The growth of saprophytic fungi on root surfaces

Summary Data are provided on the growth of non-pathogenic soil fungi on root surfaces. By growing roots from non-sterile soil into sterile vermiculite, and by regular isolation of fungi from the roots which had grown into the sterile vermiculite, it was shown that fungal growth down roots is slow. It is suggested that in the colonization of roots by non-pathogenic fungi successive lateral colonization from the soil is of greater importance than the growth of fungi down roots.     

Mobilization of phosphate in different soils by ryegrass supplied with ammonium or nitrate

Mobilization of soil P as the result of plant-induced changes of soil pH in the vicinity of plant roots was studied. Seedlings of ryegrass were grown in small containers separating roots from soil by a 30-μm meshed nylon screen which root hairs could penetrate but not roots. Two soils were used, a luvisol containing P mainly bound to calcium and an oxisol containing P mainly bound (adsorbed) to iron and aluminum. Plant-induced changes of soil pH were brought about by application of ammonium-or nitrate-nitrogen. After plants had grown for 10 d the soil was sliced in thin layers parallel to the root mat which had developed on the screen, and both soil pH and residual P determined. Mobilization of P was assessed by P-depletion profiles of the rhizosphere soil. Soil pH at the root surface decreased by up to 1.6 units as the result of ammonium N nutrition and it increased by up to 0.6 units as the result of nitrate N nutrition. These changes extended to a distance between 1 and 4 mm from the root surface depending on the type of soil and the source and level of nitrogen applied. In the luvisol, compared to zero-N treatment, P mobilization increased with the NH₄-induced decrease in pH, whereas the NO₃-induced pH increase had no effect. In contrast, in the oxisol a similar pH decrease caused by NH₄ nutrition had no effect, whereas the pH increase caused by NO₃ increased markedly the mobilization of soil P. It is concluded that in the luvisol calcium phosphates were dissolved by acidification, whereas in the oxisol adsorbed phosphate was mobilized by ligand exchange.     

Hydraulic lift: Substantial nocturnal water transport between soil layers by Artemisia tridentata roots

Summary Diel soil water potential fluctuations reflected daytime depletion and nocturnal resupply of water in upper soil layers. Transpiration suppression experiments demonstrated that water absorption by roots caused the daytime depletion. The soil water potential data and experimental results suggest that at night water absorbed from moist soil by deeper roots is transported to and lost from roots into drier upper soil layers. The deeper roots appear to absorb and transport water both day and night. Implications for the efficiency of deep roots and water storage, nutrient uptake and water parasitism in upper soil layers are discussed.     

Studies on the attractional effect of root exudates on hyphal growth of an arbuscular mycorrhizal fungus in a soil compartment-membrane system

The effects of tomato and bean rhizospheres on hyphal spreading of the arbuscular mycorrhizal (AM) fungus Glomus mosseae were studied using a soil compartment system in combination with hydrophobic polytetrafluorethylene (PTFE) membranes. Both the nylon screen and the PTFE membrane were freely permeable to hyphae but not to roots. Furthermore, the hydrophobic PTFE membrane seemed to be a barrier to the flux of soil solutions containing root exudates. The results show that water soluble exudates of tomato and bean roots greatly stimulate hyphal growth in the soil compartment system used. Moreover, water soluble root exudates of bean exert a clear attractional effect on AM hyphal growth.     

Influence of plant roots on C and P metabolism in soil

Summary A technique for studying the modification of soil by plant roots is described. Using it, soil zones differently affected by plant roots can be separated for subsequent analysis. With this method, the transfer of C from roots of¹⁴C-labelled maize plants into soil and the change in soil C and P fractions were investigated.The results show that the C released from roots to soil was 13% of the total assimilated C. The remaining root-derived C in soil was relatively small (15%). Maize roots induced a decrease in organic soil C and in both total and isotopically exchangeable soil P. On the other hand they increased the microbial biomass C, phosphatase activity, bicarbonate extractable organic P and phospholipid P and enhanced the incorporation of³²P into organic P fractions. Both root C and root influences were detectable outside the immediate root zone.These results demonstrate an intensive C turnover and P mobilization in the rhizosphere soil, including some organic P fractions, and suggest that the actual rhizosphere may be greater than is generally assumed.     

A comparison of penetrometer pressures and the pressures exerted by roots

Summary Previous work is reviewed in which the ratio of the pressures required for soil penetration by roots and penetrometers are compared. It appears that this ratio can vary from about 2 to 8 depending on conditions. However, there is very little experimental evidence and most of the work has been inferential.Direct measurements are reported for the stresses exerted by a 1 mm diameter penetrometer probe and by the roots of pea seedlings when penetrating Urrbrae fine sandy loam. Six soil conditions were used: (non-weathered remoulded soil cores + artificially weathered remoulded soil cores + undisturbed field clods) × (confined + unconfined cores or clods). The confinement treatment was to test for any effects of additional restraint to cylindrical root expansion. The weathering and field clod treatments were to test the hypothesis that root elongation is facilitated by tensile failure ahead of the root tip.The principal conclusions are as follows. The laboratory weathering treatment reduced the soil tensile strength by 25%. This resulted in a small but significant reduction in the pressure for root penetration into confined cores. Compared with remoulded non-weathered cores, field clods had a 2 to 3 fold greater penetrometer resistance and a 50% lower tensile strength. The force required for root penetration into unconfined field clods was only 10% greater than for unconfined non-weathered cores. For the former (which is closest to field conditions) the penetrometer had to exert a pressure 5.1 times greater than a root tip in order to penetrate the soil. Penetrometer penetration pressure was independent of probe diameter in the 1–2 mm range in the soil used. Core confinement restricts root radial expansion and modifies the penetration force of metal probes and plant roots.On the basis of the new results it is tentatively concluded that soil tensile failure can facilitate penetration by roots.     

Soil aggregation status and rhizobacteria in the mycorrhizosphere

Soil aggregation is a dynamic process in which plants and the soil microbiota play a major role. This experiment was conducted to determine whether the effects of mycorrhizae on the stability of water-stable soil aggregates (WSA) and on selected groups of soil microorganisms are interrelated. Soil containers consisting of four compartments were utilized. Two compartments on each side of a solid barrier were separated by a 43 m screen that permitted the passage of hyphae, but not of roots. The roots of Sorghum bicolor plants were split over the center barrier, and the roots on one side were inoculated with an arbuscular-mycorrhizal (AM) fungus. This design produced mycorrhizosphere soils (M) by AM roots or hyphosphere (H) soils by AM hyphae in the two compartments on the one side of the barrier, and rhizosphere soils (R) by nonAM roots or root- and hypha-free bulk soil (S) in the two compartments on the other side. At harvest (10 wk), there were significant differences in WSA between soils in the order: M>R>H>S, and WSA stability was significantly correlated with root or hyphal length. Numbers of colony-forming units of the microflora (total bacteria, actinomycetes, anaerobes, P solubilizers, and nonAM fungi) were in general not correlated with root or hyphal length, but in some cases were significantly correlated with WSA. Bacteria isolated from the water-stable soil-aggregate fraction tended to be more numerous than from the unstable fraction. The difference was significant in the M soil for total bacteria and P solubilizing bacteria. NonAM fungi were more numerous in the unstable fraction of the M soil. The data show that the root and fungal components of mycorrhizae enhance WSA stability individually and additively in concert, and suggest that they affect microorganism numbers indirectly by providing a favorable and protective habitat through the creation of habitable pore space in the WSA.     

The behaviour of roots encountering cracks in soil

Summary Experimental methods are described for observing the behaviour of roots encountering cracks in soil. The proportions of roots which enter a second soil block after crossing a crack of known width were measured. Soil strength was measured with a penetrometer.Results are presented for the proportions of seminal roots of wheat and primary lateral roots of pea which enter moulded soil of various strengths after crossing cracks. Results are also presented for the proportions of seminal roots of pea, rape and safflower which enter undisturbed soil after crossing cracks.It was found that, in all cases, the proportion of roots penetrating the second soil block decreased with increasing crack width and increasing soil strength. Also, a smaller proportion of thinner roots penetrated the second soil block than thicker roots under similar conditions. Root diameter in the cracks was influenced by both crack width and soil strength, and an empirical equation is presented to describe this effect.     

A biophysical analysis of root growth under mechanical stress

The factors controlling root growth in hard soils are reviewed alongside summarised results from our recent studies. The turgor in cells in the elongation zone of roots pushes the apex forward, resisted by the external pressure of the soil and the tension in the cell walls. The external pressure of the soil consists of the pressure required to deform the soil, plus a component of frictional resistance between the root and soil. This frictional component is probably small due to the continuous sloughing of root cap cells forming a low-friction lining surrounding the root. Mechanically impeded roots are not only thicker, but are differently shaped, continuing to increase in diameter for a greater distance behind the root tip than in unimpeded roots. The osmotic potential decreases in mechanically impeded roots, possibly due to accumulation of solutes as a result of the slower root extension rate. This more negative osmotic potential is not always translated into increased turgor pressure, and the reasons for this require further investigation. The persistent effect of mechanical impedance on root growth is associated with both a stiffening of cell walls in the axial direction, and with a slowing of the rate of cell production.     

Estimation of the distribution of living roots of plants under field conditions

Summary A method is described for estimating the relative distribution of living roots of graminaceous plants growing under normal field conditions and which does not involve separation of roots from soil.It has been shown that 24 hours after rubidium-86 has been injected into the base of shoots its concentration is sufficiently uniform throughout the root system for the volume of roots in different zones of soil to be inferred from their content of the tracer. During this period outward diffusion from the root system is negligible and no rubidium-86 enters dead roots.The energy of the gamma radiation from rubidium-86 enables it to be measured accurately in soil samples of appreciable size (e.g. 3kg). Consequently it is possible, using suitable analytical equipment, to obtain representative information on root distribution at different depths in soil by combining the appropriate sections of numerous cores which have been withdrawn from the soil in the neighbourhood of injected plants. The advantages of this procedure lie in the fact that losses of root in the separation of the soil are avoided and that living roots of the species under investigation are alone measured, the time taken in this operation compares favourably with that occupied in the manual separation of roots from soil provided suitable analytical equipment is available.     

Potential errors caused by roots in analyses of rhizosphere soil

Roots contain high concentrations of many elements, and have the potential to interfere with measurements of chemical change in rhizosphere soil. To assess potential interferences, maize (Zea mays L.) roots (free of soil) and soil (free of roots) were extracted separately with several extractants commonly used to assess the status of soil nutrients. The maize roots were grown within filter envelopes which prevented direct contact with soil, but permitted passage of mineral nutrients and water from the adjacent soil. Water, ammonium acetate (pH 7), DTPA (pH 7.3), Morgan's solution (pH 4.8), and dilute HCl were used as extractants. Most elements were released readily into soluble forms from roots killed by freezing to lyse the cells. Significantly lower amounts were extracted from fresh roots, with the greatest differences between fresh and killed roots for the extractants H₂O and DTPA, which were the mildest in terms of acidity and salt concentration. Extraction of P from the fresh roots by H₂O and HCL was particularly low. Contamination of rhizosphere samples with root materials would almost certainly prevent the accurate measurement of water-soluble P, K, Mn, Zn, Cu, and Na in the slightly alkaline soil used in this experiment. Large errors would be likely also for P, Mn, and Cu extracted by ammonium acetate. The DTPA extractant is normally used only for micronutrient metals or heavy metals, and the small amounts of these elements released by roots should not contribute to significant error. With Morgan's solution, errors would likely be large only for P. Dilute HCl is a reasonably strong extractant for many elements in soil, and major errors from roots contained in rhizosphere samples are unlikely. The relatively high probability of errors in extractions of soluble elements from rhizosphere soil is unfortunate, because these elements are among the most readily available to plants and the most likely to be altered by the normal activities of roots.     

Water Movement Through Plant Roots

Mathematical analysis of the hydraulics of water movement throughplant roots, in terms of radial and axial resistances, has ledto equations which provide new insights into the effects ofthe component resistan ces on water uptake by and movement throughindividual roots and root systems. The ratio of axial to radialresistance determines the optimum length of a root and its totalresistance to water movement. The equations permit direct calculationsof the plant water potentials necessary, at the base of theplant, to sustain given flow rates through root systems withgiven characteristics. Lateral spacing and the resistance ofindividual laterals are the dominant factors determining totalflux per unit area into a root. When soil water potential increases with depth (surface layersdrier) root resistance tends to decrease with increasing flowrate; the reverse occurs when the surface is wetter than thelower layers. Calculated patterns of water movement into andthrough roots, in relation to soil water potential and flowrate through the root, indicate efflux from root to soil undercertain conditions. This is considered to reflect reality, althoughthe fluxes are probably transient or intermittent. The equations presented should be combined with equations describingwater movement through soil to define the behaviour of the wholeroot-soil system adequately.     

The role of root exudates and allelochemicals in the rhizosphere

Plant roots serve a multitude of functions in the plant including anchorage, provision of nutrients and water, and production of exudates with growth regulatory properties. The root–soil interface, or rhizosphere, is the site of greatest activity within the soil matrix. Within this matrix, roots affect soil structure, aeration and biological activity as they are the major source of organic inputs into the rhizosphere, and are also responsible for depletion of large supplies of inorganic compounds. Roots are very complicated morphologically and physiologically, and their metabolites are often released in large quantities into the soil rhizosphere from living root hairs or fibrous root systems. Root exudates containing root-specific metabolites have critical ecological impacts on soil macro and microbiota as well as on the whole plant itself. Through the exudation of a wide variety of compounds, roots impact the soil microbial community in their immediate vicinity, influence resistance to pests, support beneficial symbioses, alter the chemical and physical properties of the soil, and inhibit the growth of competing plant species. In this review, we outline recent research on root exudation and the role of allelochemicals in the rhizosphere by studying the case of three plants that have been shown to produce allelopathic root exudates: black walnut, wheat and sorghum     

Mobilization of cadmium from Festuca ovina roots and its simultaneous immobilization by soil in a root-soil-extractant system (in vitro test)

Mobilization of cadmium accumulated in Festuca ovina L. roots and simultaneous immobilization of this metal by soil were studied in three chambers connected into one system containing: (1) roots in an extractant solution, (2) soil in an extractant solution, and (3) extractant solution alone. Six extractants sterilized by filtration were used: 0.1 M NaNO₃ (NA), 0.1 mM desferrioxamine B (NA + DFOB) and 1 mM citric acid (NA + CA) in NA, and a water extract of soil (SE) supplemented with the same compounds. SE mobilized 53% of the Cd introduced to the system with roots. The addition of DFOB or CA to SE increased Cd extraction from roots by 17%, while the same compounds introduced to NA did not change mobilization of Cd (60% efficiency). Regardless of the extractant used, mobilization of Cd from roots was about 25% lower when extraction was done in a control system without soil. The metal released from roots was gradually immobilized by the soils loaded into all systems during a 4-day incubation. Sequential extractions of Cd from the soils showed that the metal released from roots with NA was stabilized mainly by soil Mn and Fe oxides, while that released with SE was stabilized by soil organic matter.     

Flavin mononucleotide (FMN)-based fluorescent protein (FbFP) as reporter for gene expression in the anaerobe Bacteroides fragilis

Several outbreaks caused by pathogenic bacteria are related to the consumption of raw produce contaminated by animal manure. The majority of these outbreaks have been linked to Salmonella spp. We examined the ability of Salmonella enterica serovar Weltevreden to persist and survive in manure and soil as well as disseminate to, and persist on, spinach roots and leaves. Significantly higher numbers of S. Weltevreden inoculated into manure and applied to soil before planting spinach were found in soil than in pot cultures, where the pathogen had been inoculated directly into soil 14 days postplanting. Moreover, the pathogen seemed to disperse from manure to spinach roots, as we observed a continuous increase in the number of contaminated replicate pot cultures throughout the evaluation period. We also found that, in some cases, S. Weltevreden present in the phyllosphere had the ability to persist for the entire evaluation period (21 days), with only slight reductions in cell numbers. The results from the present study show that S. Weltevreden is capable of persisting in soil, roots and shoots for prolonged periods, indicating the importance of strict monitoring of untreated animal manure before considering its application to agricultural land.     

Root development of the Zinc-hyperaccumulator plant Thlaspi caerulescens as affected by metal origin,content and localization in soil

Penetration into and exploitation of contaminated soils by roots of hyperaccumulator plants is a prerequisite for efficient removal of heavy metals, i.e. efficacy of phytoextraction. This work was undertaken to study the development of roots of the Zn-hyperaccumulator Thlaspi caerulescens under various conditions of soil contamination. Rhizoboxes were constructed with a removable plastic front cover, and filled with soils containing different amounts and forms of metals (Zn, Cd and Pb). Treatments were: homogeneous soil profile, superposition of three layers, inclusion of contaminated soil into uncontaminated soil, or inclusion of uncontaminated soil into uniformly contaminated soil. Four seedlings were transplanted into each rhizobox, and development of the root system was periodically recorded for 133 days. At harvest, the biomass and size of the rosette of aerial parts were determined. The aerial biomass/root length fraction as well as the kinetics of root development varied according to the presence and localization of Zn. The distribution and morphology of roots at harvest were strongly dependent upon the metal content and form in soil. Roots exhibited a high affinity for the Zn-contaminated patches and showed two distinct morphologies according to the concentration of Zn in soil. This revised version was published online in July 2006 with corrections to the Cover Date.     

土壤微生物膜生理生态功能研究进展

土壤微生物膜是由土壤细菌及其分泌物积聚形成的生物群落,是生物土壤结皮的初始形态和重要组成部分。作为土壤细菌生命过程中最典型的生存形式,土壤微生物膜不仅能保护基质内细胞生存,还可黏附于土壤颗粒和植物根系表面,发挥重要的生态功能。本文在解析土壤微生物膜结构与组成的基础上,从土壤质量与植物健康两个方面总结分析了土壤微生物膜生理生态功能:土壤微生物膜代谢活性高于游离细胞,可高效分泌胞外聚合物并且具有更强的有机物质转化速率,在提升土壤肥力,吸附、固持和降解土壤污染物和促进土壤团聚体形成方面具有重要意义;土壤微生物膜可通过多种微生物间协同作用、促进分泌多种促生物质与胞外聚合物以发挥固持作用等改善植物养分利用状况,增强植物抗逆性。揭示土壤微生物膜生态功能的微观机制、筛选和应用功能性土壤微生物膜是未来重要的发展方向。     

The distribution and abundance of wheat roots in a dense,structured subsoil – implications for water uptake

We analysed the abundance, spatial distribution and soil contact of wheat roots in dense, structured subsoil to determine whether incomplete extraction of subsoil water was due to root system limitations. Intact soil cores were collected to 1.6 m below wheat crops at maturity on a red Kandosol in southern Australia. Wheat roots, remnant roots, soil pores and root–soil contact were quantified at fresh breaks in the soil cores. In surface soil layers (<0.6 m) 30–40% of roots were clumped within pores and cracks in the soil, increasing to 85–100% in the subsoil (>0.6 m), where 44% of roots were in pores with at least three other roots. Most pores contained no roots, with occupancy declining from 20% in surface layers to 5% in subsoil. Wheat roots clumped into pores contacted the surrounding soil via numerous root hairs, whereas roots in cracks were appressed to the soil surface and had very few root hairs. Calculations assuming good root–soil contact indicated that root density was sufficient to extract available subsoil water, suggesting that uptake is constrained at the root–soil interface. To increase extraction of subsoil water, genetic targets could include increasing root–soil contact with denser root hairs, and increasing root proliferation to utilize existing soil pores.     

不同土壤深度落叶松细根分解及N动态变化

细根分解受根序和土壤深度的潜在影响。使用根序法分根,将落叶松Larix gmelini根系分为两类:一级根、二级根为一类(1—2级根),即低级根;三级根和四级跟为另一类(3—4级根),即高级根。采用埋袋法对落叶松低级根和高级根在不同土壤深度(0—10、10—20 cm和20—30 cm)进行了为期862 d的分解实验,探讨不同根序细根分解规律,养分释放及其影响。结果表明:1—2级根的分解速率比3—4级分解速率慢,这种规律同时存在于不同深度的土壤中。在空间上,低级根和高级根的分解速率均随土壤深度的增加而降低,高级根的降低趋势更明显。随着分解时间的进行,各个土层之间的分解率在低级根之间差异更大。细根分解过程中,落叶松不同根序养分的释放特征不同。N释放速率总体上随细根根序增加而增大,随土壤深度的增加而降低。