• Aluminium (Al) stress reduces plant growth. However, some species such as Norway spruce (Picea abies) seem to tolerate high Al concentrations. The aim of this study was to investigate characteristics possibly involved in Al tolerance in Norway spruce seedlings. • Seedlings (10-d-old) were exposed to Al3+ concentrations of 0.5 and 5 mm for up to 168 h. The effect of Al stress on root growth, cell morphology and Al distribution, callose production, and peroxidase and chitinase activity was analysed. • Root growth decreased after 1 d and 2 d with 5 and 0.5 mm Al, respectively. Callose concentration increased strongly after 6 h treatment with 5 mm Al. The activity of many peroxidase and chitinase isoforms decreased after 1–24 h exposure of both treatments. Several isoforms increased after 48–168 h exposure to 5 mm Al. • We postulate that, with external Al concentrations 0.5 mm or lower, an increased production above constitutive levels of peroxidase or chitinase is not required for Al tolerance in young Norway spruce seedlings. High constitutive levels of peroxidase and chitinase in this species may be part of this Al tolerance.
No abstract is provided for this article.
The trivalent cation aluminum can cause chronic cytotoxicity in plants, animals and microorganisms. It has been suggested that Al interaction with cell membranes and enzyme metal binding sites may be involved in Al cytotoxicity. In this study, the binding of Al to microsomes and liposomes was found to be lipid dependent with the signal transduction element phosphatidylinositol‐4,5‐bisphosphate having the highest affinity for Al with an Al:lipid stoichiometry of 1:1. Al binding was only reduced in the presence of high concentrations of Ca 2+ (>1 mM). Both citrate and, to a lesser extent, malate were capable of preventing Al lipid binding, which is consistent with the involvement of these organic acids in a recently described Al detoxification mechanism in plants. The effects of AlCl 3 , Al‐citrate and ZnSO 4 on metal‐dependent enzyme activities (enolase, pyruvate kinase, H + ‐ATPase, myosin, Calpain, proteinase K, phospholipase A 2 and arginase) was assayed in vitro. While Zn 2+ was capable of inhibiting all the enzymes except the H + ‐ATPase, AlCl 3 and Al‐citrate had minimal effects except for with phospholipase A 2 where an interaction with AlCl 3 occurred. However, this could be negated by the addition of citrate. The results indicate that, contrary to current hypotheses, the toxic mode of Al is not through an interaction with enzymatic catalytic metal binding sites but may be through the interaction with specific membrane lipids.
To enable quantification of mycelial abundance in mixed-species environments, eight new TaqMan® real-time PCR assays were developed for five arbuscular mycorrhizal fungal (AMF, Glomeromycota) taxa. The assays targeted genes encoding 18S rRNA or actin, and were tested on DNA from cloned gene fragments, from spores, mycelia, and from root-free soil, and on reverse-transcribed rRNA templates from entire mycelia and from colonized roots. The assays showed high specificity, sensitivity, and reproducibility, enabling reliable quantitation over broad ranges of template molecules. From cultured mycelia, DNA and RNA measures both correlated with spore number rather than extraradical hyphal length, and epifluorescence microscopy identified pronounced heterogeneity in vitality and nuclear distribution in hyphae. Root colonization was also spatially heterogeneous, as shown by a mixing experiment with root fragments of different length. Therefore, although real-time PCR can reproducibly and accurately quantify AMF nucleic acids, these are poorly correlated with visual measures because of spatial heterogeneity.
No abstract is provided for this article.
Acidifying slurry with sulfuric acid (H2SO4) is practiced in some countries to help reduce ammonia (NH3) emissions during slurry storage and spreading to land. However, knowledge of how the application of acidified slurry affects soil health and nutrient cycling is lacking. This is particularly important since acidification with H2SO4 supplies an additional source of sulfur (S) to increasingly S-deficient agricultural soils. We hypothesized (i) that slurry acidification would increase the dissolved C, N, P and S content of the slurry, and (ii) that after application to soil it would reduce soil pH and promote nutrient availability. Using laboratory mesocosms we monitored nutrient dynamics (C, N, P and S) in an agricultural soil receiving cattle slurry (±acidification) in comparison to soil receiving no slurry, over a 2-month period. Measurements included greenhouse gases (CO2, CH4, N2O), solute concentrations (NO3-, NH4+, PO43-, SO42-, DOC, DON, EC, pH) in soil pore water, and soil extractions using distilled water and K2SO4. There were six treatments: (1) soil (control), (2) soil + slurry, (3) soil + acidified slurry (with H2SO4, pH =5.5), (4) soil + HCl, (5) soil + K2SO4, and (6) soil + K2SO4 + H2SO4. Our results showed that slurry acidification reduced soil pH and available P but increased electrical conductivity. In contrast, acidification did not affect the concentration of water extractable-SO42- despite evidence for microbial S consumption. Acidification treatments (3, 4 and 6) stimulated N mineralization and DOC production, likewise increased nitrification. Further, acidification did not affect N2O emissions but decreased net CO2 and CH4 emissions leading to an overall reduction in the soil’s total greenhouse gas footprint (expressed as CO2e). We conclude that application of acidified slurry to soil can reduce soil pH over a 2-month period with no negative impacts seen on soil and slurry C, N, P and S dynamics.
It has been reported that plant roots can directly utilise soil organic-N in the form of amino acids without prior mineralisation by the soil's microbial biomass. To critically assess this, however, requires a knowledge of microbial amino acid-N turnover times in soil. The effects of soil type, depth and temperature on the uptake and partitioning of a mixture of 15 14C-labelled amino acids by the soil's microbial biomass was therefore studied in 10 contrasting soil types. The results indicated that the degradation of amino acids was soil dependent but that the mean half-life in topsoils at 18°C was 1.7±0.6 h, whilst in subsoils the mean half-life was 12.2±3.3 h. On average 34% of the amino acid-C was respired as CO2 whilst 66% was utilised for new cell biomass. Amino acid decomposition increased with soil temperature, however, rapid rates of amino acid uptake and assimilation were also observed at 5°C (mean half-life in topsoil=2.9±1.5 h). Little correlation was observed between amino acid half-life and either microbial yield, soil arginase activity or organic matter content (r 2<0.40), however, decomposition did appear to be weakly related with soil respiration. The high concentration of amino acids used here (5 mM) was intended to simulate amino acid release after root cell lysis. For previously reported lower concentrations in the bulk soil solution, half lives can be predicted to be even less based on microbial amino acid transport kinetics. The significance of this previously overlooked microbial decomposition of amino acids in the utilisation of organic N by plants is discussed.
