P. H. Nye

Changes of pH across the rhizosphere induced by roots

SummaryPlants that absorb nitrogen as NO3− tend to raise the pH in the rhizosphere. Those absorbing nitrogen as NH4+ or N2 lower the pH. The change in pH near the root surface may be calculated approximately from the H+ or HCO3− efflux and radius of the root; and the pH buffering capacity, moisture content, initial pH and pCO2 of the soil. An accurate equation, solved numerically, also takes account of root hairs, mass flow and slow acid-base reaction in the soil. The pH at the root surface will often differ from the pH a few mm away by 1–2 units.

Diffusion of phosphate to plant roots in soil

SummaryImproved resolution in autoradiography, achieved by the use of the low β 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 experiment

The effect of the nutrient intensity and buffering power of a soil, and the absorbing power, size and root hairs of a root, on nutrient absorption by diffusion

SummaryA portion of a single plant root is treated as an absorbing cylindrical sink to which nutrients move by diffusion. Assuming that the rate of uptake of nutrient is proportional to its concentration at the root surface, and that the nutrient, though reacting with the solid, moves only through the soil solution, standard diffusion equations are used to calculate the effect of soil and plant characteristics on the rate of uptake. The treatment is applicable to phosphorus and potassium. Among soil properties uptake should increase directly with the soil solution concentration. It should also increase, but only slowly, with increasing buffering power. It increases with increasing soil moisture. Among plant characteristics, uptake should increase with the root absorbing power until diffusion through the soil becomes limiting. Absorption by unit surface area of root increases as the root radius decreases. A root hair is shown to interfere quickly with the uptake of adjacent hairs. The hairs increase absorption by the root because they can exploit rapidly the soil between the hairs, and they have the effect of extending the effective root surface to their tips.

Uptake of solutes by multiple root systems from soil

SummaryA procedure is put forward for calculating the plant uptake of solutes supplied by diffusion and mass flow to the randomly dispersed roots of a developing root system. The model was tested as follows: (a) for a constant root density, and both transport processes—against a more accurate numerical solution of the same system (b) for an increasing root density, and for supply by diffusion only—by electrical simulation using the analog described in Part I. In both cases, results obtained by the two types of calculation were in close agreement.A less accurate method which includes both supply mechanisms and does not require a computer is presented, and compared with an electrical simulation when there is no mass flow. Agreement is within 20 per cent.The model should be useful for predicting plant nutrient uptake from soil, and may be of special interest to modellers of the whole plant system. re]19720905

A theoretical study of the distribution of substances around roots resulting from simultaneous diffusion and mass flow

SummaryThe change in concentration of a solute in soil, moving near the surface of a root by both mass flow and diffusion, has been calculated by a numerical method with a computer. The effect of change in the plant controlled variables v0 (the solvent flux at the root surface) and k (the root absorbing power), and the soil variables b (the buffer power) and D (the diffusion coefficient) are described in turn.The concentration at the root surface, relative to the undisturbed soil solution, approaches a limiting value v0/k. As v0 is increased, the limiting value is approached more rapidly, and the zone of disturbance is more compressed. A steady state is reached if r0v0/bD>2, but if r0v0/bD<2 the disturbance continues to spread outwards even though the concentration at the root surface has nearly attained its limiting value.As k is increased, other factors being constant, the limiting relative concentration at the root surface is approached more rapidly, but the spread of the disturbance away from the root is little affected.As Db is decreased, corresponding to a decrease in soil moisture, the concentration at the root surface reaches its limit more rapidly and the zone of disturbance is compressed.If, because of increase in the concentration at the root surface, the efficiency of root absorption declines, the relative concentration will exceed v0/k, and may reach no limit — at least until the assumptions of the model used break down.

The possibility of predicting solute uptake and plant growth response from independently measured soil and plant characteristics

SummaryRape plants were grown in solutions of 10-6, 10-5, 10-4 and 10-3M phosphate in a controlled environment that gave near optimum climatic conditions for growth. Uptake and growth were followed by replicate harvests taken every five days. The relation between the mean root absorbing power, and the concentration of P in solution was derived. The relations between the % P in the shoot dry matter and the other parameters of the growth model described in paper I were also determined. Growth rates were exceptionally high, with RGR values above 0.5 g/g/d in solutions of concentration 10-5M and more during the early stages of growth. RGR was reduced to about half this value in 10-6M P. The range of response to solution concentration in these conditions therefore lay between 10-6 and 10-5M P. In solutions of 10-6 and 10-5M P root hairs were abundant but in solutions of 10-4 and 10-3M P, they were absent. Rape had a high UAR for P as a result of its high RGR, but it had a correspondingly large root surface area per unit plant weight. Onions (see Paper II of this series) had an inherently lower RGR and UAR for P, but had a comparatively low root surface area per unit plant weight. It appears that these contrasting features of rape and onions broadly compensated for each other so that the P concentration range over which the two species responded was much the same.

The Rate-limiting Step In Plant Nutrient Absorption From Soil

The rate of uptake of a nutrient may be limited by its rate of diffusion through the soil or by the ability of the root to absorb it from low concentration in the soil solution. In the growth-response range of concentration, uptake of N, P, and K appears to be limited by diffusion. Transpiration rate, in these conditions, should have little influence. The root may be able to increase the soil solution concentration of sparingly soluble nutrients.

The influence of phosphate nutrition on H ion efflux from the roots of young rape plants

Changes in pH around the roots of young rape plants were studied using a nutrient film technique which allowed either part or all of the root system to be subjected to specific nutrient treatments. The rapidity and direction of change of pH was assessed by embedding absorbing roots in a thin layer of agar containing bromocresol purple. Measurements were also made with a pH microelectrode placed next to the roots.Phosphate-fed plants were deprived of phosphate when 14 days old. Patterns of pH changes round the deprived roots were the same as with phosphate-fed plants until the plants had been deprived of P for three days, when H ion efflux started in the terminal portions of the roots. The lengths of root producing acid and amounts of H ion both increased as the plants became more P deficient. Both P fed and P deprived roots produced HCO3 ions but the net amount of HCO3 ion produced by the P deficient roots fell as did nitrate uptake rates. Cation-anion balances measured at the end of the experiment showed that uptake of all anions and K decreased in the P deprived plants but uptake of Ca and Mg were little altered. This resulted in a smaller ratio of anions to cations absorbed which was reflected in the reduced HCO3 ion efflux.

Uptake of solutes by multiple root systems from soil: II. The theoretical effects of rooting density and pattern on uptake of nutrients from soil

SummaryThe analog described in Part I is used to investigate quantitatively the the effects of pattern and density on the uptake and uptake rate of nutrients which move to plant roots by diffusion. The uptake by two roots is considered first, to illustrate the competitive effect. The results for multiple root systems are given for a variety of different soil and plant parameters at different times and demonstrate the importance of pattern and density in the uptake of different plant nutrients in both competitive and non competitive situations. Pattern can decrease the uptake by root systems by at least 75 per cent, depending on the value of the diffusion coefficient, time, and root density. Graphs of two indices of dispersion against uptake are given so that the effect of any pattern can be estimated. A procedure is outlined which enables the uptake after any time by a developing root system to be predicted and compared with a theoretical maximum. If the uptake is known, then the graphs show whether soil or plant parameters are limiting uptake.

The effect of high solute concentrations on nitrification rates in soil

SummaryA short term nitrification assay (<18 h) was used to assess the effect of high concentrations of different solutes on the rate of nitrate production. High solute concentrations were found to inhibit nitrification and the degree of inhibition was related both to the osmotic pressure of the soil solution and the osmoticum used. Ammonium chloride, ammonium sulphate and sorbitol were used as sources of osmotic pressure. The results showed that, with ammonium salts, no inhibition was observed with pressures less than 2 atm. Above these values, the severity of the inhibition followed the order ammonium chloride>ammonium sulphate>sorbitol up to the maximum osmotic pressure studied (25 atm). With ammonium chloride, a pressure of 3.5 atm. was sufficient to cause a 90% inhibition of nitrification rate.The inhibition produced by mixtures of ammonium chloride and sorbitol, each mixture generating an osmotic pressure of 5 atm. in the assay, was also investigated. The results suggest that inhibition by Cl-ion is disproportionate to its contribution to the osmotic pressure of the soil solution.The recovery of the nitrification rate, following exposure to high osmotic pressure solutions, was also investigated. It was found that the recovery of the nitrification rate was only partial, with the extent of the recovery diminishing as the severity of the initial osmotic stress applied increased. These results suggest that both reversible and irreversible mechanisms are involved in the inhibition of nitrification.