Spatial and temporal variation in organic acid anion exudation and nutrient anion uptake in the rhizosphere of Lupinus albus L.

We investigated in situ the temporal patterns and spatial extent of organic acid anion exudation into the rhizosphere solution of Lupinus albus, and its relation with the nutrient anions phosphate, nitrate and sulfate by means of a rhizobox micro suction cup method under P sufficient conditions. We compared the soil solution in the rhizosphere of cluster roots with that in the vicinity of normal roots, nodules and bulk soil. Compared to the other rhizosphere and soil compartments, concentrations of organic acid anions were higher in the vicinity of cluster roots during the exudative burst (citrate, oxalate) and nodules (acetate, malate), while concentrations of inorganic nutrient anions were highest in the bulk soil. Both active cluster roots and nodules were most efficient in taking up nitrate and phosphate. The intensity of citrate exudation by cluster roots was highly variable. The overall temporal patterns during the lifetime of cluster roots were overlaid by a diurnal pattern, i.e. in most cases, the exudation burst consisted of one or more peaks occurring in the afternoon. Multiple exudation peaks occurred daily or were separated by 1 or 2 days. Although citrate concentrations decreased with distance from the cluster root apex, they were still significantly higher at a distance of 6 to 10 mm than in the bulk soil. Phosphate concentrations were extremely variable in the proximity of cluster roots. While our results indicate that under P sufficient conditions cluster roots take up phosphate during their entire life time, the influence of citrate exudation on phosphate mobilization from soil could not be assessed conclusively because of the complex interactions between P uptake, organic acid anion exudation and P mobilization. However, we observed indications of P mobilization concurrent with the highest measured citrate concentrations. In conclusion, this study provides semiquantitative in situ data on the reactivity of different root segments of L. albus L. in terms of root exudation and nutrient uptake under nutrient sufficient conditions, in particular on the temporal variability during the lifetime of cluster roots.     

Secretion activity of white lupin’s cluster roots influences bacterial abundance,function and community structure

White lupin (Lupinus albus L. cv. Amiga) reacts to phosphate deficiency by producing cluster roots which exude large amounts of organic acids. The detailed knowledge of the excretion physiology of the different root parts makes it a good model plant to study plant-bacteria interaction. Since the effect of the organic acid exudation by cluster roots on the rhizosphere microflora is still poorly understood, we investigated the abundance, diversity and functions of bacteria associated with the cluster roots of white lupin, with special emphasis on the influence of root proximity (comparing root, rhizosphere soil and bulk soil fractions) and cluster root growth stages, which are characterized by different excretion activities. Plants were grown for five weeks in microcosms, in the presence of low phosphate concentrations, on acidic sand inoculated with a soil suspension from a lupin field. Plate counts showed that bacterial abundance decreased at the stage where the cluster root excretes high amounts of citrate and protons. In vitro tests on isolates showed that the frequencies of auxin producers were highest in juvenile and mature cluster roots and significantly decreased in senescent cluster roots. However, no significant difference in the frequency of auxin producers was found between cluster and non cluster roots. The diversity and structure of bacterial communities were investigated by DGGE of 16S rDNA and 16S rRNA. The diversity and community structure were mostly influenced by root proximity and, to a lesser extent, by cluster root stage. The richness of bacterial communities decreased with root proximity, whereas the proportion of active populations increased. The high citrate and proton excretion occurring at the mature stage of cluster roots had a strong impact on the structure and richness of the bacterial communities, both in the root and in the rhizosphere soil.     

Investigation of small-scale processes in the rhizosphere of Lupinus albus using micro push-pull tests

Background and Aims

Rhizosphere processes affect the mobility, phytoavailability and toxicity of solutes in soil. To study reactions in the rhizosphere under quasi in situ conditions, we recently developed the “micro push-pull test” (μPPT) method, combining micro-suction cups with the principle of the “push-pull test” method known from groundwater applications. Here we report the application of μPPT to investigate rhizosphere reactions in situ, i.e. degradation of deuterated citrate (citrate-d4) in the rhizosphere of Lupinus albus grown in sand-filled rhizoboxes.

Methods

In a μPPT, a solution containing reactive (citrate-d4) and non-reactive solutes (bromide) is injected into a porous medium and shortly thereafter, the pore water solution is re-extracted from the same location. Concentration (“breakthrough”) curves of extracted reactants can be compared to those of the non-reactive solute, allowing the determination of reaction rates. We applied the μPPT in rhizoboxes with Lupinus albus and sampled different types of micro-habitats: bulk soil, rhizosphere of normal roots and rhizosphere of cluster roots of different ages.

Results

Breakthrough curves of citrate-d4 varied considerably between tests adjacent to cluster roots and normal roots, and in bulk soil. Degradation of citrate-d4 in bulk soil and adjacent to normal roots was below detection, while we found strong degradation of citrate-d4 adjacent to 4 to 5-days old cluster roots. In situ degradation rate constants for citrate-d4 around cluster roots were found to be in the range from 0.38 to 0.71 h^?1.

Conclusions

We successfully applied the μPPT to the rhizosphere. The μPPT is useful to investigate local processes in microcosms and to monitor processes also over time (e.g., during cluster-root development) due to its non-destructive nature.     

Soil Phosphorus Uptake by Continuously Cropped Lupinus albus: A New Microcosm Design

When grown in soils with sparingly available phosphorus (P), white lupin (Lupinus albus L.) forms special root structures, called cluster roots, which secrete large amounts of organic acids and concomitantly acidify the rhizosphere. Many studies dealing with the understanding of this P acquisition strategy have been performed in short time experiments either in hydroponic cultures or in small microcosm designs with sand or sand:soil mixtures. In the present study, we applied an experimental design which came nearer to the natural field conditions: we performed a one-year experiment on large microcosms containing 7 kg of soil and allowing separation of rhizosphere soil and bulk soil. We planted six successive generations of lupins and analysed P uptake, organic P desorption, phosphatase activities and organic acid concentrations in different soil samples along a spatio-temporal gradient. We compared the rhizosphere soil samples of cluster (RSC) and non-cluster roots (RSNC) as well as the bulk soil (BS) samples. A total shoot biomass of 55.69 ± 1.51 g (d.w.) y⁻¹ was produced and P uptake reached 220.59 ± 5.99 mg y⁻¹. More P was desorbed from RSC than from RSNC or BS (P < 0.05). RSC and RSNC showed a higher activity of acid and alkaline phosphatases than BS samples and a higher acid phosphatase activity was observed in RSC than in RSNC throughout the one-year experiment. Fumarate was the most abundant organic acid in all rhizosphere soil samples. Citrate was only present in detectable amounts in RSC while malate and fumarate were recovered from both RSC and RSNC. Almost no organic acids could be detected in the BS samples. Our results demonstrated that over a one-year cultivation period in the absence of an external P supply, white lupin was able to acquire phosphate from the soil and that the processes leading to this P uptake took place preferentially in the rhizosphere of cluster roots.     

Organic acids differ in enhancing phosphorus uptake by Triticum aestivum L.—effects of rhizosphere concentration and counterion

Organic acids such as citrate and oxalate have been implicated in enhancing many rhizosphere processes including nutrient acquisition. This study was conducted to determine the importance of organic acid type and concentration on rhizosphere P mobilization and subsequent uptake by wheat (Triticum aestivum L.) roots and its translocation to shoots. A single wheat plant was grown in soil-filled rhizosphere microcosms and allowed to pass through a KH₂PO₄ ³³P-isotopically labeled patch of calcareous soil. Two days after ³³P-injection, citrate and oxalate at concentrations of 1 mM and 10 mM were injected into the microcosms at the same patch every day over a period of 4 days. Oxalate resulted in a several-fold enhancement in plant ³³P accumulation, while citrate had no such effect. In comparison with oxalate, high rates of citrate mineralization were observed suggesting that this reduced its potential to enhance plant ³³P acquisition. This study concludes that organic acids cause an increase in P mobilization and P uptake by wheat but that this response is highly organic acid specific.     

Bioavailability of zinc and phosphorus in calcareous soils as affected by citrate exudation

Aims

Zinc (Zn) and phosphorus (P) deficiency often occurs at the same time and limits crop production in many soils. It has been suggested that citrate root exudation is a response of plants to both deficiencies. We used white lupin (Lupinus albus L.) as a model plant to clarify if citrate exuded by roots could increase the bioavailability of Zn and P in calcareous soils.

Methods

White lupin was grown in nutrient solution and in two calcareous soils in a rhizobox. Rhizosphere soil solution was sampled to determine citrate, metals and P. Based on the measured citrate concentrations, a soil extraction experiment with citrate as extractant was done.

Results

Absence of Zn triggered neither cluster root formation nor citrate exudation of white lupin grown in nutrient solution, whereas low P supply did. The maximum citrate concentration (～1.5?mM) found in the cluster rhizosphere soil solution of one soil mobilized P, but not Zn. In the other soil the highest citrate concentration (～0.5?mM) mobilized both elements.

Conclusions

White lupin does not respond to low Zn bioavailability by increasing citrate exudation. Such a response was observed at low P supply only. Whether Zn and P can be mobilized by citrate is soil-dependent and the possible controlling mechanisms are discussed.     

Dynamics of organic acid occurrence under flooding stress in the rhizosphere of three plant species from the water fluctuation zone of the Three Gorges Reservoir, P.R. China

The effects of flooding on rhizospheric organic acid concentrations of three abundant flooding tolerant plant species (Alternanthera philoxeroides Mart., Arundinella anomala Steud., Salix variegata Franch.) from the water fluctuation zone of the Three Gorges Reservoir (TGR, Yangtze River) were investigated. Soil solution samples of eight low molecular weight organic acids were obtained from rhizotrons using micro suction cups during 3 weeks of waterlogging, after 6 weeks flooding and after a 1 week recovery. To estimate the contribution of water temperature and microbial community, plants in sterile glass bead substrate and original Yangtze sediment were submerged in laboratory at +10°, +20° and +30°C. Waterlogged plants did seldom express a significantly different pattern of rhizospheric organic acid (OA) composition compared to control plants. Flooding caused no burst of organic acid concentration in soil solution: All species express a silencing strategy. Average OA levels were higher in A. anomala rhizosphere than in the other two species, but increased again after resurfacing in all species. Temperature had a stronger influence in sediment than in sterile setup. In contrast to field measurements, succinate, malate and citrate were detected in the sterile setup. Microbial contribution appeared to have great influence on increasing OA occurrence.     

Organic acid extraction from rhizosphere soil: effect of field-moist, dried and frozen samples

This study investigates the effect of soil treatment and storage on organic acid extraction. For this study one clayey-loamy (Typic Udochrept) and one sandy-loamy (Aquic Ustifluvent) soil were selected and used to grow Lupinus albus L. plants in a climate chamber. After 4 weeks the rhizosphere soil was sampled and divided into five portions: (a) field moist, no storage; (b) air-dried; (c) oven-dried, (d) field-moist at +4°C for 8 weeks; (e) field-moist at ?20°C for 8 weeks. Organic acid extraction (1:4 w/v) was carried out for each soil portion both in water and in 10 mM NaH₂PO4. Organic acid concentration was subsequently determined by reversed-phase high performance liquid chromatography (HPLC). Oxalic, fumaric, malonic and α-ketoglutaric acid were identified in the rhizosphere of both soils but the extractable concentration was significantly higher in the sandy-loamy soil. For both soils NaH₂PO₄ extracted significantly higher organic acid concentrations than water. Oven drying increased the extractability of organic acids in both soils. Field moist samples (i.e. where no storage occurred) of the sandy-loamy soil showed a similar behaviour than ?20° stored samples whereas the one of the sandy-loamy soil were more close to the air-dried samples. These results indicate that organic acid extraction strongly depends on soil storage as well as on the soil type. Sample storage seems thus to be a crucial issue for the determination of organic acids in rhizosphere soil and needs to be considered prior analysis.     

Diversity of low-molecular weight organic acids synthesized by <Emphasis Type="Italic">Salix</Emphasis> growing in soils characterized by different Cu,Pb and Zn concentrations

The aim of the study was to evaluate the biosynthesis and exudation of 10 low-molecular weight organic acids (LMWOAs) into the rhizosphere with a simultaneous analysis of the acid contents in the roots and leaves of 9 Salix taxa growing on two experimental areas, differing in their concentrations of copper (Cu), lead (Pb) and zinc (Zn) in the soil (Area 1—low, Area 2—high concentration). The obtained results reveal a significant difference in the phytoextraction of the tested Salix taxa for the analysed metals in both areas. The highest contents of Cu, Pb and Zn were observed for all Salix collected from Area 2, especially in S. × smithiana roots (116 ± 8.76, 87.84 ± 7.30 and 203.42 ± 14.62 mg kg^?1 DW, respectively). The results obtained in Area 2 also revealed acidification of the rhizosphere and a higher concentration of acids, mainly oxalic, malic, malonic, acetic and citric acids. Contents of oxalic, malic, acetic and citric acids increased in the roots of Salix taxa from Area 2, while in the leaves formic and succinic acids were also present. S. × smithiana was the taxon with the highest concentration of acids in the rhizosphere and roots (73.48 ± 6.77 and 49.79 ± 2.65 μM 100 g^?1 DW, respectively), while in leaves a higher content was observed for S. alba and S. viminalis ‘PR’ taxa (78.12 ± 3.95 and 71.12 ± 3.75 μM 100 g^?1 DW, respectively).     

Time and substrate dependent exudation of carboxylates by Lupinus albus L. and Brassica napus L.

Root exudates influence significantly physical, chemical and biological characteristics of rhizosphere soil. Their qualitative and quantitative composition is affected by environmental factors such as pH, soil type, oxygen status, light intensity, soil temperature, plant growth, nutrient availability and microorganisms. The aim of the present study was to assess the influence of growth substrate and plant age on the release of carboxylates from Lupinus albus L. and Brassica napus L.Both plant species were studied in continuously percolated microcosms filled with either sand, soil or sand + soil (1:1) mixture. Soil solution was collected every week at 7, 14, 21, 28 and 35 days after planting (DAP). Carboxylate concentrations were determined by reversed-phase liquid chromatography - electrospray ionization - time of flight mass spectrometry (LC-ESI-TOFMS).Oxalate, citrate, succinate, malate and maleate were detected in soil solutions of both plant species. Their concentrations were correlated with the physiological status of the plant and the growth substrate. Oxalate was the predominant carboxylate detected within the soil solution of B. napus plants while oxalate and citrate were the predominant ones found in the soil solutions of L. albus plants.The sampling determination of carboxylates released by plant roots with continuous percolation systems seems to be promising as it is a non-destructive method and allows sampling and determination of soluble low molecular weight organic compounds derived from root exudation as well as the concentration of soluble nutrients, which both might reflect the nutritional status of plants.     

Cluster-root production and organic anion exudation in a group of old-world lupins and a new-world lupin

We examined the capacity of several Old-World lupin species (Lupinus luteus L., L. hispanicus Boiss. et Reuter and L. angustifolius L.) and one species of a New-World lupin (L. mutabilis Sweet) to form cluster roots under a range of conditions in solution culture. The effect of the synthetic auxin, IBA (indole-3-butyric acid), on cluster-root development in L. luteus and L. albus L. provided with an adequate phosphorus (P) supply was also investigated. In addition, the effect of a high nitrate-N (NO₃-N) supply on the efflux of citrate and malate from roots of L. angustifolius was examined to determine if specific regions of the root system exuded these organic anions. When P-deficient, L. hispanicus, L. luteus and L. mutabilis formed cluster roots that secreted organic anions. Citrate was generally the dominant organic anion exuded, although succinate was also exuded in large quantities from L. luteus. Citrate efflux by L. hispanicus and L. luteus was at least comparable to that reported for P-deficient L. albus[up to 1.092 nmol g^–1 fresh weight (FW) s^–1], but was over an order of magnitude lower in L. mutabilis (0.036 nmol g^–1 FW s^–1). Citrate and malate were not detected in significant amounts from either the lateral roots or the root tips of any species grown under P-sufficient or -deficient conditions. Citrate efflux from cluster roots of L. luteus showed a diurnal pattern, similar to that reported for L. albus, with maximum efflux during the day, and declining to a minimum before dawn. IBA added to the nutrient solution induced cluster-root formation on both L. albus and L. luteus at concentrations of P that would normally suppress the production of these roots. However, the IBA-induced cluster roots did not exude significant amounts of citrate. Although L. angustifolius did not produce cluster roots when P-deficient, it produced cluster-like root structures that exuded citrate (0.053 nmol g^–1 FW s^–1) when grown at a high nitrate-N (NO₃-N) supply. L. angustifolius did not exude significant citrate or malate from lateral roots or root tips when grown at either high or low NO₃-N supply. Our findings for L. hispanicus and L. luteus are the first reports of cluster-root formation in response to P deficiency for these Old-World species, and for L. mutabilis, it is the first report of cluster roots for a New-World lupin species. These reports indicate that evolutionary and biogeographical aspects of cluster-root formation in the genus Lupinus need to be revised. Furthermore, investigation is warranted to determine the capacity of species of the large group of New-World lupins to form cluster roots in soils of their native habitats.     

Citrate, Malate, and Succinate Concentration in Exudates from P-Sufficient and P-Stressed Medicago sativa L. Seedlings

Under certain stress conditions roots exude organic molecules, which may facilitate the uptake of nutrients. The objective of this research was to identify and measure the effect of low P upon the exudation of organic acids by roots of alfalfa (Medicago sativa L.) seedlings. Surface-sterilized alfalfa seeds were grown aseptically in sterile sand using an apparatus specially designed for the addition of ±P nutrient solutions and for the collection of root exudates. Citric, malic, and succinic acids were detected in the root exudates of 24-day-old alfalfa seedlings. Citrate exudation from the roots of P-stressed alfalfa was 182% that of plants receiving a complete nutrient solution. The increased release of citrate may provide a mechanism by which P-stressed plants enhance the availability of P in the rhizosphere.     

Exudation of carboxylates in Australian Proteaceae: chemical composition

Roots of a wide range of plant species exude carboxylates, such as citrate, into the rhizosphere. In the present study, seedlings of a range of Australian Banksia, Hakea and Dryandra species (Proteaceae) were assayed for their exudation of carboxylates. All of these species (Hakea prostrata, Hakea undulata, Hakea petiolaris, Hakea baxteri, Banksia grandis, Banksia prionotes, Banksia occidentalis and Dryandra sessilis) form cluster roots when grown in nutrient solution with a low phosphate concentration. Exudation of carboxylates was studied for cluster roots and non‐cluster roots separately, and for the entire root system. Cluster roots of these Proteaceae exuded malate, malonate, lactate, acetate, maleate, citrate, fumarate, cis‐ and trans‐aconitate. The relative contributions of each of these carboxylates differed between species. Malate, malonate, lactate, citrate and trans‐aconitate, however, were invariably present in large proportions of total carboxylate exudation. Non‐cluster roots of H. prostrata exuded a spectrum of carboxylates (mainly malonate, lactate and citrate), which differed somewhat from the exudation pattern of cluster roots (mainly malate, malonate, lactate and citrate). The rate of exudation for cluster roots of the seven species was approximately 1·6 nmol g^?1 FM s^?1, which is considerably higher than that reported for a variety of crop and native species that do or do not form cluster roots. Contrary to what occurs in the cluster roots of Lupinus albus, which release carboxylates accompanied by protons so that the rhizosphere is acidified, the present Proteaceae exude the carboxylates as anions without concomitant proton release. The role of carboxylates in the mobilization of phosphate and other nutrients from soil is discussed.     

Accumulation of low-molecular-weight organic acids in roots and leaf segments of Zea mays plants treated with cadmium and copper

Maize seedlings grown in a nutrient solution were treated with Cd (50, 100 µM) or Cu (50, 100 µM). Roots and separated leaf sections (from the youngest––basal, through the middle––mature, to the oldest––apical) were analysed. Shoot and root fresh weight, and root net growth rates were reduced significantly after Cu application in comparison to Cd. Copper (50, 100 µM) and cadmium (100 µM) decreased the sum of chlorophyll and total carotenoid pools mostly in mature and old leaf sections. The concentration of Cu and Cd increased in the old and mature leaf sections. Analysis of organic acids showed that both metals differently influenced the low-molecular-weight organic acids (LMWOAs) content in maize leaf sections. In mature sections, the excess Cu increased the succinate and tartrate contents, whereas in the young ones mainly the tartrate level was elevated. Cadmium elevated citrate accumulation in mature and old sections, compared to the control plants. Malate, the main LMWOA in maize leaves, increased only after addition of 100 µM of Cd (mature and old sections) or 50 µM of Cu (old sections). Analysis of LMWOAs in roots showed that the excess of Cd or Cu induced higher accumulation of tartrate and malate and, additionally, copper increased the citrate content.     

Investigations into the exploitation of heterogeneous soils by Lupinus albus L. and L. pilosus Murr. and the effect upon plant growth

In calcareous soils, genotypes of Lupinus albus L. generally grow poorly, resulting in stunted plants that often develop lime-induced chlorosis. In contrast, some genotypes of L. pilosus Murr. occur naturally in calcareous soils without developing any visible symptoms of stress. Some genotypic variation for tolerance to calcareous soil does exist in L. albus and the tolerance mechanisms need to be determined. The adaptation through root system morphological plasticity of L. albus and L. pilosus, to heterogeneous limed soil profiles (pH 7.8) containing either patches of acid (non-limed) soil, or vertically split between acid and limed soil, was investigated. When grown in the presence of patches of acid soil, L. albus had a 52% greater shoot dry weight and visibly greener leaves compared with plants grown in the homogeneous limed soil. Total root dry matter in the acid-soil patches was greater than in the control limed-soil patches. This was due to a four-fold increase in the cluster root mass, accounting for 95% of the root dry matter in the acid-soil patch. Although these cluster roots secreted no more citric acid per unit mass than those in the limed soil did, their greater mass resulted in a higher citrate concentration in the surrounding soil. L. pilosus responded to the patches of acid soil in a manner comparable with L. albus. When grown in the homogeneous limed soil, L. pilosus had a greater maximum net CO₂ assimilation rate (P_max) than L. albus, however, the P_max of both species increased after they had accessed a patch of acid soil. Differences were apparent between the L. albus genotypes grown in soil profiles split vertically into limed and acid soil. A genotype by soil interaction occurred in the partitioning between soils of the cluster roots. The genotype La 674 was comparable with L. pilosus and produced over 11% of its cluster roots in the limed soil, whereas the other genotypes produced only 1–3% of their cluster roots in the limed soil. These results indicate L. pilosus is better adapted to the limed soil than L. albus, but that both species respond to a heterogeneous soil by producing mainly cluster roots in an acid-soil patch. This revised version was published online in June 2006 with corrections to the Cover Date.     

Muscodor albus, a potential biocontrol agent against plant-parasitic nematodes of economically important vegetable crops in Washington State, USA

The fungus, Muscodor albus, was tested for nematicidal and nematostatic potential against four plant-parasitic nematode species with three different feeding modes on economically important vegetable crops in the Pacific Northwest. Meloidogyne chitwoodi, Meloidogyne hapla, Paratrichodorus allius, and Pratylenchus penetrans were exposed for 72 h to volatiles generated by M. albus cultured on rye grain in sealed chambers at 24 °C in the laboratory. In addition, the nematodes were inoculated into soil fumigated with M. albus, and incubated for 7 days prior to the introduction of host plants under greenhouse conditions. The mean percent mortality of nematodes exposed to M. albus in the chamber was 82.7% for P. allius, 82.1% for P. penetrans, and 95% for M. chitwoodi; mortality in the nontreated controls was 5.8%, 7%, and 3.9%, respectively. Only 21.6% of M. hapla juveniles died in comparison to 8.9% in controls. However, 69.5% of the treated juveniles displayed reduced motility and lower response to physical stimulus by probing, in comparison to the control juveniles. This is evidence of nematostasis due to M. albus exposure. The greenhouse study showed that M. albus caused significant reduction to all nematode species in host roots and in rhizosphere soil. The percent mortality caused by M. albus applied at 0.5% and 1.0% w/w in comparison to the controls was as follows: 91% and 100% for P. allius in the soil; 100% for P. penetrans in bean roots and soil; 85% and 95% for M. chitwoodi in potato roots, and 56% and 100% in the soil; 100% for M. hapla both in pepper roots and soil. In this study, M. albus has shown both nematostatic and nematicidal properties.     

Matricaria chamomilla is not a hyperaccumulator, but tolerant to cadmium stress

The influence of low (3 μM) and high (60 and 120 μM) cadmium (Cd) concentrations were studied on selected aspects of metabolism in 4-week-old chamomile (Matricaria chamomilla L.) plants. After 10 days’ exposure, dry mass accumulation and nitrogen content were not significantly altered under any of the levels of Cd. However, there was a significant decline in chlorophyll and water content in the leaves. Among coumarin-related compounds, herniarin was not affected by Cd, while its precursors (Z)- and (E)-2-β-d-glucopyranosyloxy-4-methoxycinnamic acids (GMCAs) increased significantly at all the levels of Cd tested. Cd did not have any effect on umbelliferone, a stress metabolite of chamomile. Lipid peroxidation was also not affected by even 120 μM Cd. Cd accumulation was approximately seven- (60 μM Cd treatment) to eleven- (120 μM Cd treatment) fold higher in the roots than that in the leaves. At high concentrations, it stimulated potassium leakage from the roots, while at the lowest concentration it could stimulate potassium uptake. The results supported the hypothesis that metabolism was altered only slightly under high Cd stress, indicating that chamomile is tolerant to this metal. Preferential Cd accumulation in the roots indicated that chamomile could not be classified as a hyperaccumulator and, therefore, it is unsuitable for phytoremediation.     

Growth Medium and Phosphorus Supply Affect Cluster Root Formation and Citrate Exudation by Lupinus albus Grown in a Sand/Solution Split-Root System

We examined cluster root formation and root exudation by white lupin (Lupinus albus L. cv. Kiev Mutant) in response to growth medium and phosphorus supply in a sand/solution split-root system. The split-root system consisted of a nutrient solution compartment and a siliceous sand compartment. Phosphorus was applied at 1 (low-P plants) or 50 (high-P plants) μM as KH₂PO₄ to the solution compartment and at 10, 50 or 250 mg P kg⁻¹ as hydroxyapatite (Ca-P) to the sand compartment. In contrast to the high-P plants, P concentration and P uptake in the low-P plants increased with increasing P supply to the sand compartment. The NaHCO₃-extractable P was lower in the rhizosphere of the low-P plants than the high-P ones. The proton extrusion rate by the solution-grown roots of the low-P plants was higher than that of the high-P plants at the early growth stage. For the low-P plants, the proportion of dry root biomass allocated to cluster roots was higher in the solution compartment than that in the sand compartment. The citrate exudation increased in the sand compartment and decreased in the solution compartment with time, showing a lack of synchronization in citrate exudation by two root halves grown in different media. The cluster root proportion and citrate exudation in both compartments decreased with increasing shoot P concentration. An additional experiment with no P added to either root compartment showed that the proportion of cluster roots was about 9% lower in the sand than solution compartments. The results suggest that cluster root formation and citrate exudation can be significantly affected by the root growth medium in addition to being regulated by shoot P status. More P can be exploited from sparingly available Ca-P by the low-P plants than the high-P ones due to greater citrate exudation under P deficiency.     

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.     

Physiological changes in white lupin associated with variation in root-zone CO2 concentration and cluster-root P mobilization

White lupin (Lupinus albus L.) mobilizes insoluble soil phosphorus through exudation of organic acids from ‘cluster’ roots. Organic acid synthesis requires anaplerotic carbon derived from dark CO₂ fixation involving PEP-carboxylase. We tested the hypothesis that variation in root-zone CO₂ concentration would influence organic acid synthesis and thus P mobilization. Root-zone CO₂ concentrations and soil FePO₄ concentrations supplied to sand-grown white lupin (cv. Kiev Mutant) were varied. More biomass accumulated in plants supplied with 360 µL L⁻¹ CO₂ to the root-zone, compared with those aerated with either 100 or 6000 µL L⁻¹ CO₂. Increased FePO₄ in the sand resulted in greater leaf P concentrations, but root-zone [CO₂] did not influence leaf P concentration. Suppression of cluster-root development in plants supplied with 100 µL L⁻¹ root-zone CO₂ was correlated with increased leaf [P]. However, at both 360 and 6000 µL L⁻¹ CO₂, cluster-root development was suppressed only at the highest leaf P concentration. Phloem sap [P] was significantly increased by greater [FePO₄] in the sand, but was reduced with increased root-zone [CO₂], and this may have triggered increased cluster-root initiation. Succinate was the major organic acid (carboxylate) in the phloem sap (minor components included malate, citrate, fumarate) and was increased at greater [FePO₄], suggesting that this shoot-derived carboxylate might provide an important source of organic acids for root metabolism. Since cluster root development was inhibited by increasing concentrations of FePO₄ in the sand, it is possible that succinate was utilized for the functioning of the root-nodules.