Compositional changes of selected amino acids,organic acids,and soluble sugars in the xylem sap of N,P, or K‐deficient tomato plants

Xylem sap plays a major role in long‐distance transport of water, nutrients, and metabolites. However, there is little information on the behavior of metabolites in mineral‐deficient xylem sap. For this reason, the time‐dependent changes in selected metabolites (amino acids, organic acids, and soluble sugars) from tomato xylem sap in response to nitrogen (N), phosphorus (P), or potassium (K)‐deficient condition were investigated. Tomato plants (Solanum lycopersicum L.) were grown hydroponically in liquid culture under three different mineral regimes: N‐deficient [0.5 mM Ca(NO₃)₂ and 0.5 mM KNO₃], P‐deficient (0.05 mM KH₂PO₄), and K‐deficient (0.5 mM KNO₃), respectively. Xylem sap was collected at 10:00 am after 1, 5, 15, and 30 d, and the selected metabolites were analyzed with liquid chromatography. All N, P, or K deficiencies led to a substantial increase in metabolites in the xylem sap. The predominant amino acid in the xylem sap was glutamine and, interestingly, all mineral deficiencies resulted in a substantial amount of γ‐aminobutyric acid (GABA). Additionally, organic acids (citrate and malate) and soluble sugars were strongly increased in all mineral deficiencies, and, in particular, the level of shikimate was greatly affected by N deficiency. Based on these data, it is necessary to clearly elucidate an unknown event taking place in xylem loading in a variety of environmental impacts, and we are now studying to expand our knowledge on metabolic and proteomic responses using GC‐MS and LC‐MS.     

Stickstoff-Assimilation in der Wurzel und Transport von N-Verbindungen mit dem Blutungssaft der Wurzeln in Abhängigkeit von der Mn-Versorgung

Nitrogen Assimilation in Roots and the Transport of Nitrogen Compounds in the Bleeding Sap of Roots in relation to Manganese Nutrition. The assimilation of nitrogen in the roots of 27 days old pumpkin plants was examined in relation to manganese nutrition. The transport of nitrogen compounds in the xylem was determined in roots and in the bleeding sap of roots using nitrate as the N-source. The maximum NO₃ content in the roots was observed in the Mn treatment which resulted in the highest shoot yields (0.05 ppm Mn). The bleeding sap of this treatment was lowest in nitrate concentration, but showed the highest rate of transport of organic nitrogen compounds. In experiments with ¹⁵N in the nutrient solution the isotope was found in the roots in organic and in inorganic compounds. The composition of the fraction of free amino acids differed between roots and xylem sap. In the bleeding sap glutamine was especially dominant. In the roots the amino acid composition depended on the extent of Mn-supply. Lowest glutamine concentrations were found in the xylem sap from the treatment with maximum shoot yields. A numerical difference was found in the xylem sap between organic N (N_(org)) and the amino acid nitrogen. This difference which account for more than 50 % of the organically bound nitrogen is suggested to be made up in part by low molecular weight peptides, amino sugars and other compounds. In Mn deficiency a general reduction in the intensity of nitrogen metabolism was found. With Mn toxicity the N assimilation activity was more intensive than for the low Mn supply. Simultaneously, however, the transport of organic N compounds from the root was lower.     

Diurnal variations in uptake,transport and assimilation of NO3 ‐and efflux of OH‐ in maize plants

Maize plants, grown for 7 and 21 days on a nutrient solution with NO₃ ^‐ as the sole nitrogen source showed a clear diurnal pattern with respect to the in vivo NRA. Especially in roots dark/light fluctuations of the enzyme activity were high. Also in NO₃ ^‐ uptake, OH^‐ efflux and endogenous content of water soluble carbohydrates a diurnal variation was found. The plant age did not significantly affect the daily rhythm.

Because day/night changes of the in vivo root NRA and nitrate uptake were proportional, the relative content of reduced N in the xylem sap of the plants was constant during a day/night interval. At both day 7 and day 21 about 40–50% of the N was transported via the xylem as amino N. As a result of non‐synchronous variation of the specific root and shoot NRA, root reduction capacity showed a great within‐day variation. It varied between 20 and 40% of the whole plant reduction capacity. Since the ratio N‐organic to N‐total in the xylem sap was about 0.5, cycling of organic nitrogen was very likely in these maize plants.     

Influence of potassium:rubidium ratios on the xylematic transport of solutes in cucumber plants grown with nitrate plus ammonium

The influence of three potassium:rubidium (K:Rb) ratios (6:0, 5:1, and 4:2) on the xylematic transport of solutes in cucumber plants cv. Medusa supplied with both nitrate (NO₃ ^‐) (60%) and ammonium (NH₄ ⁺) (40%) was studied in greenhouse conditions. In the xylem sap of plants grown with a K:Rb ratio of 4:2, there was an increase in the transport of NO₃ ^‐, phosphate (H₂PO₄ ^‐), calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), manganese (Mn) and boron (B) while that of organic‐N, organic‐P, K⁺, zinc (Zn), organic acids, and carbohydrates decreased, if compared with the sap of the plants supplied with K alone. The translocation of NO₃ ^‐, H₂PO₄ ^‐, Ca²⁺, Mg²⁺, and Mn was enhanced and that of K⁺ and organic acids decreased when the plants were supplied with a K:Rb ratio of 5:1. The K:Rb ratio detected in the xylem sap was the same K:Rb ratio as in the solutions. However, in the cucumber plant substituting 33% of total K by Rb resulted in an alteration in the transport of solutes, probably due to a competition between Rb and K rather than between the latter two and NH₄ ⁺.     

Effect of External Nitrogen and Potassium Supply on Xylem Sap Composition of Sugarcane

ABSTRACT

This study was conducted to evaluate the effect of nitrogen (N) and potassium (K) availability on root exudate composition of two sugarcane cultivars known to differ with regard to their resistance to drought and salinity stress. The plants were hydroponically grown in a greenhouse and subjected to three levels of N (0.1, 1.0, and 10 mM N) and three levels of K (0.02, 0.2, and 2 mM K). Nitrogen and K stress altered the xylem sap composition. Nitrogen stress significantly reduced nitrate (NO₃ ^?), ammonium (NH₄ ⁺), calcium (Ca), magnesium (Mg), and amino acid content and increased the pH, phosphorus (P), and K content. Whereas, K stress significantly decreased pH, K, NH₄ ⁺, and amino acid content but increased Ca, Mg, and P content. Nitrogen and K stress had opposing effects on xylem sap pH and osmolality. Results indicated that sugarcane plants recycle compounds between the phloem and xylem. The results also suggested that the NO₃ ^? and K concentration of xylem sap could be effectively used to estimate the N and K status of the soil solution.     

Potassium recirculation in tomato plants in relation to potassium supply

The Ben Zioni ‐ Dijkshoorn hypothesis that NO₃ uptake by roots is regulated by NO₃ assimilation in the shoot was tested using the tomato plant. Plants were grown at three K levels and the fate of anion charge from NO₃,^‐ and SO₄ ^2‐. assimilation followed in its distribution between organic acid anion accumulation and HCO₃ ^‐ efflux into the nutrient medium. For the high K treatment almost all of this charge was directed towards organic acid accumulation with HCO₃ ^‐ efflux accounting for only 3% of the total charge. On the other hand for plants supplied at the low K level, a substantial proportion of the anion charge was excreted as HCO₃ ^‐ (32%).

Xylem sap analyses and NO₃ reductase assay results indicated that in the tomato plant the upper plant parts constituted the major site for NO₃ reduction. The quantitatively most important ionic constituents in the sap were K⁺, Ca²⁺ and NO₃ ^‐.

Results have been presented that indicate that when K is in short supply in the nutrient medium, K recycling occurs within the plant to facilitate the upward transport of NO₃ ^‐ from root to shoot.     

Growth attributes,xylem sap composition,and photosynthesis in common bean as affected by nitrogen and phosphorus deficiency

The effects of nitrogen (N‐) and phosphorus (P‐) deficiency, isolatedly or in combination, on growth, nitrogenous fraction, and inorganic phosphate in xylem exudade, and photosynthesis of common bean (Phaseolus vulgaris L. cv. Negrito) were investigated. Plants were grown in nutrient solution adjusted daily to pH 5.5 and aerated continuously. Ten days after emergence mineral deficiency was imposed. Plants were then supplied with high N (7.5 mol m^‐3) or low N (0.5 mol m^‐3), and also with high P (0.5 mol m^‐3) or low P (0.005 mol m^‐3). All sampling and measurements were made 28 days after emergence. N‐ or P‐deprivation brought about large decreases in total leaf area by inhibiting the emergence of new leaves and primarily the expansion of the leaves. The specific leaf area did not change under N‐ but decreased under P‐limitation. The decreased shoot to root ratio in all deficiency treatments was a consequence of a lowering mass of above‐ground organs, especially of leaves.

The content of chlorophylls declined significantly only under N‐deficiency alone; carotenoids declined under both N‐ and combined N‐ and P‐limitation. No alteration in amino acid concentration in xylem exudate occurred in plants experiencing N‐starvation, while ureides increased by 79%, and nitrate and inorganic phosphate decreased greatly. Under P‐deprivation, amino acids and nitrate in xylem sap dropped by about half; ureides were held relatively constant, and phosphate was severely depressed. Total upward translocation of N through xylem was estimated to be about 16% higher in N‐deficient plants than in plants without mineral limitation, but leaf N levels in the former were lower as compared to control plants. The net carbon (C) assimilation decreased similarly regardless of the imposed deficiency treatment. Such a decrease was mainly determined by non‐stomatal factors. In general, no additive effect between N‐ and P‐limitation on any of measured parameters was observed.     

绿茶施用铵态氮和硝态氮后木质部导管中N运移模拟

An experiment was carried out to study the transport process of nitrogen （N） assimilation from tea roots by monitoring the dynamic composition of N compounds in xylem sap after 15^N-NO3 and 15^N-NH4 were fed to the root of tea plants （Camellia sinensis L.）. Results showed that the main amino acids were glutamine, theanine, axginine, asparic acid and glutamic acid, which accounted for 49%, 17%, 8%, 7%, and 4%, respectively, of the total amino acids in the xylem sap. After the tea plants were fed with 15^N-NO3 and 15^N-NH4 for 48 h, the amount of total amino acids in xylem sap significantly increased and those fed with 15^N-NH4 had higher increment than those with 15^N-NOa. Two hours after 15^N- NO3 and 15^N-NH4 were fed, 15N abundance in glutamine, asparagine, glutamic acid, alanine, and arginine were detected and increased quickly over time. This indicated that it took less than 2 h for NO3-N and NH4-N to be absorbed by tea roots, incorporated into the above amino acids and transported to the xylem sap. Rapid increase in 15^N-NO3 in the xylem sap of tea plants fed with 15^N-NO3 indicated that nitrate could be directly transported to the xylem sap. Glutamine, theanine, and alanine were the main amino acids transported in xylem sap of tea plants fed with both 15^N-NO3 and 15^N-NH4.     

Fruit Yield of Tomato Cultivated on Media with Bicarbonate and Nitrate/Ammonium as the Nitrogen Source

ABSTRACT

This article presents the effects of nitrate/ammonium (NO₃ ^?/NH₄ ⁺), applied at different proportions to the root media with or without 5 mmol bicarbonate (HCO₃ ^?), on the yield and chemical composition of tomato fruit. Tomato plants were grown hydroponically (pH 6.9) in glasshouse conditions. The yield of fruit fresh matter from four clusters obtained from plants grown on the medium with NH₄ ⁺ was about 25% lower than from the plants grown on the medium containing NO₃ ^? as the nitrogen (N) source. Supplying NO₃ ^?/NH₄ ⁺ at a ratio of 4:1 increased the fruit yield by about 20% in comparison with the value recorded for NO₃ ^??plants. The enrichment of the medium with HCO₃ ^? stimulated the bearing, while the result depended on the ratio of NO₃ ^?/NH₄ ⁺. A combined treatment of HCO₃ ^? with NO₃ ^? or NH₄ ⁺ in the medium increased yields by about 28% and 11%, respectively, in comparison to plants cultivated without HCO₃ ^?. The application of NO₃ ^?/NH₄ ⁺ at ratios of 4:1 and 1:1 with HCO₃ ^? increased the respective yields by about 16% and 10% in comparison with plants grown without HCO₃ ^?. Modifications in the composition of the media affected the accumulation of organic solutions in the fruit. The NH₄ ⁺ nutrition effected a 20% decrease in the accumulation of reducing sugars in the fruit in comparison to the fruit of plants grown in media with NO₃ ^?. In the cultivation of plants in media with various NO₃ ^?/NH₄ ⁺ proportions the intermediate values of the reduced sugar concentrations were recorded in comparison with the values obtained for NO₃ ^??plants and NH₄ ⁺?plants. The enrichment of media with HCO₃ ^? increased the concentration of sugars in fruit from about 28% (for NO₃ ^??plants) to about 10% (for NH₄ ⁺?plants).

Malate and citrate are the main constituents of carboxylates in tomato fruit. The form of nitrogen applied to the medium did not significantly affect the concentration of carboxylates in fruit. Significant differences in carboxylate concentrations appeared in fruit grown on media enriched with HCO₃ ^? ions. In comparison with the cultivation without HCO₃ ^?, increases in the accumulation of carboxylates varied from about 22% to 30% depending on the form of the applied nitrogen. The concentration of amino acids in the fruit of plants grown with NH₄ ⁺ exceeded that in NO₃ ^??plants by about 55%. In the plants grown on media of modified NO₃ ^?/NH₄ ⁺ proportions, the concentration of amino acids in fruits were positively correlated with the level of NH₄ ⁺ in the medium. The enrichment of media with HCO₃ ^? stimulated a further increase in amino acid concentration in fruit by about 9% in NO₃ ^? plants and about 21% in NH₄ ⁺ plants compared with the respective control (without HCO₃ ^?).     

增铵营养对番茄植株伤流液组分及含量的影响

在总氮(N)浓度相等的条件下,研究全硝营养(100% NO3-)和25% 增铵营养(NH4+∶NO3- = 25%∶75%)对开花期和幼果期番茄植株伤流液各组分含量的影响.结果表明,增铵营养显著增加幼果期伤流液中 K的含量,对Ca、Mg、P元素含量没有显著影响;增铵营养下伤流液中 NO3- 的含量下降、NH4+ 含量增加,氨基酸、苹果酸等的含量均显著增加,氨基酸/硝态氮含量之比显著提高,表明喜硝作物适当增铵不仅能够提高根系活力,显著促进K的吸收以供果实发育之需,而且提高了植株整体同化N素的能力.     

Host-rhizobia interaction for effective inoculation: evaluation of the potential use of the ureide assay to monitor the symbiotic performance of tepary bean (Phaseolus acutifolius A. Gray)

Domesticated and wild-type tepary beans (Phaseolus acutifolius A. Gray) were grown with or without inoculation with rhizobia in pots under bacteriologically controlled conditions in a temperature-controlled glasshouse. Seeds were inoculated with a mixture of seven strains isolated from nodules collected from domesticated field-grown tepary bean in Arizona, USA, or with a commercial inoculant strain for Phaseolus vulgaris (CC511). Different degrees of plant reliance upon N₂ fixation for growth were generated by supplying the inoculated plants throughout growth with nutrients containing a range of concentrations of ¹⁵N-labeled NO₃ (0, 1, 2, 5 or 10 mM). An uninoculated treatment that received 10 mM ¹⁵N-labeled NO₃ was included to provide data for plants solely dependent upon NO₃ for growth. Six weeks after sowing, shoots were harvested for dry matter determination and subsequent ¹⁵N analysis, root-bleeding xylem sap was collected, and nodulation assessed. With regard to shoot biomass production, domesticated lines were more responsive to inoculation, but less responsive to applied N than wild types. All inoculated plants were nodulated, but the field isolates from tepary bean were more effective in N₂ fixation than strain CC511. It was concluded that tepary bean requires a specific inoculant to benefit from fixation of atmospheric N₂. Xylem sap samples were analysed for ureides (allantoin and allantoic acid), amino acid content (α-amino-N), and NO₃ concentration. The amount of ureide-N present in xylem sap was expressed as a percentage of total solute N, described as the relative abundance of ureide-N (RUN), for each N treatment and was compared to the proportion of plant N derived from N₂ fixation (%Ndfa) calculated using a ¹⁵N dilution technique. The RUN values ranged from 8% for saps collected from uninoculated plants provided with 10 mM NO₃ in the nutrient solution (%Ndfa=0) to 86-91% for nodulated plants grown in the absence of externally supplied NO₃ (%Ndfa=100). These data indicated that ureides were the principal product of N₂ fixation exported from the nodules to the shoot in xylem sap. Since RUN values were closely related to %Ndfa, it was proposed that N-solute analysis of xylem sap could provide a valuable analytical tool to monitor the symbiotic performance of tepary bean.     

Responses of Root Growth and Nitrogen Transfer Metabolism to Uniconazole,a Growth Retardant,during the Seedling Stage of Soybean under Relay Strip Intercropping System

Relay strip intercropping of soybean has been widely developed in the southwest of China to secure China's soybean production. However, due to the shading from maize, soybean plants are thin and have a poor root system. Uniconazole is a plant-growth retardant that could enhance root vigor; increase root length, root volume, and root dry weight; and affect nitrogen (N) metabolism. To understand the effects of uniconazole on the root growth and N-transfer metabolism of soybean seedlings under relay strip intercropping, the changes in some morphological characteristics of root, dry-matter weight, root vigor, nitrate (NO₃ ^?)-N, ammonium (NH₄ ⁺)-N, and amino acid of xylem sap after seed treatment with uniconazole powder (0, 2, 4, and 8 mg kg^?1 seed) were investigated. Main root length, total lateral root lengths, first lateral root numbers, root nodule numbers, root vigor together with bleeding sap, bleeding sap–top ratio, root dry weight, and root/shoot ratio were increased, indicating uniconazole improved soybean root system in relay strip intercropping. Uniconazole powder treatment could increase NO₃ ^?-N, NH₄ ⁺-N, and total amino acid of xylem sap, to increase the potential of leaf and root N reduction and assimilation, and increase of leaf and root N contents. Thus, results suggested that uniconazole treatment can improve root growth and N transfer mechanism of soybean to support its further growth.     

Growth of tomato seedlings under different HCO‐ 3 concentration in the medium

An increase in the concentration of HCO^‐ ₃ ions in liquid media surrounding the root system significantly affected the biomass production of tomato seedlings in early stages of growth. This effect depended upon HCO^‐ ₃ concentration. The cultivation of seedlings during a period of 24 days (from 21–45 days after sowing) on a medium enriched to 5.68 mM HCO₃ (0.025% CO₂ after the computed dissociation) increased the production of dry matter (DW) plant"¹ to about 179% as compared with the respective control. Various tomato organs showed different values of DW increase, the greatest one being noted in leaf blades. This result was correlated with an increase in leaf blade area to about 176% in relation to the control. With an increase in the concentration to 22.72 mM HCO^‐ ₃ (0.1% CO₂ after the computed dissociation) a general tendency of changes was maintained, however, the absolute values of growth were diminished. In media of an enriched HCO₃ content the length of shoots, and roots, was not significantly modified. The values of other growth parameters computed for the respective three experimental series with HCO₃ concentration of 0.0, 5.68, and 22.72 mM, respectively, in the medium also showed correlations with the biomass production in tomato organs. The enrichment of liquid media with HCO₃ affected the processes of absorption, distribution, and accumulation of such elements as nitrogen, potassium, and calcium. No statistically significant differences in the content of phosphorus were obtained. The data presented here are the continuation of the study whose results were published in 1992 (J. Plant Nutr. 15: 293–312).     

Effect Of Mineral Nutrition on Metabolic Change Induced in Crop Plant Roots (I)

Attempts have long been made to study the effect of mineral nutrition on the metabolic substances in excised roots from a numcer of plant species, but very little attention has teen given to an approach to the problem by using the bleeding sap from crop plants. Recently, however, an increasing number of reports about the occurrence of organic substances in bleeding sap from crop plants has completely revised an old view that xylem sap was essentially a rather dilute aqueous solution of inorganic salts. Evidence has already been obtained which suggests a significant role for the root system as a centre of metabolism and an upward transport of metabolites from the root via the xylem to the leaf. Although analyses of the nitrogenous compounds present in bleeding sap from herbaceous plants have shown glutamine and asparagine to be the most important constituents, in some species of plants nitrate nitrogen may be a predominant nitrogenous compound. Besides ami des and nitrate, the presence of amino acids in bleeding sap has ceen shown by Kulayeva, Silina, and Kursanov ¹⁾ for pumpkins, Wieringa and Bakhuis ²¹ for Lupins, and DIE³⁾ for cucumbers and tomatoes.     

Jahreszeitliche Veränderung der NO3−-Konzentrationen im Xylemsaft des unteren Stammteiles von Buchen (Fagus sylvatica L.)

Seasonal variation of NO₃^? concentration in xylem sap of the lower trunk part of beeches (Fagus sylvatica L.) From October 1988 to October 1989 five beech trees from a 35-year-old and a 42-year-old stand were felled in 14 day intervals. Xylem sap was extracted from the lower 100 cm of the trunk by means of liquid displacement. In general there was an increase of NO₃^? xylem sap concentrations in summer. Higher xylem sap nitrate concentrations were accompanied by an almost equal but opposite pH decrease. It is assumed that the rapid surge in NO₃^? concentration of the xylem sap was due to summer acidification pushes in the forest soil.     

Nitrogen partitioning in greenhouse‐grown broccoli in response to varying NH+ 4:NO− 3 ratios

Abstract

Broccoli (Brassica oleracea var. italica) plants were grown in the greenhouse and supplied continuously with 18 mM N in the following NH⁺ ₄:NO^? ₃ ratios: 100:0, 75:25, 50:50, 25:75, 0:100. At commercial maturity, the plant characteristics and partitioning of nitrogen in xylem and phloem saps and in plant tissue were determined. Plants fed solely with NH⁺ ₄ were stunted, exhibited signs of marginal necrosis on the lower leaves, and accumulated NH⁺ ₄ in the foliage. The maximum yield and shortest harvest time, together with minimal NO^? ₃ and NH⁺ ₄ accumulation were found at a ratio of 75:25. Ammonium concentrations in xylem sap decreased linearly with decreasing NH⁺ ₄ portion in the nutrient solution, whereas the NO^? ₃ concentrations reached a maximum when NO^? ₃ constituted 50% of the N supply. The glutamate family dominated the amino acid composition of both xylem and phloem saps, but did not vary much with NH⁺ ₄:NO^? ₃ ratio. It is suggested that the NH⁺ ₄ concentration in xylem sap may be used to assess NH⁺ ₄ vs NO^? ₃ utilization by broccoli grown under field conditions.     

YIELD,FRUIT QUALITY AND NITROGEN UPTAKE OF ORGANICALLY AND CONVENTIONALLY GROWN MUSKMELON WITH DIFFERENT INPUTS OF NITROGEN,PHOSPHORUS, AND POTASSIUM

The effects of varied amounts of fertilization on yield, fruit quality, and nitrogen (N) uptake of muskmelons (Cucumis melo L. var reticulatus Naud) grown under both organic and conventional farming conditions were evaluated. Organic fertilizer (0.0, 0.55, 1.1, and 2.2 kg m^?2) and mineral fertilizers containing the same amounts of estimated plant available nutrients [N, phosphorus (P), and potassium (K)] were applied to organic and conventional farming plots, respectively, in both the spring and autumn seasons of 2005. In comparison to conventional farming conditions, muskmelons grown under organic farming conditions had the same yield, total soluble solids (TSS) and soluble sugar contents in both growing seasons, and fruit pulp nitrate content was significantly reduced by 12% on average in spring and 16% on average in autumn. At harvest maturity the aboveground plant N concentration was significantly higher in the conventional treatments than in the organic treatments. At the vine growth stage, the plant N concentrations were similar in all treatments in both seasons. The ratios of nitrate N to total N amount in aboveground biomass were higher in conventional and high fertilized organic treatments than in low or not fertilized organic treatments under limited N supply from the soil. Muskmelon plants absorbed mainly inorganic N, and the protein N fraction in the xylem sap was larger than the amino acid N fraction. Plants grown in the organic system had a higher proportion of organic N in their xylem sap, especially when manure input was low.     

Growth and chlorophyll,mineral, and total amino acid composition of tomato and wheat plants in relation to nitrogen and iron nutrition II. Chlorophyll content and total amino acid composition

Studies of the amino acids distribution in plants subjected to nutrient regimes are limited. The present study investigated the effect of NO₃‐N and FeSO₄‐Fe regimes on chlorophyll and total amino acids composition of tomato and wheat plants. Also the distribution of 17 amino acids between the different plant parts was studied. Increasing the NO₃‐N level up to 200 mg kg^‐1 greatly increased the total amino acids content of tomato plants. The total amino acids content of wheat plants continued to increase with addition of NO₃‐N up to 400 mg kg^‐1. The response of chlorophyll content to NO₃‐N supply was highly dependent on Fe level both in tomato and wheat plants. The interaction between NO₃‐N and FeSO₄‐Fe had a great effect on the total amino acids content and distribution. Iron increased the translocation of proline from roots to leaves. The overall amino acids contents of leaves was higher than that of stems or roots.     

Content and bioavailability of organic forms of nitrogen in the O horizon of a podzol

It is often thought that the most important source of nitrogen for plants and microorganisms comes from amino acids and amino sugars when they are hydrolysed in acid conditions. We did a microcosm experiment to test the hypothesis. In the experiment spruce seedlings (Picea abies L. Karst) were grown for 145 days in soil taken from a podzol Oa horizon under a long-term nitrogen fertilization experiment (control and N-treated soil). Net changes in different pools of organic N were determined using standard fractionation (acid hydrolysis and pyrophosphate extraction). During the experiment the amino acid and amino sugar pools decreased significantly (14% and 15% for the control and 10% and 17% for the N treatment), whereas no significant change was observed in the non-amino acid plus non-amino sugar fraction. On a per organic C basis there was even a significant increase in the non-amino acid plus non-amino sugar fraction of 11% for the control and 8% for the N treatment. Pyrophosphate extractions suggest that amino acids or amino sugars associated with the humin fraction were more accessible to microbes and plants than those associated with the humic acid, fulvic acid and hydrophilic substances. The long-term N fertilization (about 73 kg N ha⁻¹ was added annually as NH₄NO₃ during a 24-year period) resulted in an enrichment of all major fractions of organic N, i.e. amino acids, amino sugars and non-amino acids plus non-amino sugars. This enrichment was largely the result of small increases in all of the amino acids rather than large increases in just a few.     

Amino acid interference with ammonium determination in soil extracts using the automated indophenol method

Abstract

The formation of a colored indophenol complex is commonly used as a quantitative measure of the ammonium content of soil extracts. The potential interference with ammonium determination from co‐extracted amino acids was examined. The extent of color development was examined for 22 amino acids by subjecting pure solutions to ammonium determination by both the indophenol method and steam distillation. Apparent detection of amino acid as ammonium ranged from 0 to 94 % of total nitrogen for the indophenol procedure, whereas steam distillation resulted in little apparent ammonium recovery. With the exception of threonine, the extent of color development was inversely related to amino acid molecular weight. The range in recoveries for the indophenol procedure suggests both size and composition of the co‐extracted amino acid pool is important in determining the extent of interference.

Significantly (p=0.001) greater estimates, averaging 0.4 μg mL^‐1, were found in indophenol estimates of mineral‐N content of moist, fresh soil samples. Air drying, oven drying or chloroform fumigation significantly increased the difference (0.3 ‐ 0.7 μg mL^‐1) in estimates of ammonium content. At 10: 1 extract: soil ratios this could cause ? to be overestimated by 3–7 μg g^‐1soil. The increased interference was attributed to a release of amino acid as a result of pretreatment. The difference between distillation and indophenol estimates of ammonium content of 0.5 M K₂SO₄was found to be dependent upon ammonium content. The use of procedures employing a distillation step (manual or automated) is recommended to avoid amino acid interference when precise NH₄+‐N determinations are needed on dried or fumigated samples.