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.     

Screening rough-seeded lupins (Lupinus pilosus Murr. and Lupinus atlanticus Glads.) for tolerance to calcareous soils

Soil- and solution-based screening methods were used to identify interspecific and intraspecific variation in lupins for tolerance to calcareous soils. Plants were grown for 21 days in a calcareous soil (pH 8.2; 50% CaCO₃; moisture content 90% of field capacity) for soil-based screening and in nutrient solution containing 15 mM KHCO₃ for solution-based screening. Chlorosis as an indicator of tolerance was recorded. Lupinus pilosus Murr. had the most tolerant genotypes and had the greatest range of intraspecific variation. Most genotypes of Lupinus atlanticus Glads. and Lupinus angustifolius L. were moderately intolerant, although two genotypes of L. atlanticus appeared to be tolerant. Lupinus albus L. had moderately tolerant to moderately intolerant genotypes, whilst the single genotypes of Lupinus cosentinii Guss. and Lupinus digitatus Forsk. appeared tolerant. In a field study six genotypes of L. pilosus identified in the soil-based screening as differing in their tolerance to the calcareous soil were grown on comparable calcareous (pH 8.3; topsoil 3% CaCO₃, subsoil 13% CaCO₃) and non-calcareous (pH 7.3) soils within a paddock. Chlorosis and nutrient concentrations in the youngest leaves were measured 53 days after sowing, whilst grain yield was estimated at harvest. Despite the soil containing a much lower CaCO₃ content than used in the screening method, the field study confirmed that moderately intolerant to intolerant genotypes had lower relative grain yields than more tolerant genotypes. Chlorosis rankings of the genotypes were correlated between field and the screening studies. It is suggested that the incorporation of genes conferring tolerance to calcareous soils into high yielding, agronomically suitable genotypes of L. pilosus should be an important objective in a lupin breeding program for calcareous soils.     

pH above 6.0 reduces nodulation in Lupinus species

Lupinus angustifolius and Lupinus pilosus differ substantially in root growth in response to high solution pH with L. angustifolius showing much greater sensitivity to pH above 6. This study examined the effect of pH above 6 on nodulation of these two species in buffered solution. Shoot weight and root weight and length in L. pilosus was not significantly affected by pH, whereas the growth of shoots and roots of L. angustifolius was markedly impaired by increasing pH. Total root length, the number of lateral roots, and the length of individual lateral roots were greatly decreased, resulting in decreased uptake of iron and phosphorus. In addition, L. angustifolius had a higher internal requirement for iron than L. pilosus. A solution pH above 6 decreased the number of nodule initials and nodules similarly in both species but decreased nodule mass much more in L. angustifolius. The effect of high pH on nodule formation occurred prior to that on host shoot growth. High pH also decreased nitrogen concentration and content in both species but to a greater extent for L. angustifolius. The results suggested that pH above 6 has a specific effect in the impairment of nodulation in lupins.     

The effect of soil moisture on the tolerance of Lupinus pilosus genotypes to a calcareous soil

Commercial narrow-leafed lupins (Lupinus angustifolius L.) grown on calcareous soils commonly display chlorotic symptoms resembling Fe deficiency. The severity of chlorosis increases with concurrent increases in soil moisture content. Our research has indicated that the rough-seeded lupin species, Lupinus pilosus Murr., has a range of adaptation to calcareous soils, from tolerant to intolerant. A pot experiment was conducted comparing a tolerant, a moderately tolerant and a moderately intolerant genotype of L. pilosus. Plants were grown for 35 days in a calcareous soil (50% CaCO₃) at three moisture contents (80%, 100% and 120% of field capacity); the growth was compared with that on a fertile black cracking clay control soil at 70% of field capacity. Visual chlorosis score, chlorophyll meter readings, number of leaves and shoot dry weights were recorded at 14, 21, 28 and 35 days after sowing. Concentrations of chlorophyll, active Fe and nutrients in the youngest fully expanded leaves were also measured. Results showed that increased soil moisture increased the severity of chlorotic symptoms (increased chlorosis score) in all genotypes. The tolerant genotype showed significantly less symptoms than other genotypes at all moisture contents. All genotypes were able to recover from chlorosis symptoms at 80% moisture in the calcareous soil. Chlorosis score negatively correlated with chlorophyll meter readings, chlorophyll concentration and foliar active and total Fe, and Mn concentrations. Visual chlorosis score appeared to be a cost effective, accurate and efficient method enabling classification of the tolerance of genotypes. The chlorotic symptoms were likely to be due to HCO₃ ^- induced nutrient deficiencies or a direct effect of HCO₃ ^- on chlorophyll synthesis. This study indicates that the most probable mechanism of tolerance is related to an ability to prevent uptake of HCO₃ ^- or efficiently sequester it once inside the root which prevents increases in internal pH and transport to the shoots.     

Physiological responses of lupin roots to high pH

High pH seems to be a major constraint limiting the production of narrow-leafed lupin (Lupinus angustifolius L.) on alkaline soils. Whereas there has been much interest in soil acidity, relatively little is known about the effect of high pH on the growth of roots of higher plants.Elongation of roots of L. angustifolius was particularly sensitive to pH6.0 compared with other species. The effect of high pH in decreasing root elongation in L. angustifolius occurred within one hour. It was via an effect on cell elongation and not cell division and the effect was readily reversible. The mechanisms of the adverse effect of high pH are unknown. The permeability ratio of K⁺ to Na⁺ in the plasma-membrane of the root cortical cells was similar in solutions of both low pH and high pH. Reduced cell growth at high pH was not associated with an inefficiency of proton extrusion to the bulk solution by roots of this species. Nevertheless, increasing buffer concentration in the external solution decreased root elongation more in L. angustifolius than in Lupinus pilosus and Pisum sativum.     

Lupinus species differ in their requirements for iron

The effect of iron supply on the growth and nodulation ofLupinus angustifolius L. (Gungurru),Lupinus luteus L. (R-1171) andLupinus pilosus Murr. (P20957) was studied in acid solutions. Plants of the three species were grown together in the same solution and inoculated withBradyrhizobium (Lupinus) WU 425. Plants were then grown with or without applied NH₄NO₃. The lupin species differed greatly in their sensitivity to low iron concentrations in solution withL. pilosus being most tolerant andL. luteus most sensitive.L. pilosus had the highest iron concentration in tissues and had a higher ratio of iron concentration in the youngest fully expanded leaf blades (YEB) to that in roots than the other two species.L. luteus had higher iron concentrations in roots but lower iron concentration in YEB and shoots than didL. angustifolius. The requirements of internal iron for the maximal chlorophyll synthesis in YEB were 65 μg g^-1 forL. angustifolius andL. luteus, and 52 μg g^-1 forL. pilosus. In contrast to effects on growth, the three species had similar external iron requirements for nodule formation in roots and for maximal nitrogen concentrations in shoots. The results indicate that iron tolerant lupin species require lower internal and external iron supply and have a greater ability than sensitive species to translocate iron from roots to shoots.     

Mineral-element composition ofLupinus albus andLupinus angustifolius in relation to manganese accumulation

Summary Lupinus albus andK. angustifolius were grown in the field, harvested prior to leaf fall and the Ca, Mg, Na, K, Fe, Mn, Zn, Cu, P and Si content of the leaflets, petioles, stems, young shoots and inflorescences determined. Element levels inL. albus parts had changed little from those at an earlier harvest, except in the inflorescences in which most element levels had decreased. Both species accumulated manganese;L. angustifolius only in, the leaflets andL. albus in most parts. The leaflets of both were enriched with iron and this with manganese accumulation in a soil of neutral pH is considered to result from reducing conditions in the rhizosphere increasing their availability to the lupins. Silica, present at trace levels, is unlikely to be important for tolerance of lupins to manganese.     

Effects of pH and bicarbonate on the nutrient status and growth of three Lupinus species

Aims

High pH, and high bicarbonate (HCO₃⁻) and calcium (Ca) availability characterise calcareous soils. High [Ca] only partially explains why some Lupinus species are calcifuge, so we explored high [HCO₃⁻] and high pH.

Methods

We grew six Lupinus genotypes in hydroponics with pH 5, 6.5 and 8^a (adjusted by KOH), and 8^b (adjusted by KHCO₃). Leaf symptoms and areas, root appearance and biomass were recorded; whole leaf and root nutrient concentrations, and leaf cellular phosphorus (P), Ca and potassium (K) concentrations were determined using elemental X-ray microanalysis.

Results

Chlorosis was observed in young leaves at high pH for L. angustifolius and L. cosentinii, and P deficiency at high pH for all genotypes. High pH decreased iron (Fe) and zinc (Zn) uptake in all genotypes. It also decreased lateral root growth, the uptake of P, K, Ca, and manganese (Mn) by all sensitive species; and translocation of P, Fe, Zn, Mn, and Ca to leaves in most sensitive species. However, leaf [Ca], leaf [K], [K] within each measured cell type, and translocation of K and Ca to leaves of L. pilosus and L. cosentinii at pH 8 were greater than at pH 5 and 6.5. Compared with pH 8^a, all L. angustifolius genotypes translocated more P, Fe, Zn, Mn and K from roots to leaves at pH 8^b. High pH did not affect the leaf cell types that accumulated P and Ca, but decreased the leaf cellular [P].

Conclusions

Lupinus angustifolius and L. cosentinii were sensitive to high [HCO₃⁻] and/or high pH; L. pilosus was relatively tolerant. High pH decreased lateral root growth and nutrient uptake, inhibiting growth of sensitive species. High [HCO₃⁻] diminished the negative effect of pH 8 on nutrient translocation to leaves in most L. angustifolius genotypes. This knowledge provides critical insights into the habits of Lupinus species to guide breeding of calcicole plants.

     

Sensitivity of different Lupinus species to calcium under a low phosphorus supply

To study mechanism underpinning the calcifuge habit of some Lupinus species, especially under low‐phosphorus (P) conditions, Lupinus species that were likely to respond differently to calcium (Ca) availability were assembled, and the sensitivity to Ca under a low‐P supply was assessed. Seven Lupinus species (9 genotypes, L. albus L. cv Kiev, L. albus L. P26766, L. angustifolius L. cv Mandelup, L. angustifolius L. P26723, L. luteus L. cv Pootalong, L. hispanicus ssp. bicolor Boiss. and Reut. P22999, L. pilosus Murr. P27440, L. cosentinii Guss. P27225, and L. atlanticus Gladst. P27219) were grown hydroponically at 10 or 6000 μM Ca. Leaf symptoms, gas exchange and biomass were recorded; leaf and root nutrient concentrations were analysed, and the leaf cell types in which Ca and P accumulated were determined using elemental X‐ray microanalyses. Calcium toxicity was demonstrated for L. angustifolius P26723, L. hispanicus ssp. bicolor. P22999, and L. cosentinii P27225, whereas the other species were tolerant of a high Ca supply under low‐P conditions. In addition, genotypic differences in Ca toxicity were found within L. angustifolius. Most Ca accumulated in the mesophyll cells in all species, whereas most P was located in epidermal cells.     

Interactions Between Nitrogen and Manganese Nutrition of Barley Genotypes Differing in Manganese Efficiency

Ammonium-fed plants may acidify the rhizosphere and thus increaseavailability of Mn in calcareous alkaline soils. The importanceof N nutrition in the differential expression of tolerance toMn deficiency among cereal genotypes is not yet clear. Two factorialexperiments testing effects of the NH₄-N/NO₃-N ratio and Mnfertilization on growth of barley genotypes differing in toleranceto Mn deficiency were conducted in two calcareous alkaline soilsin pots in a controlled environment. In the soil containing80% CaCO₃at pH 8.5, better root and shoot growth and highershoot Mn concentrations were achieved with nitrate supply, especiallyat lower rates of Mn fertilization. The Mn-efficient genotypeWeeah (tolerant of Mn deficiency) achieved better root and shootgrowth than Mn-inefficient Galleon barley (sensitive to Mn deficiency)regardless of experimental treatment. Fertilization with Mndid not influence total N concentration in barley roots andshoots. In the soil containing 5% CaCO₃at pH 7.8, ammonium-fedplants had better root and shoot growth and, at shoot Mn concentrationsabove the critical level, Mn-inefficient Galleon performed betterthan Mn-efficient Weeah barley. It appears that differentialexpression of Mn efficiency among barley genotypes is not associatedwith differences in Mn availability expected to be producedby differential rhizosphere acidification as a response to differentforms of N supply. There is an apparent preference of locallyselected barley genotypes for nitrate nutrition when grown onthe highly calcareous alkaline soils of southern Australia. Ammonium; calcareous soil; Hordeum vulgare ; manganese; nitrate; nitrogen form; nutrient efficiency; rhizosphere     

Effects of soil temperature on root and shoot growth traits and iron deficiency chlorosis in sorghum genotypes grown on a low iron calcareous soil

Iron deficiency chlorosis (FeDC) is a common disorder for sorghum [Sorghum bicolor (L.) Moench] grown on alkaline calcareous soils. Four sorghum genotypes were grown in growth chambers on a low Fe (1.3 g/g DTPA-extractable), alkaline (pH 8.0), calcareous (3.87% CaCO₃ equivalent) Aridic Haplustoll to determine effects of different soil temperatures (12, 17, 22 and 27°C at a constant 27°C air temperature) on various root and shoot growth traits and development of FeDC. As soil temperature increased, leaf chlorosis became more severe, and shoot and root dry weights, root lengths, and leaf areas increased markedly. Shoot/root ratios, shoot weight/root length, leaf area/shoot weight and leaf area/root weight and root length also increased while root length/root weight decreased as soil temperature increased. Severe FeDC developed in all genotypes even though genotypes had previously shown different degrees of resistance to FeDC. Genotypes differed in most growth traits, especially dry matter yields, root lengths, and leaf areas, but most traits did not appear to be related to genotype resistance to FeDC. The most FeDC resistant genotype had the slowest growth rate and this may be a mechanism for its greater resistance to FeDC.     

Interactions Between High pH and Iron Supply on Nodulation and Iron Nutrition of Lupinus albus L. Genotypes Differing in Sensitivity to Iron Deficiency

Poor growth of white lupin (Lupinus albus L.) on alkaline soils may result from its sensitivity to iron deficiency and poor nodulation. This study examined interactive effects of iron supply and high pH on the growth and nodulation of three genotypes differing in their sensitivity to iron deficiency. Three genotypes (P27486, Ultra and WTD180) were grown for 17 days in buffered solutions with Fe supply of 0.2, 2 and 20 μM. Solution pH was adjusted to 5.2, 6.5 or 7.5. Plant growth, nodulation and nutrient concentrations in plants were measured. Decreasing Fe supply decreased chlorophyll concentration in young leaves by up to 92%. Increasing pH decreased chlorophyll concentration by an average of 40% at pH 6.5 and by 47% at pH 7.5. The decrease of chlorophyll was less obvious in P27485 than in Ultra or WTD180. Shoot biomass was reduced by up to 18% by Fe deficiency, with such decrease being less for P27486. Increasing pH exacerbated the effect of Fe deficiency on shoot biomass only of Ultra. Decreasing Fe supply decreased nodule number by an average of 54%, and increasing pH decreased nodule number by 80%. P27486 formed the greatest number of nodules while WTD180 the least. P27486 had high Fe uptake and low internal requirement. Irrespective of genotype, leaf chlorosis positively correlated with cluster root formation. The results suggest that a combination of Fe deficiency and high pH impaired nodulation in L. albus, and that selection of genotypes for both tolerance of iron deficiency and good nodulation at high pH is important for a successful lupin crop on alkaline soils.     

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.     

Soil aluminium effects on uptake,influx, and transport of nutrients in sorghum genotypes

Sorghum [Sorghum bicolor (L.) Moench] is the fifth most important cereal crop of the world. In South America, it is grown mainly on acid soils, and its production on these soils is limited by deficient levels of available P, Ca, Mg, and micronutrients, and toxic levels of Al and Mn. A greenhouse experiment was undertaken to evaluate the genotypic differences in sorghum for uptake (U), inhibition (IH), influx (IN) into roots, and transport (TR) to shoot for nutrients at three levels of soil Al saturation (2, 41, 64%). Overall shoot nutrient U, IN, and TR showed a significant inverse correlation with soil Al saturation and shoot Al concentration, and a significant positive correlation with shoot and root dry weight. The nutrient uptake parameters differentiated genotypes into most and least efficient categories at various levels of soil Al saturation. The nutrient uptake parameters showed significant differences with respect to soil Al saturation, genotypes, and their interactions. In the current study, Al tolerant genotypes recorded higher IN and TR for P, K, Ca, Mg, Zn, and Fe than Al-sensitive genotypes. Therefore, these U, IN, and TR traits could be used in selection of sorghum plants adaptable to acid soils. Sorghum genotypes used in this study showed intraspecific genetic diversity in U, IN, and TR for essential nutrients. It was concluded that selection of acid soil tolerant genotypes and further breeding of acid (Al) tolerant sorghum cultivars are feasible.IICA/EMBRAPA/World BankIICA/EMBRAPA/World BankIICA/EMBRAPA/World Bank     

Cluster-root formation and carboxylate release in three Lupinus species as dependent on phosphorus supply,internal phosphorus concentration and relative growth rate

Background and Aims

Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status. The cause of variation in cluster-root formation among cluster-root-forming Lupinus species is unknown. The aim of this study was to investigate if cluster-root formation is, in part, dependent on different relative growth rates (RGRs) among Lupinus species when they show similar shoot P status.

Methods

Three cluster-root-forming Lupinus species, L. albus, L. pilosus and L. atlanticus, were grown in washed river sand at 0, 7·5, 15 or 40 mg P kg⁻¹ dry sand. Plants were harvested at 34, 42 or 62 d after sowing, and fresh and dry weight of leaves, stems, cluster roots and non-cluster roots of different ages were measured. The percentage of cluster roots, tissue P concentrations, root exudates and plant RGR were determined.

Key Results

Phosphorus treatments had major effects on cluster-root allocation, with a significant but incomplete suppression in L. albus and L. pilosus when P supply exceeded 15 mg P kg⁻¹ sand. Complete suppression was found in L. atlanticus at the highest P supply; this species never invested more than 20 % of its root weight in cluster roots. For L. pilosus and L. atlanticus, cluster-root formation was decreased at high internal P concentration, irrespective of RGR. For L. albus, there was a trend in the same direction, but this was not significant.

Conclusions

Cluster-root formation in all three Lupinus species was suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among the three species cannot be explained by species-specific variation in RGR or leaf P concentration.     

Effects of solution pH and bicarbonate on the growth and nodulation of a range of grain legume species

Defining plant adaptation to soil conditions is critical for the successful introduction of grain legume species into farming systems. This nutrient solution study examined the effects of pH (4, 5, 6, 7 and 8) and bicarbonate (5 mM KHCO₃) on the growth and nodulation of 14 grain legume species supplied with N or reliant on N₂-fixation. Species includedPisum sativum L.,Cicer arietinum L.,Lens culinaris Med., and a range ofLupinus, Vicia andLathyrus species. Species differed greatly in response to solution pH. For both N-fertilized and N₂-fixing plants, shoot growth ofL. culinaris was very sensitive to low pH (pH < 7), whereas shoot growth ofLupinus angustifolius L. andLupinus albus L. was sensitive to higher pH (pH ≥ 6). Other species had a broader optimal pH range for growth when supplied with N, but were generally sensitive to low pH (pH < 7 forC. arietinum andVicia sativa L., pH < 6 forP. sativum, Vicia faba L.,Lathyrus sativus L. andLathyrus cicera L., and pH < 5 forVicia benghalensis L. andVicia narbonensis L.) when reliant on N₂-fixation. For these other species, symbiotic N₂-fixation appeared to be more sensitive than host plant growth to low pH. This finding was supported by lower nodule numbers and mass, and lower N concentrations in shoots of sensitive species at low pH relative to higher pH. ForL. culinaris, nodule numbers and mass were relatively unaffected by pH 5–8, N concentrations in shoots were high at low pH and plants developed symptoms relating to H⁺ toxicity at pH as high as 7. These results indicate that host plant growth ofL. culinaris is more sensitive to low pH than theRhizobium symbiosis. ForL. albus andL. angustifolius, both host plant growth and symbiotic N₂-fixation appeared to be equally sensitive to pH ≥ 6.Lupinus pilosus Murr. was more tolerant of high pH than the otherLupinus species. At pH 4, two genotypes ofC. arietinum had better early nodulation than other species.Vicia ervilia L. nodulated poorly at all levels of solution pH, indicating that the commercial Group E inoculum (Rhizobium leguminosurum bv.viceae SU303) may not be effective for this species in solution culture. Addition of bicarbonate decreased shoot growth, nodulation and N concentrations in shoots of most species. Early nodulation (nodule number) ofLathyrus ochrus (L.) DC was not affected by the bicarbonate treatment.     

Assessing variability in root traits of wild Lupinus angustifolius germplasm: basis for modelling root system structure

Background and aims

Intra-specific variation in root system architecture and consequent efficiency of resource capture by major crops has received recent attention. The aim of this study was to assess variability in a number of root traits among wild genotypes of narrow-leafed lupin (Lupinus angustifolius L.), to provide a basis for modelling of root structure.

Methods

A subset of 111 genotypes of L. angustifolius was selected from a large germplasm pool based on similarity matrices calculated using Diversity Array Technology markers. Plants were grown for 6?weeks in the established semi-hydroponic phenotyping systems to measure the fine-scale features of the root systems.

Results

Root morphology of wild L. angustifolius was primarily dominated by the taproot and first-order branches, with the presence of densely or sparsely distributed second-order branches in the late growth stage. Large variation in most root traits was identified among the tested genotypes. Total root length, branch length and branch number in the entire root system and in the upper roots were the most varied traits (coefficient of variation CV >0.50). Over 94% of the root system architectural variation determined from the principal components analysis was captured by six components (eigenvalue >1). Five relatively homogeneous groups of genotypes with distinguished patterns of root architecture were separated by k-means clustering analysis.

Conclusions

Variability in the fine-scale features of root systems such as branching behaviour and taproot growth rates provides a basis for modelling root system structure, which is a promising path for selecting desirable root traits in breeding and domestication of wild and exotic resources of L. angustifolius for stressful or poor soil environments.     

Stress-induced changes in the root architecture of white lupin (Lupinus albus) in response to pH, bicarbonate, and calcium in liquid culture

Current agronomic cultivars of white lupin (Lupinus albus) are intolerant of calcareous or limed soils. In these soils, high pH, bicarbonate (HCO₃^?), and calcium (Ca) concentrations are the major chemical stresses to the root system. To determine the responses of the root system to these factors, evaluate root architecture, and compare genotypes for tolerance, a series of liquid culture experiments was completed using root chambers that allowed the study of the root system in two dimensions. Each stress condition caused changes in different parts of the root system and there was no generalised stress response. HCO₃^? (5 mM) had the greatest effect on cultivars intolerant of calcareous soil; it decreased the dry weight of the shoot and caused the highest percentage of tap root deaths. HCO₃^? also discriminated between short (determinate) and long (indeterminate) roots, as it decreased the number and density of the determinate roots only. Calcium (3 mM) affected all parts of the root system. The tap root was shortened and showed an increased tortuousness in its path compared with 1 mM Ca, although no plants suffered tap root death. The numbers and densities of the two lateral root forms were also decreased, as were the lengths of the indeterminate roots. Stress from alkaline pH (7.5) media caused a lower number and density of determinate lateral roots to be produced than at pH 6.5. The experiments demonstrated that each culture condition elicited a definable stress response. Stress conditions altered the root architecture of genotypes reported to be tolerant of calcareous soil less than in intolerant genotypes. Although soil is more complex than liquid culture, it is possible that in a calcareous or limed soil each stress condition examined may affect the overall stress of the plant, and increased tolerance may result from tolerance to a single stress.     

LaALMT1 mediates malate release from phosphorus-deficient white lupin root tips and metal root to shoot translocation

Under phosphorus (P) deficiency, Lupinus albus (white lupin) releases large amounts of organic acid anions from specialized root structures, so-called cluster or proteoid roots, to mobilize and acquire sparingly soluble phosphates from a restricted soil volume. The molecular mechanisms underlying this release and its regulation are, however, poorly understood. Here, we identified a gene belonging to the aluminium (Al)-activated malate transporter (ALMT) family that specifically contributes to malate, but not citrate release. This gene, LaALMT1, was most prominently expressed in the root apices under P deficiency, including those of cluster roots and was also detected in the root stele. Contrary to several ALMT homologs in other species, the expression was not stimulated, but moderately repressed by Al. Aluminium-independent malate currents were recorded from the plasma membrane localized LaALMT1 expressed in Xenopus oocytes. In composite lupins with transgenic roots, LaALMT1 was efficiently mutated by CRISPR-Cas9, leading to diminished malate efflux and lower xylem sap malate concentrations. When grown in an alkaline P-deficient soil, mutant shoot phosphate concentrations were similar, but iron and potassium concentrations were diminished in old leaves, suggesting a role for ALMT1 in metal root to shoot translocation, a function that was also supported by growth in hydroponics.     

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.