The effect of phosphorus on cluster-root formation and functioning of Embothrium coccineum (R. et J. Forst.)

Background and aims

Embothrium coccineum (R. et J. Forst.) is a Proteaceae species from the southern part of South America. South-central Chilean soils are younger and contain more phosphorus (P) than soils in Australia and South Africa, where Proteaceae are common. Phosphorus deficiency is the main factor promoting cluster-root formation in Proteaceae. It is not known, however, whether this also applies to E. coccineum, which grows on soils with higher P content.

Methods

Four-month-old seedlings were grown for 4 weeks in hydroponic cultures with 1 μM P or 50 μM P. The number of cluster roots, relative height increment, biomass distribution, cluster root/total root biomass ratio, foliar P concentration, root acid phosphatase activity and root carboxylate-exudation rates were determined.

Results

Seedlings growing at 50 μM P showed a 10?, 1.3? and 3.3-fold greater increase in relative height, total dry mass and foliar P concentration, respectively, compared with those grown at1 μM P. However, seedlings grown at 1 μM P showed a 5?, 16?, 1.7? and 1.3-fold greater number of cluster roots, cluster root/total root biomass ratio, phosphatase activity and total carboxylate exudation, respectively, as compared with those grown at 50 μM P.

Conclusions

A low P supply promotes the initiation, growth and metabolic activity of cluster roots which is in accordance with reports on Proteaceae species occurring in ancient and highly weathered soils.     

Phosphorus-mobilization ecosystem engineering: the roles of cluster roots and carboxylate exudation in young P-limited ecosystems

Background

Carboxylate-releasing cluster roots of Proteaceae play a key role in acquiring phosphorus (P) from ancient nutrient-impoverished soils in Australia. However, cluster roots are also found in Proteaceae on young, P-rich soils in Chile where they allow P acquisition from soils that strongly sorb P.

Scope

Unlike Proteaceae in Australia that tend to proficiently remobilize P from senescent leaves, Chilean Proteaceae produce leaf litter rich in P. Consequently, they may act as ecosystem engineers, providing P for plants without specialized roots to access sorbed P. We propose a similar ecosystem-engineering role for species that release large amounts of carboxylates in other relatively young, strongly P-sorbing substrates, e.g. young acidic volcanic deposits and calcareous dunes. Many of these species also fix atmospheric nitrogen and release nutrient-rich litter, but their role as ecosystem engineers is commonly ascribed only to their diazotrophic nature.

Conclusions

We propose that the P-mobilizing capacity of Proteaceae on young soils, which contain an abundance of P, but where P is poorly available, in combination with inefficient nutrient remobilization from senescing leaves allows these species to function as ecosystem engineers. We suggest that diazotrophic species that colonize young soils with strong P-sorption potential should be considered for their positive effect on P availability, as well as their widely accepted role in nitrogen fixation. Their P-mobilizing activity possibly also enhances their nitrogen-fixing capacity. These diazotrophic species may therefore facilitate the establishment and growth of species with less-efficient P-uptake strategies on more-developed soils with low P availability through similar mechanisms. We argue that the significance of cluster roots and high carboxylate exudation in the development of young ecosystems is probably far more important than has been envisaged thus far.     

A re‐assessment of sucrose signaling involved in cluster‐root formation and function in phosphate‐deficient white lupin (Lupinus albus)

Apart from substrate functions, a signaling role of sucrose in root growth regulation is well established. This raised the question whether sucrose signals might also be involved in formation of cluster‐roots (CRs) under phosphate (Pi) limitation, mediating exudation of phosphorus (P)‐mobilizing root exudates, e.g. in Lupinus albus and members of the Proteaceae. Earlier studies demonstrated that CR formation in L. albus was mimicked to some extent by external application of high sucrose concentrations (25 mM) in the presence of extremely high P supply (1–10 mM), usually suppressing CR formation. In this study, we re‐addressed this question using an axenic hydroponic culture system with normal P supply (0.1 mM) and a range of sucrose applications (0.25–25 mM). The 2.5 mM sucrose concentration was comparable with internal sucrose levels in the zone of CR initiation in first‐order laterals of P‐deficient plants (3.4 mM) and induced the same CR morphology. Similar to earlier studies, high sucrose concentrations (25 mM) resulted in root thickening and inhibition of root elongation, associated with a 10‐fold increase of the internal sucrose level. The sucrose analog palatinose and a combination of glucose/fructose failed to stimulate CR formation under P‐sufficient conditions, demonstrating a signal function of sucrose and excluding osmotic or carbon source effects. In contrast to earlier findings, sucrose was able to induce CR formation but had no effect on CR functioning with respect to citrate exudation, in vitro activity and expression of genes encoding phosphoenolpyruvate carboxylase, secretory acid phosphatase and MATE transporters, mediating P‐mobilizing functions of CRs.     

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.     

Cluster Roots: A Curiosity in Context

Cluster roots are an adaptation for nutrient acquisition from nutrient-poor soils. They develop on root systems of a range of species belonging to a number of different families (e.g., Proteaceae, Casuarinaceae, Fabaceae and Myricaceae) and are also found on root systems of some crop species (e.g., albus, Macadamia integrifoliaandCucurbita pepo). Their morphology is variable but typically, large numbers of determinate branch roots develop over very short distances of main root axes. Root clusters are ephemeral, and continually replaced by extension of the main root axes. Carboxylates are released from cluster roots at very fast rates for only a few days during a brief developmental window termed an ‘exudative burst’. Most of the studies of cluster-root metabolism have been carried out using the crop plant L. albus, but results on native plants have provided important additional information on carbon metabolism and exudate composition. Cluster-root forming species are generally non-mycorrhizal, and rely upon their specialised roots for the acquisition of phosphorus and other scarcely available nutrients. Phosphorus is a key plant nutrient for altering cluster-root formation, but their formation is also influenced by N and Fe. The initiation and growth of cluster roots is enhanced when plants are grown at a very low phosphate supply (viz. ≤1 μM P), and cluster-root suppression occurs at relatively higher P supplies. An important feature of some Proteaceae is storage of phosphorus in stem tissues which is associated with the seasonality of cluster-root development and P uptake (winter) and shoot growth (summer), and also maintains low leaf [P]. Some species of Proteaceae develop symptoms of P toxicity at relatively low external P supply. Our findings with Hakea prostrata (Proteaceae) indicate that P-toxicity symptoms result after the capacity of tissues to store P is exceeded. P accumulation in H. prostrata is due to its strongly decreased capacity to down-regulate P uptake when the external P supply is supra-optimal. The present review investigates cluster-root functioning in (1) L.albus (white lupin), the model crop plant for cluster-root studies, and (2) native Proteaceae that have evolved in phosphate-impoverished environments.     

Developmental physiology of cluster-root carboxylate synthesis and exudation in harsh hakea. Expression of phosphoenolpyruvate carboxylase and the alternative oxidase

Harsh hakea (Hakea prostrata R.Br.) is a member of the Proteaceae family, which is highly represented on the extremely nutrient-impoverished soils in southwest Australia. When phosphorus is limiting, harsh hakea develops proteoid or cluster roots that release carboxylates that mobilize sparingly soluble phosphate in the rhizosphere. To investigate the physiology underlying the synthesis and exudation of carboxylates from cluster roots in Proteaceae, we measured O2 consumption, CO2 release, internal carboxylate concentrations and carboxylate exudation, and the abundance of the enzymes phosphoenolpyruvate carboxylase and alternative oxidase (AOX) over a 3-week time course of cluster-root development. Peak rates of citrate and malate exudation were observed from 12- to 13-d-old cluster roots, preceded by a reduction in cluster-root total protein levels and a reduced rate of O2 consumption. In harsh hakea, phosphoenolpyruvate carboxylase expression was relatively constant in cluster roots, regardless of developmental stage. During cluster-root maturation, however, the expression of AOX protein increased prior to the time when citrate and malate exudation peaked. This increase in AOX protein levels is presumably needed to allow a greater flow of electrons through the mitochondrial electron transport chain in the absence of rapid ATP turnover. Citrate and isocitrate synthesis and accumulation contributed in a major way to the subsequent burst of citrate and malate exudation. Phosphorus accumulated by harsh hakea cluster roots was remobilized during senescence as part of their efficient P cycling strategy for growth on nutrient impoverished soils.     

Shoot P status regulates cluster-root growth and citrate exudation in Lupinus albus grown with a divided root system

The present study was carried out to investigate whether the P concentration in the roots or the shoots controls the growth and citrate exudation of cluster roots in white lupin (Lupinus albus L). Foliar P application indicated that low P concentration in the shoots enhanced cluster‐root growth and citrate‐exudation rate more so than low P concentration in the roots. In the split‐root study, the P concentration in the shoots increased with increased P supply (1, 25 or 75 mmol m^?3 P), to the ‘privileged’ root halves. Roots ‘deprived’ of P invariably had the same low P concentrations, whereas those in the ‘privileged’ roots increased with increasing P supply (1, 25 or 75 mmol m^?3 P). Nevertheless, the proportion of the total root mass allocated to cluster roots, and the citrate‐exudation rates from the root halves were always similar on both root halves, irrespective of P supply, and decreased with increasing shoot P concentrations. Peak citrate exudation rates from developing cluster roots were significantly faster from cluster roots on the ‘deprived’ root halves when the ‘privileged’ half was exposed to 1 mmol m^?3 P as compared with 25 or 75 mmol m^?3 P. The possibility that changes in the concentrations of P fractions in the root halves influenced cluster‐root growth and citrate exudation was discounted, because there were no significant differences in insoluble organic P, ester‐P and inorganic P among all ‘deprived’ root halves. The results indicate that cluster‐root proportions and citrate exudation rates were regulated systemically by the P status of the shoot, and that P concentrations in the roots had little influence on growth and citrate exudation of cluster roots in L. albus.     

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.     

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.     

Phosphorus deficiency affects the allocation of below-ground resources to combined cluster roots and nodules in Lupinus albus

Lupins can rely on both cluster roots and nodules for P acquisition and biological nitrogen fixation (BNF), respectively. The resource allocation (C, N and P) between cluster roots and nodules has been largely understudied during P-deficient conditions. The aim of this investigation was therefore to determine the changes in resource allocation between these organs during fluctuations in P supply. Lupinus albus was cultivated in sand culture for 3 weeks, with either sufficient (2 mM high) or limiting (0.1 mM low) P supply. Although variation on P supply had no effect on the total biomass, there were significant differences in specialised below-ground organ allocation to cluster roots and nodule formation. Cluster root formation and the associated C-costs increased during low P supply, but at sufficient P-supply the construction and growth respiration costs of cluster roots declined along with their growth. In contrast to the cluster root decline at high P supply, there was an increase in nodule growth allocation and corresponding C-costs. However, this was not associated with an increase in BNF. Since cluster roots were able to increase P acquisition under low P conditions, this below-ground investment may also have benefited the P nutrition of nodules. These findings provide evidence that when lupins acquire N via BNF in their nodules, there may be a trade-off in resource allocation between cluster roots and nodules.     

The effect of nitrogen nutrition on cluster root formation and proton extrusion by Lupinus albus

Nitrogen nutrition can influence cluster root formation in many wild species, but the effect of N form on cluster root formation and root exudation by white lupin is not known. In a solution culture study, we examined the effect of N nutrition (ammonium, nitrate, both or N2 fixation) on cluster root formation and H+ extrusion by white lupin plants under deficient and adequate P supply. The number of cluster roots increased greatly when plants were supplied with I microM P compared with 50 microM P, the increase being 7.8-fold for plants treated with (NH4)2SO4, 3-fold for plants treated with KNO3 and NH4NO3, and 2-4-fold for N2-fixing plants. Under P deficiency. NH4+-N supply resulted in production of a greater number and biomass of cluster roots than other N sources. Dry weight of cluster roots was 30 % higher than that of non-cluster roots in P-deficient plants treated with (NH4)2SO4 and NH4NO3. In plants treated with sufficient P (50 microM), the weight of non-cluster roots was approx. 90 % greater than that of cluster roots. Both total (micromol per plant h(-1)) and specific (micromol g(-1) root d. wt h(-1)) H+ extrusions were greatest from roots of plants supplied with (NH4)2SO4, followed by those supplied with NH4NO3 and N2 fixation, whereas plants receiving KNO3 had negative net H+ extrusion between the third and fifth week of growth (indicating uptake of protons or release of OH- ions). The rate of proton extrusion by NH4+-N-fed plants was similar under P-deficient and P-sufficient conditions. In contrast, proton exudation by N2-fixing plants and KNO3-treated plants was ten-fold greater under P deficiency than under P sufficiency. In comparison with P deficiency, plants treated with 50 microM P had a significantly higher concentration of P in roots, shoots and youngest expanded leaves (YEL). Compared with the N2 fixation and KNO3 treatments, total N concentration was highest in roots, shoots and YEL of plants supplied with (NH4)2SO4 and NH4NO3, regardless of P supply. Under P deficiency, K concentrations in roots decreased at all N supplies, especially in plants treated with (NH4)2SO4 and NH4NO3, which coincided with the greatest H+ extrusion at these P and N supplies. In conclusion, NH4-N nutrition stimulated cluster root formation and H+ extrusion by roots of P-deficient white lupin.     

Manganese accumulation in leaves of Hakea prostrata (Proteaceae) and the significance of cluster roots for micronutrient uptake as dependent on phosphorus supply

When grown at a low P supply, Hakea prostrata R.Br. (Proteaceae) develops dense clusters of determinate branch roots, termed 'proteoid' or 'cluster' roots and accumulates Mn in its leaves. The aim of this study was to vary the production of cluster roots and assess the relationship between Mn uptake and cluster-root mass. We collected native soil from a location inhabited by H . prostrata and amended this with 'high' and 'low' amounts of insoluble or soluble P. After 14 months, we measured the impact of the treatments on cluster-root development and the [P], [Mn], [Fe], [Zn] and [Cu] in young (expanding) and mature leaves. Dry mass and leaf area increased with increasing P availability in the soil, but growth decreased at the highest soluble [P], which caused symptoms of P toxicity. The [P] in young leaves (1.3–2.7 mg g⁻¹ DM) exceeded that in older leaves (0.28–0.85 mg g⁻¹ DM), except when plants were grown with soluble P (3.2–21 mg g⁻¹ DM). Cluster-root formation was inhibited when leaf [P] increased; [P] in young leaves, rather than that in old leaves, appeared to be the factor that determined the proportion of the root mass invested in cluster roots. Old leaves of all treatments had [Mn] from 90 to 120 µg g⁻¹ DM, except for plants grown at high levels of soluble P, when [Mn] decreased below 30 µg g⁻¹ DM. The [Mn] and [Zn] in old leaves and the [Cu] in young leaves were positively correlated with the fraction of roots invested in cluster roots. These findings support our hypothesis that cluster roots play a significant role in micronutrient acquisition, and also provide an explanation for Mn accumulation in leaves of H . prostrata , and presumably Proteaceae in general.     

Specialized 'dauciform' roots of Cyperaceae are structurally distinct, but functionally analogous with 'cluster' roots

When grown in nutrient solutions of extremely low [P] (相似文献   

Role of phosphorus nutrition in development of cluster roots and release of carboxylates in soil-grown Lupinus albus

The present study examined the effect of phosphorus (P) limitation on cluster root formation and exudation of carboxylates by N₂-fixing white lupin (Lupinus albus L. cv. Kiev) grown in a P-deficient sandy soil. Plants received 10 (limited P) or 200 g P g^–1 soil as FePO₄ (adequate P) and were grown in a phytotron at 20/12 °C (12/12 h) for 76 days in soil columns. Cluster root formation was assessed and root exudates were collected at 9-day intervals. Shoot and root dry weights were higher in plants grown in the adequate-P compared to the limited-P treatment for 67 days. No clear difference in the total root length was observed between two P treatments before day 58. However, the specific root length increased rapidly from 17 m g^–1 DW at day 40 to 28 m g^–1 at day 49 in the P-limited plants, but decreased in the P-adequate plants. The effect of P limitation on enhancement of cluster root formation was observed from day 40 and reached the maximum at day 58. The number of cluster roots was negatively correlated with the P concentration in both roots and shoots. Phosphorus limitation increased exudation of citrate from day 40. The exudation of citrate displayed a cyclic pattern throughout the experiment, and appeared related to internal P concentration in plants, particularly P concentration in shoots. The sorption of exogenously added citrate in the soil was also examined. The amount of extractable citrate remained unchanged for 2 h, but decreased thereafter, suggesting that the soil had a low capacity to sorb citrate, and the rate of its decomposition by microorganisms was slow. Collecting solution leached through a soil column is a simple and reliable method to acquire root exudates from white lupin grown in soil. The results suggest that formation of cluster roots and exudation of citrate in white lupin are regulated by P concentration in shoots.     

Characterisation of arbuscular mycorrhizal fungi colonisation in cluster roots of shape Hakea verrucosa F. Muell (Proteaceae), and its effect on growth and nutrient acquisition in ultramafic soil

Ultramafic soils at Bandalup Hill (Western Australia) are characterised by high concentrations of Ni and low levels of P. Amongst the plant species that can sustain such hostile conditions, Hakea verrucosa F. Muell from a non-mycorrhizal family (Proteaceae) would be expected to rely on cluster roots to access P. However, the acidification of ultramafic soils by cluster roots might increase the dissolution of soil Ni, and therefore its availability to plants. Symbiosis with mycorrhizal fungi, on the other hand, might help to reduce the uptake of Ni by H. verrucosa. Therefore, the aim of this study was to investigate the mycorrhizal status of H. verrucosa, and assess any contribution from mycorrhizal fungi to its growth and nutrient status. Seedlings of H. verrucosa were first grown in undisturbed ultramafic soil cores from Bandalup Hill for 8 weeks to assess the presence of mycorrhizal fungi in their roots. In a second experiment, H. verrucosa seedlings were grown in the same ultramafic soil that was either steamed or left untreated. Seedlings were inoculated with an arbuscular mycorrhizal (AM) fungal consortium from Bandalup Hill. Fungal hyphae, vesicles, as well as intracellular arbuscules and hyphal coils were observed in the cluster roots of H. verrucosa in both experiments. In the first experiment, 57% of the root length was colonized by AM fungi. Seedlings had high (between 1.4 and 1.9) shoot to root ratios and their roots had very few root hairs, despite growing in P-deficient soil. Steaming of the ultramafic soil increased the growth of seedlings and their nutrient uptake. Inoculation with AM fungi reduced the seedling growth in steamed ultramafic soil; however, it increased their shoot P and K concentration and also the shoot K content. The shoot Ni concentration of seedlings was not affected by the presence of AM fungi.     

Convergence of a specialized root trait in plants from nutrient-impoverished soils: phosphorus-acquisition strategy in a nonmycorrhizal cactus

In old, phosphorus (P)-impoverished habitats, root specializations such as cluster roots efficiently mobilize and acquire P by releasing large amounts of carboxylates in the rhizosphere. These specialized roots are rarely mycorrhizal. We investigated whether Discocactus placentiformis (Cactaceae), a common species in nutrient-poor campos rupestres over white sands, operates in the same way as other root specializations. Discocactus placentiformis showed no mycorrhizal colonization, but exhibited a sand-binding root specialization with rhizosheath formation. We first provide circumstantial evidence for carboxylate exudation in field material, based on its very high shoot manganese (Mn) concentrations, and then firm evidence, based on exudate analysis. We identified predominantly oxalic acid, but also malic, citric, lactic, succinic, fumaric, and malonic acids. When grown in nutrient solution with P concentrations ranging from 0 to 100 μM, we observed an increase in total carboxylate exudation with decreasing P supply, showing that P deficiency stimulated carboxylate release. Additionally, we tested P solubilization by citric, malic and oxalic acids, and found that they solubilized P from the strongly P-sorbing soil in its native habitat, when the acids were added in combination and in relatively low concentrations. We conclude that the sand-binding root specialization in this nonmycorrhizal cactus functions similar to that of cluster roots, which efficiently enhance P acquisition in other habitats with very low P availability.     

Excess cation uptake, and extrusion of protons and organic acid anions by Lupinus albus under phosphorus deficiency

In symbiotically-grown legumes, rhizosphere acidification may be caused by a high cation/anion uptake ratio and the excretion of organic acids, the relative importance of the two processes depending on the phosphorus nutritional status of the plants. The present study examined the effect of P deficiency on extrusions of H⁺ and organic acid anions (OA⁻) in relation to uptake of excess cations in N₂-fixing white lupin (cv. Kiev Mutant). Plants were grown for 49 days in nutrient solutions treated with 1, 5 or 25 mmol P m⁻³ Na₂HPO₄ in a phytotron room. The increased formation of cluster roots occurred prior to a decrease in plant growth in response to P deficiency. The number of cluster roots was negatively correlated with tissue P concentrations below 2.0 g kg⁻¹ in shoots and 3 g kg⁻¹ in roots. Cluster roots generally had higher concentrations of Mg, Ca, N, Cu, Fe, and Mn but lower concentrations of K than non-cluster roots. Extrusion of protons and OA⁻ (90% citrate and 10% malate) from roots was highly dependent on P supply. The amounts of H⁺ extruded per unit root biomass decreased with time during the experiment. On the equimolar basis, H⁺ extrusion by P-deficient plants (grown at 1 and 5 mmol P m⁻³) were, on average, 2–3-fold greater than OA⁻ exudation. The excess cation content in plants was generally the highest at 1 mmol P m⁻³ and decreased with increasing P supply. The ratio of H⁺ release to excess cation uptake increased with decreasing P supply. The results suggest that increased exudation of OA⁻ due to P deficiency is associated with H⁺ extrusion but contributes only a part of total acidification.     

Carbon Cost of the Fungal Symbiont Relative to Net Leaf P Accumulation in a Split-Root VA Mycorrhizal Symbiosis

Translocation of ¹⁴C-photosynthates to mycorrhizal (+ +), half mycorrhizal (0+), and nonmycorrhizal (00) split-root systems was compared to P accumulation in leaves of the host plant. Carrizo citrange seedlings (Poncirus trifoliata [L.] Raf. × Citrus sinensis [L.] Osbeck) were inoculated with the vesicular-arbuscular mycorrhizal fungus Glomus intraradices Schenck and Smith. Plants were exposed to ¹⁴ CO₂ for 10 minutes and ambient air for 2 hours. Three to 4% of recently labeled photosynthate was allocated to metabolism of the mycorrhiza in each inoculated root half independent of shoot P concentration, growth response, and whether one or both root halves were colonized. Nonmycorrhizal roots respired more of the label translocated to them than did mycorrhizal roots. Label recovered in the potting medium due to exudation or transport into extraradical hyphae was 5 to 6 times greater for (+ +) versus (00) plants. In low nutrient media, roots of (0+) and (+ +) plants transported more P to leaves per root weight than roots of (00) plants. However, when C translocated to roots utilized for respiration, exudation, etc., as well as growth is considered, (00) plant roots were at least as efficient at P uptake (benefit) per C utilized (cost) as (0+) and (+ +) plants. Root systems of (+ +) plants did not supply more P to leaves than (0+) plants in higher nutrient media, yet they still allocated twice the ¹⁴C-photosynthate to the mycorrhiza as did (0+) root systems. This indicates there is an optimal level of mycorrhizal colonization above which the plant receives no enhanced P uptake yet continues to partition photosynthates to metabolism of the mycorrhiza.     

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.     

A comparison of structure,development and function in cluster roots of Lupinus albus L. under phosphate and iron stress

Cluster roots are adaptations for nutrient acquisition, found throughout the world in many different plant families and habitats. They arise from changes in root initiation, meristem maintenance and physiology. In Lupinus albus cluster roots form under low internal plant phosphate and low internal plant iron levels. In this study, we compare morphology, structure and physiology of cluster roots formed under –P and –Fe conditions. –Fe cluster roots had a lower density of shorter rootlets than –P roots, and were yellow in colour, probably because of increased phenolics due to down-regulation of peroxidase. Rootlet length and width was reduced in –Fe conditions. The change in exudation of citrate, over time, of –P and –Fe cluster roots shared identical temporal dynamics, with an exudative burst occurring in day 3. However, the –Fe cluster roots displayed much higher rates of exudation than the –P cluster roots. Results are discussed within the context of structural and functional control.