A. Jungk

Phosphatase activity in the rhizosphere and its relation to the depletion of soil organic phosphorus

SummaryThe distribution of phosphatase activity and of phosphate fractions of the soil in the proximity of roots was studied in order to evaluate the significance of phosphatases in P nutrition of various plants (Brassica oleracea, Allium cepa, Triticum aestivum, Trifolium alexandrinum). A considerable increase in both acid and alkaline phosphatase activity in all the four soil-root interfaces was observed. Maximum distances from the root surface at which activity increases were observed ranged from 2.0 mm to 3.1 mm for acid phosphatase and from 1.2 mm to 1.6 mm for alkaline phosphatase. The increase in phosphatase activity depended upon plant age, plant species and soil type. A significant correlation was noticed between the depletion of organic P and phosphatase activity in the rhizosphere soil of wheat (r = 0.99**) and clover (r = 0.97**). The maximum organic P depletion was 65% in clover and 86% in wheat, which was observed within a distance from the root of 0.8 mm in clover and 1.5 mm in wheat. Both the phosphatases in combination appear to be responsible for the depletion of organic P.

Phosphorus efficiency of plants

Föhse et al. (1988) have shown that P influx per unit root length in seven plant species growing in a low-P soil varied from 0.6×10-14 to 4.8×10-14 mol cm-1s-1. The objective of this work was to investigate the reasons for these differences. No correlation was found between P influx and root radius, root hairs, cation-anion balance and Ca uptake. However, when root hairs were included in mathematical model calculations, the differences of P influx could be accounted for. These calculations have shown that in soils low in available P, contribution to P uptake by root hairs was up to 90% of total uptake.The large contribution of root hairs to P uptake was partly due to their surface area, which was similar to that of the root cylinder. However, the main reason for the high P uptake efficiency of root hairs was their small radius (approx. 5×10-4 cm) and their perpendicular growth into the soil from the root axis. Because of the small radius compared to root axes, P concentration at root hair surfaces decreased at a slower pace and therefore P influx remained higher. Under these conditions higher Imax (maximum influx) or smaller Km values (Michaelis constant) increased P influx. The main reasons for differences found in P influx among species were the size of Imax and the number and length of root hairs. In a soil low in available P, plant species having more root hairs were able to satisfy a higher proportion of their P demand required for maximum growth.

Phosphorus efficiency of plants I. External and internal P requirement and P uptake efficiency of different plant species

Plant species differ in their P efficiency,i.e. the P content in soil needed to reach their maximum yield. The differences in external P requirements can be atributed to either a lower internal P requirement for optimum growth or higher uptake efficiency of the plant. The objective of this research was to investigate the reasons for different P efficiencies of seven plant species.Onion, ryerass, wheat, rape, spinach, tomato and bean were grown in a P-deficient subsoil fertilized with 0, 2, 5, 10, 20, 40 and 80 mg P 100 g−1. All species showed a strong yield increase due to P fertilization. To reach 80% of maximum yield onion and tomato needed 17 and 11 mg P 100 g−1 respectively, corresponding to a soil solution concentrations of 6.9 and 5.7 μmol P l−1, whereas ryegrass, wheat and rape needed about 5 mg P 100g−1 corresponding to only 1.4 μmol P l−1 in soil solution. These differences in external P requirement cannot be explained by differences in their internal P requirement since onion, with the highest external P requirement, only contained 0.14% P in the shoot at 80% of maximum yield, while wheat, as the most P efficient species, contained 0.28%.P efficiency was related to the uptake efficiency of the plant which is determined by both root-shoot ratio and absorption rate per unit of root (influx). Species of low efficiency such as onion, tomato and bean had low influx rates and low root-shoot ratios, whereas species of medium to high efficiency had either high influx rates (rape and spinach) or high root-shoot ratios (ryegrass and wheat). The combination of both high influx rate and high root-shoot ratio was not found in any of the species studied.

Influence of phosphate and nitrate supply on root hair formation of rape, spinach and tomato plants

SummaryExperiments with tomato, rape and spinach in nutrient solutions have shown that the formation of root hairs is strongly influenced by phosphate and nitrate supply. Decreasing the phosphate concentration of the nutrient solution from 100 to 2 μM P resulted in an increase of root hair length from 0.1–0.2 to 0.7 mm of the three plant species. Root hair density also increased by a factor of 2–4 when the P concentration was lowered from 1000 to 2 μM. The variation of these two root properties raised the root surface area by a factor of 2 or 3 compared to plants well supplied with P. Root hair length was closely related to the phosphate content of the root and shoot material. On the other hand, spinach plants grown in a split-root experiment produced root hairs in solutions of high P concentration (1000μM P) if the major part of the total root system was exposed to low P concentration (2 μM P). It is therefore concluded that the formation of root hairs does not depend on directly the P concentration at the root surface but on the P content of the plant.Similar experiments with nitrate also resulted in an increase in length and density of root hairs with the decrease of concentration below 1000 μM. In this case marked differences between plant species occurred. At 2 μM compared to 1000 μM NO3 root hair length of tomato increased by a factor of 2, of rape by a factor of 5 and of spinach by a factor of 9. Root hair length was correlated, but not very closely, to the total nitrogen content of the plants. It is concluded, that the influence of nutrient supply on the formation of root hairs is a mechanism for regulating the nutrient uptake of plants.ZusammenfassungVersuche in Nährlösungen ergaben, dass die Behaarung der Wurzeln von Raps-, Spinat- und Tomatenpflanzen von der Phosphat- und Nitratversorgung abhängig ist. Mit abnehmender Phosphat-Konzentration der Nährlösung nahm die Länge der Wurzelhaare der drei Pflanzenarten von 0,1–0,2 mm bei 100 μM auf 0,7mm bei 2 μM zu. Auch die Wurzelhaardichte stieg mit abnehmender P-Konzentration im Bereich von 2 bis 1000μM P um den Faktor 2 bis 4 an. Die Veränderung dieser beiden Eigenschaften führte zur Zunahme der Wurzeloberfläche auf das Doppelte bis zum Dreifachen der reichlich mit P versorgten Pflanzen.Die Wurzelhaarlänge stand mit dem Phosphorgehalt des Sprosses und der Wurzeln in einer engen linearen Beziehung. Mit Hilfe der split-root-Technik wurde bei Spinatpflanzen ausserdem festgestellt, dass sich Wurzelhaare auch an solchen Wurzeln verstärkt bilden, die von hoher P-Konzentration (1000 μM) umgeben sind, wenn sich der grössere Teil des Wurzelsystems in Parmer Lösung (2 μM) befindet. Daraus wird der Schluss gezogen, dass nicht die P-Konzentration an der Wurzeloberfläche unmittelbar die Wurzelhaarbildung auslöst, sondern auf dem Weg über den Phosphorgehalt in der Pflanze. Entsprechende Versuche zur Wirkung von Nitrat führten mit abnehmender Konzentration ebenfalls zur Verlängerung der Wurzelhaare, jedoch in unterschiedlichem Masse bei den drei Arten. In Lösungen von 2 μM NO3 war die Länge der Wurzelhaare im Vergleich zu 1000μM bei Tomate um den Faktor 2, bei Raps um den Faktor 5 und bei Spinat um den Faktor 9 erhöht. Die Wurzelhaarlänge korrelierte dabei mit dem Gesamt-N-Gehalt der Pflanzenmasse, jedoch nicht sehr eng.Die Beeinflussung der Wurzelhaarbildung durch die Nährelement-Versorgung wird als ein Mechanismus angesehen, mit dem die Pflanze ihre Nährelementaufnahme reguliert.

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.

Verification of a mathematical model by simulating potassium uptake from soil

SummaryThis work develops the mathematical models suggested by various authors to simulate nutrient uptake of plants from soil. The simulation is based on ion transport from the soil to the roots by mass flow and diffusion and on Michaelis-Menten kinetics of nutrient uptake from soil solution by plant roots. For this purpose a differential equation is numerically integrated. Inter-root competition is allowed for by the choice of the boundary conditions. The integration procedure used makes it possible to take into account a variable buffer power which depends on soil solution concentration.The model calculates the change of nutrient concentrations in soil as a function of distance from the root surface for preestablished periods of time. Furthermore, the rate of uptake and the quantity of nutrients taken up per cm of root length is obtained. If the growth function of the root is known, nutrient uptake of a growing root system can be calculated.In order to verify the model two experiments were made:1.Potassium distribution was measured in a soil in the vicinity of rape roots under three different K levels. The calculated values agreed with the measured data.2.Potassium uptake of maize plants was measured in pot experiments with three different soils at two K levels each. Calculated K uptake agreed satisfactorily with measured K uptake. It is therefore concluded that the theoretical conception of the model is realistic and that the parameters have correctly been measured. The model thus appears to be useful to simulate such aspects of nutrient uptake of plants from soil which cannot be measured.ZusammenfassungDie Arbeit hat das Ziel, die von verschiedenen Autoren vorgeschlagenen Rechenmodelle zur Beschreibung der Nährstoffaufnahme aus dem Boden zu entwickeln. Sie basieren auf dem Nährstofftransport vom Boden zur Wurzel durch Massenfluss und Diffusion sowie auf der Kinetik der Nährstoffaufnahme von Wurzeln aus der Bodenlösung nach der Michaelis-Menten-Kinetik. Dabei wird eine Differentialgleichung numerisch integriert. Die Konkurrenz der Wurzeln um Mineralstoffe wird durch die Wahl der Randbedingungen berücksichtigt. Das gewählte Integrationsverfahren ermöglicht die Einbeziehung einer variablen, von der Konzentration der Bodenlösung abhängigen Pufferung.Das Modell errechnet die Änderung der Konzentration von Mineralstoffen im Boden mit zunehmender Entfernung von der Wurzeloberfläche in beliebigen Zeitspannen. Weiterhin wird die Aufnahmerate und die Nährstoffaufnahme pro cm Wurzel ermittelt. Its die Wachstumsfunktion der Wurzeln bekannt, so kann auch die Nährstoffaufnahme eines wachsenden Wurzelsystems errechnet werden.Zur Verifizierung des Modells wurden zwei Versuche durchgeführt:1.Ein Boden wurde auf drei K-Gehalte gebracht und die Verteilung der K-Konzentration in der Umgebung von Rapswurzeln gemessen. Die Modellrechnung ergab eine gute Übereinstimmung zwischen der gemessenen und der errechneten Konzentrationsverteilung im wurzelnahen Boden.2.In einem Gefässversuch mit drei Böden und je zwei K-Stufen wurde die K-Aufnahme von Maispflanzen gemessen. Auch hier stimmete die gemessene K-Aufnahme mit dem Ergebnis der Modellrechnung befriedigend überein Hieraus wird der Schluss gezogen, dass die theoretischen Vorstellungen, die dem Modell zugrundeliegen, realistisch sind und die verwendeten Parameter richtig gemessen wurden. Das Rechenmodell erscheint daher geeignet, um auch solche Aspekte der Mineralstoffaufnahme aus dem Boden zu untersuchen, die man nicht messen kann.

Influence of phosphate status on phosphate uptake kinetics of maize (Zea mays) and soybean (Glycine max).

To obtain plants of different P status, maize and soybean seedlings were grown for several weeks in flowing nutrient solution culture with P concentrations ranging from 0.03–100 µmol P L-1 kept constant within treatments. P uptake kinetics of the roots were then determined with intact plants in short-term experiments by monitoring P depletion of a 3.5 L volume of nutrient solution in contact with the roots. Results show maximum influx, Imax, 5-fold higher in plants which had been raised in solution of low compared with high P concentration. Because P concentrations in the plants were increased with increase in external P concentration, Imax was negatively related to % P in shoots. Michaelis constants, Km, were also increased with increased pretreatment P concentration, only slightly with soybean, but by a factor of 3 with maize. The minimum P concentration, Cmin, where net influx equals zero, was found between 0.06 and 0.3 µmol L-1 with a tendency to increase with pretreatment P concentration. Filtration of solutions at the end of the depletion experiment showed that part of the external P was associated with solid particles.It was concluded that plants markedly adapt P uptake kinetics to their P status, essentially by the increase of Imax, when internal P concentration decreases. Changes of Km and Cmin were of minor importance.

Potassium availability in relation to soil moisture II. Calculations by means of a mathematical simulation model

SummaryIn order to study the influence of soil moisture on the availability of potassium a simulation model was used. The model is designed to describe the transport of a nutrient from the soil to plant roots and its distribution around a root. From a pot experiment, the measured K uptake of onion plants, grown in soil under different moisture levels, agreed satisfactorily with the calculated K uptake. The model is therefore regarded as a valid means of quantifying the dynamics of K in the soil around plant roots.Calculations from a loess soil have shown that decreasing water content resulted in- a strong decrease of K transport from the soil to the root,- a faster decrease of the K concentration at the root surface and therefore- increasingly steep gradients of the K concentration around the root With the root density found in this experiment the K concentration of the moist soil (θ∼ 0.4) decreased almost equally in the total soil volume whereas in the dry soil (θ ∼ 0.1) not much change occurred in the middle between two roots.Therefore, the rate of K uptake per unit of root decreased much faster in the dry than in the moist soil. Calculations for sandy and loess soils, which have different water tension curves, have shown that the availability of K in the sandy soil is much more sensitive to changes in water tension than in the loess soil.The simulation technique can thus be used to analyzed the influence of single factors on the availability of K and to estimate the extent of this influence.ZusammenfassungUm den Einfluss des Wassergehaltes des Bodens auf die Verfügbarkeit von Kalium zu untersuchen, wurde ein Rechenmodell angewendet, das den Transport eines Nährstoffs vom Boden zur Wurzel und dessen Verteilung in der Umgebung der Wurzel beschreiben soll. An einem Gefässversuch mit Zwiebelpflanzen bei unterschiedlicher Bodenfeuchte ergab die Rechnung eine befriedigende Übereinstimmung mit der gemessenen K-Aufnahme der Pflanzen. Daraus wird geschlossen dass das Modell realistisch genug ist, um auch die K-Dynamik im wurzelnahen Boden zu quantifizierenSolche Rechnungen haben an einem Lössboden gezeigt, dass abnehmendem Wassergehalt- der K-Transport aus dem Boden zur Wurzel stark abnimmt,- dei K-Konzentration an der Wurzeloberfläche rascher sinkt, und daher- zunehmend steilere K-Konzentrations-Gradienten in Wurzelnähe enstehen. Bei der gegebenen Wurzeldichte sinkt die K-Konzentration des feuchten Bodens im gesamten Volumen nahezu gleichmässig ab, während sie im trockenen Boden in der Mitte zwischen zwei Wurzeln nur wenig abnimmt. Die K-Aufnahmerate pro Einheit Wurzel sinkt daher in trockenem Boden viel rascher als in feuchtem Boden ab. Rechnungen an Sand- und Lössböden, die sich durch ihre Wasserspannungskurve deutlich unterscheiden, zeigen sinngemäss, dass ein Sandboden in seiner K-Verfügbarkeit auf Änderungen der Wasserspannung viel empfindlicher als ein Lössboden reagiert.Die Modellrechnung ermöglicht es demnach, die Wirkung einzelner Faktoren der K-Verfügbarkeit zu erkennen und in ihrem Ausmass abzuschätzen.

Potassium availability in relation to soil moisture I. Effect of soil moisture on potassium diffusion, root growth and potassium uptake of onion plants

SummaryThe objective of this research is to evaluate the influence of soil water content on- the mobility of potassium in soil,- plant growth and- K uptake of plants. The mobility of K increased with soil moisture. Increasing the volumetric water content (θ) from the 0.1 to 0.4 resulted in a rise of the effective diffusion coefficient (De) by a factor of about 10. This is mainly due to the increase of the tortuosity or impedance factor with higher soil moisture.In order to relate K mobility in soil to the availability of K for plant uptake, onion plants were grown in special containers under constant water content in the range of 0.1 to 0.4 cm3 H2O cm−3 of soil. Results are- both K content and growth of the plants increased with soil moisture,- water content below θ=0.1 reduced root growth- K inflow per unit of root surface increased with soil moisture. Maximum rate of inflow occurred with θ=0.25 in the soil used. It is therefore concluded that soil moisture affected K availability by affecting both K mobility and root growth.ZusammenfassungDie Arbeit hat das Ziel, den Einfluss des Wassergehaltes des Bodens auf- die Mobilität der Kaliumionen im Boden,- das Pflanzenwachstum und- die K-Aufnahme Zu bestimmen. Hierzu wurden einerseits Messungen der Mobilität von Kalium im Boden durch-geführt. Sie ergaben eine Erhöhung des effektiven Diffusionskoeffizienten (De) mit ansteigendem volumetrischen Wassergehalt (θ). De nahm um mehr als das Zehnfache zu während θ von 0,1 auf 0,4 anstieg. Dies ist der Erhöhung des Tortuositäts-oder Widerstands-faktors mit steigendem Wassergehalt zuzuschreiben. Um zu prüfen, in welchem Masse die Diffusionsbedingungen im Boden die Pflanzenverfügbarkeit von Kalium beeinflussen, wurde ein Vegetationsversuch durchgeführt. Hierzu wurden Zwiebelpflanzen in speziellen Versuchsgefässen bei konstanten Wassergehalten zwischen 0,1 und 0,4 cm3 H2O/cm3 Boden kultiviert. Die Ergebnisse sind:- K-Konzentration und Ertrag der Pflanzen wurden mit zunehmedem Bodenwassergehalt erhöht.- Der Wassergehalt des Bodens beeinflusste das Wurzelwachstum; unter θ=0,1 nahm die Wurzellänge stark ab.- Die K-Aufnahmerate eines Wurzelabschnitts stieg mit dem Wassergehalt an; bei θ=0,25 war die maximale Aufnahmerate in diesem Boden erreicht. Bie niedrigem Wassergehalt des Bodens wird die Kalium-Verfügbarkeit demnach beeinträchtigt sowohl durch den Rückgang der Mobilität von Kalium im Boden als auch die Verringerung des Wurzelwachstums.

Utilization of organic phosphorus in calcium chloride extracts of soil by barley plants and hydrolysis by acid and alkaline phosphatases

Organic phosphorus is often a major part of total phosphorus in soil solution. The role of this fraction as a P source for plants and the mechanism involved in its transfer from soil to plant is still unclear. We studied the utilization of organic phospharus in 0.01 M calcium chloride extracts by barley and its hydrolysis by isolated acid and alkaline phosphatases. Calcium chloride extracts were used as a nutrient solution in 24 hrs assays. Concentration of organic and inorganic P in equilibrium calcium chloride extracts was 7.8 and 1.8 µmol P L-1, respectively, which was similar to the soil solution P concentration. When soil microbial biomass was destroyed by autoclaving, organic P concentration increased to 64.8 µmol P L-1 whereas the inorganic P was hardly changed. Inoculation of the autoclaved soil with non-sterile soil and incubation for 5 days decreased the organic P concentration to 27.9 µmol P L-1 but did not change inorganic P. In this study barley plants utilized organic P from all extracts. The greatest reduction of organic P concentration occurred in fresh extracts of the autoclaved soil. Inorganic P was depleted to traces in all extracts. Organic P was hydrolyzed by isolated acid and alkaline phosphatases. We conclude that organic P in soil solution is a heterogeneous pool of organic P compounds originating from microbial biomass. Its initial availability to plants was nigh but its susceptibility to phosphatase hydrolysis was quickly reduced but not completely lost.