Modelling of the Partitioning, Assimilation and Storage of Nitrate within Root and Shoot Organs of Castor Bean (Ricinus communis L.)

An experimentally-based modelling technique was developed todescribe quantitatively the uptake, flow, storage and utilizationof NO₃-N over a 9 d period in mid-vegetative growth of sandcultured castor bean (Ricinus communis L.) fed 12 mol m^–3nitrate and exposed to a mean salinity stress of 128 mol m^–3NaCl. Model construction used information on increments or lossesof NO₃-N or total reduced N in plant parts over the study periodand concentration data for NO₃-N and reduced (amino acid) Nin phloem sap and pressure-induced xylem exudates obtained fromstem, petiole and leaf lamina tissue at various levels up ashoot. The resulting models indicated that the bulk (87%) of incomingnitrate was reduced, 51% of this in the root, the remainderprincipally in the laminae of leaves. The shoot was 60% autotrophicfor N through its own nitrate assimilation, but was oversuppliedwith surplus reduced N generated by the root and fed to theshoot through the xylem. The equivalent of over half (53%) ofthis N returned to the root as phloem translocate and, mostly,then cycled back to the shoot via xylem. Nitrate comprised almosthalf of the N of most xylem samples, but less than 1% of phloemsap N. Laminae of leaves of different age varied greatly inN balance. The fully grown lower three leaves generated a surplusof reduced N by nitrate assimilation and this, accompanied byreduced N cycling by xylem to phloem exchange, was exportedfrom the leaf. Leaf 4 was gauged to be just self-sufficientin terms of nitrate reduction, while also cycling reduced N.The three upper leaves (5–7) met their N balance to varyingextents by xylem import, phloem import (leaves 6 and 7 only)and assimilation of nitrate. Petioles and stem tissue generallyshowed low reductase activities, but obtained most of theirN by abstraction from xylem and phloem streams. The models predictedthat nodal tissue of lower parts of the stem abstracted reducedN from the departing leaf traces and transferred this, but notnitrate, to xylem streams passing further up the shoot. As aresult, xylem sap was predicted to become more concentratedin N as it passed up the shoot, and to decrease the ratio ofNO₃-N to reduced N from 0·45 to 0·21 from thebase to the top of the shoot. These changes were reflected inthe measured N values for pressure-induced xylem exudates fromdifferent sites on the shoot. Transfer cells, observed in thexylem of leaf traces exiting from nodal tissue, were suggestedto be involved in the abstraction process. Key words: Ricinus communis, nitrogen, nitrate, nitrate reduction, partitioning, phloem, xylem, flow models     

Nodule Nitrate Reductase as a Source of Reduced Nitrogen in Soybean, Glycine max

Hydroponically grown soybeans were fed ¹⁵N-enriched NaNO₃ at nine reproductive stages of development. The stem exudates contained excess ¹⁵N in the fully reduced nitrogen fraction. The soybean nodules had high nitrate reductase activity, whereas the roots had no detectable nitrate reductase activity. Based on these results, we concluded that the nodule nitrate reductase system has the potential of contributing significantly to the nitrogen economy of the plant.     

Reduction of plant tissue nitrate to nitric oxide for mass spectrometric 15N analysis.

A procedure based on that of Van Slyke and LoMonte (Microchem. J.14, 608–626, 1969) is described for the mass spectrometric analysis of ¹⁵N in nitrate extracted from plant tissue. The extract is evaporated to dryness in a disposable borosilicate test tube, which forms the basal part of a reaction flask. After evacuation of the flask, nitrate is reduced to nitric oxide by sonication with mercury in 18 n H₂SO₄. The atom percentage ¹⁵N is calculated from the relative intensities of the ¹⁴NO and ¹⁵NO peaks of the mass spectrum. The method is unaffected by reduced nitrogen compounds which are present in plant tissue extracts.     

Total nitrogen determining for plant material containing nitrate

A nonfoaming method for semimicro Kieldahl determination of total nitrogen in plant samples containing appreciable amounts of nitrate was developed for use with a digestion block utilizing test tubes for digestion flasks. The sample (30 to 200 mg) is treated with 5 ml of a sulfuric acid:salicylie acid (30:1 $\text{v}\text{w}$) mixture at room temperature for 1 hr. Catalyst [K₂SO₄:CuSeO₃·2H₂O:pumice 970:19:11 w/w/w)] is added and the mixture is digested at 360 to 380°C for 1 hr after the mixture clears. Ammonium in the digest is determined by a suitable method. This semimicro Kjeldahl procedure results in a 95% or better recovery of nitrate, either from KNO₃ or from KNO₃ added to plant material.     

芘对黑麦草根系几种低分子量有机分泌物的影响

植物根系释放分泌物与有机污染物的植物修复机制密切相关,研究具有有机污染物修复潜力植物在污染胁迫条件下的根系分泌物特征有助于揭示其修复机制.以多环芳烃修复研究中常用的黑麦草为材料(Lolium perenneL.)为材料,在营养液栽培方式下研究了在芘胁迫处理下,黑麦草根系几种低分子量有机物分泌特征.结果表明,黑麦草对芘具有较强的耐受能力,芘胁迫处理下,生物量无显著变化.黑麦草根系分泌的低分子量有机酸主要为草酸.3、6 mg/L和9 mg/L芘胁迫处理下,低分子量有机酸的组成无明显变化,但含量随芘胁迫处理浓度上升而显著增加(P＜0.05);总糖分泌量随着芘胁迫处理浓度升高而呈现先略微上升后下降的趋势,相对高峰值出现在芘胁迫处理浓度3 mg/L,但差异不显著;氨基酸分泌总量随着芘胁迫处理浓度的增加而显著增多,芘胁迫浓度在3、6 mg/L和9 mg/L时,根系氨基酸的分泌总量分别是空白的1.37、2.02倍和2.65倍,但根系分泌的氨基酸组成无明显变化,19种常见氨基酸分泌的数量变化情况却不相同,分泌量总体均随着芘胁迫处理浓度的提高而增加,其中苏氨酸、丝氨酸、脯氨酸、甘氨酸、丙氨酸、亮氨酸、组氨酸和鸟氨酸的分泌量显著增多,差异显著(P＜0.05).     

Effects of aqueous root extracts and hydrophobic root exudates of cucumber (Cucumis sativus L.) on nuclei DNA content and expression of cell cycle-related genes in cucumber radicles

Allelopathic interactions implicate the inhibition of cell division by allelochemicals. To examine the effects of autotoxic agents on cell cycle and plant growth, germinated cucumber seeds (Cucumis sativus L.) were incubated in solutions containing the aqueous root extracts of cucumber at 1:100, 1:50, 1:25 and 1:10 (w:v), or the hydrophobic root exudates of cucumber at 25, 50 and 100 mg·L^?1. Aqueous root extracts and hydrophobic root exudates inhibited radicle elongation by 36.47–60.18% and 38.24–62.50%, respectively. The mitosis-specific genes were down-regulated in roots exposed to aqueous root extracts and hydrophobic root exudates. Meanwhile, exposure to either aqueous root extracts or hydrophobic root exudates decreased the proportion of 2C (C-value) and increased the proportion of 8C, leading to an increased mean C-value. We conclude that autotoxic agent-induced inhibition of radicle growth was partly attributed to the down-regulation of cell cycle-related genes and endoreduplication was enhanced under our experimental condition.     

A metabolomic approach to study the rhizodeposition in the tritrophic interaction: tomato,Pochonia chlamydosporia and Meloidogyne javanica

A combined chemometrics-metabolomics approach [excitation–emission matrix (EEM) fluorescence spectroscopy, nuclear magnetic resonance (NMR) and high performance liquid chromatography–mass spectrometry (HPLC–MS)] was used to analyse the rhizodeposition of the tritrophic system: tomato, the plant-parasitic nematode Meloidogyne javanica and the nematode-egg parasitic fungus Pochonia chlamydosporia. Exudates from M. javanica roots were sampled at root penetration (early) and gall development (late). EMM indicated that late root exudates from M. javanica treatments contained more aromatic amino acid compounds than the rest (control, P. chlamydosporia or P. chlamydosporia and M. javanica). ¹H NMR showed that organic acids (acetate, lactate, malate, succinate and formic acid) and one unassigned aromatic compound (peak no. 22) were the most relevant metabolites in root exudates. Robust principal component analysis (PCA) grouped early exudates for nematode (PC1) or fungus presence (PC3). PCA found (PC1, 73.31 %) increased acetate and reduced lactate and an unassigned peak no. 22 characteristic of M. javanica root exudates resulting from nematode invasion and feeding. An increase of peak no. 22 (PC3, 4.82 %) characteristic of P. chlamydosporia exudates could be a plant “primer” defence. In late ones in PC3 (8.73 %) the presence of the nematode grouped the samples. HPLC–MS determined rhizosphere fingerprints of 16 (early) and 25 (late exudates) m/z signals, respectively. Late signals were exclusive from M. javanica exudates confirming EEM and ¹H NMR results. A 235 m/z signal reduced in M. javanica root exudates (early and late) could be a repressed plant defense. This metabolomic approach and other rhizosphere -omics studies could help to improve plant growth and reduce nematode damage sustainably.     

Formation of unidentified nitrogen in plants: an implication for a novel nitrogen metabolism

Plants take up inorganic nitrogen and store it unchanged or convert it to organic forms. The nitrogen in such organic compounds is stoichiometrically recoverable by the Kjeldahl method. The sum of inorganic nitrogen and Kjeldahl nitrogen has long been known to equal the total nitrogen in plants. However, in our attempt to study the mechanism of nitrogen dioxide (NO₂) metabolism, we unexpectedly discovered that about one-third of the total nitrogen derived from ¹⁵N-labeled NO₂ taken up by Arabidopsis thaliana (L.) Heynh. plants was converted to neither inorganic nor Kjeldahl nitrogen, but instead to an as yet unknown nitrogen compound(s). We here refer to this nitrogen as unidentified nitrogen (UN). The generality of the formation of UN across species, nitrogen sources and cultivation environments for plants has been shown as follows. Firstly, all of the other 11 plant species studied were found to form the UN in response to fumigation with ¹⁵NO₂. Secondly, tobacco (Nicotiana tabacum L.) plants fed with ¹⁵N-nitrate appeared to form the UN. And lastly, the leaves of naturally fed vegetables, grass and roadside trees were found to possess the UN. In addition, the UN appeared to comprise a substantial proportion of total nitrogen in these plant species. Collectively, all of our present findings imply that there is a novel nitrogen mechanism for the formation of UN in plants. Based on the analyses of the exhaust gas and residue fractions of the Kjeldahl digestion of a plant sample containing the UN, probable candidates for compounds that bear the UN were deduced to be those containing the heat-labile nitrogen–oxygen functions and those recalcitrant to Kjeldahl digestion, including organic nitro and nitroso compounds. We propose UN-bearing compounds may provide a chemical basis for the mechanism of the reactive nitrogen species (RNS), and thus that cross-talk may occur between UN and RNS metabolisms in plants. A mechanism for the formation of UN-bearing compounds, in which RNS are involved as intermediates, is proposed. The important broad impact of this novel nitrogen metabolism, not only on the general physiology of plants, but also on plant substances as human and animal food, and on plants as an integral part of the global environment, is discussed.Abbreviations NO Nitric oxide - NO₂ Nitrogen dioxide - RNS Reactive nitrogen species - UN Unidentified nitrogen - TNNAT, RNNAT, INNAT and UNNAT Total, Kjeldahl, inorganic and unidentified nitrogen in naturally fed plants, respectively - TNNIT, RNNIT, INNIT and UNNIT Total, Kjeldahl, inorganic and unidentified nitrogen derived from nitrate, respectively - TNNO2, RNNO2, INNO2 and UNNO2 Total, Kjeldahl, inorganic and unidentified nitrogen derived from NO₂, respectively     

Routine analysis of N15 in plant material by mass-spectrometry

Summary A routine analysis of N¹⁵ in plant material by means of the Dumas reaction in vacuum was compared with the classical one consisting of Kjeldahl digestion, distillation of ammonia and oxydation of it with NaBrO to N₂. It was found that the systematic error introduced by non random pairing of N¹⁴ and N¹⁵ below 5% N¹⁵ atom excess is negligable.In a series of 52 samples the Kjeldahl method gave results 2% lower than those obtained by the Dumas procedure but this difference will be of little significance in greenhouse or field experiments.     

Organic Acid contents of soybean: age and source of nitrogen

The organic acid content of soybean (Glycine max v. Hodgson) root, stem, and leaf tissue was followed for 33 days after germination. Malonate was the predominant acid in leaf and root tissue, whereas fumarate was predominant in the stem. The malonate concentrations of the stem and root showed similar variations with time, but the leaf response was quite different. In nodules from 33-day-old plants, malonate was the predominant acid. Malonate levels in root and nodule tissue of 33-day-old plants were depressed in response to the addition of either nitrate or ammonia. Nodule tissue had a higher malonate concentration on nitrate nitrogen than it did on ammonium nitrogen, whereas root tissue had the higher malonate concentration on ammonium nitrogen. Analysis of organic acid concentrations of roots as a function of age and distance from the root tip in young soybean seedlings revealed a zone consistently high in malate. The malonate level in the entire root rose dramatically in tissue of age 96 to 120 hours.     

Specificity for nicotinamide adenine dinucleotide by nitrate reductase from leaves

Preliminary work revealed that nitrate reductase in crude extracts prepared from leaves of certain corn genotypes as well as soybeans could utilize NADPH as well as NADH as the electron donor. Isoelectric focusing and diethylaminoethyl cellulose chromatography confirmed previous findings that NADH and NADPH activities could not be separated, which suggests the involvement of a single enzyme. Nitrate reduction with both cofactors varies with plant species, plant age, and assay conditions. The ability of the nitrate reductase from a given genotype to utilize NADPH was associated with the amount of NADPH-phosphatase in the extract. While diethylaminoethyl cellulose chromatography of plant extracts separated nitrate reductase from the bulk (90%) of the phosphatase and caused a decrease in the NADPH activity, the residual level of phosphatase was sufficient to account for the apparent NADPH nitrate reductase activity. Addition of KH₂PO₄ and KF, inhibitors of NADPH-phosphatase activity in in vitro assays, caused a drastic reduction or abolishment of NADPH-mediated nitrate reductase activity but were without effect on NADH nitrate reductase activity. It is concluded that NADPH-nitrate reduction, in soybean and certain corn genotypes, is an artifact resulting from the conversion of NADPH to NADH by a phosphatase and that the enzyme in leaf tissue is NADH-dependent (E.C.1.6.6.1).     

Effects of nitrogen dioxide and nitrate nutrition on nodulation, nitrogenase activity, growth, and nitrogen content of bean

The influence of nutrient nitrate level (0-20 millimolar) on the effects of NO₂ (0-0.5 parts per million) on nodulation and in vivo acetylene reduction activity of the roots and on growth and nitrate and Kjeldahl N concentration in shoots was studied in bean (Phaseolus vulgaris L. cv Kinghorn Wax) plants. Exposing 8-day old seedlings for 6 hours each day, for 15 days, to 0.02 to 0.5 parts per million NO₂ decreased total nodule weight at 0 and 1 millimolar nitrate, and nitrogenase (acetylene reduction) activity at all concentrations of nitrate. The pollutant had little effect on root fresh or dry weights. Shoot growth was inhibited by NO₂. The NO₂ exposure increased nitrate concentration in roots only at 20 millimolar nutrient nitrate. Exposure to NO₂ markedly increased Kjeldahl N concentration in roots but generally decreased that in shoots. The experiments demonstrated that nutrient N level and NO₂ concentration act jointly in affecting nodulation and N fixing capability, plant growth and composition, and root/shoot relationships of bean plants.     

The effect of oxygen on nitrate and nitrite assimilation in leaves of Zea mays L. under dark conditions

The assimilation of nitrate under dark-N₂ and dark-O₂ conditions in Zea mays leaf tissue was investigated using colourimetric and ¹⁵N techniques for the determination of organic and inorganic nitrogen. Studies using ¹⁵N indicated that nitrate was assimilated under dark conditions. However, the rate of nitrate assimilation in the dark was only 28% of the rate under non-saturating light conditions. No nitrite accumulated under dark aerobiosis, even though nitrate reduction occurred under these conditions. The pattern of nitrite accumulation in leaf tissue in response to dark-N₂ conditions consisted of three phases: an initial lag phase, followed by a period of rapid nitrite accumulation and finally a phase during which the rate of nitrite accumulation declined. After a 1-h period of dark-anaerobiosis, both nitrate reduction and nitrite accumulation declined considerably. However, when O₂ was supplied, nitrate reduction was stimulated and the accumulated nitrite was rapidly reduced. Anaerobic conditions stimulated nitrate reduction in leaf tissue after a period of dark-aerobic pretreatment.     

Nitrate Reduction in Roots and Shoots of Barley (Hordeum vulgare L.) and Corn (Zea mays L.) Seedlings: I. N Study

Nitrate reduction in roots and shoots of 7-day-old barley seedlings, and 9-day-old corn seedlings was investigated. The N-depleted seedlings were transferred for 24 h or 48 h of continuous light to a mixed nitrogen medium containing both nitrate and ammonium. Total nitrate reduction was determined by ¹⁵N incorporation from ¹⁵NO₃⁻, translocation of reduced ¹⁵N from the roots to the shoots was estimated with reduced ¹⁵N from ¹⁵NH₄⁺ assimilation as tracer, and the translocation from the shoots to the roots was measured on plants grown with a split root system. A model was proposed to calculate the nitrate reduction by roots from these data. For both species, the induction phase was characterized by a high contribution of the roots which accounted for 65% of the whole plant nitrate reduction in barley, and for 70% in corn. However, during the second period of the experiment, once this induction process was finished, roots only accounted for 20% of the whole plant nitrate reduction in barley seedlings, and for 27% in corn. This reversal in nitrate reduction localization was due to both increased shoot reduction and decreased root reduction. The pattern of N exchanges between the organs showed that the cycling of reduced N through the plant was important for both species. In particular, the downward transport of reduced N increased while nitrate assimilation in roots decreased. As a result, when induction was achieved, the N feeding of the roots appeared to be highly dependent on translocation from the leaves.     

Organic Constituents and Complexation of Nickel(II), Iron(III), Cadmium(II), and plutonium(IV) in Soybean Xylem Exudates

The xylem exudates of soybean (Glycine max cv Williams), provided with fixed N, were characterized as to their organic constituents and in vivo and in vitro complexation of plutonium, iron, cadmium, and nickel. Ion exchange fractionation of whole exudates into their compound classes (organic acid, neutral, amino acid, and polyphosphate), followed by thinlayer electrophoresis, permitted evaluation of the types of ligands which stabilize each element. The polyvalent elements plutonium(IV) and iron(III) are found primarily as organic acid complexes, while the divalent elements nickel(II) and cadmium(II) are associated primarily with components of the amino acid/peptide fraction. For plutonium and cadmium, it was not possible to fully duplicate complexes formed in vivo by back reaction with whole exudates or individual class fractions, indicating the possible importance of plant induction processes, reaction kinetics, and/or the formation of mixed ligand complexes. The number and distribution of specific iron- and nickel-containing complexes varies with plant age and appears to be related to the relative concentration of organic acids and amino acids/peptides being produced and transported in the xylem as the plant matures.     

Ureide assay for measuring nitrogen fixation by nodulated soybean calibrated by N methods

We report experiments to quantify the relationships between the relative abundance of ureide-N in root-bleeding sap, vacuum-extracted sap, and hot water extracts of stems and petioles of nodulated soybean (Glycine max [L.] Merrill cv Bragg) and the proportion of plant N derived from nitrogen fixation. Additional experiments examined the effects of plant genotype and strain of rhizobia on these relationships. In each of the five experiments reported, plants of cv Bragg (experiment 1), cv Lincoln (experiments 3, 4, 5), or six cultivars/genotypes (experiment 2) were grown in a sand:vermiculite mixture in large pots in a naturally lit, temperature-controlled glasshouse during summer. Pots were inoculated at sowing with effective Bradyrhizobium japonicum CB1809 (USDA 136) or with one of 21 different strains of rhizobia. The proportions of plant N derived from nitrogen fixation were determined using ¹⁵N dilution. In one experiment with CB1809, plants were supplied throughout growth with either N-free nutrients or with nutrients supplemented with 1, 2, 4, or 8 millimolar ¹⁵N-nitrate and harvested on eight occasions between V6 and R7 for root-bleeding sap, vacuum-extracted sap, stems (including petioles), and whole plant dry matter. Analyses of the saps and stem extracts for ureides (allantoin plus allantoic acid), α-amino-N, and nitrate, and of dry matter for N and ¹⁵N, indicated a positive effect of nitrate supply on concentrations of nitrate in saps and extracts and a negative effect on ureides and on the proportion of plant N derived from nitrogen fixation. The relative abundance of ureide-N in root-bleeding sap, vacuum-extracted sap (100 [ureide-N]/[ureide-N+ α-amino-N + nitrate-N]) and stem extracts (100 [ureide-N]/[ureide-N + nitrate-N]) and the proportion of plant N, derived from nitrogen fixation between successive samplings were highly correlated (r = 0.97-1.00). For each variable, two standard curves were prepared to account for the shifts in the compositions of N solutes of xylem saps and extracts after flowering which were not related to a change in nitrogen fixation. Relationships between relative ureide-N and the proportion of plant N derived from nitrogen fixation were not affected by plant genotype or by strain of rhizobia. Therefore, assessment of nitrogen fixation by soybean using the ureide technique should now be possible with the standard curves presented, irrespective of genotype or strain of rhizobia occupying the nodules.     

Arthrobacter pokkalii sp nov,a Novel Plant Associated Actinobacterium with Plant Beneficial Properties,Isolated from Saline Tolerant Pokkali Rice,Kerala, India

A novel yellow colony-forming bacterium, strain P3B162^T was isolated from the pokkali rice rhizosphere from Kerala, India, as part of a project study aimed at isolating plant growth beneficial rhizobacteria from saline tolerant pokkali rice and functionally evaluate their abilities to promote plant growth under saline conditions. The novel strain P3B162^T possesses plant growth beneficial traits such as positive growth on 1-aminocyclopropane-1-carboxylic acid (ACC), production of indole acetic acid (IAA) and siderophore. In addition, it also showed important phenotypic characters such as ability to form biofilm and utilization of various components of plant root exudates (sugars, amino acids and organic acids), clearly indicating its lifestyle as a plant rhizosphere associated bacterium. Taxonomically, the novel strain P3B162^T was affiliated to the genus Arthrobacter based on the collective results of phenotypic, genotypic and chemotaxonomic analyses. Moreover, molecular analysis using 16S rRNA gene showed Arthrobacter globiformis NBRC 12137^T, Arthrobacter pascens DSM 20545^T and Arthrobacter liuii DSXY973^T as the closely related phylogenetic neighbours, showing more than 98% 16S rRNA similarity values, whereas the recA gene analysis displayed Arthrobacter liuii JCM 19864^T as the nearest neighbour with 94.7% sequence similarity and only 91.7% to Arthrobacter globiformis LMG 3813^T and 88.7% to Arthrobacter pascens LMG 16255^T. However, the DNA-DNA hybridization values between strain P3B162^T, Arthrobacter globiformis LMG 3813^T, Arthrobacter pascens LMG 16255^T and Arthrobacter liuii JCM 19864^T was below 50%. In addition, the novel strain P3B162^T can be distinguished from its closely related type strains by several phenotypic characters such as colony pigment, tolerance to NaCl, motility, reduction of nitrate, hydrolysis of DNA, acid from sucrose, cell wall sugars and cell wall peptidoglycan structure. In conclusion, the combined results of this study support the classification of strain P3B162^T as a novel Arthrobacter species and we propose Arthrobacter pokkalii sp.nov.as its name. The type strain is P3B162^T (= KCTC 29498^T = MTCC 12358^T).     

Allantoin and Allantoic Acid in Tissues and Stem Exudate from Field-grown Soybean Plants

Samples of stem exudate and plant tissue collected from field-grown soybean (Glycine max [L.] Merr.) plants were analyzed for allantoin and allantoic acid. Nitrogen in nitrate plus amino acids exceeded ureide N concentration in stem exudate prior to flowering. During all of reproductive development (from about 40 days after planting until maturity), ureide N concentration was two to six times greater than amino acid plus nitrate N concentration. Allantoin and allantoic acid, not asparagine, are the principal forms of nitrogen transported from nodulated roots to shoots of the soybean plant. During pod and seed development ureide N comprised as high as 2.3, 37.7, and 15.8% of total N in leaf blades, stems + petioles, and fruits, respectively. The concentration of ureide in stems and fruits declined to nearly zero at maturity.     

Utilization of organic phosphorus in calcium chloride extracts of soil by barley plants and hydrolysis by acid and alkaline phosphatases

Organic phosphorus is often a major part of total phosphorus in soil solution. The role of this fraction as a P source for plants and the mechanism involved in its transfer from soil to plant is still unclear. We studied the utilization of organic phospharus in 0.01 M calcium chloride extracts by barley and its hydrolysis by isolated acid and alkaline phosphatases. Calcium chloride extracts were used as a nutrient solution in 24 hrs assays. Concentration of organic and inorganic P in equilibrium calcium chloride extracts was 7.8 and 1.8 µmol P L^-1, respectively, which was similar to the soil solution P concentration. When soil microbial biomass was destroyed by autoclaving, organic P concentration increased to 64.8 µmol P L^-1 whereas the inorganic P was hardly changed. Inoculation of the autoclaved soil with non-sterile soil and incubation for 5 days decreased the organic P concentration to 27.9 µmol P L^-1 but did not change inorganic P. In this study barley plants utilized organic P from all extracts. The greatest reduction of organic P concentration occurred in fresh extracts of the autoclaved soil. Inorganic P was depleted to traces in all extracts. Organic P was hydrolyzed by isolated acid and alkaline phosphatases. We conclude that organic P in soil solution is a heterogeneous pool of organic P compounds originating from microbial biomass. Its initial availability to plants was nigh but its susceptibility to phosphatase hydrolysis was quickly reduced but not completely lost.     

Root exudates as mediators of mineral acquisition in low-nutrient environments

Plant developmental processes are controlled by internal signals that depend on the adequate supply of mineral nutrients by soil to roots. Thus, the availability of nutrient elements can be a major constraint to plant growth in many environments of the world, especially the tropics where soils are extremely low in nutrients. Plants take up most mineral nutrients through the rhizosphere where micro-organisms interact with plant products in root exudates. Plant root exudates consist of a complex mixture of organic acid anions, phytosiderophores, sugars, vitamins, amino acids, purines, nucleosides, inorganic ions (e.g. HCO₃ ^–, OH^–, H⁺), gaseous molecules (CO₂, H₂), enzymes and root border cells which have major direct or indirect effects on the acquisition of mineral nutrients required for plant growth. Phenolics and aldonic acids exuded directly by roots of N₂-fixing legumes serve as major signals to Rhizobiaceae bacteria which form root nodules where N₂ is reduced to ammonia. Some of the same compounds affect development of mycorrhizal fungi that are crucial for phosphate uptake. Plants growing in low-nutrient environments also employ root exudates in ways other than as symbiotic signals to soil microbes involved in nutrient procurement. Extracellular enzymes release P from organic compounds, and several types of molecules increase iron availability through chelation. Organic acids from root exudates can solubilize unavailable soil Ca, Fe and Al phosphates. Plants growing on nitrate generally maintain electronic neutrality by releasing an excess of anions, including hydroxyl ions. Legumes, which can grow well without nitrate through the benefits of N₂ reduction in the root nodules, must release a net excess of protons. These protons can markedly lower rhizosphere pH and decrease the availability of some mineral nutrients as well as the effective functioning of some soil bacteria, such as the rhizobial bacteria themselves. Thus, environments which are naturally very acidic can pose a challenge to nutrient acquisition by plant roots, and threaten the survival of many beneficial microbes including the roots themselves. A few plants such as Rooibos tea (Aspalathus linearis L.) actively modify their rhizosphere pH by extruding OH^– and HCO₃ ^– to facilitate growth in low pH soils (pH 3 – 5). Our current understanding of how plants use root exudates to modify rhizosphere pH and the potential benefits associated with such processes are assessed in this review.