Development and replenishment of the P-depletion zone around the primary root of maize during the vegetation period

The dynamics of the development and replenishment of P-depletion zones around the primary root of maize (Zea mays L. cv ‘Garbo’) was studied during a vegetation period (80 days) under greenhouse conditions in a loamy sand of low P-availability. A recently described freeze-cutting technique was used to determine radial diffusion of labelled phosphate to the primary root. The development of the depletion zone was biphasic. In the initial phase after two days of growth of the primary root in a soil layer labelled with³³P a minimum of isotopically exchangeable P (EP) was observed which had decreased to about 30% of its original amount at the root surface. At that time the corresponding P-concentration in the soil solution was calculated to be as low as 5×10⁻⁷ M. The depletion zone had already spread 0.4 mm from the root surface. During the second phase, between the 10th and 20th day of plant growth the concentration of EP at the root surface increased slowly but did not change markedly. However, the depletion zone continued to spread and after the 20th day of growth reached its maximal diameter (1.07 mm from the root surface) but remained completely within the root hair cyclinder; the single root hairs never exceeded 1.14 mm in length. The biphasic growth of the depletion zone was probably caused by proton extrusion of the root tip. Acidification of the soil solution from pH 5.8 to about 3.9 results in an about 3-fold rise of the concentration of desorbed phosphate and might also have activated acidophilic P-translocators of the root during the initial phase. Anion over cation uptake normally prevailing during the later stage of root development might resulted in a rise of the soil pH within the root hair zone. Consequently P-availability, as well as P-uptake capacity declined, but P-uptake by the seminal root still continued until the 20th day. Subsequently, the P-concentration within the depletion zone increased again while simultaneously its extent was reduced until it was almost completely replenished after 60 days indicating a loss of P-uptake capacity of the primary root. Within the root tissue³³P was accumulated to about twice the concentration of that in the undepleted soils. This accumulation corresponded to periods of high uptake due to the development of root laterals. In the root cortex a high P-content was observed during the first 30 days of growth. At the onset of the reproductive stage of the plant the P-content of the shoot and especially in the developing seeds rose considerably at the cost of phosphate stored in the root cortex. The accumulation of³³P in the root tissue indicated that nutrient gain was mainly achieved during the early stages of plant development and that P was temporarily stored to some extent within the root system.     

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     

Transformations and movement of P in the rhizosphere

Summary Wheat plants labelled with³³P were grown in thin layers of soil amended with³²P-labelled fertiliser. Roots were separated from the soil during plant growth by a porous membrane to overcome difficulties in measuring microbial P in rhizosphere soil. Over the 22 day growth period, net movement of³³P out of healthy growing roots varied from 0.9–4.9% of the total³³P translocated to the root. Over the same period the plants took up 12.0% and the microbial biomass 14.1% of the fertiliser³²P. On drying and rewetting of the soil after the plants were harvested, a large proportion of root P moved into soil fractions while³²P appeared to accumulate in the biomass and stable P forms.     

In situ-determination of the phosphate-gradient around a root by radioautography of frozen soil sections

Summary A radioautographic method is described which allows the determination of phosphate concentration profiles around a root in situ,i.e. under conditions of radial diffusional flow. The device consisted of a soil column containing a distinct layer labelled with³³P which was kept separated from the rest of the content of a (modified) Kich-Brauckmann vessel. The primary root of a maize plant was directed into the special soil core whereas the other roots were allowed to develop into the unlabelled portion of the pot. Two or five days after the roots had penetrated the labelled soil sections the soil blocks were immediately frozen in liquid nitrogen and, ommitting any further embedding procedures, sliced perpendicular to the growth direction of the root by means of a stone cutting saw. From the frozen soil slices radioautograms were prepared and densitometrically analysed for phosphate content within and outside the root. The P-depletion zones around the root as well as areas of P-accumulation within the root coincided well with anatomical and morphological root parameters as determined with maize plants grown under similar conditions thus mutually corroborating the findings. Interestingly, the P-depletion zone around the primary root did not exceed the area of the root hair cylinder. Although soil composition and the extent of water supply to the pot somewhat limit the applicability of the presented technique, it should be appropriate for the investigation of a variety of agricultural soils. Since laterals did not interfere with the analysis this method should also allow long-term studies to be performed.     

Role of root hairs in phosphorus depletion from a macrostructured soil

One rape (Brassica napus cv. Wesroona) plant and four cotton (Gossypium hirsutum cv. Sicot 3) plants were grown in plastic cells containing soil labelled with 407 kBq of³³P g⁻¹ soil. After 5–8 days of growth, the³³P depletion zones of all plants were autoradiographed and³³P uptake by plants was measured. The autoradiographs were scanned with a microdensitometer and the optical densities at several places within the³³P depletion zones of roots were obtained. The volume of soil explored by root hairs was estimated from measurements of root diameters and lengths of roots and root hairs. About half of the total³³P depleted by cotion roots came from outside the root hair cylinder whereas most of³³P taken up by rape was from within the root hair cylinder. Plants grown in a macrostructured soil may have roots growing in voids, within aggregates or on the surfaces of aggregates. The results of this study demonstrate that root hairs have a strong influence on the accessibility of phosphorus to roots in such a soil, and thus on the phosphorus nutrition of plants.     

Extension of the phosphorus depletion zone in VA-mycorrhizal white clover in a calcareous soil

To examine the influence of vesicular-arbuscular (VA) mycorrhizal fungi on phosphorus (P) depletion in the rhizosphere, mycorrhizal and non-mycorrhizal white clover (Trifolium repens L.) were grown for seven weeks in a sterilized calcareous soil in pots with three compartments, a central one for root growth and two outer ones for hyphae growth. Compartmentation was accomplished by a 30-μm nylon net. The root compartment received a uniform level of P (50 mg kg⁻¹ soil) in combination with low or high levels of P (50 or 150 mg kg⁻¹ soil) in the hyphal compartments. Plants were inoculated withGlomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or remained uninfected. Mycorrhizal inoculation doubled P concentration in shoot and root, and increased dry weight, especially of the shoot, irrespective of P levels. Mycorrhizal contribution accounted for 76% of total P uptake at the low P level and 79% at the high P level, and almost all of this P was delivered by the hyphae from the outer compartment. In the non-mycorrhizal plants, the depletion of NaHCO₃-extractable P (Olsen-P) extended about 1 cm into the outer compartment, but in the mycorrhizal plants a uniform P depletion zone extended up to 11.7 cm (the length of the hyphal compartment) from the root surface. In the outer compartment, the mycorrhizal hyphae length density was high (2.5–7 m cm⁻³ soil) at the various distances (0–11.7 cm) from the root surface. Uptake rate of P by mycorrhizal hyphae was in the range of 3.3–4.3×10⁻¹⁵ mol s⁻¹ cm⁻¹.     

Phosphorus (P) uptake and growth of a root hairless barley mutant (bald root barley, brb) and wild type in low- and high-P soils

The recently isolated root‐hairless mutant of barley (Hordeum vulgare L), bald root barley, brb offers a unique possibility to quantify the importance of root hairs in phosphorus (P) uptake from soil. In the present study the ability of brb and the wild‐type, barley genotype Pallas producing normal root hairs to deplete P in the rhizosphere soil was investigated and the theory of diffusion and mass flow applied to compare the predicted and measured depletion profiles of diffusible P. Pallas depleted twice as much P from the rhizosphere soil as brb. The P depletion profile of Pallas uniformly extended to 0.8 mm from the root surface, which was equal to the root hair length (RHL). The model based on the theory of diffusion and mass flow explained the observed P‐depletion profile of brb, and the P depletion outside the root‐hair zone of Pallas, suggesting that the model is valid only for P movement in rhizosphere soil outside the root‐hair zone. In low‐P soil (P in soil solution 3 µm ) brb did not survive after 30 d, whereas Pallas continued to grow, confirming the importance of root hairs in plant growth in a P‐limiting environment. In high‐P soil (P in soil solution 10 µm ) both brb and Pallas maintained their growth, and they were able to produce seeds. At the high‐P concentration, RHL of the Pallas was reduced from 0.80 ± 0.2 to 0.68 ± 0.14 mm. In low‐P soil, P‐uptake rate into the roots of Pallas was 4.0 × 10^?7 g mm^?1 d^?1 and that of brb was 1.9 × 10^?7 g mm^?1 d^?1, which agreed well with the double amount of P depleted from the rhizosphere soil of Pallas in comparison with that of brb. In high‐P soil, the P uptake rates into the roots of brb and Pallas were 3.3 and 5.5 × 10^?7 g mm^?1 d^?1, respectively. The results unequivocally confirmed that in a low‐P environment, root hairs are of immense importance in P acquisition and plants survival, but under high‐P conditions they may be dispensable. The characterization of phenotypes brb and Pallas and the ability to reproduce seeds offers a unique possibility of molecular mapping of QTLs and candidate genes conferring root‐hair formation and growth of barley.     

Effect of soil compaction on root growth and uptake of phosphorus

Summary Zea mays L. andLolium rigidum Gaud. were grown for 18 and 33 days respectively in pots containing three layers of soil each weighing 1 kg. The top and bottom layers were 100 mm deep and they had a bulk density of 1200 kg m^–3, while the central layer of soil was compacted to one of 12 bulk densities between 1200 and 1750 kg m^–3. The soil was labelled with³²P and³³P so that the contribution of the different layers of soil to the phosphorus content of the plant tops could be determined. Soil water potential was maintained between –20 and –100 kPa.Total dry weight of the plant tops and total root length were slightly affected by compaction of the soil, but root distribution was greatly altered. Compaction decreased root length in the compacted soil but increased root length in the overlying soil. Where bulk density was 1550 kg m^–3, root length in the compacted soil was about 0.5 of the maximum. At that density, the penetrometer resistance of the soil was 1.25 and 5.0 MPa and air porosity was 0.05 and 0.14 at water potentials of –20 and –100 kPa respectively, and daytime oxygen concentrations in the soil atmosphere at time of harvest were about 0.1 m³m^–3. Roots failed to grow completely through the compacted layer of soil at bulk densities 1550 kg m^–3. No differences were detected in the abilities of the two species to penetrate compacted soil.Ryegrass absorbed about twice as much phosphorus from uncompacted soil per unit length of root as did maize. Uptake of phosphorus from each layer of soil was related to the length of root in that layer, but differences in uptake between layers existed. Phosphorus uptake per unit length of root was higher from compacted than from uncompacted soil, particularly in the case of ryegrass at bulk densities of 1300–1500 kg m^–3.     

Contribution by two arbuscular mycorrhizal fungi to P uptake by cucumber (Cucumis sativus L.) from32P-labelled organic matter during mineralization in soil

An experiment was set up to investigate the role of arbuscular mycorrhiza (AM) in utilization of P from organic matter during mineralization in soil. Cucumber (Cucumis sativus L.) inoculated with one of two AM fungi or left uninoculated were grown for 30 days in cross-shaped PVC pots. One of two horizontal compartments contained 100 g soil (quartz sand: clay loam, 1:1) with 0.5 g ground clover leaves labelled with³²P. The labelled soil received microbial inoculum without AM fungi to ensure mineralization of the added organic matter. The labelling compartment was separated from a central root compartment by either 37 m or 700 m nylon mesh giving only hyphae or both roots and hyphae, respectively, access to the labelled soil. The recovery of³²P from the hyphal compartment was 5.5 and 8.6% for plants colonized withGlomus sp. andG. caledonium, respectively, but only 0.6 % for the non-mycorrhizal controls. Interfungal differences were not related to root colonization or hyphal length densities, which were lowest forG. caledonium. Both fungi depleted the labelled soil of NaHCO₃-extractable P and³²P compared to controls. A 15–25% recovery of³²P by roots was not enhanced in the presence of mycorrhizas, probably due to high root densities in the labelled soil. The experiment confirms that AM fungi differ in P uptake characteristics, and that mycorrhizal hyphae can intercept some P immobilization by other microorganisms and P-sorbing clay minerals.     

Salinization of the soil solution decreases the further accumulation of salt in the root zone of the halophyte Atriplex nummularia Lindl. growing above shallow saline groundwater

Water use by plants in landscapes with shallow saline groundwater may lead to the accumulation of salt in the root zone. We examined the accumulation of Na⁺ and Cl^? around the roots of the halophyte Atriplex nummularia Lindl. and the impacts of this increasing salinity for stomatal conductance, water use and growth. Plants were grown in columns filled with a sand–clay mixture and connected at the bottom to reservoirs containing 20, 200 or 400 mM NaCl. At 21 d, Na⁺ and Cl^? concentrations in the soil solution were affected by the salinity of the groundwater, height above the water table and the root fresh mass density at various soil depths (P  < 0.001). However, by day 35, the groundwater salinity and height above the water table remained significant factors, but the root fresh mass density was no longer significant. Regression of data from the 200 and 400 mM NaCl treatments showed that the rate of Na⁺ accumulation in the soil increased until the Na⁺ concentration reached ~250 mM within the root zone; subsequent decreases in accumulation were associated with decreases in stomatal conductance. Salinization of the soil solution therefore had a feedback effect on further salinization within the root zone.     

Phosphorus depletion in the rhizosphere as influenced by soil moisture

To study the influence of soil moisture on phosphorus (P) depletion in the rhizosphere, maize (Zea mays cv. Trak) was pre-grown in vermiculite filled-PVC tubes for 9 days and then the plants with the tubes were transplanted into soil columns maintained at two soil moisture levels () of 0.14 and 0.20 cm³ cm^–3 for 10 days. The soil columns were separated at 1 cm depth by a nylon screen of 53 m inner mesh size, into 1 cm soil layer above and 3 cm soil column below screen. A root mat developed over the screen, but root hairs only could penetrate it. Regardless of the soil moisture level in the columns, and adequate and equal water and nutrients supply was maintained via wicks from an external nutrient solution to the plant roots in vermiculite. After 10 days, the soil columns were separated from the root mats, quickly frozen in liquid nitrogen and sliced into thin layers (0.2mm) using a refrigerated microtome to give soil samples at defined distances from the root mats for analyses. Lower soil moisture (=0.14) resulted in narrower and steeper depletion profile of 0.5 M NaHCO₃ extractable P (NaHCO₃-P_i) as compared to higher soil moisture (=0.20). Depletion of P in soil solution in the immediate vicinity of root mats did not differ much but the extension of the depletion zones was 0.10 cm at =0.14 and 0.20 cm at =0.20. The depletion up to 0.05cm with =0.14 and up to 0.07 cm with =0.20 was uniform, and may be attributed to the depletion in the root hair zone. Beyond the root hair zones, the theory of diffusion and mass flow was able to explain the observed differences in shape and extent of the P depletion profiles at the two soil moisture levels.     

Phosphorus acquisition characteristics of cotton (Gossypium hirsutum L.), wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.) under P deficient conditions

A rhizobox experiment was conducted to examine the P acquisition characteristics of cotton (Gossypium hirsutum L.), wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.) under P-deficient conditions. We aimed to identify whether cotton is physiologically efficient at acquiring P through release of protons, phosphatases or carboxylates. Plants were pre-grown in the upper compartment of rhizoboxes filled with a sand and soil mixture to create a dense root mat against a 53 μm polyester mesh. For each species, two P treatments (0 and 20 mg P kg^?1) were applied to the upper compartment in order to create P-deficient and P-sufficient plants. At harvest, the upper compartment with intact plants was used for collection of root exudates while the lower soil compartment was sliced into thin layers (1 mm) parallel to the rhizoplane. Noticeable carboxylates release was only detected for white lupin. All P-deficient plants showed a capacity to acidify their rhizosphere soil to a distance of 3 mm. The activity of acid phosphatase was significantly enhanced in the soil-root interfaces of P-stressed cotton and wheat. Under P-deficient conditions, the P depletion zone of cotton from the lower soil compartment was narrowest (<2 mm) among the species. Phosphorus fractionation of the rhizosphere soil showed that P utilized by cotton mainly come from NaHCO₃–P_i and NaOH–P_o pools while wheat and white lupin markedly depleted NaHCO₃–P_i and HCl–P pools, and the depletion zone extended to 3 mm. Wheat also depleted NaOH–P_o to a significant level irrespective of P supply. The study suggests that acquisition of soil P is enhanced through P mobilization by root exudates for white lupin, and possibly proton release and extensive roots for wheat under P deficiency. In contrast, the P acquisition of cotton was associated with increased activity of phosphatases in rhizosphere soil.     

地下滴灌条件下三倍体毛白杨根区土壤水分动态模拟

在根系分布试验观测的基础上,提出了三倍体毛白杨一维根系吸水模型,在考虑根系吸水情况下利用HYDRUS模型模拟了地下滴灌条件下三倍体毛白杨根区的土壤水分动态,通过田间试验对模型进行验证,并利用HYDRUS研究了不同灌水技术参数对土壤湿润模式的影响.结果表明：在灌溉结束和水分再分布24 h后,土壤含水量模拟结果的相对平均绝对误差(RMAE)分别为7.8%和6.0%,均方根误差(RMSE)分别为0.036和0.026 cm³·cm^-3,说明HYDRUS模型能很好地模拟地下滴灌条件下三倍体毛白杨根区的短期土壤水分动态,且所建根系吸水模型合理;与2、4 L·h^-1的滴头流速和连续性灌溉相比,流速1 L·h^-1和脉冲式灌溉(每隔30 min灌水30 min)能增大土壤湿润体体积,且可以减少水分深层渗漏量,因此,对试验地三倍体毛白杨根区进行地下滴灌应首选流速1 L·h^-1的脉冲式灌溉.     

A sensitive RNase protection assay using33P labeled antisense riboprobes

This article presents for the first time a modified protocol for RNase protection analysis that allows the substitution of³²P with³³P without loss of the high sensitivity of this method achieved with³²P. With this protocol, we were able to detect at least 1 pg of specific mRNA. In the RNase protection analysis³³P labeled riboprobes are more advantageous with regard to an easier handling and better resolution.     

Linking root traits and competitive success in grassland species

Background and aims

Measures of phosphorus (P) in roots recovered from soil underestimate total P accumulation below-ground by crop species since they do not account for P in unrecovered (e.g., fine) root materials. ³³P-labelling of plant root systems may allow more accurate estimation of below-ground P input by plants.

Methods

Using a stem wick-feeding technique ³³P-labelled phosphoric acid was fed in situ to canola (Brassica napus) and lupin (Lupinus angustifolius) grown in sand or loam soils in sealed pots.

Results

Recovery of ³³P was 93 % in the plant-soil system and 7 % was sorbed to the wick. Significantly more ³³P was allocated below-ground than to shoots for both species with 59–90 % of ³³P measured in recovered roots plus bulk and rhizosphere soil. ³³P in recovered roots was higher in canola than lupin regardless of soil type. The proportion of ³³P detected in soil was greater for lupin than canola grown in sand and loam (37 and 73 % lupin, 20 and 23 % canola, respectively). Estimated total below-ground P accumulation by both species was at least twice that of recovered root P and was a greater proportion of total plant P for lupin than canola.

Conclusion

Labelling roots using ³³P via stem feeding can empower quantitative estimates of total below-ground plant P and root dry matter accumulation which can improve our understanding of P distribution in soil-plant systems.

     

沙地云杉幼苗根表土体中NPK的梯度分布

采用水平根和垂直根两种处理方法对6年生沙地云杉幼苗进行栽培实验,应用分层取样方法对幼苗根表不同距离土体进行取样,并测定不同层次土体中速效N、速效P、速效K的含量。结果表明,在沙地云杉根表不同距离的土体中,速效N、P、K呈现有规律的梯度分布,即在根表近距离土体中营养元素由于根系的吸附作用而含量较高,同时根系生命活动对营养元素的大量消耗又使得营养元素随即出现严重的亏缺区,再向外延伸营养元素含量又逐渐上升而达到土壤本底值,在水平根处理中,由于沙地云杉对N、P、K吸收和利用的强度不同,亏缺区出现的位置不同,速效N和速效K的亏缺区出现在距离根表1cm处;速效P出现在距离根表0．5cm处,在垂直根处理中,速效N、速效P、速效K的梯度变化与水平根处理的相似速效N和速效K亏缺区出现在距离根表大约1cm处,而速效P在根表附近土壤中的含量始终少于根表远处,说明沙地云杉幼苗对速效P的吸收和利用强度大,速效P可能成为沙地云杉生长发育的限制因子。因此,在沙地云杉引种栽培时,应该选择含P丰富的土壤,或者对林地适当施用一些P肥。     

Growth effects of the vesicular-arbuscular mycorrhizal fungusGlomus constrictum on maize plants in pot trials

Maize plants were inoculated withGlomus constrictum in soil of low phosphorus content amended with five rates of P in the form of Ca₃(PO₄)₂. Maize dry matter yield was increased by addition of P up to 30 and/or 60 mg P/kg soil, above that it began to decrease to reach at 100 mg P/kg a value similar to that of the control. At all P levels used, the shoot and root (total plant) dry mass of inoculated plants was significantly increased compared with the non-inoculated controls and this increment ranged in some cases between 50 and 70%. Development of vesicular-arbuscular mycorrhizal fungus (VAM) monitored in terms of P contents in dry matter of maize revealed that the P content of plants not inoculated withG. constrictum was not influenced by P addition to soil. On the other hand, P content of maize plants inoculated with VAM was dramatically increased by increasing P levels of soil and was maximum at 30 mg P; above that it began to decline. Mycorrhizal root infection (expressed as percentage of root length infected) increased by increasing the P concentrations above the soil basal level up to 80 mg P where the infected root length was 72% of the total root length after 28 d of planting. The increase in VAM spore formation in soil was similar to that of root infection except that the highest spore number was sieved from soil at 60 mg P/kg soil.     

Response of onion plants to arbuscular mycorrhizae

Onion (Allium cepa) plants were grown in pots in two types of irradiated soil, mineral and organic. Onion development was observed under two or three levels of P fertilization, and three methods of arbuscular mycorrhizal fungus inoculation with two fungus species. In mineral soil, preinoculated onion plants had a higher biomass than non-inoculated control plants or plants inoculated with either colonized root segments or spores. Fungus species had no differential effect on dry biomass or final bulb diameter. Preinoculated onion plants reached marketable size (>25 mm bulb diameter) 2-3 weeks earlier than those inoculated by either of the other two methods. Non-inoculated onion plants remained stunted. Bulbs of onions inoculated with Glomus versiforme were firmer than those inoculated with G. intraradices. Increasing P fertilizer rates had a significant positive linear effect on the P tissue concentration of plants inoculated with G. intraradices or G. versiforme, but no effect on bulb firmness. The P tissue concentration of inoculated plants was significantly higher than that of non-inoculated controls, and in inoculated plants, it differed among inoculation methods. The P tissue concentration was higher in onion plants inoculated with G. versiforme than in those inoculated with G. intraradices. In organic soil, the dry biomass of preinoculated plants was higher than that of plants inoculated by root segments. The highest root colonization levels were obtained under a low soil P level with G. intraradices, and with the root segment method of inoculation with G. versiforme.     

The importance of root gravitropism for inter-root competition and phosphorus acquisition efficiency: results from a geometric simulation model

We have observed that low soil phosphorus availability alters the gravitropic response of basal roots in common bean (Phaseolus vulgaris L.), resulting in a shallower root system. In this study we use a geometric model to test the hypotheses that a shallower root system is a positive adaptive response to low soil P availability by (1) concentrating root foraging in surface soil horizons, which generally have the highest P availability, and (2) reducing spatial competition for P among roots of the same plant. The growth of nine root systems contrasting in gravitropic response over 320 h was simulated in SimRoot, a dynamic three-dimensional geometric model of root growth and architecture. Phosphorus acquisition and inter-root competition were estimated with Depzone, a program that dynamically models nutrient diffusion to roots. Shallower root systems had greater P acquisition per unit carbon cost than deeper root systems, especially in older root systems. This was due to greater inter-root competition in deeper root systems, as measured by the volume of overlapping P depletion zones. Inter-root competition for P was a significant fraction of total soil P depletion, and increased with increasing values of the P diffusion coefficient (De), with root age, and with increasing root gravitropism. In heterogenous soil having greater P availability in surface horizons, shallower root systems had greater P acquisition than deeper root systems, because of less inter-root competition as well as increased root foraging in the topsoil. Root P acquisition predicted by SimRoot was validated against values for bean P uptake in the field, with an r ² between observed and predicted values of 0.75. Our results support the hypothesis that altered gravitropic sensitivity in P-stressed roots, resulting in a shallower root system, is a positive adaptive response to low P availability by reducing inter-root competition within the same plant and by concentrating root activity in soil domains with the greatest P availability. This revised version was published online in August 2006 with corrections to the Cover Date.     

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