Phosphorus acquisition from soil by white lupin (Lupinus albus L.) and soybean (Glycine max L.), species with contrasting root development

White lupin and soybean have contrasting root morphologies: white lupin develops proteoid or cluster roots, roots with discreet clusters of short, determinate branch roots (rootlets) while soybean develops a more fibrous root system with evenly distributed, longer branch roots. Growth and P acquisition by white lupin and soybean were compared in a soil high in bound, total P, with or without additional inorganic P applied in solution. Additional P increased biomass by 25% and doubled total P in soybean. In contrast, white lupin did not respond to additional P in biomass or total P. However added P decreased cluster development on proteoid roots indicating that white lupin sensed the added P. The reduction in cluster weight per plant was exactly countered by an increase in dry weight of other roots. Soybean root development responded to P application, proliferating branch roots with active meristems in the upper portion of the soil profile where P was applied, and reducing root weight to plant weight by 13%. White lupin did not proliferate roots in response to P application. When P was not added to soil, soybean and lupin acquired similar P per unit root dry weight. However, white lupin accumulated 4.8 times more P per unit root length, suggesting that P acquisition in these plants involved other mechanisms such as the exudation of P solubilizing compounds. Soybean accessed P by developing more root length thus colonising more soil volume than white lupin and, therefore, was better able to take advantage of the added P. Pericycle and root tip meristem activities were critical to the differences in root development between white lupin and soybean, and therefore their responses to plant and soil P.     

Citric acid excretion and precipitation of calcium citrate in the rhizosphere of white lupin (Lupinus albus L.)

Abstract. White lupin ( Lupinus albus L.) was grown for 13 weeks in a phosphorus (P) deficient calcareous soil (20% CaCO₃, pH(H₂O)7.5) which had been sterilized prior to planting and fertilized with nitrate as source of nitrogen. In response to P deficiency, proteoid roots developed which accounted for about 50% of the root dry weight. In the rhizosphere soil of the proteoid root zones, the pH dropped to 4.8 and abundant white precipitates became visible. X-ray spectroscopy and chemical analysis showed that these precipitates consisted of calcium citrate. The amount of citrate released as root exudate by 13-week-old plants was about 1 g plant⁻¹, representing about 23% of the total plant dry weight at harvest. In the rhizosphere soil of the proteoid root zones the concentrations of available P decreased and of available Fe, Mn and Zn increased. The strong acidification of the rhizosphere and the cation/anion uptake ratio of the plants strongly suggests that proteoid roots of white lupin excrete citric acid, rather than citrate, into the rhizosphere leading to intensive chemical extraction of a limited soil volume. In a calcareous soil, citric acid excretion leads to dissolution of CaCO₃ and precipitation of calcium citrate in the zone of proteoid roots.     

Effect of phosphorus supply on the formation and function of proteoid roots of white lupin (Lupinus albus L.)

We investigated (1) the effect of constant and altered inorganic phosphate (P_i) supply (1–100 mmol m^–3) on proteoid root production by white lupin ( Lupinus albus L.); and (2) the variation in citrate efflux, enzyme activity and phosphate uptake along the proteoid root axis in solution culture. Proteoid root formation was greatest at P_i solution concentrations of 1–10 mmol m^–3 and was suppressed at 25 mmol m^–3 P_i and higher. Except at 1 mmol m^–3 P_i, the formation of proteoid roots did not affect plant dry matter yields or shoot to root dry matter ratios, indicating that proteoid roots can form under conditions of adequate P supply and not at the expense of dry matter production. Plants with over 50% of the root system as proteoid roots had tissue P concentrations considered adequate for maximum growth, providing additional evidence that proteoid roots can form on P-sufficient plants. There was an inverse relationship between the P_i concentration in the youngest mature leaf and proteoid root formation. Citrate efflux and the activities of enzymes associated with citric acid synthesis (phosphoenolpyruvate carboxylase and malate dehydrogenase) varied along the proteoid root axis, being greatest in young proteoid rootlets of the 1–3 cm region from the root tip. Citrate release from the 0–1 and 5–9 cm regions of the proteoid root was only 7% (per unit root length) of that from the 1–3 cm segment. Electrical potential and ³²P_i uptake measurements showed that P_i uptake was more uniform along the proteoid root than citrate efflux.     

Changes in root morphology of white lupin (Lupinus albus L.) and its adaptation to soils with heterogeneous alkaline/acid profiles

The ability of Lupinus albus L. to adapt to a heterogeneous soil profile containing acid subsoil below limed topsoil of the same type, and to utilize nutrients by significantly altering its root system structure, was investigated using specially constructed soil profile tubes. Plants grown in homogeneous acid profiles had the fastest growth while those grown in homogeneous limed-soil profiles showed the slowest growth and exhibited some chlorosis after 19 days. Limed topsoil combined with an acid subsoil profile initially retarded plant growth similar to that in a homogeneous limed soil. However, after 68 days significantly greater growth had occurred in the limed/acid soil treatment relative to the homogeneous limed soil, indicating plants had benefited from the acid subsoil stratum. Plants in the homogeneous limed soil profile had lower concentrations of P, Fe and Mn in shoots compared with those in heterogeneous soils. In contrast, the concentration of Ca increased by 74%, due mainly to an increase in the water-soluble Ca fraction. When grown in a heterogeneous limed/acid soil profile, concentrations of P, Ca, K, Mg, Fe, Mn and Zn in shoots were comparable to those grown in a soil with a homogeneous acid profile. Although total root production was lower in the homogeneous limed-soil profile compared to the acid-soil containing profiles, cluster root mass was maintained at a level comparable with that in acid soil. The roots in heterogeneous soil profiles exhibited extensive plasticity, demonstrating a root-type specific, morphological response to the soil conditions. Within the acid subsoil of a heterogeneous profile, there was a large increase in cluster root mass compared with non-cluster roots. The proliferation of cluster roots in acid soil below limed topsoil may enhance the plant's ability to exploit this soil and facilitate the cultivation of L. albus on limed soil. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interactions between atmospheric CO2 concentration and phosphorus nutrition on the formation of proteoid roots in white lupin (Lupinus albus L.)

Atmospheric [CO₂] affects photosynthesis and therefore should affect the supply of carbon to roots. To evaluate interactions between carbon supply and nutrient acquisition, the [CO₂] effects on root growth, proteoid root formation and phosphorus (P) uptake capacity were studied in white lupin (Lupinus albus L.) grown hydroponically at 200, 410 and 750 µmol mol^?1 CO₂, under sufficient (0·25 mm P) and deficient (0·69 µm P) phosphorus. Plant size increased with increasing [CO₂] only at high P. Both P deficiency and increasing [CO₂] increased the production of proteoid clusters; the increase in response to increased [CO₂] was proportionally greater from low to ambient [CO₂] than from ambient to high. The activity of phosphoenol pyruvate carboxylase in the proteoid root, the exudation of organic acids from the roots, and the specific uptake of P increased with P deficiency, but were unaffected by [CO₂]. Increasing [CO₂] from Pleistocene levels to those predicted for the next century increased plant size and allocation to proteoid roots, but did not change the specific P uptake capacity per unit root mass. Hence, rising [CO₂] should promote nutrient uptake by allowing lupins to mine greater volumes of soil.     

Abscisic acid concentration,root pH and anatomy do not explain growth differences of chickpea (Cicer arietinum L.) and lupin (Lupinus angustifolius L.) on acid and alkaline soils

The ABA concentrations of leaves, roots, soils and transport fluids of chickpea and lupin plants growing in acid (pH=4.8) and alkaline (pH=8.0) soils and an acid soil with an alkaline subsoil and an alkaline soil with an acid subsoil were measured with the aim of explaining the poor growth of narrow-leafed lupins in alkaline soil. The ABA concentration in the leaves was higher in lupin than chickpea, but did not differ when the plants were grown in alkaline compared to acid soil. The ABA concentration of the roots and xylem sap of lupin did not differ significantly when grown in acid or alkaline soil. Chickpea roots and xylem sap had, however, lower ABA concentrations in acid soil. The ABA concentration in the soil solution was higher in the acid than in the alkaline soil. Roots of lupin and chickpea showed no suberization of the hypodermis or exodermis whether grown aeroponically or hydroponically and the pH of the cytoplasm did not change significantly when root cells of lupin and chickpea were exposed to external pHs of 4.8 or 8.0. The chickpea roots had greater suberization of the endodermal cells adjacent to radial xylem rays and maintained a slightly higher vacuolar pH than lupin in both acid and alkaline external media, but these small differences are insufficient to explain the reductions in lupin growth in alkaline soil.     

Fractal geometry of root systems: Field observations of contrasting genotypes of common bean (Phaseolus vulgaris L.) grown under different phosphorus regimes

Root growth and architecture are important for phosphorus acquisition due to the relative immobility of P in the soil. Fractal geometry is a potential new approach to the analysis of root architecture. Substantial genetic variation in root growth and architecture has been observed in common bean. Common bean (Phaseolus vulgaris L.) genotypes with contrasting root architecture were grown under moderate and low P conditions in a field experiment. Linear and planar fractal dimension were measured by tracing root intercepts with vertical planes. Linear fractal dimension increased over time in efficient genotypes, but remained fairly constant over time in inefficient genotypes. Planar fractal dimension increased over time for all genotypes, but was higher in efficient than inefficient genotypes at the end of the experiment. Planar fractal dimension of medium P plants was found to correlate with shoot P content indicating fractal dimension to be a possible indicator for root P uptake. The increasing fractal dimension over time indicates that fractal analysis is a sensitive measure of root branching intensity. A less destructive method for acquisition of data that allows for continuous analysis of fractal geometry and thereby screening for more P efficient genotypes in the field is suggested. This method will allow the researcher to conduct fractal analysis and still complete field trials with final yield evaluation.     

The effect of soil freezing on the survial of winter-sown white lupins (Lupinus albus L.)

Specially constructed soil-freezing growth boxes were used to study the effects of the intensity and duration of soil freezing on root injury and the survival of white lupin seedlings of different ages under controlled conditions. The extent of root damage depended on both the intensity of soil freezing and the stage of seedling development (measured as the extent of lignification of the central stele of the primary root). Seedlings whose secondary root development was well advanced, and in which the endodermis was completely lignified, survived intense soil freezing intact. Young seedlings with weakly lignified roots were damaged by moderate soil freezing (> 5 days at ?1°C) and killed by more intense freezing (5 days at ?2°C). The extent of root development and ligmfkation was correlated with the number of leaf primordia produced at the shoot apex so that the susceptibility to soil freezing damage could be accurately predicted by a simple physiological/leaf production model.     

Active chitinases in the apoplastic fluids of healthy white lupin (Lupinus albus L.) plants

Acidic exocellular class III chitinase (EC 3.2.1.14) was previously identified in healthy white lupin (Lupinus albus L.) plants and suspension-cultured cells by N-terminal microse-quencing. In this study, the detection of chitinase activity with Remazol Brilliant Violet 5R (RBV)-labelled chitin derivatives is described. Chitinase activity was observed in protein fractions of cytoplasmic or exocellular origin from roots, hypocotyls, cotyledons, and leaves of healthy white lupin plants. Using isoelectrofocusing followed by a new overlay technique with carboxymethyl chitin-RBV conjugate-containing gel, up to six different chitinase isoforms were visualised. Their activity was distributed fairly evenly within a plant with acidic isoforms predominating in cell walls and basic (or neutral) ones found intracellularly. Exocellular location of some chitinase isoforms were also confirmed by detection of their activities in intercellular washing fluids from white lupin tissues. Chitinase activity was demonstrated in culture filtrates and cell walls of suspension-cultured white lupin cells.     

Interactive effects of phosphorus deficiency and exogenous auxin on root morphological and physiological traits in white lupin (Lupinus albus L.)

White lupin (Lupinus albus) exhibits strong root morphological and physiological responses to phosphorus (P) deficiency and auxin treatments, but the interactive effects of P and auxin in regulating root morphological and physiological traits are not fully understood. This study aimed to assess white lupin root traits as influenced by P (0 or 250 μmol L^?1) and auxin (10^?8 mol L^?1 NAA) in nutrient solution. Both P deficiency and auxin treatments significantly altered root morphological traits, as evidenced by reduced taproot length, increased number and density of first-order lateral roots, and enhanced cluster-root formation. Changes in root physiological traits were also observed, i.e., increased proton, citrate, and acid phosphatase exudation. Exogenous auxin enhanced root responses and sensitivity to P deficiency. A significant interplay exists between P and auxin in the regulation of root morphological and physiological traits. Principal component analysis showed that P availability explained 64.8% and auxin addition 21.3% of the total variation in root trait parameters, indicating that P availability is much more important than auxin in modifying root responses of white lupin. This suggests that white lupin can coordinate root morphological and physiological responses to enhance acquisition of P resources, with an optimal trade-off between root morphological and physiological traits regulated by external stimuli such as P availability and auxin.     

Phosphorus supply and the growth of frequently defoliated white clover (Trifolium repens L.) in dry soil

A previous study found that increased phosphorus (P) supply to frequently defoliated white clover plants, growing in a low-P, dry soil, alleviated water stress symptoms and increased plant recovery on rewatering. In this study we determined how these stresses influence white clover growth. Measurements were made of the leaf canopy, stolon infrastructure and root system of the white clover plants growing in a low-P soil. Treatments included the factorial combination of four levels of P supply, two defoliation frequencies and two soil water treatments. White clover growth declined markedly when P-deficient plants were exposed to frequent defoliation and dry soil conditions. Leaf area was more affected than other parameters, in that the combination of stresses reduced leaf area to 2% of maximum observed for infrequently defoliated plants growing in high-P soil, with adequate water. Increased P supply generally increased the growth of all plant parts. Frequently defoliated plants growing in dry soil produced similar or greater leaf mass and leaf area as plants from similar treatments growing in wet soil, when the P supply increased to 50 mg P kg-1 soil. Higher P rates were able to negate the effect of dry soil on these frequently defoliated plants, as a result of larger water and P uptake. Also, the frequently defoliated plants with restricted root growth did not respond to a small increase in P supply (17 mg P kg-1 soil) for the leaf growth, irrespective of whether they were growing in wet or dry soil. Infrequently defoliated plants with greater root growth, compared to frequently defoliated plants, more than doubled their leaf mass with this P treatment.     

The development of potential screens based on shoot calcium and iron concentrations for the evaluation of tolerance in Egyptian genotypes of white lupin (Lupinus albus L.) to limed soils

European cultivars of white lupin (Lupinus albus L.) grow poorly in limed or calcareous soils. However, Egyptian genotypes are grown successfully in highly calcareous soil and show no stress symptoms. To examine their physiological responses to alkaline soil and develop potential screens for tolerance, three experiments were conducted in limed and non-limed (neutral pH) soil. Measurements included net CO2 uptake, and the partitioning of Fe2+ and Fe3+ and soluble and insoluble Ca in stem and leaf tissue. Intolerant plants showed clear symptoms of stress, whereas stress in the Egyptian genotypes and in L pilosus Murr. (a tolerant species) was less marked. Only the intolerant plants became chlorotic and this contributed to their reduced net CO2 uptake in the limed soil. In contrast, Egyptian genotypes and L pilosus showed no change in net CO2 uptake between the soils. The partitioning of Ca and Fe either resulted from the stress responses, or was itself a stress response. L pilosus and some Egyptian genotypes differed in soluble Ca concentrations compared with the intolerant cultivars, although no significant difference was apparent in the Ca partitioning of the Egyptian genotype Giza 1. In a limed soil, Giza 1 maintained its stem Fe3+ concentration at a level comparable with that of plants grown in non-limed soil, whereas stem [Fe3+] of an intolerant genotype increased. Gizal increased the percentage of plant Fe that was Fe2+ in its leaf tissue under these conditions; that of the intolerant genotype was reduced. The potential tolerance of the Egyptian genotypes through these mechanisms and the possibility of nutritional-based screens are discussed.     

Variation in the growth of lupin species and genotypes on alkaline soil

Commercial lupins grow poorly on alkaline and neutral fine-textured soils. Genotypic variation exists among lupins. The present study compared the growth of 13 lupin genotypes, including introduced cultivars and wild types, in an alkaline loamy soil and an acid loamy soil.Plants grown in the alkaline and acid soils did not show obvious symptoms of iron deficiency at any stage. There was however a large variation of shoot fresh weight among genotypes in response to the alkaline soil with L. atlanticus and L. pilosus being more tolerant than L. luteus, L. cosentinii, L. albus and L. angustifolius. Some variation also existed among genotypes of L. angustifolius. In addition, root growth was retarded on the alkaline soil except for L. atlanticus, L. pilosus P20955 and L. albus Kiev mutant. In the alkaline soil, root growth at week 2 correlated well with the shoot fresh weight at week 12. The results suggest that early root elongation may be useful for screening tolerant genotypes for alkaline soils.     

Biosynthesis, accumulation and secretion of isoflavonoids during germination and development of white lupin (Lupinus albus L.)

A combination of ¹⁴C labelling experiments of white lupin seedlings(Lupinus albus L.) and high pressure liquid chromatography oftissue extracts indicated active biosynthesis of isoflavonoidsduring the first 2–4 d of seed germination. These weresynthesized mostly as glucosides and to a lesser extent as aglycones,with trace amounts of prenylated derivatives. There was a generaldecrease in total isoflavonoids during later stages of germination(c. until d 13), which may be ascribed to their turnover and/orexudation into the rhizosphere. In addition, exudation of isoflavonoidsby lupin seeds germinated under sterile conditions continuesfor 12 d with a preferential release of monoprenylated compounds.The relationship between the specific developmental events duringseed germination and the accumulation of certain groups of isoflavonoidsis discussed in relation to their possible role in the growthprocesses of lupin. Key words: Isoflavonoids, glucosides, aglycones, prenylated derivatives, root exudates, white lupin     

Effect of water stress on abscisic acid levels in white lupin (Lupinus albus L.) fruit,leaves and phloem exudate

Abscisic acid (ABA) was identified by combined gas liquid chromatography-mass spectrometry in sieve-tube exudate collected from the cut stylar ends of white lupin fruit. Water stress caused an increase in ABA levels in leaf, seed and pod tissues and phloem exudate. When compared with levels in extracts of these tissues, the concentration of ABA in sieve-tube sap was very high. It is suggested that ABA is actively transported out of mature leaves in the phloem and this finding is discussed in terms of the ABA balance of the plant.Abbreviations ABA abscisic acid - GLC gas liquid chromatography     

Nitric oxide is the shared signalling molecule in phosphorus- and iron-deficiency-induced formation of cluster roots in white lupin (Lupinus albus)

Background and Aims

Formation of cluster roots is one of the most specific root adaptations to nutrient deficiency. In white lupin (Lupinus albus), cluster roots can be induced by phosphorus (P) or iron (Fe) deficiency. The aim of the present work was to investigate the potential shared signalling pathway in P- and Fe-deficiency-induced cluster root formation.

Methods

Measurements were made of the internal concentration of nutrients, levels of nitric oxide (NO), citrate exudation and expression of some specific genes under four P × Fe combinations, namely (1) 50 µm P and 10 µm Fe (+P + Fe); (2) 0 P and 10 µm Fe (–P + Fe); (3) 50 µm P and 0 Fe (+P–Fe); and (4) 0 P and 0 Fe (–P–Fe), and these were examined in relation to the formation of cluster roots.

Key Results

The deficiency of P, Fe or both increased the cluster root number and cluster zones. It also enhanced NO accumulation in pericycle cells and rootlet primordia at various stages of cluster root development. The formation of cluster roots and rootlet primordia, together with the expression of LaSCR1 and LaSCR2 which is crucial in cluster root formation, were induced by the exogenous NO donor S-nitrosoglutathione (GSNO) under the +P + Fe condition, but were inhibited by the NO-specific endogenous scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl- 3-oxide (cPTIO) under –P + Fe, +P–Fe and –P–Fe conditions. However, cluster roots induced by an exogenous supply of the NO donor did not secrete citrate, unlike those formed under –P or –Fe conditions.

Conclusions

NO plays an important role in the shared signalling pathway of the P- and Fe-deficiency-induced formation of cluster roots in white lupin.     

The formation,morphology and anatomy of cluster root of Lupinus albus L. as dependent on soil type and phosphorus supply

The effect of phosphorus supply on the formation, morphology and anatomy of cluster roots of Lupinus albus L. cv Ultra grown in a loam and two sandy soils was examined relative to its effect on total root length, shoot weight and the phosphorus concentration of the shoots. The loam soil was most conducive to the formation of cluster roots. Cluster roots growing in the sandy soils developed to a lesser extent on plants of an equivalent phosphorus status, suggesting that some biotic or abiotic factors independent of phosphorus supply were also operating. The presence of mature cluster rootlets on a length of lateral root increased the root surface area by 14–22 times of an equal length of lateral roots not bearing cluster rootlets. The application of phosphorus decreased cluster-root length, whereas total root length showed a steady increase. There was an inverse relationship between cluster-root production and phosphorus concentration in shoots ranging from 2 to 8.5 mg g^–1 with the critical phosphorus level for maximum shoot growth being around 2.5 mg g^–1. Cluster roots formed in solution culture were not well developed in comparison with those grown in the loam soil or nutrient solution with added loam soil. The organisation of the cluster rootlet was similar to that of the lateral roots. Mature rootlets lacked an apical meristem and a vascular cambium with a reduced root cap and cortical tissue.     

Enhancing white lupin (Lupinus albus L.) adaptation to calcareous soils through selection of lime-tolerant plant germplasm and Bradyrhizobium strains

Aims

This study aimed to determine whether white lupin adaptation to moderately calcareous soils could be enhanced by lime-tolerant plants and Bradyrhizobium strains.

Methods

Fourteen landraces from Italy, Morocco and Egypt and some cultivars were grown in moderate-lime (ML) and low-lime (LL) soil with each of two inoculants, one commercial and one including three Bradyrhizobium strains well-nodulating under ML soil (isolated from other lupin species). Grain yield and above-ground biomass were assessed in large artificial environments that mimicked field conditions. Shoot, root and nodulation traits at onset of flowering were studied in a pot experiment.

Results

ML soil severely reduced plant yield, growth and nodulation but increased the harvest index relative to LL. Top-yielding genotypes for grain yield displayed significant rank inversion across soil types (P < 0.05). Lime-tolerant genotypes reduced their nodulation in ML soil less than limesusceptible ones. Some landraces outperformed the reference lime-tolerant cultivar Giza 1 in ML soil. One Italian landrace had a lime-tolerant response across agricultural locations. The Moroccan inoculant provided greater nodulation, more shoot residues but similar grain yield in ML soil, and less grain and shoot residues in LL soil, compared with the commercial inoculant.

Conclusions

Lupin adaptation to ML soil can be improved mainly through selection of lime-tolerant plants.     

Spatial and temporal dynamics of the microbial community structure in the rhizosphere of cluster roots of white lupin (Lupinus albus L.)

White lupin was grown in a quartz sand–soil mix with poorly available Ca phosphate. The plants were harvested on days 21, 35 and 51 and DNA was extracted from the non-cluster roots, the young, mature and senescent cluster roots with adhering soil. Bacterial community structure was examined by PCR-DGGE of 16S rDNA, digitisation of the band patterns and multivariate analyses. In all root zones the bacterial community structure changed with plant age. The communities in the rhizosphere of the non-cluster roots were always different from those of the cluster roots. The bacterial communities of the cluster roots were cluster age and plant age dependent. The differences in bacterial community structure between the cluster root age classes were significant on days 35 and day 51 but not on d 21. A separate experiment, in which root exudates and samples for PCR-DGGE were collected simultaneously, showed that both bacterial and eukaryotic (18S rDNA) community structures change with organic acid exudation. While eukaryotic community structure of the cluster roots was correlated with citric acid exudation, bacterial community structure was correlated with cis-acconitic, citric and malic acid exudation.