Secretion of malate and citrate from roots is related to high Al-resistance in Lespedeza bicolor

Lespedeza bicolor (Lespedeza bicolor Turcz. cv. Jiangxi) is a leguminous shrub that is well adapted to acid infertile soils. However, the mechanisms of aluminum resistance in this species have not been established. This study aimed to assess the possible resistance mechanisms of this plant to Al. An Al-sensitive species of Lespedeza, sericea lespedeza [Lespedeza cuneata (Dum.-Cours.) G. Don cv. Zhejiang], was used as a reference. The roots of L. bicolor secreted both malate and citrate after exposure to Al, but roots of L. cuneata did not. The secretion of organic acids from L. bicolor was specific to Al; neither 15-day P starvation nor 50 μM lanthanum induced the secretion of these organic acid anions. Secretion of organic acid anions in L. bicolor was detected after 3–6 h exposure to Al, and the amount increased significantly after 6 h exposure, suggesting that this plant shows a pattern II-type organic acid secretion. This is supported by the finding that the secretion was significantly inhibited by a protein-synthesis inhibitor, cycloheximide. Two kinds of anion-channel inhibitors had different effects on Al-induced secretion of organic acids: 9-anthracene carboxylic acid completely inhibited secretion, phenylglyoxal had no effect. Root elongation in L. bicolor was more severely inhibited by Al in the presence of 9-anthracene carboxylic acid. All these results indicated that the secretion of malate and citrate is a specialized response to Al stress in L. bicolor roots, which might be one of the Al-resistance mechanisms in this species.     

Invasive Lespedeza cuneata and native Lespedeza virginica experience asymmetrical benefits from rhizobial symbionts

Background and Aims

Lespedeza cuneata (Dum. Cours.) G. Don is an invasive legume that displaces populations of native N. American congeners. Our aims are to determine the growth benefits of different rhizobacterial strains for L. cuneata and native Lespedeza virginica (L.) Britton, and to determine if these strains influence competition between these plants.

Methods

Plants were grown under nitrogen-limiting conditions in sterilized soil in pairs consisting of two L. cuneata, two L. virginica, or one of each species, and then plants were inoculated with one of seven rhizobial isolates, or with a no-strain control. After 3 months, plants were harvested for determination of biomass and nodulation rate.

Results

Five of the assayed stains improved L. cuneata biomass over uninoculated controls, but none of the strains benefited L. virginica. L. cuneata plants had more biomass and root nodules when grown in competition with L. virginica than with a conspecific.

Conclusions

Asymmetrical benefits from these symbionts accrued to invasive L. cuneata but not to native L. virginica, and this may provide the invader with a growth advantage in the field. Changes in the availability of effective symbionts in the soils of invaded sites can shape performance of native and invasive plants.     

LC–MS-based chemotaxonomic classification of wild-type Lespedeza sp. and its correlation with genotype

In this study, 39 specimens belonging to Lespedeza species (Lespedeza cyrtobotrya, L. bicolor, L. maximowiczii, and Lespedeza cuneata) (Leguminosae) were classified phenotypically and genotypically. We constructed a phylogenetic tree based on the combined nrDNA (internal transcribed spacer; ITS) and cpDNA (trnL-trnF) sequences with the aim of classifying the genotypes. Samples were mainly divided into three genotypes. Samples of L. cyrtobotrya and L. bicolor were mixed in a single branch, whereas samples of L. maximowiczii and L. cuneata were clustered within species, respectively. We performed a liquid chromatography–electrospray ionization–mass spectrometry-based metabolite profiling analysis to classify the phenotypes. Multivariate statistical analyses such as principal component analysis (PCA) and hierarchical clustering analysis (HCA) were used for the clustering pattern analysis and distance analysis between species, respectively. According to the PCA and HCA results, leaves were classified into four phenotypes according to species. In both the genetic and chemotaxonomic classification methods, the distance between L. cyrtobotrya and L. bicolor was the closest between species, and L. cuneata was the farthest away from the other three species. Additionally, orthogonal partial least squares-discriminant analysis was employed to identify significantly different phytochemicals between species. We classified L. cyrtobotrya and L. bicolor by identifying significantly different phytochemicals. Interestingly, leaves and stems showed different phenotypic classifications based on the chemotaxonomic classification. Stem samples of the other three species were mixed regardless of species, whereas L. cyrtobotrya stem samples were clustered within species. The phenotypic classification of leaves coincided more with the genotypic classification than that of stems. Key message We classified four wild-type Lespedeza sp. by analyzing the combined nrDNA (ITS) and cpDNA (trnL-trnF) sequences. We also classified leaves and stems of Lespedeza sp. by applying liquid chromatography–mass spectroscopy-based metabolite profiling.     

Genotypic differences in Al resistance and the role of cell-wall pectin in Al exclusion from the root apex in Fagopyrum tataricum

Background and Aims

Aluminium (Al) toxicity is one of the factors limiting crop production on acid soils. However, genotypic differences exist among plant species or cultivars in response to Al toxicity. This study aims to investigate genotypic differences among eight cultivars of tatary buckwheat (Fagopyrum tataricum) for Al resistance and explore the possible mechanisms of Al resistance.

Methods

Al resistance was evaluated based on relative root elongation (root elongation with Al/root elongation without Al). Root apex Al content, pectin content and exudation of root organic acids were determined and compared.

Key Results

Genotypic differences among the eight cultivars were correlated with exclusion of Al from the root apex. However, there was a lack of correlation between Al exclusion and Al-induced oxalate secretion. Interestingly, cell-wall pectin content of the root apex was generally lower in Al-resistant cultivars than in Al-sensitive cultivars. Although we were unable to establish a significant correlation between Al exclusion and pectin content among the eight cultivars, a strong correlation could be established among six cultivars, in which the pectin content in the most Al-resistant cultivar ‘Chuan’ was significantly lower than that in the most Al-sensitive cultivar ‘Liuku2’. Furthermore, root apex cell-wall pectin methylesterase activity (PME) was similar in ‘Chuan’ and ‘Liuku2’ in the absence of Al, but Al treatment resulted in increased PME activity in ‘Liuku2’ compared with ‘Chuan’. Immunolocalization of pectins also showed that the two cultivars had similar amounts of either low-methyl-ester pectins or high-methyl-ester pectins in the absence of Al, but Al treatment resulted in a more significant increase of low-methyl-ester pectins and decrease of high-methyl-ester pectins in ‘Liuku2’.

Conclusions

Cell-wall pectin content may contribute, at least in part, to differential Al resistance among tatary buckwheat cultivars.     

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.     

Competition overwhelms the positive plant–soil feedback generated by an invasive plant

Invasive plant species can modify soils in a way that benefits their fitness more than the fitness of native species. However, it is unclear how competition among plant species alters the strength and direction of plant–soil feedbacks. We tested how community context altered plant–soil feedback between the non-native invasive forb Lespedeza cuneata and nine co-occurring native prairie species. In a series of greenhouse experiments, we grew plants individually and in communities with soils that differed in soil origin (invaded or uninvaded by L. cuneata) and in soils that were live vs. sterilized. In the absence of competition, L. cuneata produced over 60% more biomass in invaded than uninvaded soils, while native species performance was unaffected. The absence of a soil origin effect in sterile soil suggests that the positive plant–soil feedback was caused by differences in the soil biota. However, in the presence of competition, the positive effect of soil origin on L. cuneata growth disappeared. These results suggest that L. cuneata may benefit from positive plant–soil feedback when establishing populations in disturbed landscapes with few interspecific competitors, but does not support the hypothesis that plant–soil feedbacks influence competitive outcomes between L. cuneata and native plant species. These results highlight the importance of considering whether competition influences the outcome of interactions between plants and soils.     

Effects of phosphorus supply on growth, phosphate concentration and cluster-root formation in three Lupinus species

Background and Aims

In some lupin species, phosphate deficiency induces cluster-root formation, which enhances P uptake by increasing root surface area and, more importantly, the release of root exudates which enhances P availability.

Methods

Three species of Lupinus, L. albus, L. atlanticus and L. micranthus, with inherently different relative growth rates were cultivated under hydroponics in a greenhouse at four phosphate concentrations (1, 10, 50 and 150 µm) to compare the role of internal P in regulating cluster-root formation.

Key Results

The highest growth rate was observed in L. atlanticus, followed by L. albus and L. micranthus. At 1 µm P, cluster-root formation was markedly induced in all three species. The highest P uptake and accumulation was observed in L. micranthus, followed by L. atlanticus and then L. albus. Inhibition of cluster-root formation was severe at 10 µm P in L. atlanticus, but occurred stepwise with increasing P concentration in the root medium in L. albus.

Conclusions

In L. atlanticus and L. albus cluster-root formation was suppressed by P treatments above 10 µm, indicating a P-inducible regulating system for cluster-root formation, as expected. By contrast, production of cluster roots in L. micranthus, in spite of a high internal P concentration, indicated a lower sensitivity to P status, which allowed P-toxicity symptoms to develop.     

Characterization of Transposable Elements in the Ectomycorrhizal Fungus Laccaria bicolor

Background

The publicly available Laccaria bicolor genome sequence has provided a considerable genomic resource allowing systematic identification of transposable elements (TEs) in this symbiotic ectomycorrhizal fungus. Using a TE-specific annotation pipeline we have characterized and analyzed TEs in the L. bicolor S238N-H82 genome.

Methodology/Principal Findings

TEs occupy 24% of the 60 Mb L. bicolor genome and represent 25,787 full-length and partial copy elements distributed within 171 families. The most abundant elements were the Copia-like. TEs are not randomly distributed across the genome, but are tightly nested or clustered. The majority of TEs exhibits signs of ancient transposition except some intact copies of terminal inverted repeats (TIRS), long terminal repeats (LTRs) and a large retrotransposon derivative (LARD) element. There were three main periods of TE expansion in L. bicolor: the first from 57 to 10 Mya, the second from 5 to 1 Mya and the most recent from 0.5 Mya ago until now. LTR retrotransposons are closely related to retrotransposons found in another basidiomycete, Coprinopsis cinerea.

Conclusions

This analysis 1) represents an initial characterization of TEs in the L. bicolor genome, 2) contributes to improve genome annotation and a greater understanding of the role TEs played in genome organization and evolution and 3) provides a valuable resource for future research on the genome evolution within the Laccaria genus.     

Phylogenetic analysis of the genus Sorghum based on combined sequence data from cpDNA regions and ITS generate well-supported trees with two major lineages

Background and Aims

Wild Sorghum species provide novel traits for both biotic and abiotic stress resistance and yield for the improvement of cultivated sorghum. A better understanding of the phylogeny in the genus Sorghum will enhance use of the valuable agronomic traits found in wild sorghum.

Methods

Four regions of chloroplast DNA (cpDNA; psbZ-trnG, trnY-trnD, trnY-psbM and trnT-trnL) and the internal transcribed spacer (ITS) of nuclear ribosomal DNA were used to analyse the phylogeny of sorghum based on maximum-parsimony analyses.

Key Results

Parsimony analyses of the ITS and cpDNA regions as separate or combined sequence datasets formed trees with strong bootstrap support with two lineages: the Eu-sorghum species S. laxiflorum and S. macrospermum in one and Stiposorghum and Para-sorghum in the other. Within Eu-sorghum, S. bicolor-3, -11 and -14 originating from southern Africa form a distinct clade. S. bicolor-2, originally from Yemen, is distantly related to other S. bicolor accessions.

Conclusions

Eu-sorghum species are more closely related to S. macrospermum and S. laxiflorum than to any other Australian wild Sorghum species. S. macrospermum and S. laxiflorum are so closely related that it is inappropriate to classify them in separate sections. S. almum is closely associated with S. bicolor, suggesting that the latter is the maternal parent of the former given that cpDNA is maternally inherited in angiosperms. S. bicolor-3, -11 and -14, from southern Africa, are closely related to each other but distantly related to S. bicolor-2.     

Volatile compounds from beneficial or pathogenic bacteria differentially regulate root exudation,transcription of iron transporters,and defense signaling pathways in <Emphasis Type="Italic">Sorghum bicolor</Emphasis>

Key message

Our results show that Sorghum bicolor is able to recognize bacteria through its volatile compounds and differentially respond to beneficial or pathogens via eliciting nutritional or defense adaptive traits.

Abstract

Plants establish beneficial, harmful, or neutral relationships with bacteria. Plant growth promoting rhizobacteria (PGPR) emit volatile compounds (VCs), which may act as molecular cues influencing plant development, nutrition, and/or defense. In this study, we compared the effects of VCs produced by bacteria with different lifestyles, including Arthrobacter agilis UMCV2, Bacillus methylotrophicus M4-96, Sinorhizobium meliloti 1021, the plant pathogen Pseudomonas aeruginosa PAO1, and the commensal rhizobacterium Bacillus sp. L2-64, on S. bicolor. We show that VCs from all tested bacteria, except Bacillus sp. L2-64, increased biomass and chlorophyll content, and improved root architecture, but notheworthy A. agilis induced the release of attractant molecules, whereas P. aeruginosa activated the exudation of growth inhibitory compounds by roots. An analysis of the expression of iron-transporters SbIRT1, SbIRT2, SbYS1, and SbYS2 and genes related to plant defense pathways COI1 and PR-1 indicated that beneficial, pathogenic, and commensal bacteria could up-regulate iron transporters, whereas only beneficial and pathogenic species could induce a defense response. These results show how S. bicolor could recognize bacteria through their volatiles profiles and highlight that PGPR or pathogens can elicit nutritional or defensive traits in plants.

     

Root cortical aerenchyma inhibits radial nutrient transport in maize (Zea mays)

Background and Aims

Formation of root cortical aerenchyma (RCA) can be induced by nutrient deficiency. In species adapted to aerobic soil conditions, this response is adaptive by reducing root maintenance requirements, thereby permitting greater soil exploration. One trade-off of RCA formation may be reduced radial transport of nutrients due to reduction in living cortical tissue. To test this hypothesis, radial nutrient transport in intact roots of maize (Zea mays) was investigated in two radiolabelling experiments employing genotypes with contrasting RCA.

Methods

In the first experiment, time-course dynamics of phosphate loading into the xylem were measured from excised nodal roots that varied in RCA formation. In the second experiment, uptake of phosphate, calcium and sulphate was measured in seminal roots of intact young plants in which variation in RCA was induced by treatments altering ethylene action or genetic differences.

Key Results

In each of three paired genotype comparisons, the rate of phosphate exudation of high-RCA genotypes was significantly less than that of low-RCA genotypes. In the second experiment, radial nutrient transport of phosphate and calcium was negatively correlated with the extent of RCA for some genotypes.

Conclusions

The results support the hypothesis that RCA can reduce radial transport of some nutrients in some genotypes, which could be an important trade-off of this trait.     

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.     

Adaptive shoot and root responses collectively enhance growth at optimum temperature and limited phosphorus supply of three herbaceous legume species

Background and Aims

Studies on the effects of sub- and/or supraoptimal temperatures on growth and phosphorus (P) nutrition of perennial herbaceous species at growth-limiting P availability are few, and the impacts of temperature on rhizosphere carboxylate dynamics are not known for any species.

Methods

The effect of three day/night temperature regimes (low, 20/13 °C; medium, 27/20 °C; and high, 32/25 °C) on growth and P nutrition of Cullen cinereum, Kennedia nigricans and Lotus australis was determined.

Key Results

The highest temperature was optimal for growth of C. cinereum, while the lowest temperature was optimal for K. nigricans and L. australis. At optimum temperatures, the relative growth rate (RGR), root length, root length per leaf area, total P content, P productivity and water-use efficiency were higher for all species, and rhizosphere carboxylate content was higher for K. nigricans and L. australis. Cullen cinereum, with a slower RGR, had long (higher root length per leaf area) and thin roots to enhance P uptake by exploring a greater volume of soil at its optimum temperature, while K. nigricans and L. australis, with faster RGRs, had only long roots (higher root length per leaf area) as a morphological adaptation, but had a higher content of carboxylates in their rhizospheres at the optimum temperature. Irrespective of the species, the amount of P taken up by a plant was mainly determined by root length, rather than by P uptake rate per unit root surface area. Phosphorus productivity was correlated with RGR and plant biomass.

Conclusions

All three species exhibited adaptive shoot and root traits to enhance growth at their optimum temperatures at growth-limiting P supply. The species with a slower RGR (i.e. C. cinereum) showed only morphological root adaptations, while K. nigricans and L. australis, with faster RGRs, had both morphological and physiological (i.e. root carboxylate dynamics) root adaptations.     

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.     

Localized application of soil organic matter shifts distribution of cluster roots of white lupin in the soil profile due to localized release of phosphorus

Background and Aims

Phosphorus (P) is a major factor controlling cluster-root formation. Cluster-root proliferation tends to concentrate in organic matter (OM)-rich surface-soil layers, but the nature of this response of cluster-root formation to OM is not clear. Cluster-root proliferation in response to localized application of OM was characterized in Lupinus albus (white lupin) grown in stratified soil columns to test if the stimulating effect of OM on cluster-root formation was due to (a) P release from breakdown of OM; (b) a decrease in soil density; or (c) effects of micro-organisms other than releasing P from OM.

Methods

Lupin plants were grown in three-layer stratified soil columns where P was applied at 0 or 330 mg P kg⁻¹ to create a P-deficient or P-sufficient background, and OM, phytate mixed with OM, or perlite was applied to the top or middle layers with or without sterilization.

Key Results

Non-sterile OM stimulated cluster-root proliferation and root length, and this effect became greater when phytate was supplied in the presence of OM. Both sterile OM and perlite significantly decreased cluster-root formation in the localized layers. The OM position did not change the proportion of total cluster roots to total roots in dry biomass among no-P treatments, but more cluster roots were concentrated in the OM layers with a decreased proportion in other places.

Conclusions

Localized application of non-sterile OM or phytate plus OM stimulated cluster-root proliferation of L. albus in the localized layers. This effect is predominantly accounted for by P release from breakdown of OM or phytate, but not due to a change in soil density associated with OM. No evidence was found for effects of micro-organisms in OM other than those responsible for P release.     

Aluminium tolerance and high phosphorus efficiency helps Stylosanthes better adapt to low-P acid soils

Backgrond and Aims

Stylosanthes spp. (stylo) is one of the most important pasture legumes used in a wide range of agricultural systems on acid soils, where aluminium (Al) toxicity and phosphorus (P) deficiency are two major limiting factors for plant growth. However, physiological mechanisms of stylo adaptation to acid soils are not understood.

Methods

Twelve stylo genotypes were surveyed under field conditions, followed by sand and nutrient solution culture experiments to investigate possible physiological mechanisms of stylo adaptation to low-P acid soils.

Key Results

Stylo genotypes varied substantially in growth and P uptake in low P conditions in the field. Three genotypes contrasting in P efficiency were selected for experiments in nutrient solution and sand culture to examine their Al tolerance and ability to utilize different P sources, including Ca-P, K-P, Al-P, Fe-P and phytate-P. Among the three tested genotypes, the P-efficient genotype ‘TPRC2001-1’ had higher Al tolerance than the P-inefficient genotype ‘Fine-stem’ as indicated by relative tap root length and haematoxylin staining. The three genotypes differed in their ability to utilize different P sources. The P-efficient genotype, ‘TPRC2001-1’, had superior ability to utilize phytate-P.

Conclusions

The findings suggest that possible physiological mechanisms of stylo adaptation to low-P acid soils might involve superior ability of plant roots to tolerate Al toxicity and to utilize organic P and Al-P.Key words: Stylosanthes, phosphorus, P efficiency, organic P, Al toxicity, acid soil     

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

Aims

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

Methods

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

Results

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

Conclusions

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

Host range and selectivity of the hemiparasitic plant Thesium chinense (Santalaceae)

Background and Aims

Thesium chinense is a hemiparasitic plant that is common in grassland habitats of eastern Asia. Although the physiology of Thesium has been well studied in attempts to control its weedy habit, there have been few ecological investigations of its parasitic life history. Thesium chinense is thought to parasitize species of Poaceae, but evidence remains circumstantial.

Methods

A vegetation survey was conducted to test whether any plant species occurs significantly more often in plots with T. chinense than expected. In addition, haustorial connections were examined directly by excavating the roots and post-attachment host selectivity was evaluated by comparing the observed numbers of haustoria on different hosts against those expected according to the relative below-ground biomass. Haustorium sizes were also compared among host species.

Key Results

Only two of the 38 species recorded, Lespedeza juncea and Eragrostis curvula, occurred more often in plots with Thesium than expected. In contrast to this, T. chinense parasitized 22 plant species in 11 families, corresponding to 57·9 % of plant species found at the study site. Haustoria were non-randomly distributed among host species, suggesting that there is some post-attachment host selectivity. Thesium chinense generally preferred the Poaceae, although haustoria formed on the Fabaceae were larger than those on other hosts.

Conclusions

This is the first quantitative investigation of the host range and selectivity of hemiparasitic plants of the Santalales. The preference for Fabaceae as hosts may be linked to the greater nutrient availability in these nitrogen-fixing plants.Key words: Haustorium, hemiparasite, host range, host selectivity, Santalaceae, Thesium chinense