term uptake of 15N-labelled sources and 15N-enriched amino acids were detected by gas chromatography mass spectrometry to explore the mechanism by which Cr stress affects glycine uptake and metabolism, which showing that Cr stress hindered the uptake of ammonium and glycine but increased significantly the uptake of nitrate. Cr stress did not decrease the active or passive uptake of glycine, but it inhibited the conversion of glycine to serine in pak choi roots, indicating that the metabolism of glycine to serine in roots, rather than the root uptake, was the limiting step in glycine contribution to total N uptake in pak choi. Since Cr affects the relative uptake of different N sources, a feasible way to reduce Cr-induced stress is application of selective fertilization, in particular nitrate, in pak choi cultivation on Cr-polluted soil.

Nitric oxide synthase-mediated early nitric oxide-burst alleviates drought-induced oxidative damage in ammonium supplied-rice roots

Ammonium (NH4+) can enhance rice drought tolerance in comparison to nitrate (NO3-). The mechanism underpinning this relationship was investigated based on the time-dependent nitric oxide (NO) production and its protective role in oxidative stress of NH4+-/NO3--supplied rice under drought. An early burst of NO was induced by drought 3h after root NH4+ treatment but not after NO3- treatment. Root oxidative damage induced by drought was significantly higher in NO3- than in NH4+-treatment due to its reactive oxygen species accumulation. Inducing NO production by applying NO donor 3h after NO3- treatment alleviated the oxidative damage, while inhibiting the early NO burst increased root oxidative damage in NH4+ treatment. Application of nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) completely suppressed NO synthesis in roots 3h after NH4+ treatment and aggravated drought-induced oxidative damage, indicating the aggravation of oxidative damage might have resulted from changes in NOS-mediated early NO burst. Drought also increased root antioxidant enzymes activities, which were further induced by NO donor but repressed by NO scavenger and NOS inhibitor in NH4+-treated roots. Thus, the NOS-mediated early NO burst plays an important role in alleviating oxidative damage induced by drought by enhancing antioxidant defenses in NH4+-supplied rice roots. Highlight NOS-mediated early NO burst plays an important role in alleviating oxidative damage induced by water stress, by enhancing the antioxidant defenses in roots supplemented with NH4+

Elevated CO2 levels enhance the uptake and metabolism of organic nitrogen

The effects of elevated CO2 (eCO2 ) on the relative uptake of inorganic and organic nitrogen (N) are unclear. The uptake of different N sources by pak choi (Brassica chinensis L.) seedlings supplied with a mixture of nitrate, glycine and ammonium was studied using 15 N-labelling under ambient CO2 (aCO2 ) (350 ppm) or eCO2 (650 ppm) conditions. 15 N-labelled short-term uptake and 15 N-gas chromatography mass spectrometry (GC-MS) were applied to measure the effects of eCO2 on glycine uptake and metabolism. Elevated CO2 increased the shoot biomass by 36% over 15 days, but had little effect on root growth. Over the same period, the N concentrations of shoots and roots were decreased by 30 and 2%, respectively. Elevated CO2 enhanced the uptake and N contribution of glycine, which accounted for 38-44% and 21-40% of total N uptake in roots and shoots, respectively, while the uptake of nitrate and ammonium was reduced. The increased glycine uptake resulted from the enhanced active uptake and enhanced metabolism in the roots. We conclude that eCO2 may increase the uptake and contribution of organic N forms to total plant N nutrition. Our findings provide new insights into plant N regulation under eCO2 conditions.

Concentration and composition of soil amino compounds in major Chinese croplands

ABSTRACT The importance of organic nitrogen (N) in soil N cycle is difficult to assess because of uncertainties regarding its composition, sources, and bioavailability. The objective of this study was to characterise the distribution of soil inorganic N and amino compounds in major Chinese croplands. Results demonstrated that soil extractable N composition was dominated by inorganic N (), and the contents of different N forms widely varied across the 11 different soils. Free amino acid contents ranged from 0.26 to 1.03 mg N kg−1. In contrast, soil-adsorbed amino acids were approximately seven times greater than the free amino acids, which likely represent a potential important pool for free amino acids. Pearson analysis showed that soil free and -adsorbed amino acids were negatively related to soil pH (r = −.39 to −.81, p < .05), while extractable total N, , and hydrolysable amino acids showed the opposite trends. Irrespective of the significant differences in soil properties and environment, soil major amino acid composition was similar, with seven dominant amino acids: alanine, glycine, glutamic acid, aspartic acid, serine, threonine, and isoleucine. The similar composition suggests that the production and utilisation of amino acids likely rely on the same biochemistry or similar amino acid constituents.

Effects of ammonium application rate on uptake of soil adsorbed amino acids by rice

In recent years, excessive use of chemical nitrogen (N) fertilizers has resulted in the accumulation of excess ammonium (NH4+) in many agricultural soils. Though rice is known as an NH4+-tolerant species and can directly absorb soil intact amino acids, we still know considerably less about the role of high exogenous NH4+ content on rice uptake of soil amino acids. This experiment examined the effects of the exogenous NH4+ concentration on rice uptake of soil adsorbed glycine in two different soils under sterile culture. Our data showed that the sorption capacity of glycine was closely related to soils;’ physical and chemical properties, such as organic matter and cation exchange capacity. Rice biomass was significantly inhibited by the exogenous NH4+ content at different glycine adsorption concentrations. A three-way analysis of variance demonstrated that rice glycine uptake and glycine nutritional contribution were not related to its sorption capacity, but significantly related to its glycine:NH4+ concentration ratio. After 21-d sterile cultivation, the rice uptake of adsorbed glycine accounted for 8.8%;–22.6% of rice total N uptake, which indicates that soil adsorbed amino acids theoretically can serve as an important N source for plant growth in spite of a high NH4+ application rate. However, further studies are needed to investigate the extent to which this bioavailability is realized in the field using the 13C, 15N double labeling technology.概要目 的通过采用无菌土培培养方法, 阐明外源高铵态氮施用量与水稻幼苗生长、 土壤吸附态氨基酸吸收之间的关系。创新点借助无菌培养和 15N 同位素示踪方法, 揭示高铵态氮浓度条件下土壤吸附态氨基酸对水稻幼苗生长发育及其氮营养贡献的影响。方 法采集两种不同生态系统的土壤 A 和 B, 经 0.5 mol/L K2SO4 连续淋洗 5 次, 121 °C 灭菌 30 min, 15N-甘氨酸处理后, 根据甘氨酸吸附曲线 (图 1) 确定甘氨酸吸附饱和点和吸附半饱和点, 然后向土壤中添加一些不同浓度的铵态氮, 水稻幼苗无菌培养 21 天后, 用 MAT-271 质谱仪测定水稻幼苗氨基酸吸收量。结 论实验结果表明土壤甘氨酸吸附能力大小与土壤理化性质紧密相关, 有机质和阳离子交换量。 外源高铵态氮水平显著抑制水稻幼苗生长发育 (P<0.05;), 但甘氨酸吸收及其氮营养贡献与甘氨酸吸附能力大小无关, 而与土壤吸附态甘氨酸和铵态氮的浓度比值显著相关 (P<0.05;)。 经过 21 天的无菌培养, 土壤吸附态氨基酸对水稻的氮营养贡献率达 8.8%∼22.6%, 表明土壤吸附态氨基酸理论上可能作为植物的一种潜在重要营养氮源。