Acid phosphatase role in chickpea/maize intercropping

• Background and aims Organic P comprises 30–80 %of the total P in most agricultural soils. It has been proventhat chickpea facilitates P uptake from an organic P sourceby intercropped wheat. In this study, acid phosphatase excretedfrom chickpea roots is quantified and the contribution of acidphosphatase to the facilitation of P uptake by intercroppedmaize receiving phytate is examined. • Methods For the first experiment using hydroponics, maize(Zea mays ‘Zhongdan No. 2’) and chickpea (Cicerarietinum ‘Sona’) were grown in either the sameor separate containers, and P was supplied as phytate, KH₂PO₄at 0·25 mmol P L^–1, or not at all. The second experimentinvolved soil culture with three types of root separation betweenthe two species: (1) plastic sheet, (2) nylon mesh, and (3)no barrier. Maize plants were grown in one compartment and chickpeain the other. Phosphorus was supplied as phytate, Ca(H₂PO₄)₂at 50 mg P kg^–1, or no P added. • Key results In the hydroponics study, the total P uptakeby intercropped maize supplied with phytate was 2·1-foldgreater than when it was grown as a monoculture. In the soilexperiment, when supplied with phytate, total P uptake by maizewith mesh barrier and without root barrier was 2·2 and1·5 times, respectively, as much as that with solid barrier.In both experiments, roots of both maize and chickpea suppliedwith phytate and no P secreted more acid phosphatase than thosewith KH₂PO₄ or Ca(H₂PO₄)₂. However, average acid phosphataseactivity of chickpea roots supplied with phytate was 2–3-foldas much as maize. Soil acid phosphatase activity in the rhizosphereof chickpea was also significantly higher than maize regardlessof P sources. • Conclusions Chickpea can mobilize organic P in both hydroponicand soil cultures, leading to an interspecific facilitationin utilization of organic P in maize/chickpea intercropping.     

Growth Consequences of Soil Nutrient Heterogeneity for two Old-field Herbs,Ambrosia artemisiifoliaandPhytolacca americana, Grown Individually and in Combination

Plant species can respond to small scale soil nutrient heterogeneityby proliferating roots or increasing nutrient uptake kineticsin nutrient-rich patches. Because root response to heterogeneitydiffers among species, it has been suggested that the distributionof soil resources could influence the outcome of interspecificcompetition. However, studies testing how plants respond toheterogeneity in the presence of neighbours are lacking. Inthis study, individuals of two species,Phytolacca americanaL.andAmbrosia artemisiifoliaL. were grown individually and incombination in soils with either a homogeneous or heterogeneousnutrient distribution. Above-ground biomass of individuallygrown plants of both species was greater when fertilizer waslocated in a single patch than when the same amount of fertilizerwas distributed evenly throughout the soil. Additionally, bothspecies proliferated roots in high-nutrient patches.A. artemisiifoliaexhibitedlarger root:shoot ratios, increased nitrogen depletion fromnutrient patches, and a higher growth rate thanP. americana,suggestingA. artemisiifoliais better suited to find and rapidlyexploit nutrient patches. In contrast to individually grownplants, soil nutrient distribution had no effect on final above-groundplant biomass for either species when grown with neighbours,even though roots were still concentrated in high nutrient patches.This study demonstrates that increased growth of isolated plantsas a consequence of localized soil nutrients is not necessarilyan indication that heterogeneity will affect interspecific encounters.In fact, despite a significant below-ground response, soil nutrientheterogeneity was inconsequential to above-ground performancewhen plants were grown with neighbours.Copyright 1999 Annalsof Botany Company Phytolacca americana, pokeweed,Ambrosia artemisiifolia, ragweed, nutrient heterogeneity, root proliferation, plasticity, foraging, nutrient patches.     

Soil Water Extraction, Measured by Computer-Assisted Tomography, in Seedling Lupinus angustifolius cv. Yandee when Healthy and Infected with Phytophthora cinnamomi

The water-mould fungus Phytophthora cinnamomi Rands causes drought-likesymptoms on many hosts, and yet the mechanisms by which infectionleads to wilting are not fully understood. This is the firststudy to describe in detail changes in soil water around theroot with infection. Computer-assisted tomography (CAT) wasused with Lupinus angustifolius L. cv. Yandee to examine drawdowns(removal of soil water) around a central root infected by P.cinnamomi in a white sand. No growth differences in roots or shoots were found betweenhealthy and diseased plants during the 8 d of the experiment.However,drawdowns failed at high levels of inoculum (8–16 /Pc-infectedmillet seeds/plant) by 8 d. Water contents in pots with uninfectedplants were in the range 0·09–0·12 cm³ watercm^–3 soil in the centre of the pot, while water contentsin pots with infected plants at 16 millet seeds applied werein the range of 0·16–0·19 cm³ water cm^–3soil in the centre of the pot. A higher transpirational demand produced lower soil water contentsnear the root but this effect was confounded with infection:disease was more pronounced with higher transpirational demand,and disease led to an increase in water content. Key words: Root disease, Phytophthora cinnamomi, water uptake, soil-root interface, computer-assisted tomography     

The relationship between silicon availability,and growth and silicon concentration of the salt marsh halophyte Spartina anglica

Incorporation of cover crops into cropping systems may contribute to a more efficient utilization of soil and fertilizer P by less P-efficient crops through exudation of P-mobilizing compounds by the roots of P-efficient plant species. The main objective of the present work was to test this hypothesis. First a method has been developed which allows the quantification of organic anion exudation from individual cluster roots formed by P-deficient white lupin (Lupinus albus L.). Lupin plants were grown in nutrient solution at 1 μM P and in a low P loess in small rhizotrons. Organic anions exuded from intact plants grown in nutrient solution were collected from individual cluster roots and root tips sealed in small compartments by an anion-exchange resin placed in nylon bags (resin-bags). Succinate was the dominant organic anion exuded followed by citrate and malate. The mean of citrate exudation-rate was 0.06 pmol mm⁻¹ s⁻¹ with exudation highly dependent on the citrate concentration and on the age of the cluster roots. Exudates from cluster roots and root tips grown at the soil surface (rhizotron-grown plants) were collected using overlayered resin–agar (resin mixed with agar). Citrate exudation from cluster roots was 10 times higher than that from root tips. Fractionation of P in the cluster root rhizosphere-soil indicates that white lupin can mobilize P not only from the available and acid-soluble P, but also from the stable residual soil P fractions. In pot experiments with an acid luvisol derived from loess low in available P, growth of wheat was significantly improved when mixed-cropped with white lupin due to improved P uptake. Both in mixed culture and in rotation wheat could benefit from the P mobilization capacity of white lupin, supporting the hypothesis above. Nine tropical leguminous cover crops and maize were grown in a pot experiment using a luvisol from Northern Nigeria low in available P. All plant species derived most of their P from the resin and bicarbonate-extractable inorganic P. Organic P (P_o) accumulated particularly in the rhizosphere of all plant species. There was a significant negative correlation between the species-specific rhizosphere acid phosphatase activity and P_o accumulation. Growth and P uptake of maize grown in rotation after legumes were enhanced indicating that improved P nutrition was a contributing factor. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interaction between root growth allocation and mycorrhizal fungi in soil with patchy P distribution

Aims and Background

Many plants preferentially grow roots into P-enriched soil patches, but little is known about how the presence of arbuscular mycorrhizal fungi (AMF) affects this response.

Methods

Lotus japonicus (L.) was grown in a low-P soil with (a) no additional P, (b) homogeneous P (28 mg pot^?1), (c) low heterogeneous P (9.3 mg pot^?1), and (d) high heterogeneous P (28 mg pot^?1). Each P treatment was combined with one of three mycorrhiza treatments: no mycorrhizae, Glomus intraradices, indigenous AMF. Real-time PCR was used to assess the abundance of G. intraradices and the indigeneous AMF G. mosseae and G. claroideum.

Results

Mycorrhization and P fertilization strongly increased plant growth. Homogeneous P supply enhanced growth in both mycorrhizal treatments, while heterogeneous P fertilization increased biomass production only in treatments with indigenous AMF inoculation. Preferential root allocation into P-enriched soil was significant only in absence of AMF. The abundance of AMF species was similar in P-enriched and unfertilized soil patches.

Conclusion

Mycorrhization may completely override preferential root growth responses of plants to P- patchiness in soil. The advantage of this effect for the plants is to give roots more freedom to forage for other resources in demand for growth and to adapt to variable soil conditions.     

Root growth and plant biomass in <Emphasis Type="Italic">Lolium perenne</Emphasis> exploring a nutrient-rich patch in soil

We investigated soil exploration by roots and plant growth in a heterogeneous environment to determine whether roots can selectively explore a nutrient-rich patch, and how nutrient heterogeneity affects biomass allocation and total biomass before a patch is reached. Lolium perenne L. plants were grown in a factorial experiment with combinations of fertilization (heterogeneous and homogeneous) and day of harvest (14, 28, 42, or 56 days after transplanting). The plant in the heterogeneous treatment was smaller in its mean total biomass, and allocated more biomass to roots. The distributions of root length and root biomass in the heterogeneous treatment did not favor the nutrient-rich patch, and did not correspond to the patchy distribution of inorganic nitrogen. Specific root length (length/biomass) was higher and root elongation was more extensive both laterally and vertically in the heterogeneous treatment. These characteristics may enable plants to acquire nutrients efficiently and increase the probability of encountering nutrient-rich patches in a heterogeneous soil. However, heterogeneity of soil nutrients would hold back plant growth before a patch was reached. Therefore, although no significant selective root placement in the nutrient-rich patch was observed, plant growth before reaching nutrient-rich patches differed between heterogeneous and homogeneous environments.     

Aerenchyma and an Inducible Barrier to Radial Oxygen Loss Facilitate Root Aeration in Upland, Paddy and Deep-water Rice (Oryza sativa L.)

The present study evaluated waterlogging tolerance, root porosityand radial O₂ loss (ROL) from the adventitious roots, of sevenupland, three paddy, and two deep-water genotypes of rice (Oryzasativa L.). Upland types, with the exception of one genotype,were as tolerant of 30 d soil waterlogging as the paddyand deep-water types. In all but one of the 12 genotypes, thenumber of adventitious roots per stem increased for plants grownin waterlogged, compared with drained, soil. When grown in stagnantdeoxygenated nutrient solution, genotypic variation was evidentfor root porosity and rates of ROL, but there was no overalldifference between plants from the three cultural types. Adventitiousroot porosity increased from 20–26 % for plants grownin aerated solution to 29–41 % for plants grown instagnant solution. Growth in stagnant solution also induceda ‘tight’ barrier to ROL in the basal regions ofadventitious roots of five of the seven upland types, all threepaddy types, and the two deep-water types. The enhanced porosityprovided a low resistance pathway for O₂ movement to the roottip, and the barrier to ROL in basal zones would have furtherenhanced longitudinal O₂ diffusion towards the apex, by diminishinglosses to the rhizosphere. The plasticity in root physiology,as described above, presumably contributes to the ability ofrice to grow in diverse environments that differ markedly insoil waterlogging, such as drained upland soils as well as waterloggedpaddy fields.     

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.     

Fine root responses to temporal nutrient heterogeneity and competition in seedlings of two tree species with different rooting strategies

There is little direct evidence for effects of soil heterogeneity and root plasticity on the competitive interactions among plants. In this study, we experimentally examined the impacts of temporal nutrient heterogeneity on root growth and interactions between two plant species with very different rooting strategies: Liquidambar styraciflua (sweet gum), which shows high root plasticity in response to soil nutrient heterogeneity, and Pinus taeda (loblolly pine), a species with less plastic roots. Seedlings of the two species were grown in sandboxes in inter‐ and intraspecific combinations. Nutrients were applied in a patch either in a stable (slow‐release) or in a variable (pulse) manner. Plant aboveground biomass, fine root mass, root allocation between nutrient patch and outside the patch, and root vertical distribution were measured. L. styraciflua grew more aboveground (40% and 27% in stable and variable nutrient treatment, respectively) and fine roots (41% and 8% in stable and variable nutrient treatment, respectively) when competing with P. taeda than when competing with a conspecific individual, but the growth of P. taeda was not changed by competition from L. styraciflua. Temporal variation in patch nutrient level had little effect on the species’ competitive interactions. The more flexible L. styraciflua changed its vertical distribution of fine roots in response to competition from P. taeda, growing more roots in deeper soil layers compared to its roots in conspecific competition, leading to niche differentiation between the species, while the fine root distribution of P. taeda remained unchanged across all treatments. Synthesis. L. styraciflua showed greater flexibility in root growth by changing its root vertical distribution and occupying space of not occupied by P. taeda. This flexibility gave L. styraciflua an advantage in interspecific competition.     

Foraging for nutrients, responses to changes in light, and competition in tropical deciduous tree seedlings

We evaluated (1) the responses of two co-occurring tropical tree species, Heliocarpuspallidus and Caesalpiniaeriostachys, to changes in light, (2) the ability of these species to search for and exploit a fertilized soil patch, (3) the relationship between the capacity to forage for a fertilized patch and the capacity to respond to changes in light availability and (4) how the relationship between light and nutrient acquisition influenced the competitive interactions between these species. Plants of the two species were exposed to a factorial combination of high (H) and low (L) light intensity and fertilized (+Fp) and unfertilized (−Fp) nutrient patches for 50 days. Half of the plants from H were then transferred to L (HL treatment), and half of the plants from L were transferred to H (LH). The remaining plants were kept in their original light condition and grown for another 50 days. Plants were grown in these light and patch treatments alone (one plant per pot) and in interspecific competition (one plant per species resulting in two plants per pot). Both species exploited fertilized patches by increasing root biomass and length in the patch. This enhanced plant productivity and growth rate mainly under LH and HH conditions for Heliocarpus and the HH condition for Caesalpinia). When plants in the HH light environment were grown with an unfertilized patch, plant biomass and relative growth rates (RGRs) were even lower than␣under the LL light environment [(HH–Fp)<LL]. However, the combined activity of shoot and roots when above- and below-ground resources were temporally and spatially heterogeneous influenced plant productivity and growth rate. The benefit from light increase (LH) was reduced when grown with an unfertilized patch. Larger reductions in root biomass, length and density in the patch, and in plant biomass and RGR, were exhibited by Heliocarpus than by Caesalpinia. These results suggest a close relationship between root foraging and light capture, where the benefit of the exploitation of the patch will be reflected in whole-plant benefit, if enough light is captured above-ground. In addition, the results suggest a change in the expected plant responses to light due to heterogeneity in soil nutrients, even though the fertilized patch was only a small proportion of the total soil volume. Leaf characteristics such as specific leaf area responded only to light conditions and not to patchily distributed nutrients. Root characteristics responded more strongly to nutrient heterogeneity. Competition modified the pattern of foraging under both high- and low-light conditions in Heliocarpus by 50 days, and the ability to forage for a fertilized patch under LL after 100 days of growth for Caesalpinia. Even though plant growth and productivity are greatly reduced under low-light conditions (HL and LL), competition modifies the ability of species to forage for a rich patch (especially for the fast-growing species Heliocarpus). Received: 24 November 1997 / Accepted: 15 June 1998     

Influences of Water Status, Temperature, and Root Age on Daily Patterns of Root Respiration for Two Cactus Species

Daily patterns of root respiration measured as CO₂, efflux werestudied at various soil water potentials, temperatures, androot ages for individual, attached roots of the barrel cactusFerocactus acanthodes and the platyopuntia Opuntia ficus-indica.The daily patterns of root respiration for both establishedroots and rain roots followed the daily patterns of root temperature.Root respiration increased when root temperature was raisedfrom 5 °C to 50 °C for F. acanthodes and from 5 °Cto 55 °C for O. ficus-indica; at 60 °C root respirationdecreased 50° from the maximum for F. acanthodes and decreased25° for O. ficus-indica. Root respiration per unit d. wtdecreased with root age for both species, especially for rainroots. Root respiration rates for rain roots were reduced tozero at a soil water potential (     

Tree species, root decomposition and subsurface denitrification potential in riparian wetlands

Patches of organic matter have been found to be important `hotspots' of denitrification in both surface and subsurface soils, but the factors controlling the formation and maintenance of these patches are not well established. We compared the concentration of patches of organic matter and root biomass in the subsurface (saturated zone) beneath poorly drained riparian wetland soils at four sites in Rhode Island, USA - two dominated by red maple (Acer rubrum) and two dominated by white pine (Pinus strobus). Denitrification enzyme activity (DEA) and carbon (C) content of patch material were compared between sites and between patches with different visual characteristics. Root decomposition was measured in an 8-week ex-situ incubation experiment that compared the effects of water content, root species, and soil matrix origin on CO₂ evolution. We observed significantly greater concentrations of patches at 55 cm at one red maple site than all other sites. DEA and percent C in patches was generally higher in patches than matrix soil and did not vary between sites or by patch type. White pine roots decomposed at a faster rate than red maple roots under unsaturated conditions. Our results suggest that faster root decomposition could result in lower concentrations of patches of organic material in subsurface soils at sites dominated by white pine. Tree species composition and root decomposition may play a significant role in the formation of patches and the creation and maintenance of groundwater denitrification hotspots in the subsurface of riparian wetlands. Abbreviations: DEA – denitrification enzyme activity; DOC – dissolved organic carbon; PD – poorly drained; RM-1 – red maple-1 site; RM-2 – red maple-2 site; WP-1 – white pine-1 site; WP-2 – white pine-2 site.     

Thermal and Water Relations of Roots of Desert Succulents

Two succulent perennials from the Sonoran Desert, Agave desertiEngelm. and Ferocactus acanthodes (Lem.) Britton and Rose, loselittle water through their roots during drought, yet respondrapidly to light rainfall. Their roots tend to be shallow, althoughabsent from the upper 20 mm or so of the soil. During 12–15d after a rainfall, new root production increased total rootlength by 47 per cent to 740 m for A. deserti and by 27 percent to 230 m for F. acanthodes; root dry weight then averagedonly 15 per cent of shoot dry weight. The annual carbon allocatedto dry weight of new roots required 11 per cent of shoot carbondioxide uptake for A. deserti and 19 per cent for F. acanthodes.Elongation of new roots was greatest near a soil temperatureof 30°C, and lethal temperature extremes (causing a 50 percent decrease in root parenchyma cells taking up stain) were56°C and -7°C. Soil temperatures annually exceeded themeasured tolerance to high temperature at depths less than 20mm, probably explaining the lack of roots in this zone. Attached roots immersed in solutions with osmotic potentialsabove -2·6 MPa could produce new lateral roots, with50 per cent of maximum elongation occurring near -1·4MPa for both species. Non-droughted roots lost water when immersedin solutions with osmotic potentials below -0·8 MPa,and root hydraulic conductance decreased markedly below about-1·2 MPa. Pressure-volume curves indicated that, fora given change in water potential, non-droughted roots lostthree to five times more water than droughted roots, non-droughtedleaves, or non-droughted stems. Hence, such roots, which couldbe produced in response to a rainfall, will lose the most tissuewater with the onset of drought, the resulting shrinkage beingaccompanied by reduced root hydraulic conductance, less contactwith drying soil, and less water loss from the plant to thesoil. Agave deserti, Ferocactus acanthodes, roots, soil, temperature, water stress, drought, Crassulacean acid metabolism, succulents     

Mean annual temperature influences local fine root proliferation and arbuscular mycorrhizal colonization in a tropical wet forest

Mean annual temperature (MAT) is an influential climate factor affecting the bioavailability of growth‐limiting nutrients nitrogen (N) and phosphorus (P). In tropical montane wet forests, warmer MAT drives higher N bioavailability, while patterns of P availability are inconsistent across MAT. Two important nutrient acquisition strategies, fine root proliferation into bulk soil and root association with arbuscular mycorrhizal fungi, are dependent on C availability to the plant via primary production. The case study presented here tests whether variation in bulk soil N bioavailability across a tropical montane wet forest elevation gradient (5.2°C MAT range) influences (a) morphology fine root proliferation into soil patches with elevated N, P, and N+P relative to background soil and (b) arbuscular mycorrhizal fungal (AMF) colonization of fine roots in patches. We created a fully factorial fertilized root ingrowth core design (N, P, N+P, unfertilized control) representing soil patches with elevated N and P bioavailability relative to background bulk soil. Our results show that percent AMF colonization of roots increased with MAT (r² = .19, p = .004), but did not respond to fertilization treatments. Fine root length (FRL), a proxy for root foraging, increased with MAT in N+P‐fertilized patches only (p = .02), while other fine root morphological parameters did not respond to the gradient or fertilized patches. We conclude that in N‐rich, fine root elongation into areas with elevated N and P declines while AMF abundance increases with MAT. These results indicate a tradeoff between P acquisition strategies occurring with changing N bioavailability, which may be influenced by higher C availability with warmer MAT.     

Increased Synthesis of ABA in Partially Dehydrated Root Tips and ABA Transport from Roots to Leaves

Zhang, J. and Davies, W. J. 1987. Increased synthesis of ABAin partially dehydrated root tips and ABA transport from rootsto leaves.—J. exp. Bot. 38: 2015–2023. Isolated root tips of pea (Pisum sativum L. cv. Feltham First)and Commelina communis L. were air-dried until they lost between10% and 40% of their fresh weight, followed by a period of incubationat these reduced water contents. These treatments resulted inincreased ABA production, suggesting that root tips of bothspecies have the capacity to synthesize ABA in increased amountswhen water deficits develop in the root. The ABA concentrationin pea roots increased linearly as turgors fell below about0·15 M Pa and relative water contents (R WC) fell below90%. Commelina roots produced more ABA when RWC fell below asimilar value but the threshold turgor for increased ABA productionin Commelina roots was around 0·30 MPa. Roots of intact plants loaded with ABA as a result of incubationin solutions of varying concentrations provided ABA to leaveswhich resulted in increased ABA concentrations in the leaveswhen these were assayed several hours later. This occurred whenthese roots were not contributing substantially to transpirationalflux. Leaves on shoots that were enclosed and darkened and thereforenot transpiring, did not accumulate ABA from ‘loaded’roots. A role for root-sourced ABA in root-to-shoot communication ofthe effects of soil drying is discussed. Key words: ABA, roots, water relations     

Enhanced formation of aerenchyma and induction of a barrier to radial oxygen loss in adventitious roots of Zea nicaraguensis contribute to its waterlogging tolerance as compared with maize (Zea mays ssp. mays)

Enhancement of oxygen transport from shoot to root tip by the formation of aerenchyma and also a barrier to radial oxygen loss (ROL) in roots is common in waterlogging‐tolerant plants. Zea nicaraguensis (teosinte), a wild relative of maize (Zea mays ssp. mays), grows in waterlogged soils. We investigated the formation of aerenchyma and ROL barrier induction in roots of Z. nicaraguensis, in comparison with roots of maize (inbred line Mi29), in a pot soil system and in hydroponics. Furthermore, depositions of suberin in the exodermis/hypodermis and lignin in the epidermis of adventitious roots of Z. nicaraguensis and maize grown in aerated or stagnant deoxygenated nutrient solution were studied. Growth of maize was more adversely affected by low oxygen in the root zone (waterlogged soil or stagnant deoxygenated nutrient solution) compared with Z. nicaraguensis. In stagnant deoxygenated solution, Z. nicaraguensis was superior to maize in transporting oxygen from shoot base to root tip due to formation of larger aerenchyma and a stronger barrier to ROL in adventitious roots. The relationships between the ROL barrier formation and suberin and lignin depositions in roots are discussed. The ROL barrier, in addition to aerenchyma, would contribute to the waterlogging tolerance of Z. nicaraguensis.     

The Effects of Partially Drying Part of the Root System of Helianthus annuus on the Abscisic Acid Content of the Roots, Xylem Sap and Leaves

Plants of Helianthus annuus were grown in soil in pots suchthat approximately 30% of the root system protruded throughthe base of the pot. After 7 d further growth in aerated nutrientsolution, the attached, protruding roots were air-dried for10–15 min and thereafter surrounded with moist still air,in the dark, for 49 h, whilst the soil was kept at field capacity.The roots of the control plants remained in the nutrient solutionthroughout the experiment. This treatment rapidly reduced the water content of protrudingroots from 20.5 to 17.8 g g^–1 dry mass (DM), which remainedless than that of the control roots for the rest of the experiment.This treatment also reduced root turgor and water potential.The abscisic acid (ABA) concentrations in the protruding roots,xylem sap and leaves of the treated plants increased significantly,compared to values recorded for control plants. In treated roots, the ABA concentration was significantly increased4 h after treatment, with a maximum of 4.4+0.1 nmol g^–1(DM) after 25 h. The ABA concentration in the xylem sap of thetreated plants was significantly greater than in the controls25 h, 30 h, and 49 h after the partial drying of the roots,with a maximum concentration of approximately 970 pmol ABA cm-3at 49 h. Initially, the ABA concentration in the leaves was0.45 nmol g^–1 (DM) which increased significantly to 1.1±0.1 nmol g^–1 at 25 h, to 1.7±0.3 nmol g^–1at 49 h. Leaf conductance was significantly less in plants with air-driedroots than in the controls 8 h after the start of the treatmentand thereafter. The water relations of the leaves of the treatedplants did not differ from those of the control plants. These results confirm previous reports that ABA is rapidly generatedin partially-dried and attached root systems and demonstratesa concomitant large increase in the ABA content of the xylemsap. It is suggested that partial dehydration of some of theroots of Helianthus annuus, increases ABA concentration in thetranspiration stream and decreases leaf conductance in the absenceof changes in leaf water status. As these responses were initiatedin free-growing roots the stimulus is independent of any increasesin soil shear strength that are associated with soil drying. Key words: Soil drying, roots, ABA, leaf conductance, water relations     

Sagebrush (Artemisia tridentata Nutt.) roots maintain nutrient uptake capacity under water stress

Phosphorus and nitrogen uptake capacities were assessed during36–58 d drying cycles to determine whether the abilityof sagebrush (Artemisia tridentata Nutt.) to absorb these nutrientschanged as the roots were subjected to increasing levels ofwater stress. Water was withheld from mature plants in large(6 I) containers and the uptake capacity of excised roots insolution was determined as soil water potentials decreased from–0.03 MPa to –5.0 MPa. Phosphorus uptake rates of excised roots at given substrateconcentrations increased as preharvest soil water potentialsdecreased to –5.0 MPa. V_max and K_m also increased as soilwater potentials declined. Declining soil water potentials depressednitrogen uptake at set substrate concentrations, but uptakecapacity, calculated as the sum V_max for both NH⁺₄+NO^–₃,did not change significantly with drying. The sum V_max correlatedwith root nitrogen concentration. Root uptake capacity for nitrogen and phosphorus was extremelystable under severe water stress in this aridland shrub. Maintenanceof uptake capacity, coupled with a previously demonstrated abilityto conduct hydraulic lift, may enable A. tridentata better tomaintain nitrogen and phosphorus uptake as soil water availabilitydeclines. These mechanisms may be important in the ability ofA. tridentata to maintain growth, complete reproduction, andgain an advantage against competitors late in the season whenthe soil layers with higher nutrient availability are dry. Key words: Kinetics, nitrogen, phosphorus, roots, water stress     

Influence of the fungicide pentachloronitrobenzene on VA-mycorrhizal and total root length and phosphorus uptake of oats (Avena sativa)

The effect of application of the fungicide pentachloronitrobenzene (PCNB) at levels between 2 and 50 mg kg^–1 soil on root growth, mycorrhizal infection and P uptake was studied in pot culture with oats (Avena sativa cv. Alfred) growing in a rendzina soil low in available P. The soil had been partially sterilized by X-ray, and half of the pots were inoculated with spores of the VAM-fungusGlomus mosseae (indigenous species).Soil irradiation (0.5 Mrad) did not decrease the levels of infection by VAM. Application of PCNB decreased the VAM-infected root length, at 50 mg PCNB kg^–1 soil VAM-infected root length was about 12% of the controls. Total root length, however, increased to about 126% of control values at PCNB rates up to 20 mg kg^–1 soil, but decreased to 89% of the controls at 50 mg kg^–1 soil. Total P-uptake decreased with increasing levels of PCNB and was linearly correlated with infected root length (r=0.92).The stimulation of root growth by PCNB at rates up to 20 mg kg^–1 soil is regarded as an indirect effect, brought about by suboptimal P-supply due to inhibition of VA-mycorrhiza. Conversely, the reduction of total root length at 50 mg PCNB kg^–1 soil is most likely a direct effect. Due to the phytotoxicity of the fungicide, the contribution of the indigenous VA-mycorrhiza to plant P uptake under field conditions cannot be determined by soil application of PCNB at rates sufficient for complete inhibition of VAM.As inhibition or absence of VAM may lead to compensatory root growth, mycorrhizal dependency ought to be calculated from the amounts of P taken up per unit root length in mycorrhizal and nonmycorrhizal plants, respectively.     

Heterogeneous nutrient supply promotes maize growth and phosphorus acquisition: additive and compensatory effects of lateral roots and root hairs

Background and AimsRoot proliferation is a response to a heterogeneous nutrient distribution. However, the growth of root hairs in response to heterogeneous nutrients and the relationship between root hairs and lateral roots remain unclear. This study aims to understand the effects of heterogeneous nutrients on root hair growth and the trade-off between root hairs and lateral roots in phosphorus (P) acquisition.MethodsNear-isogenic maize lines, the B73 wild type (WT) and the rth3 root hairless mutant, were grown in rhizoboxes with uniform or localized supply of 40 (low) or 140 (high) mg P kg⁻¹ soil.ResultsBoth WT and rth3 had nearly two-fold greater shoot biomass and P content under local than uniform treatment at low P. Significant root proliferation was observed in both WT and rth3 in the nutrient patch, with the WT accompanied by an obvious increase (from 0.7 to 1.2 mm) in root hair length. The root response ratio of rth3 was greater than that of WT at low P, but could not completely compensate for the loss of root hairs. This suggests that plants enhanced P acquisition through complementarity between lateral roots and root hairs, and thus regulated nutrient foraging and shoot growth. The disappearance of WT and rth3 root response differences at high P indicated that the P application reduced the dependence of the plants on specific root traits to obtain nutrients.ConclusionsIn addition to root proliferation, the root response to a nutrient-rich patch was also accompanied by root hair elongation. The genotypes without root hairs increased their investment in lateral roots in a nutrient-rich patch to compensate for the absence of root hairs, suggesting that plants enhanced nutrient acquisition by regulating the trade-off of complementary root traits.