Diffusion of phosphate to plant roots in soil

Summary Improved resolution in autoradiography, achieved by the use of the low energy isotope, P³³, as tracer for soil phosphorus, enables the exchangeable phosphorus in a soil block to be measured quantitatively. A technique is described for the autoradiography of the P-depletion zone around the roots growing in soil, from which the P gradients are measured by microdensitometry.The amounts of P taken up by rape (Brassica napus) on a P-treated Begbroke Sandy Loam compared well with that removed from the soil as measured from the autoradiograph of the depletion zone. The P gradient around the roots suggests intense root hair activity; but the zone of depletion extended well beyound the tips of root hairs. The experimentally observed gradient is much closer to the one predicted from diffusion theory considering uniform depletion from within the equivalent root hair cylinder, than to the one obtained assuming the root hairs are inactive.A rapid depletion of up to about 60 per cent of the exchangeable P was observed within the root hair cylinder during the initial 3 days of absorption. The corresponding concentration of P in solution within the cylinder determined from a desorption isotherm, is hence brought down to a low level very rapidly, and is held at or near this level at later periods. The amounts transferred into the root hair cylinder from outside as calculated from a diffusion model were lower than the experimental values. It is suggested that the discrepancy may lie in the calculation of the effective diffusion coefficients for P in the soil from a P-desorption isotherm, owing to difficulties involved in simulating the root environment in the desorption isotherm experimentSoil Science Laboratory, Department of Agricultural Science, University of Oxford     

Diffusion of phosphate to plant roots in soil

Summary Autoradiographs of rape (Brassica napus L.) seedlings growing in a Begbroke Sandy Loam treated to different P levels showed P accumulations near root apices of primary and lateral roots, without corresponding depletion from the adjacent soil, indicating marked translocation.Laterals less than 2 days old did not deplete the soil despite considerable P accumulations in them. Their growth and P uptake were enhanced when the growth of the primary root was checked. The length of root hairs decreased markedly with increasing P supply.The P depletion zones developed in the same way at all points along the primary axis (except for a short length behind the apex). At the highest P level the concentration of exchangeable P at the root surface was lowered by about 30% on day 2, by about 40% on day 4 and rose slowly after day 8.Whereas in P treated soils the depletion from within the root hair cylinder was fairly uniform, in the low P soil there was a continuous decrease in P concentrations toward the root surface, within the root hair zone.Soil Science Laboratory, Department of Agricultural Science, University of Oxford     

Effect of phosphate fertiliser and plant density on phosphate inflow into ryegrass roots in soil

Summary Ryegrass plants were grown in pots of Horotiu sandy loam, either singly or as a dense sward, with a range of P fertiliser rates and regular harvests. Plants were non-mycorrhizal. P inflows into roots increased with P fertiliser rate. Sward plants absorbed up to 14.3% of the P fertiliser added, and single plants up to 7.6%. Sward plants absorbed most of the fertiliser P available to them within two weeks of germinating. After that, root growth ceased except at the highest P fertiliser rate used, and P inflow into roots decreased from 10^–15 to 10^–16 mole/cm/sec. Single plants, however, had continuous root growth and P uptake throughout the two month experiment, except for those at the two lowest fertiliser rates. Singly grown plants absorbed much more P than each plant in the dense swards. In single plants, root length/root weight ratio increased with increasing P fertiliser. Plant growth was dependant on continued P uptake by the roots, which only occurred if new roots were continally produced and new volumes of soil tapped for P. Non-growing root systems absorbed very little P. re]19760201     

The supply of nutrient ions by diffusion to plant roots in soil

Summary Observation of soil grown roots of rye-grass shows that an approximately cylindrical volume of soil, the root hair cylinder, is densely occupied by root hairs. Estimates are given of the concentration of labile and solution potassium within the root hair cylinder during experiments measuring potassium uptake from two soils by single roots. Calculations, using a diffusion model, suggest that labile potassium concentrations may be reduced to between 99.3 and 53 per cent of the initial, depending on the diffusion characteristics of the soil and nutrient demand by the root. Of the total potassium absorbed by a root in 4 days, the proportion which is supplied from within the root hair cylinder is small (0.8 to 6.3 per cent) indicating that diffusion to the root from the soil outside the root hair cylinder is of paramount importance. When root demand is high, diffusion appears to limit uptake to between 71 and 59 per cent of that which roots of comparable physiology would be expected to absorb from stirred solution of the same concentration. Nevertheless, the presence of root hairs is calculated to have enhanced uptake by up to 77 per cent compared with roots without hairs because they virtually increase the root diameter. Diffusion does not appear to be a limiting factor when root demand is low and hairs can then add little to the efficiency of the root system in potassium absorption.     

The supply of nutrient ions by diffusion to plant roots in soil

Summary Measurements were made of the diffusion of P³²-labelled phosphate to single roots of onion, leek and rye-grass growing in an Upper Greensand sandy loam (UGS) and a Coral Rag Clay (CRC) to which different amounts of phosphate had been added. Concentration-dependent diffusion coefficients for phosphate ions in the soils were calculated from phosphate desorption isotherms in calcium chloride. The experimental uptake by roots of known dimensions was compared with supply expected by diffusion to a cylindrical model root of the same dimensions. Allowance was made for absorption by the root hairs on rye-grass roots. Phosphate absorption by a cm length of intact root was found to continue for at least 16 days for onion, 10 days for leek and 5 days for rye-grass. Over a wide range of conditions (phosphate concentrations, soils, plant species), experimental uptake was close to the maximum calculated to be possible for the diffusion model except on one soil at a high level of phosphate. Although the concentration of phosphate in the soil solution at the root boundary appeared to be reduced to a small fraction of the initial concentration, because of the extreme non-linear form of the desorption isotherm less than 1/2 of the P³² exchangeable pool of P was considered to contribute to diffusion. Phosphate uptake by rye grass could only be accounted for if the root hairs were active. Although only a small fraction of the uptake is derived from inside the root hair cylinder, this increases the efficiency of the central root 2.3 fold by providing a zone close to the central root through which phosphate moves very readily.     

Non-uniform soil water extaction by plant roots

Using the technique of Computer Assisted Tomography applied to gamma ray attenuation measurement of soil water content, it has been shown that the assumption of uniform absorption of soil water along a plant root is clearly erroneous and that drawdown distance is a function of time. The results suggest that the plant sequentially removes water from the top to the bottom of the root as soil hydraulic resistance becomes a major limiting factor in the upper layers, even at the high soil water potential (–0.30 MPa) used.     

Cultivation of mesophilic soil crenarchaeotes in enrichment cultures from plant roots

Because archaea are generally associated with extreme environments, detection of nonthermophilic members belonging to the archaeal division Crenarchaeota over the last decade was unexpected; they are surprisingly ubiquitous and abundant in nonextreme marine and terrestrial habitats. Metabolic characterization of these nonthermophilic crenarchaeotes has been impeded by their intractability toward isolation and growth in culture. From studies employing a combination of cultivation and molecular phylogenetic techniques (PCR-single-strand conformation polymorphism, sequence analysis of 16S rRNA genes, fluorescence in situ hybridization, and real-time PCR), we present evidence here that one of the two dominant phylotypes of Crenarchaeota that colonizes the roots of tomato plants grown in soil from a Wisconsin field is selectively enriched in mixed cultures amended with root extract. Clones recovered from enrichment cultures were found to group phylogenetically with sequences from clade C1b.A1. This work corroborates and extends our recent findings, indicating that the diversity of the crenarchaeal soil assemblage is influenced by the rhizosphere and that mesophilic soil crenarchaeotes are found associated with plant roots, and provides the first evidence for growth of nonthermophilic crenarchaeotes in culture.     

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 method to separate plant roots from soil and analyze root surface area

Analysis of the effects of soil management practices on crop production requires knowledge of these effects on plant roots. Much time is required to wash plant roots from soil and separate the living plant roots from organic debris and previous years’ roots. We developed a root washer that can accommodate relatively large soil samples for washing. The root washer has a rotary design and will accommodate up to 24 samples (100 mm diam. by 240 mm long) at one time. We used a flat-bed scanner to digitize an image of the roots from each sample and used a grid system with commercially-available image analysis software to analyze each sample for root surface area. Sensitivity analysis and subsequent comparisons of ‘dirty’ samples containing the roots and all the organic debris contained in the sample and ‘clean’ samples where the organic debris was manually removed from each sample showed that up to 15% of the projected image could be coveredwith debris without affecting accuracy and precision of root surface area measurements. Samples containing a large amount of debris may need to be partitioned into more than one scanning tray to allow accurate measurements of the root surface area. Sample processing time was reduced from 20 h, when hand separation of roots from debris was used, to about 0.5 h, when analyzing the image from an uncleaned sample. The method minimizes the need for preprocessing steps such as dying the roots to get better image contrast for image analysis. Some information, such as root length, root diameter classes and root weights, is not obtained when using this technique. Root length measurements, if needed, could be made by hand on the digital images. Root weight measurement would require sample cleaning and the advantage of less processing time per sample with this method would be lost. The significance of the tradeoff between information not obtained using this technique and the ability to process a greater number of samples with the time and personnel resources available must be determined by the individual researcher and research objectives.     

The effect of plant hormones on phosphate uptake and translocation in maize roots

In short-term (1 h) uptake experiments GA₃(10^-5M) stimulated P_i uptake into maize root cortex cells by 28.7 %, Ethrel (10^-3M) inhibited it by 18.5 % and BA, IAA, and ABA were inactive. In long-term (5 h) experiments ABA remained inactive, GA₃ lost its stimulatory effect, and BA (5. 10^-6M), IAA (10^-4 -10^-5M), and Ethrel (10^-3 -5. 10^-4M) decreased P_i uptake. When the hormones were present only during 3 h preincubation (“augmentation”) period ABA was inactive, GA₃ slightly raised and BA, IAA, and Ethrel slowed down subsequent P_i uptake. BA(10^-7 –10^-5M) decreased xylem sap volume flow and P_i translocation. ABA in all tested concentrations (10^-8 –10^-5M) reduced exudation rate and P_i translocation, its effect declining with time. IAA effect strongly depended on concentration used and on application time and varied from strong inhibition to moderate stimulation of both volume flow and P_i translocation. GA₃ (10^-7M) slightly stimulated xylem volume flow but inhibited phosphate translocation. Ethrel (10^-4 and 10^-5M) increased both parameters, but P_i transloeation much more than volume flow. IAA, BA, and ABA influenced volume flow and P transloeation to the same extent leaving P_i concentration in the xylem sap unchanged. GA₃ and Ethrel influence P_i concentration in the xylem sap and it is thus probable that these hormones regulate release of phosphate ions into the xylem sap.     

Belowground volatiles facilitate interactions between plant roots and soil organisms

Many interactions between organisms are based on the emission and perception of volatiles. The principle of using volatile metabolites as communication signals for chemo-attractant or repellent for species-specific interactions or mediators for cell-to-cell recognition does not stop at an apparently unsuitable or inappropriate environment. These infochemicals do not only diffuse through the atmosphere to process their actions aboveground, but belowground volatile interactions are similarly complex. This review summarizes various eucaryotes (e.g., plant (roots), invertebrates, fungi) and procaryotes (e.g., rhizobacteria) which are involved in these volatile-mediated interactions. The soil volatiles cannot be neglected anymore, but have to be considered in the future as valuable infochemicals to understand the entire integrity of the ecosystems.     

Automated image analyses for separating plant roots from soil debris elutrated from soil cores

Historically, destructive root sampling has been labor intensive and requires manual separation of extraneous organic debris recovered along with the hydropneumatic elutriation method of separating plant roots from soils. Quantification of root system demographics by public domain National Institute of Health (NIH-Image) and Root Image Processing Laboratory (RIPL) image processing algorithms has eliminated much of the labor-intensive manual separation. This was accomplished by determining the best length to diameter ratio for each object during image analyses. Objects with a length to diameter ratio less than a given threshold are considered non-root materials and are rejected automatically by computer algorithms. Iterative analyses of length to diameter ratios showed that a 15:1 ratio was best for separating images of maize (Zea mays L.) roots from associated organic debris. Using this threshold ratio for a set of 24 soil cores, a highly significant correlation (r² = 0.89) was obtained between computer image processed total root length per core and actual root length. A linear relationship (r² = 0.80) was observed between root lengths determined by NIH-Image analyses and lengths determined independently by the RIPL imaging system, using the same maize root + debris samples. This correlation demonstrates that computer image processing provides opportunities for comparing root length parameters between different laboratories for samples containing debris.     

Organic matter contributed to soil by plant roots during the growth and decomposition of maize

Maize (Zea mays var. Caldera) plants were grown under sterile and not sterile conditions in soil in an atmosphere continuously enriched with ¹⁴CO₂ for 36 days. At harvest the above ground parts of the maize were cut off and the roots were separated from the soil by washing with water. The soil was dispersed using ultrasonics and separated into soluble clay silt and sand fraction. Roots were included in the coarse sand fraction. 25% of the total label present in the soil 5.5% of that in the soil-plant system, was water soluble. Very little label was present in the clay and silt fractions (5% in each) and most (65%) was in the sand fraction as root material.Rapid extraction of soil after the removal of roots without ultrasonic treatment released soluble matter which amounted to <0.5% of the total activity in the soil-plant system.Isolated roots steeped in water released about 18% of their activity. Much of the soluble fraction may therefore be root lysate.The soil and roots accounted for 22% of the total activity in the soil-plant system. Glucose accounted for 89% of the sugars in the soluble fraction of the soil.78% or more of the ¹⁴C present in glucose, arabinose and xylose constituents of the root-soil mixture occurred in the coarse and fine sand fractions, which also included root material. For mannose and galactose the value was 70% and for rhamnose, 50%.After reinoculation of the soil-root mixture and decomposition for 56 weeks, the water soluble material obtained on fractionation of the soil decreased to less than 1% of the total activity. A much greater proportion, 25%, was present in the clay fraction as a result of decomposition.     

The fate of18O labelled phosphate in soil/plant systems

KH₂PO₄ labelled with¹⁸O and³²P was mixed with soil that was placed in pots in which grass seed was sown. Grass samples were taken after 5, 7, and 12 weeks. The dilution factor (DF) for¹⁸O in the first cut was much greater than the DF for³²P, indicating that the bulk of the¹⁸O in the absorbed phosphate was lost. The DFs for¹⁸O and³²P determined in phosphate extracted from the soil at the end of the pot experiment indicated that half the¹⁸O excess in the applied phosphate was lost.A succeeding experiment showed no loss of¹⁸O when the treated soil was shaken for 3 months with water to which a germicide was added. Thus, the loss of¹⁸O was presumably caused by biochemical processes which brought about the replacement of¹⁸O by¹⁶O. We suggest that the loss of¹⁸O from applied labelled phosphate may be used as a measure of biological activity in soil.     

Effect of soil moisture and phosphate level on root hair growth of corn roots

Summary Root hairs have been shown to enhance P uptake by plants growing in low P soil. Little is known of the factors controlling root hair growth. The objective of this study was to investigate the influence of soil moisture and P level on root hair growth of corn (Zea mays L.). The effect of volumetric soil moistures of 22% (M₀), 27% (M₁), and 32% (M₂) and soil (Raub silt loam, Aquic Argiudoll) P levels of, 0.81 (P₀), 12.1 (P₁), 21.6 (P₂), 48.7 (P₃), and 203.3 (P₄) mol P L^–1 initially in the soil solution, on shoot and root growth, P uptake, and root hair growth of corn was studied in a series of pot experiments in a controlled climate chamber. Root hair growth was affected more by soil moisture than soil P. The percentage of total root length with root hairs and the density and length of root hairs on the root sections having root hairs all increased as soil moisture was reduced from M₂ to M₀. No relationship was found between root hair length and soil P. Density of root hairs, however, was found to decrease with an increase in soil P. No correlation was found between root hair growth parameters and plant P content, further suggesting P plays a secondary role to moisture in regulating root hair growth in soils. The increase in root hair growth appears to be a response by the plant to stress as yield and P uptake by corn grown at M₀ were only 0.47 to 0.82, and 0.34 to 0.74, respectively, of that measured at M₁ across the five soil P levels. The increase in root hair growth at M₀, which represents an increase of 2.76 to 4.03 in root surface area, could offset, in part, the reduced rate of root growth, which was the primary reason for reduced P uptake under limited soil moisture conditions.Journal Paper No. 10,066 Purdue Univ. Agric. Exp. Stn., W. Lafayette, IN 47907. Contribution from the Dep. of Agron. This paper was supported in part by a grant from the Tennessee Valley Authority.