Availability of phosphate and potassium as the result of interactions between root and soil in the rhizosphere

A number of findings are summarized in order to show the significance of individual plant properties and soil factors on the availability of phosphate and potassium to plants growing in soil. The flux of a nutrient into a given plant root depends directly on the concentration of the nutrient in the adjacent solution. In nutrient solution, P and K influx follows Michaelis-Menten kinetics. Almost maximum rates of influx have been observed in the range of soil solution concentrations usually found in German arable soils. Roots exhaust P and K from solutions to about 0.2 μmol P and 1 μmol K 1^?1 if not replenished. At the root surface P and K concentrations in soil decrease rapidly within one day; small changes occur after this period. Initially, the extent of the depletion zone is very small but it extends radially with time. After the initial phase therefore, P and K supply to the plant depends on transport from more remote parts of the soil and also on release from undissolved sources. The degree of depletion and the extent of the depletion zone are related to the diffusion coefficient; they decrease with increasing clay content of soil. Root hairs penetrate the soil and extend the volume of soil supplying nutrients to a unit of root. P and K influx therefore increase with the length of root hairs. Proton release of roots mobilize P and K in soil. This is clearly detected by the HCl-soluble P and K fractions within 2 mm of the root surface. The activity of acid and alkaline phosphatases strongly increase in the soil in the vicinity of the root surface of several plant species. It is supposed that organic P compounds can therefore be utilized by plants. P and K influx per unit of root length and root length per unit of shoot weight differed widely between species. The product of these two parameters however was closely related to the P and K concentration of the shoots. Calculations from a mathematical model were in good agreement with measured K depletion profiles and K uptake by plants. It is therefore concluded that the main factors influencing the P and K availability of plants growing in soil have been accounted for in the mathematical model and that the parameters have been accurately measured.     

Influence of soil temperature and soil compaction on growth and P uptake of sugar beet

Sugar beet growth is often impaired by cold and compacted soil. The aim of this study was to determine the effect of soil temperature and soil compaction on the growth and function of sugar beet roots. For this purpose a pot experiment with sugar beet (Beta vulgaris) was conducted in a growth chamber in which the soil temperature was kept constant either at 10°C or 20°C and air temperature at 20°C. The soil was uncompacted (1.30 g cm^?3) or compacted to a bulk density of 1.65 g cm^?3. In order to find out whether growth restriction was caused by insufficient P supply of the plant the experiment was run without and with P application (300 mg per kg soil). Root growth was much smaller at 10°C compared to 20°C, whereas root/shoot ratio was not affected by soil temperature. Hence, root and shoot growth was inhibited to the same extent. P content of the plants was not reduced, neither by cold nor by compacted soil, although parameters of acquisition such as root length and morphological root properties were altered. Soil temperature strongly affected P influx, whereas compaction did not. The calculation with a simulation model showed that at 10°C soil temperature the predicted P uptake of the plants agreed with the measured P uptake irrespective of compaction and P application. However, at 20°C the model underestimated the P influx at low soil P availability even if allowance was made for root hairs. It is concluded that under conditions of high shoot P demand and low P availability in soil P has been mobilized by mechanisms not taken into account by the model.     

Bedeutung von Kaliumaufnahmerate,Wurzelwachstum und Wurzelhaaren für das Kaliumaneignungsvermögen verschiedener Pflanzenarten

Effect of K uptake rate, root growth and root hairs on potassium uptake efficiency of several plant species Pot experiments with maize, rape, tomato, rye-grass and onion plants were carried out to evaluate the influence of – rate of K uptake per cm of root, – cm root per mg shoot dry weight and – mean root age (as a measure of the time roots absorb potassium) on potassium uptake efficiency of these plants. Percent K in shoot dry matter was used to indicate K uptake efficiency. No close correlation was observed between one of these factors to K concentration in shoot dry matter. The product of K uptake rate and root-shoot ratio was closely related to the K concentration of shoots. However, regression lines for maize, rape and onion were different. One single regression line was found when K concentration in shoot was related to the product of K uptake rate, root-shoot ratio and mean root age. It is therefore concluded that K uptake of plants depends on all three of these factors. In different species the proportion of these factors were markedly different. The plant factors in turn were affected by the K nutritional status of the plants. K uptake rate increased whereas root-shoot ratio and mean root age decreased with increasing K supply of the soil. K uptake rate per cm root was strongly affected by root hairs. The radial distance of the K (Rb) depletion zone of the soil adjacent to the root surface also increased with the length of the root hairs. It is therefore concluded that root hairs substantially affect the spatial access of potassium in soil by the plant.     

辣椒丛枝菌根真菌吸磷机制研究

A pot experiment was conducted to investigate the action mechanisms of arbuscular mycorrhizal (AM) fungi in phosphorus (P) uptake of Capsicum annuum L.in a sterilized fossil Oxisol.Three P levels of 0,10 and 200 mg kg-1 soil (P0,P10 and P200,respectively) without and with AM fungal inoculation were applied as Ca(H2PO4)2·H2O.Shoot dry matter yields and shoot P uptake increased significantly (P > 0.05) by the inoculation of AM fungi at P0 and P10.Root length and P concentration in soil solution increased with the inoculation of AM fungi but the root:shoot ratio decreased or remained constant.Around 50% roots of inoculated plants were infected by AM and the external hyphae amounted to 20 m g-1 soil at P10 and P200.The hyphae surface area of the infected root cylinder amounted to 11 and 2 cm-2 cm-2 root at P0 and P10,respectively.The increased P uptake of inoculated plants was mainly because of an up to 5 times higher P influx of the infected root.Model calculations showed that the root alone could not have achieved the measured P influx in both infected and non-infected roots.But the P influx for hyphae calculated by the model was even much higher than the measured one.The P uptake capacity of hyphae introduced in the model was too high.Model calculations further showed that the depletion zone around roots or hyphae was very narrow.In the case of the root only 7% of the soil volume would contribute P to the plant,while in the case of hyphae it would be 100%.The results together with the model calculations showed that the increased P uptake of AM inoculated plants could be explained partly by the increased P concentration in the soil solution and by the increased P absorbing surface area coming from the external hyphae.     

Phosphatverarmung des wurzelnahen Bodens und Phosphataufnahme von Mais und Raps

Phosphate depletion at the soil — root interface and the phosphate uptake of maize and rape Maize and rape plants were grown in flat containers in a ³3P-labelled sandy soil and the distribution of soil phosphate near roots was determined by using densitometric scans of autoradiographs. The concentration of isotopically exchangeable phosphate at the root surface decreased within a few days by 42 per cent with rape and by 50–65 per cent with maize. Initially the width of the depletion zone is very small. Within six days the depletion zone extended to the final distance from the surface of the root cylinders of about 2 mm for maize and 2.6 mm for rape. The soil within the range of the mean length of root hairs (0.7 mm for maize and 1.3 mm for rape) is almost equally depleted. This indicates that root hairs are very important for P-uptake from soil. This is further supported by higher P-uptake rates per cm root length of rape than of maize. The P-concentration of the soil solution was estimated by means of the phosphate desorption curve. Within the root hair cylinder the P-concentration of the soil solution decreased from 0.8 to 0.03 mg P/l. Changes of the P-depletion profile with time were used to calculate P-uptake rates for roots of different age. The results indicate that for the first 3–5 days P-uptake rates remained near maximum, even though the P-concentration of the soil solution at the root surface had strongly decreased within two days. Phosphate uptake rates per cm root length did not decrease unless the whole root hair cylinder had been depleted.     

Phosphorus efficiency of ornamental plants in peat substrates

A pot experiment was conducted to investigate factors contributing to phosphorous (P) efficiency of ornamental plants. Marigold (Tagetes patula) and poinsettia (Euphorbia pulcherima) were cultivated in a peat substrate (black peat 80% + mineral component 20% on a volume basis), treated with P rates of 0, 10, 35, 100, and 170 mg (L substrate)^–1. During the cultivation period, plants were fertigated with a complete nutrient solution (including 18 mg P L^–1) every 2 d. Both poinsettia and marigold attained their optimum yield at the rate of 35 mg P (L substrate)^–1 and the critical level of P in shoot dry matter of both crops was 5–6 mg g^–1. After planting, plant‐available P increased at lower P rates to a higher level for poinsettia than for marigold, but no significant change was observed at higher P rates. Balance sheet calculations indicated that at lower P rates more P was fertigated than was taken up by the plants. Root‐length density, root‐to‐shoot ratio, and root‐hair length of marigold were doubled compared to that of poinsettia. Root‐length density increased with crop growth, and 10 d after planting the mean half distance between roots exceeded the P‐depletion zone around roots by a factor of 3 and 1.5 for poinsettia and marigold, respectively. Thus, at this early stage poinsettia exploited only 10% of the substrate volume whereas marigold utilized 43%. Later in the cultivation period, the depletion zones around roots overlapped for both crops. Taking into account P uptake via root hairs, the simulation revealed that this was more important for marigold compared to poinsettia especially at low P‐supply levels. However, increase of P uptake due to root hairs was only 10%–20% at optimum P supply. For the two lower P levels, the P‐depletion profile around roots calculated for 10 d after planting showed that after 2 d of depletion the concentration at the root surface was below the assumed K_m value (5 μM) and the concentration gradient was insufficient to fit the demand. A higher content of plant‐available P in the substrate was observed for poinsettia compared to marigold in the treatment with P application adequate for optimum growth, because more fertigated P was accumulated during early stages of cultivation due to lower root‐length density of poinsettia. The observed difference of root morphological parameters did not contribute significantly to P‐uptake efficiency, since P mobility in the peat substrate was high.     

在富营养土壤斑块中根增值对玉米养分吸收和生长的贡献

Root proliferation can be stimulated in a heterogeneous nutrient patch; however, the functions of the root proliferation in the nutrient-rich soil patches are not fully understood. In the present study, a two-year field experiment was conducted to examine the comparative effects of localized application of ammonium and phosphorus (P) at early or late stages on root growth, nutrient uptake, and biomass of maize (Zea mays L.) on a calcareous soil in an intensive farming system. Localized supply of ammonium and P had a more evident effect on shoot and root growth, and especially stimulated fine root development at the early seedling stage, with most of the maize roots being allocated to the nutrient-rich patch in the topsoil. Although localized ammonium and P supply at the late stage also enhanced the fine root growth, the plant roots in the patch accounted for a low proportion of the whole maize roots in the topsoil at the flowering stage. Compared with the early stage, fine root length in the short-lived nutrient patch decreased by 44%-62% and the shoot dry weight was not different between heterogeneous and homogeneous nutrient supply at the late growth stage. Localized supply of ammonium and P significantly increased N and P accumulation by maize at 35 and 47 days after sowing (DAS); however, no significant difference was found among the treatments at 82 DAS and the later growth stages. The increased nutrient uptake and plant growth was related to the higher proportion of root length in the localized nutrient-enriched patch. The results indicated that root proliferation in nutrient patches contributed more to maize growth and nutrient uptake at the early than late stages.     

Wirkung der Kaliumdüngung auf die Kaliumverfügbarkeit in der Rhizosphäre von Raps

Influence of potassium supply on the availability of potassium in the rhizosphere of rape (Brassica napus) Potassium depletion of the soil in the proximity of roots was studied in order to obtain quantitative information on the availability of potassium. For this purpose rape seedlings were grown in pots which separate roots from soil by a fine meshed screen; root hairs penetrated the screen. The soil adjacent to the screen was sliced by microtome into layers about 0.1 mm thick which were separately analysed for k. Plant roots strongly depleted the soil in their proximity; further distant ranges remained unchanged. A loess derived loam, brought to different levels of exchangeable K by precropping or K application, was equally depleted to 150 μmoles/100g soil at the root surface. Therefore, the quantity of K released from this source increased with initial K level. In addition, the distance of the depletion zone extended with K level from 4,6 to 6.3 mm from the surface of the root cylinder. Hence, the volume of soil contributing K to the root increased from 0.7 to 1.2 cm³ per cm root length. The combination of these two parameters, i.e. the quantity of exchangeable soil K released per unit of root length increased by a factor of 20 whereas exchangeable soil K was raised by a factor of 4.5 only. K uptake of the plants after 4 days was in agreement with the depletion of exchangeable soil K in the high K treatment only. The other treatments had obviously taken up considerable percentages of nonexchangeable K. This fraction was released from the soil ranging less than 1.5 mm from the root. The distance of the K depletion zone was also extended by application of NaCl and MgCl². Because of cation exchange, K concentration of the soil solution was increased, K buffer power decreased and, therefore, K diffusion was enhanced. It is concluded that plants in the field do not uniformly deplete the total rooted soil volume. Whereas roots in their proximity strongly deplete the soil including parts of nonexchangeable K they do not even use exchangeable K in a slightly greater distance. The quantity of K available per unit of root length is, therefore, determined by both - the degree of soil K depletion at the root surface and -the distance of the depletion zone, i.e. the volume of soil that contributes K to the root. Either factor was markedly affected by the level of soil K and thus by K application.     

Root to shoot translocation of macronutrients in relation to shoot demand in maize (Zea mays L.) grown at different root zone temperatures

Root growth and nutrient uptake rates of maize (Zea mays L.) are decreased at low root zone temperatures (RZT) and thus, shoot growth may be limited by nutrient deficiency. The objectives of this research were to characterize the shoot demand for nutrients per unit root at suboptimal RZT and to relate net translocation rates of N, P, K, and Ca from the roots to the shoot to shoot demand. Maize plants were grown for 11 days in soil or 8 days in nutrient solution at uniform shoot (24°/20°C, day/night) but different RZT (12°, 18°, and 24°C). The shoot base of the plants (apical shoot meristem and zone of leaf extension) was either kept within or above the cooled root zone. Shoot and root growth were significantly reduced at suboptimal RZT (12°, 18°). Lifting the shoot base above the cooling zone increased shoot growth markedly, whereas root growth was not significantly influenced. Thus, the shoot fresh weight increment day^?1 g^?1 root fresh weight (i.e. the shoot demand per unit root) was increased by a factor of up to 9 for plants with their shoot base above as compared to within the cooling zone. At suboptimal RZT, translocation rates of N, K, and Ca to the shoot remained low in plants with the shoot base in the cooling zone but were higher than in 24°C-grown plants, when the shoot base was above the cooling zone. In both nutrient solution- and soil-grown plants translocation rates of N, K, and Ca were closely correlated with the shoot demand per unit root but less to RZT. In contrast, the translocation rate of P was mainly affected by RZT but insensitive to shoot demand and, therefore, was always higher at a RZT of 24° than of 12°C. From these results it is suggested, that at low RZT the root-to-shoot translocation rates of N, K, and Ca are mainly determined by the shoot demand, whereas the translocation rate of P, regardless of the shoot demand, is reduced by a direct effect of low temperature on the roots.     

Phosphorus Efficiency of Buckwheat (Fagopyrum esculentum)

The ability of buckwheat (Fagopyrum esculentum) roots to acquire phosphorus (P) was characterized by investigating P uptake, morphological features, and chemical changes in the rhizosphere. Over a range of nutrient solution P concentrations (5–500 μmol · L^?1), maximum shoot growth was achieved with a P supply between 5 and 100 μmol · L^?1. Root weight and root length, as well as length and frequency of root hairs, were higher at low P levels. Root surface and the root surface/shoot dry weight ratio reached high values. Though P uptake rates were only moderate (0.15 pmol · cm^?1 root · sec^?1), shoot P concentrations were high (1.8% of dry weight with 100 μM P) predominantly being inorganic (80%). Phosphorus efficiency was characterized by a high specific absorption rate (810 mmol P · kg^?1 root dry wt · d^?1) rather than by an efficient utilization for dry weight production. Root exudates of low-P plants had lower pH values than exudates of high-P plants and increased the solubility of FePO₄ and MnO₂ to a greater extent. Amounts of exuded organic acids and phenolics were low and could not account for the observed solubilization of FePO₄ and MnO₂. Enhanced hydrolysis of glucose-6-phosphate by exudates from low-P plants was due to an increased “soluble” acid phosphatase activity, and root surface phosphatase activity was also slightly enhanced with P deficiency. In the rhizosphere soil of buckwheat, some depletion of organic P forms was observed, and in pot trials with quartz sand, buckwheat utilized glucose- 6-phosphate as a P source at the same rate as inorganic P.     

Interaction of soil compaction, phosphorus and zinc on clover growth and accumulation of phosphorus

Soil compaction generally reduces crop performance because of degraded soil physical and biological properties, and possibly inappropriate soil nutritional status. The effects of varying compaction, and phosphorus (P) and zinc (Zn) supplies on the growth of Berseem or Egyptian clover (Trifolium alexandrimum), and accumulation of P and Zn in shoots and roots were investigated in a pot experiment using a surface layer of a Typic Torrifluvent (USDA), Calcaric Fluvisols (FAO) soil. Plants were treated with three soil compaction levels, three rates of P and three rates of Zn in a factorial combination. Phosphorus accumulation in shoots did not change up to bulk densities of 1.65 Mg m⁻³ and declined at bulk density of 1.80 Mg m⁻³. Increasing the levels of Zn and P resulted in a significant increase in shoot dry mass (from 0.3 to 0.8 g pot⁻¹), and root length (from 11.4 to 32.5 m pot⁻¹). Shoot and root growth were reduced by soil compaction particularly at low P and Zn application rates. Shoot dry mass was reduced from 0.8 to 0.3 g pot⁻¹, and root length from 43 to 5 m pot⁻¹ at bulk densities of 1.4 and 1.8 Mg m⁻³, respectively. However, the accumulation of P (from 0.06 to 0.15 g kg⁻¹) and Zn per unit length of roots (from 0.8 to 1.8 μg pot⁻¹) increased as soil compaction increased. As the Zn supply increased, Zn accumulation per unit length of roots, and total Zn accumulation increased. Severe compaction reduced P and Zn accumulation in shoots and also decreased shoot dry mass, and root length compared to lower soil compaction levels. The present study suggests that Zn and P supply can moderate the adverse effect of soil compaction on clover performance.     

Root hairs and the acquisition of plant nutrients from soil

The literature on the role of root hairs when plants acquire mineral nutrients from soil is reviewed. After a short outline of the root properties affecting the acquisition of nutrients, the roles of root hairs are discussed in four sections, entitled: morphological properties of root hairs, mode of action of root hairs, factors affecting the formation of root hairs, and relationship between root hair formation and plant nutrient uptake. The formation of root hairs depends on both genetic and environmental factors, particularly the supply of phosphate and nitrate. It is concluded that root hairs may substantially contribute to the acquisition of nutrients, mainly those of low mobility in soil and high demand in plants. The percentage of a nutrient acquired by root hairs varies widely, from almost zero to approximately 80 % of the total uptake of the nutrient. The contribution of root hairs depends on plant species and the genetic variability of root hair formation on the one hand, and the kind of nutrient and its availability in soil on the other. According to the published reports, essentially only phosphorus and potassium were considered.     

On the extent to which root properties and transport through the soil limit nitrogen uptake by lowland rice

Measurements of N uptake by rice plants growing in a puddled flooded soil and the corresponding changes in soil solution NH: concentrations and the soil NH: diffusion coefficient are used to calculate how far root uptake properties and transport to the roots limit acquisition of N by rice. With root uptake parameters assigned values such that influx was maximal within realistic ranges, the minimum root length densities required to explain uptake were similar to measured root length densities in both N-fertilized and unfertilized soil. This suggests that most if not all of the root length was active in uptake and that uptake per unit root length was near maximal. A sensitivity analysis showed that the necessary minimum root length is very sensitive to root uptake properties within appropriate ranges. Transport to the roots was mainly by diffusion. Rates of diffusion will generally not limit uptake in well-puddled soils, but they may greatly limit uptake in puddled soils that have been drained and re-flooded and in unpuddled flooded soils. Uptake of fertilizer N broadcast into rice field floodwater and absorbed by roots in the floodwater or soil near the floodwater is not likely to be limited by root uptake properties or transport.     

旱后复水条件下磷添加对白羊草根系形态及氮磷累积的影响

植物生长及养分利用特征可揭示半干旱区植物对多变水肥环境条件的适应策略。在白羊草分蘖期设置2个供水条件(正常供水和干旱胁迫21天后复水)和2个磷添加水平(复水当日1 kg干土添加0,0.2 g P_2O_5),2周后测定其根冠生物量、根系形态以及氮磷含量。结果表明,旱后复水条件下,磷添加后白羊草根冠生物量、总生物量和根冠比无显著变化,总根长和根表面积显著增加27.1%和24.1%,比根长和比根面积分别显著增加18.3%和15.9%,根系平均直径显著降低1.3%;白羊草地上部、根系和整株磷含量分别显著增加61.1%,35.8%和49.6%,磷累积量分别显著增加68.6%,52.0%和61.3%,氮磷比显著降低。除地上部氮累积量外,各水分和磷处理下白羊草地上部、根系和整株氮磷累积量与总根长和根表面积呈显著正相关关系。本研究表明,根长和根表面积增加是白羊草响应水肥环境条件改善的主要策略。     

Estimation of phosphorus uptake capacity by different zones of the primary root of soil-grown maize (Zea mays L.)

A method is described which allows determination of the nutrient uptake capacity by different zones of individual roots of soil-grown plants. Examples are given for phosphorus uptake by different zones of the primary root of maize. Agar strips with labelled phosphorus (³²P) are placed on the soil-root interface of root zones of different age. After 24 h autoradiographs are made of the soil-root interface and phosphorus uptake rates determined by plant analysis. Along the primary root the relative phosphorus uptake capacity declines per unit root length from 100% in 1 day old root zones to about 25–30% in 26 day old root zones. The change in the extension of the phosphorus (³²P) depletion zone along the root axis indicates that the uptake capacity for phosphorus of root hairs was maximal upto 4 days and declined thereafter. The decline in capacity for phosphorus uptake from apical to basal root zones probably reflects the decline in root hair viability. The relative high uptake capacity in basal root zones is therefore at least partially due to the uptake capacity of the cortical cells.     

Phosphorus uptake of maize as affected by ammonium and nitrate nitrogen - Measurements and model calculations -

Phosphorus uptake is often enhanced by ammonium compared to nitrate nitrogen nutrition of plants. A decrease of pH at the soil-root interface is generally assumed as the cause. However, an alteration of root growth and the mobilization of P by processes other than net release of protons induced by the source of nitrogen may also be considered. To study these alternatives a pot experiment was conducted with maize using a fossil Oxisol high in Fe/Al-P with low soil solution P concentration. Three levels of phosphate (0, 50, 200 mg P kg^?1) in combination with either ammonium or nitrate nitrogen (100 mg N kg^?1) were applied. Plants were harvested 7 and 21 d after sowing, P uptake measured and root and shoot growth determined. To assess the importance of factors involved in the P transfer from soil into plants, calculations were made using a model of Barber and Claassen. In the treatments with no and low P supply NH₄-N compared to NO₃-N nutrition increased the growth of the plants by 25 % and their shoot P content by 38 % while their root growth increased by 6 % only. The rhizosphere pH decreased in the NH₄-N treatments by 0.1 to 0.6 units as compared to the bulk soil while in the NO₃-N treatments it increased by 0.1 to 0.5 units. These pH changes had a minor influence on P uptake only, as was demonstrated by artificially altering the soil pH to 4.7 and 6.3 respectively. At the same rhizosphere pH, however, P influx was doubled by the application of NH₄-compared to NO₃-N. It is concluded that in this soil the enhancement of P uptake of maize plants after ammonium application cannot be attributed to the acidification of the rhizosphere but to effects mobilizing soil phosphate or increasing P uptake efficiency of roots. Model calculation showed that these effects accounted for 53 % of the P influx per unit root length in the NO₃-N and 72 % in the NH₄-N supplied plants if no P was applied. With high P application the respective figures were only 18 and 19%.     

Effect of external phosphorus and seed phosphorus supply on the shoot and root growth of yellow serradella,burr medic,and subterranean clover

Glasshouse studies were undertaken to compare the responses of yellow sertadella, subterranean clover, and burr medic to changes in phosphorus (P) supply. Shoot growth at 35–50 days after sowing was related to differences in their physical root characteristics and their ability to take up P under conditions of limiting and adequate P supply. Subterranean clover and burr medic were also compared in their shoot response to applied P when seed with differing concentrations of P was sown. Where P supply limited the growth of subterranean clover shoots by at least 40%, yellow sertadella produced more shoot weight, had a lower root and shoot ratio, and longer, thinner roots than both subterranean clover and burr medic. Burr medic produced less shoot weight, had a higher root and shoot ratio and shorter, thicker roots than the other two species. Yellow serradella was also able to take up more P (greater shoot P content per gram root and per meter length root) than subterranean clover, whereas burr medic had the lowest ability to take up P. When P supply was adequate for near maximum growth, yellow serradella and burr medic had similar shoot weight to subterranean clover. Yellow serradella still had the lowest root and shoot ratio, the thinnest roots and a greater shoot P content per gram and per meter roots than subterranean clover, but subterranean clover had similar or slightly longer roots. Burr medic also had the highest root and shoot ratio, but now had roots longer and as thick or thicker than subterranean clover roots. The shoot P content per gram and per meter root in burr medic relative to subterranean clover varied with experiment. Sowing seed with a higher initial concentration of P increased the curvature coefficient of the response to applied P, reduced the absolute response to applied P, and altered the comparative requirement of subterranean clover and burr medic for applied P at 90% maximum yield.     

Growth and uptake rates of P,K, Ca and Mg in wheat 1

The relation between plant age and nutrient absorption properties of red winter wheat (Triticum aestivum L.) roots were investigated. Understanding the change in ion uptake parameters with increasing plant age is helpful in devising efficient fertilization systems. Such information can be used to determine the nutrient levels needed in the soil to supply nutrients rapidly enough to the root surface to minimize deficiencies. Wheat was grown for periods up to 40 days in solution culture in a controlled climate chamber. Sequential harvest and nutrient influx measurements were made. Shoot growth was exponential with time to 30 days and linear thereafter. Root dry weight increased linearly with time at a slower rate than shoot dry weight. Root length increased linearily with time. With increasing plant age there was a reduction in average P and K uptake rate while average uptake rates for Ca and Mg remained relatively unchanged. With increasing plant age, the maximum influx, I_max. for P and Mg remained constant, but for K and Ca, there was a decrease. For the Michael is constant, Km, no change was observed for P, an increase occurred for K, and a decrease for Ca and Mg, as the wheat plant grew from 5 to 40 days.     

Root trait plasticity and plant nutrient acquisition in phosphorus limited soil

To overcome soil nutrient limitation, many plants have developed complex nutrient acquisition strategies including altering root morphology, root hair formation or colonization by arbuscular mycorrhizal fungi (AMF). The interactions of these strategies and their plasticity are, however, affected by soil nutrient status throughout plant growth. Such plasticity is decisive for plant phosphorus (P) acquisition in P‐limited soils. We investigated the P acquisition strategies and their plasticity of two maize genotypes characterized by the presence or absence of root hairs. We hypothesized that in the absence of root hairs plant growth is facilitated by traits with complementary functions, e.g., by higher root mycorrhizal colonization. This dependence on complementary traits will decrease in P fertilized soils. At early growth stages, root hairs are of little benefit for nutrient uptake. Regardless of the presence or absence of root hairs, plants produced average root biomass of 0.14 g per plant and exhibited 23% root mycorrhizal colonization. At later growth stages of maize, contrasting mechanisms with functional complementarity explained similar plant biomass production under P limitation: the presence of root hairs versus higher root mycorrhizal colonization (67%) favored by increased fine root diameter in absence of root hairs. P fertilization decreased the dependence of plant on specific root traits for nutrient acquisition. Through root trait plasticity, plants can minimize trade‐offs for developing and maintaining functional traits, while increasing the benefit in terms of nutrient acquisition and plant growth. The present study highlights the plasticity of functional root traits for efficient nutrient acquisition strategies in agricultural systems with low nutrient availability.     

田间玉米和蚕豆对低磷胁迫响应的差异比较

【目的】植物在长期进化过程中形成了一系列适应机制,以应对低磷胁迫。本文提出玉米主要通过根系形态变化适应低磷胁迫的假设,并通过与蚕豆植株在根系形态与生理方面对低磷胁迫反应的比较试验加以验证。【方法】在中国农业大学上庄长期定位试验田进行两年田间实验,玉米和蚕豆分别单作,重复3次。在玉米抽雄前的拔节至大喇叭口期和蚕豆的初花至盛花期两次取样(两年的两次取样时间间隔10~12天),比较研究了不供磷和供磷100 kg/hm2下玉米和蚕豆生长和磷素吸收、根系在0—40 cm土层中分布、以及根际p H值和酸性磷酸酶活性的差异。【结果】1)玉米植株的生物量和含磷量远远高于蚕豆;第一次取样时蚕豆的根冠比高于玉米,而且两种植物低磷下的根冠比高于供磷充足处理。两次取样时玉米的总根长大于蚕豆,两种植物的大部分根系分布在0—20 cm表层土壤,玉米根系在0—10 cm土层的分布更多。2)蚕豆根系的比根长明显大于玉米,但单位根长吸磷量低于玉米,两种植物间的上述差异不受取样时间和供磷水平的影响。3)两次取样时,蚕豆根表的酸性磷酸酶活性均明显高于玉米。玉米根表的酸性磷酸酶活性在两个供磷水平下没有差异。第一次取样时,缺磷蚕豆根表的酸性磷酸酶活性高于供磷充足的蚕豆植株。4)缺磷蚕豆的根际土壤p H值明显低于供磷充足蚕豆;但玉米根际土壤p H值在缺磷和供磷充足条件下无显著差异。【结论】低磷条件下两种植物的根冠比均明显增加。玉米根系单位根长的吸磷量高于蚕豆,并且在含磷量丰富的表层土壤分布有更多根系,但缺磷条件下玉米没有增加根系的质子和酸性磷酸酶的分泌,主要以根系形态变化来适应低磷胁迫。结果支持本文提出的玉米主要通过根系形态变化适应低磷胁迫的假设。但蚕豆在低磷条件下除了增加根系生长外,还具有通过增加质子分泌和根表酸性磷酸酶活性提高根际土壤有效磷浓度的潜力。