Exudation Of Organic Compounds Research Articles

Cluster roots of Embothrium coccineum modify their metabolism and show differential gene expression in response to phosphorus supply

Embothrium coccineum produces cluster roots (CR) to acquire sparingly soluble phosphorus (P) from the soil through the exudation of organic compounds. However, the physiological mechanisms involved in carbon drainage through its roots, as well as the gene expression involved in the biosynthesis of carboxylates and P uptake, have not been explored. In this work, we evaluated the relationship between carboxylate exudation rate and phosphoenolpyruvate carboxylase (PEPC) activity in roots of E. coccineum seedlings grown in a nutrient-poor volcanic substrate. Second, we evaluated CR formation and the expression of genes involved in the production of carboxylates (PEPC) and P uptake (PHT1) in E. coccineum seedlings grown under three different P supplies in hydroponic conditions. Our results showed that the carboxylate exudation rate was higher in CR than in non-CR, which was consistent with the higher PEPC activity in CR. We found higher CR formation in seedlings grown at 5 μM of P supply, concomitant with a higher expression of EcPEPC and EcPHT1 in CR than in non-CR. Overall, mature CR of E. coccineum seedlings growing on volcanic substrates poor in nutrients modify their metabolism compared to non-CR, enhancing carboxylate biosynthesis and subsequent carboxylate exudation. Additionally, transcriptional responses of EcPEPC and EcPHT1 were induced simultaneously when E. coccineum seedlings were grown in P-limited conditions that favored CR formation. Our results showed, for the first time, changes at the molecular level in CR of a species of the Proteaceae family, demonstrating that these root structures are highly specialized in P mobilization and uptake.

Biological Nitrification Inhibition (BNI): Phenotyping of a Core Germplasm Collection of the Tropical Forage Grass Megathyrsus maximus Under Greenhouse Conditions.

Modern intensively managed pastures that receive large external nitrogen (N) inputs account for high N losses in form of nitrate (NO3–) leaching and emissions of the potent greenhouse gas nitrous oxide (N2O). The natural plant capacity to shape the soil N cycle through exudation of organic compounds can be exploited to favor N retention without affecting productivity. In this study, we estimated the relationship between biological nitrification inhibition (BNI), N2O emissions and plant productivity for 119 germplasm accessions of Guineagrass (Megathyrsus maximus), an important tropical forage crop for livestock production. This relation was tested in a greenhouse experiment measuring BNI as (i) rates of soil nitrification; (ii) abundance of ammonia-oxidizing bacteria (AOB) and archaea (AOA); and (iii) the capacity of root tissue extracts to inhibit nitrification in vitro. We then measured N2O emissions, aboveground biomass and forage nutrition quality parameters. Reductions on nitrification activity ranging between 30 and 70% were found across the germplasm collection of M. maximus. Accessions with low nitrification rates showed a lower abundance of AOB as well as a reduction in N2O emissions compared to accessions of high nitrification rates. The BNI capacity was not correlated to N uptake of plants, suggesting that there may be intraspecific variation in the exploitation of different N sources in this grass species. A group of accessions (cluster) with the most desirable agronomic and environmental traits among the collection was identified for further field validation. These results provide evidence of the ability of M. maximus to suppress soil nitrification and N2O emissions and their relationship with productivity and forage quality, pointing a way to develop N conservative improved forage grasses for tropical livestock production.

Analysis of wild sunflower (Helianthus annuus L.) root exudates using gas chromatography‐mass spectrometry

AbstractPlant root systems mediate ecological processes in the rhizosphere through the exudation of organic compounds. Although exudate composition is thought to depend strongly on plant nutrient status, little is known about the influence of multi‐nutrient stresses. In this study, we examined responses to short‐term (3 d) nutrient limitation in Helianthus annuus (common sunflower), and root exudates were collected for 2, 4, or 6 h with the trap‐solution method. Root exudates, analyzed by means of gas chromatography‐mass spectrometry, consisted of over 60 sugars, sugar alcohols, amino acids, organic acids, and phosphates, with sugars and organic acids generally detected in the highest quantities. Twenty‐five of the detected metabolites, including half of the organic acids, sugars, and sugar alcohols, differed in relative abundance among the three sampling intervals, exhibiting higher abundance in sampling intervals greater than 2 h. Similarly, 24 of the detected metabolites, including half of the amino acids, phosphates, and sugar alcohols, were affected by nutrient supply, with 20 exhibiting higher abundance in the high‐nutrient treatment. Fumaric acid, quinic acid, and glucose were detected at significantly higher levels in the low‐nutrient treatment, potentially representing an adaptive response to nutrient limitation in sunflower. However, as sampling interval exerted a strong influence on the apparent effects of nutrient supply, future studies should consider the potential impacts of sampling‐interval length in comparative analyses of genotypes or treatments.

Improving potassium acquisition and utilisation by crop plants

AbstractTo avoid loss of yield, crops must maintain tissue potassium (K) concentrations above 5–40 mg K (g DM)–1. The supply of K from the soil is often insufficient to meet this demand and, in many agricultural systems, K fertilisers are applied to crops. However, K fertilisers are expensive. There is interest, therefore, in reducing applications of K fertilisers either by improving agronomy or developing crop genotypes that use K fertilisers more efficiently. Agronomic K fertiliser use efficiency is determined by the ability of roots to acquire K from the soil, which is referred to as K uptake efficiency (KUpE), and the ability of a plant to utilise the K acquired to produce yield, which is referred to as K utilisation efficiency (KUtE). There is considerable genetic variation between and within crop species in both KUpE and KUtE, and chromosomal loci affecting these characteristics have been identified in Arabidopsis thaliana and several crop species. Plant traits that increase KUpE include (1) exudation of organic compounds that release more non‐exchangeable soil K, (2) high root K uptake capacity, (3) early root vigour, high root‐to‐shoot ratios, and high root length densities, (4) proliferation of roots throughout the soil volume, and (5) high transpiration rates. Plant traits that increase KUtE include (1) effective K redistribution within the plant, (2) tolerance of low tissue K concentrations, and, at low tissue K concentrations, (3) maintenance of optimal K concentrations in metabolically active cellular compartments, (4) replacement of K in its non‐specific roles, (5) redistribution of K from senescent to younger tissues, (6) maintenance of water relations, photosynthesis and canopy cover, and (7) a high harvest index. The development of crop genotypes with these traits will enable K fertiliser applications to be reduced.

Influence of mature Douglas fir roots on the solid soil phase of the rhizosphere and its solution chemistry

Plants can induce significant changes in the rhizosphere through the uptake of water and ions, the exudation of organic compounds and the activities of micro-organisms. The aim of the present study was to assess the influence of tree roots on the chemistry (pH, exchangeable cations, total organic carbon) of both the solid phase of the soil and the soil solutions, extracted by centrifugation, under a mature Douglas fir stand over two distinct seasons (March and September, 1999). The chemical characteristics of either the solid soil phase or the soil solutions of the rhizosphere were found to be different from those of the bulk soil. The cation exchange capacity, base saturation and organic C were all greater in the rhizosphere than in the bulk soil, as a possible result of rhizodeposition, incorporation of decaying root material and micro-organism activity. The concentration of all elements increased in the rhizosphere solutions as compared to the bulk soil solutions, except for P. The pH was lower in the rhizosphere than in the bulk soil for both the solid soil phase and the soil solutions. Despite the greater overall Al concentration of the rhizosphere solutions, as compared to the bulk soil solutions, we suggest that in both, Al toxicity was efficiently restricted by both high Ca + Mg contents and Al complexation with various ligands.

Importance of rhizodeposition in the coupling of plant and microbial productivity

SummaryPlant roots influence the biological, chemical and physical properties of rhizosphere soil. These effects are a consequence of their growth, their activity and the exudation of organic compounds from them. In natural ecosystems, the linkages between inputs of carbon from plants and microbial activity driven by these inputs are central to our understanding of nutrient cycling in soil and the productivity of these systems. This coupling of plant and microbial productivity is also of increasing importance in agriculture, where the shift towards low‐input systems increases the dependence of plant production on nutrient cycling, as opposed to fertilizers. This review considers the processes by which plants can influence the cycling of nutrients in soil, and in particular the importance of organic inputs from roots in driving microbially mediated transformations of N. This coupling of plant inputs to the functioning of the microbial community is beneficial for acquisition of N by plants, particularly in low‐input systems. This occurs through stimulation of microbes that produce exoenzymes that degrade organic matter, and by promoting cycling of N immobilized in the microbial biomass via predation by protozoa. Also, plants increase the cycling of N by changes in exudation in response to nitrogen supply around roots, and in response to browsing by herbivores. Plants can release compounds in exudates that directly affect the expression of genes in microbes, and this may be an important way of controlling their function to the benefit of the plant.

Effects of defoliation and atmospheric CO2 depletion on nitrate acquisition, and exudation of organic compounds by roots of Festuca rubra

The aim of this study was to investigate the physiological basis of increased root exudation from Festuca rubra, in response to defoliation. The hypothesis, that assimilate supply to roots is a key determinant of the response of root exudation to defoliation was tested by imposing CO2-deplete (< 50 μmol mol−1) atmospheres to F. rubra. This was done as a non-destructive means of preventing supply of new assimilate to roots of intact and defoliated plants. F. rubra was grown in axenic sand systems, with defoliation and CO2-depletion treatments applied to plants at 14 and 35 days after planting. Root exudation and NO3 − uptake were quantified throughout, and post-treatment uptake and allocation of N were determined from the distribution of 15N label, supplied as 15NO3 −. Defoliation of F. rubra resulted in significantly (P <0.01) increased root exudation, CO2-depletion did not result in increased exudation from plants of either age. When treatments were applied to F. rubra after 14 days, defoliation and CO2-depletion each reduced NO3 − uptake significantly (P <0.05). However, in older plants, uptake of NO3 − was less sensitive to defoliation and CO2-depletion. The results indicate that increased root exudation following defoliation is not related directly to reduced assimilate supply to roots. This was evident from the lack of effect of CO2-depletion on root exudation, and the absence of correlation between root-C efflux and the rate of NO3 − uptake. The physiological basis of increased exudation following defoliation remains uncertain, but may be dependent on physical damage, either directly or as a consequence of systemic responses to wounding.

Amino acid sequence of proteins induced in Fe-deficient stressed alfalfa (Medicago sativa L.) roots

Alfalfa (Medicago sativa L.) grows well in soils with a moderately high pH and dissolves insoluble iron in the rhizosphere. We have investigated active uptake mechanisms under Fe-deficient nutrient conditions and the effects of Fe-deficiency on plants. Previously, we observed that Fe-deficient alfalfa roots exuded many compounds (Masaoka et al. 1993) such as fiavonoids. We also identified a new compound “alfafuran” which is a phenol compound and is different from organic acids or phytosiderophore-type amino acid derivatives exuded by Fe-deficient plant roots. This compound is also very effective in dissolving ferric phosphate (Noguchi et al. 1994), suggesting that alfalfa may have developed several strategies against Fe-deficient stress including the exudation of organic compounds like alfafuran which accelerate the Fe3+-reducing activity on the root cell membrane to dissolve insoluble iron compounds. Suzuki et al. (1995, 1997) observed that in barley several peptide spots obtained by electrophoresis were induced under Fe-deficient stress when mugineic acid-family phytosiderophores were secreted from the roots. They suggested that these peptides control the mugineic acid synthesis and secretion. We examined the peptides induced in Fe-deficient alfalfa roots.

Mangrove forest productivity and biomass accumulation in Hinchinbrook Channel, Australia

Data on stand structure and rates of photosynthesis were used to estimate net canopy carbon fixation and carbon accumulation as living biomass in mangrove forests in Hinchinbrook Channel, Australia. Total annual canopy net carbon fixation was estimated to be about 29 t C ha−1 yr−1. This equates to about 204,000 t C yr−1 for all mangrove forests in Hinchinbrook Channel. Of this, only about 12% was stored as living plant biomass. Although it is not yet possible to present a robust carbon balance for mangrove trees, the remainder is presumably lost through plant respiration, litter fall, root turnover and exudation of organic compounds from roots.

Combined Effect of Inorganic Carbon and Different Nitrogen Sources in the Growth Media on Biomass Production and Nitrogen Uptake in Young Willow and Birch Plants

Summary The combined effects of inorganic carbon in the nutrient solution and different nitrogen sources on biomass production were studied in young plants of birch ( Betula pendula Roth.) and willow ( Salix sp., clone no. DK 6351) grown in hydroponic culture media for 35 (birch) and 37 (willow) days. The plants were randomly assigned to six different treatments, in which three different nitrogen sources (ammonium, nitrate or both ions in a combination of 1:1) were used with or without bicarbonate. In all treatments, inorganic carbon increased the biomass production of birch by ca 10 % compared with the controls. In willow, when nitrate was the source of nitrogen, the biomass production was ca. 50 % greater with bicarbonate than without bicarbonate. When ammonium was the nitrogen source, the effect of inorganic carbon on growth of willow was smaller, and the dry mass production was only 19 % greater than that of the controls. When ammonium and nitrate were given as 1:1 combination, growth of willow was 19 % less with bicarbonate than without bicarbonate. In willows, the large increase in biomass production obtained with the combination of bicarbonate and nitrate was associated with a great loss of inorganic carbon from the roots and decreased exudation of organic compounds.

EXUDATION OF 14C‐LABELLED COMPOUNDS FROM WHEAT ROOTS: INFLUENCE OF NUTRIENTS, MICRO‐ORGANISMS AND ADDED ORGANIC COMPOUNDS

SUMMARYWhen tops of 6‐day wheat seedlings grown in nutrient solution, sterile or inoculated with soil, received 14CO2, it was found that the radioactivity of the solution measured after 24 and 48 hours was reduced by the presence of micro‐organisms. Omission of phosphate from the sterile growth medium reduced the radioactivity of the root solution to 47% of the control. The addition of acetate buffer at pH 5 greatly increased exudation whereas the adjustment to a similar pH with H2SO4 or universal buffer or the addition of sodium acetate at pH 7 significantly reduced exudation.These results demonstrate that exudation of organic compounds from roots can be affected by the addition of substrates and butters used in root enzyme studies. From these results, it appears that some organic compounds in soil could influence the loss of carbon compounds from roots growing under natural conditions.

Diffusion of Carbon Compounds Away from Wheat Roots

The exudation of organic compounds from intact roots has been reported for many plant species, but most of these results record only the water-soluble exudates and provide no information on the sites of the exudation. Pearson and Parkinson (1961) and Schroth and Snyder (1961) reported greater exudation of compounds which reacted with ninhydrin and silver nitrate from the root tip and the elongating zone of the roots of seedlings of broad bean and French bean than from the older parts of the roots; while McDougall (1968) has reported greater exudation of 14C-labelled compounds from the lateral root zones of wheat than from the tip region. Bowen (1968) found that with Pinus radiata seedlings proportionately greater loss of 36Cl occurred from the apical region of the roots than from the basal region.