Carboxylate composition of root exudates does not relate consistently to a crop species' ability to use phosphorus from aluminium, iron or calcium phosphate sources

* The relationship between carboxylate release from roots and the ability of the species to utilize phosphorus from sparingly soluble forms was studied by comparing Triticum aestivum, Brassica napus, Cicer arietinum, Pisum sativum, Lupinus albus, Lupinus angustifolius and Lupinus cosentinii. * Plants were grown in sand and supplied with 40 mg P kg(-1) in the sparingly soluble forms AlPO(4), FePO(4) or Ca(5)OH(PO(4))(3), or as soluble KH(2)PO(4); control plants received no P. * The ability to utilize sparingly soluble forms of P differed between forms of P supplied and species. Pisum sativum and C. arietinum did not access AlPO(4) or FePO(4) despite releasing carboxylates into the rhizosphere. * Species accessed different forms of sparingly soluble P, but no species was superior in accessing all forms. We conclude that a single trait cannot explain access to different forms of sparingly soluble P, and hypothesize that in addition to carboxylates, rhizosphere pH and root morphology are key factors.     

Maximum axial growth pressures of the lateral roots of pea and eucalypt

Although lateral roots may contribute significantly towards growth and nourishment of plants, the mechanics of their elongation behaviour in strong soils is not well known. The aim of this study is to report maximum axial growth pressures (p) and maximum elongation rates (E) of the lateral roots of an annual herbaceous plant (pea) and a woody perennial (eucalypt). As such measurements have not been reported previously, measurements of P and E for lateral roots were compared with the primary roots of pea for which reports are widespread. Values of P were estimated from the measured maximum values of axial force and root diameter on single, intact roots of seedlings in the laboratory. Additional measurements of both P and E were made for the lateral roots of pea when the growth of the remaining root axes was stopped (with removal of tips) to determine the overall effects of root-growth-inhibition on P and E of single roots.Values of P and E for lateral roots of pea were significantly greater than those for the lateral roots of eucalypt. Although root diameter for the primary roots of pea were similar to those for the lateral roots of eucalypt, the former exerted nearly twice as much pressure as the latter. The lateral roots of pea elongated significantly slower than the primary roots; however, P of lateral roots was significantly lower than the primary roots when elongation of all other roots was inhibited during the measurements. Production and/or development of lateral roots increased when elongation of the remaining roots (both primary and lateral roots) of pea seedlings was restricted due to the removal of root tips and exposure of one of the lateral roots to high strength. In general, maximum axial force exerted by primary and lateral roots was similar for roots of <1 mm diameter. However, primary roots exerted greater maximum axial force than the lateral roots when root diameter was >1 mm. As axial pressure of lateral roots was independent of root diameter, thickening of root tips is less likely to assist penetration of lateral roots in strong soils.     

Aluminum exclusion from root zone and maintenance of nutrient uptake are principal mechanisms of Al tolerance in <Emphasis Type="Italic">Pisum sativum</Emphasis> L.

Our study aimed to evaluate intraspecific variability of pea (Pisum sativum L.) in Al tolerance and to reveal mechanisms underlying genotypic differences in this trait. At the first stage, 106 pea genotypes were screened for Al tolerance using root re-elongation assay based on staining with eriochrome cyanine R. The root re-elongation zone varied from 0.5 mm to 14 mm and relationships between Al tolerance and provenance or phenotypic traits of genotypes were found. Tolerance index (TI), calculated as a biomass ratio of Al-treated and non-treated contrasting genotypes grown in hydroponics for 10 days, varied from 30% to 92% for roots and from 38% to 90% for shoots. TI did not correlate with root or shoot Al content, but correlated positively with increasing pH and negatively with residual Al concentration in nutrient solution in the end of experiments. Root exudation of organic acid anions (mostly acetate, citrate, lactate, pyroglutamate, pyruvate and succinate) significantly increased in several Al-treated genotypes, but did not correlate with TI. Al-treatment decreased Ca, Co, Cu, K, Mg, Mn, Mo, Ni, S and Zn contents in roots and/or shoots, whereas contents of several elements (P, B, Fe and Mo in roots and B and Fe in shoots) increased, suggesting that Al toxicity induced substantial disturbances in uptake and translocation of nutrients. Nutritional disturbances were more pronounced in Al sensitive genotypes. In conclusion, pea has a high intraspecific variability in Al tolerance and this trait is associated with provenance and phenotypic properties of plants. Transformation of Al to unavailable (insoluble) forms in the root zone and the ability to maintain nutrient uptake are considered to be important mechanisms of Al tolerance in this plant species.     

类芦根际溶磷真菌的筛选、鉴定及其溶磷能力分析

为揭示类芦(Neyraudia reynaudiana)等水土保持植物的耐低磷机制,开发溶磷菌种质资源,提高赤红壤磷素利用率,从类芦根际土壤中筛选到一株溶磷能力较强的真菌FP1,经形态学和ITS序列分析,鉴定为黑曲霉(Aspergillus niger)。3种难溶性磷酸盐液体培养基接种菌株FP1后,其pH值和溶磷量的动态变化显示,培养液的pH值均呈显著下降趋势。溶磷量与培养时间有关,除磷酸三钙外,菌株FP1对其他难溶性磷酸盐的溶磷趋势均为先上升再下降并趋于稳定。菌株FP1对不同磷源的最大溶磷率顺序为:磷酸铝(92.02%)磷酸三钙(41.62%)3种磷酸盐的混合物(35.86%)磷酸铁(19.20%)。FP1对磷酸铝和磷酸铁都具有较强的溶磷能力,表明抗逆性强的水土保持植物类芦根际土壤蕴藏着高效的溶磷微生物资源。     

Influence of an arbuscular mycorrhizal fungus on the interaction of a binucleate Rhizoctonia species with Myc+ and Myc- pea roots

A binucleate Rhizoctonia (BNR) species was isolated from a clay loam soil on the Epoisses experimental station of INRA, Dijon and identified as belonging to the anastomosis Group A (AG-A). The BNR was inoculated to a Myc- Pisum sativum mutant (P53, sym30 locus) and its wild-type parent (cv Frisson) in the presence or absence of the arbuscular mycorrhizal fungus Glomus mosseae. The BNR had no significant effect on plant weight. Myc+ and Myc- roots were equally susceptible towards BNR and showed no localized cellular defense responses. The presence of BNR decreased significantly the percentage of root length colonized by G. mosseae and, inversely, G. mosseae reduced the number of BNR monilioid chains formed in root epidermal cells of the two pea genotypes. The pisatin concentration was increased significantly by BNR in both Myc+ and Myc- roots and by G. mosseae in the wild-type pea plants. The highest accumulation of pisatin was observed in Myc+ roots when both fungi were present.     

Solubilization of insoluble inorganic phosphates by a novel salt- and pH-tolerant Pantoea agglomerans R-42 isolated from soybean rhizosphere

To develop environment-friendly biofertilizer solubilizing insoluble phosphates, salt- and pH-tolerant, insoluble inorganic phosphate-solubilizing bacterium was isolated from soybean rhizosphere. On the basis of its physiological characteristics and Vitek analysis, this bacterium was identified as Pantoea agglomerans. The optimal medium composition and cultural conditions for the solubilization of insoluble phosphate by P. agglomerans R-42 were 3% (w/v) of glucose, 0.1% (w/v) of NH4NO3, 0.02% (w/v) of MgSO4 x 7H2O, and 0.06% (w/v) of CaCl2 x 2H2O along with initial pH 7.5 at 30 degree C. The soluble phosphate production under optimal condition was around 900 mg/l, which was approximately 4.6-fold higher than the yield in the MPVK medium. The solubilization of insoluble phosphate was associated with a drop in the pH of the culture medium. P. agglomerans R-42 showed resistance against different environmental stresses like 5-45 degrees C temperature, 1-5% salt concentration and 3-11 pH range. Insoluble phosphate solubilization was highest from CaHPO4 (1367 mg/l), hydroxyapatite (1357 mg/l) and Ca3(PO4)2 (1312 mg/l). However, the strain produced soluble phosphate to the culture broth with the concentrations of 28 mg/l against FePO4, and 19 mg/l against AlPO4, respectively.     

Effect of phosphorus deficiency on growth angle of basal roots in Phaseolus vulgaris

Root architectural plasticity might be an important factor in the acquisition by plants of immobile nutrients such as phosphorus (P). In this study, we examined the effect of P availability on the orientation of basal roots with respect to gravity, and thereby on the growth angle of these roots of common bean (Phaseolus vulgaris L.). In one set of studies the growth angle of basal roots of bean seedlings was measured over time. Sixteen bean genotypes were examined; six showed a decrease in root orientation with respect to gravity in low P media, one increased orientation, and nine showed no difference within 5 d of basal root emergence. Bean taproots also showed decreased root orientation with respect to gravity in low P. Growth angle after 5 d was correlated with field performance of contrasting genotypes in low P tropical soils. Mineral deficiencies other than P did not cause changes in root angle. In a split pouch system that provided high or low P solution to different parts of the root system, the decrease in root angle in low P was found to be a response to global P availability, and not local to the portion of the root system in low P. Effects of P availability on root angle were associated with reduced shoot P concentration, but preceded effects on plant biomass accumulation and leaf area expansion. Results from growth pouches for genotype G 19833 were confirmed using a solid-phase buffered sand-culture system supplying P at three levels. Pea (Pisum sativum), soybean (Glycine max Williams), chickpea (Cicer arietinum), lima bean (Phaseolus lunatus), and lentil (Lens culinaris) were grown with and without P; soybean and pea also showed decreased basal root angles in low P.     

Cadmium accumulation and buffering of cadmium-induced stress by arbuscular mycorrhiza in three Pisum sativum L. genotypes

The role of arbuscular mycorrhiza in reducing Cd stress was investigated in three genotypes of Pisum sativum L. (cv. Frisson, VIR4788, VIR7128), grown in soil/sand pot cultures in the presence and absence of 2-3 mg kg(-1) bioavailable Cd, and inoculated or not with the arbuscular mycorrhizal fungus Glomus intraradices. Shoot, root and pod biomass were decreased by Cd in non-mycorrhizal plants. The presence of mycorrhiza attenuated the negative effect of Cd so that shoot biomass and activity of photosystem II, based on chlorophyll a fluorescence, were not significantly different between mycorrhizal plants growing in the presence or absence of the heavy metal (HM). Total P concentrations were not significantly different between mycorrhizal and non-mycorrhizal plants treated with Cd. From 20-50-fold more Cd accumulated in roots than in shoots of Cd-treated plants, and overall levels were comparable to other metal-accumulating plants. Genetic variability in Cd accumulation existed between the pea genotypes. Concentration of the HM was lowest in roots of VIR4788 and in pods of VIR4788 and VIR7128. G. intraradices inoculation decreased Cd accumulation in roots and pods of cv. Frisson, whilst high concentrations were maintained in roots and pods of mycorrhizal VIR7128. Shoot concentrations of Cd increased in mycorrhizal cv. Frisson and VIR4788. Sequestration of Cd in root cell walls and/or cytoplasm, measured by EDS/SEM, was comparable between non-mycorrhizal pea genotypes but considerably decreased in mycorrhizal cv. Frisson and VIR7128. Possible mechanisms for mycorrhiza buffering of Cd-induced stress in the pea genotypes are discussed.     

Effect of ferritin overexpression in tobacco on the structure of bacterial and pseudomonad communities associated with the roots

The genetic structures of total bacterial and pseudomonad communities were characterized in rhizosphere soil and rhizoplane+root tissues of tobacco wild type and a ferritin overexpressor transgenic line (P6) by a cultivation-independent method using directly extracted DNA at the end of three consecutive plant cultures. The structure of total bacterial communities was characterized by automated ribosomal intergenic spacer analysis (A-RISA), and that of pseudomonad communities was characterized by PCR-restriction fragment length polymorphism (PCR-RFLP) from DNA amplified with specific primers. The structure of total bacterial communities was significantly modified in the rhizosphere soil by the overaccumulation of iron in the tobacco transgenic P6 line at the first culture, to a lesser extent at the second culture, and not at all at the third culture. No significant difference was recorded between the total communities associated with the roots (rhizoplane+root tissues) of the two plant genotypes in any of the cultures. In contrast, the difference in pseudomonad structure between the two plant genotypes increased with successive culture at the root level, but was not detected at a significant level in the rhizosphere soil. The impact of iron overaccumulation by the tobacco transgenic P6 line on pseudomonads supports previous findings on the importance of iron competition among fluorescent pseudomonads.     

一株嗜松青霉JP-NJ4的解磷特性

【目的】土壤中磷素供应不足是造成马尾松林地力衰退的原因之一。本研究对前期从马尾松根际土样中分离筛选出的一株解磷能力较强的嗜松青霉JP-NJ4的解无机磷及解有机磷能力进行探讨。【方法】探究嗜松青霉JP-NJ4对4种无机磷源及2种有机磷源的降解能力,并对其分泌的有机酸和酶类进行测定,对其解磷特性进行初步分析。【结果】表明JP-NJ4菌株可在4种不同无机磷源的培养基中生长,其中对磷酸钙[Ca3(PO4)2]的解磷效果最好,对4种磷源的解磷能力大小为:磷酸钙磷酸铝磷酸氢钙磷酸铁;其分泌的有机酸种类主要为葡萄糖酸、草酸及丙二酸;JP-NJ4菌株的磷酸酶活性较高,并具有一定的植酸酶活性;同时对草甘膦具有较好的生物降解功能,降解率达49.6%。【结论】嗜松青霉JP-NJ4解磷能力受磷源的结构组成影响,且解磷能力与发酵液pH值呈负相关关系;该菌株分泌的葡萄糖酸和草酸对磷酸钙及磷酸铝的溶解效果较明显。本研究供试菌株嗜松青霉JP-NJ4具有良好的解磷功能,在作为林业生物菌肥方面具有极大的应用潜力。     

Genotypic variability in phosphorus solubilizing activity of root exudates by pigeonpea grown in low-nutrient environments

A pot experiment confirmed that pigeonpea could efficiently utilize various sources of phosphorus (P) (aluminium phosphate, iron phosphate and apatite), irrespective of genotype. A qualitative assay method for iron (Fe)-P solubilizing activity showed that root exudates collected from P-deficient pigeonpea contained Fe-P solubilizing substances and that they were released mainly from root tips. Citric, malic, malonic, succinic and piscidic acids were identified in root exudates. Citric and piscidic acids release from roots was increased by low-P treatment in all the genotypes tested. The release rates of citric and piscidic acids were affected by the P concentration of shoots rather than that of roots. The pigeonpea roots released approximately 5–100 times more piscidic acid than citric acid depending on P stress status, plant age and genotype. When organic acids were added to Alfisols, citric acid was most capable of mobilizing P from the soil, followed by piscidic acid and malic acid. No correlation was found between genotypic variability in the release rates of citric and piscidic acids from the roots under low-P treatment at hydroponic culture and in the growth and P uptake of plants on Alfisols. Although citric and piscidic acids released from pigeonpea roots may play a partial role in solubilizing unavailable insoluble P in soils, the releases were thought to be an unsatisfactory strategy for explaining genotypic variation in low P availability of pigeonpea.     

Variation in phosphorus uptake efficiency by genotypes of cowpea (Vigna unguiculata) due to differences in root and root hair length and induced rhizosphere processes

The lengths of roots and root hairs and the extent of root-induced processes affect phosphorus (P) uptake efficiency by plants. To assess the influence of variation in the lengths of roots and root hairs and rhizosphere processes on the efficiency of soil phosphorus (P) uptake, a pot experiment with a low-P soil and eight selected genotypes of cowpea (Vigna unguiculata (L) WALP) was conducted. Root length, root diameter and root hair length were measured to estimate the soil volume exploited by roots and root hairs. The total soil P was considered as a pool of Olsen-P, extractable with 0.5 M NaHCO₃ at pH 8.5, and a pool of non-Olsen-P. Model calculations were made to estimate P uptake originated from Olsen-P in the root hair zone and the Olsen-P moving by diffusion into the root hair cylinder and non-Olsen-P uptake. The mean uptake rate of P and the mean rate of non-Olsen-P depletion were also estimated. The genotypes differed significantly in lengths of roots and root hairs, and in P uptake, P uptake rates and growth. From 6 to 85% of total P uptake in the soil volume exploited by roots and root hairs was absorbed from the pool of non-Olsen-P. This indicates a considerable activity of root-induced rhizosphere processes. Hence the large differences show that traits for more P uptake-efficient plants exist in the tested cowpea genotypes. This opens the possibility to breed for more P uptake-efficient varieties as a way to bring more sparingly soluble soil P into cycling in crop production and obtain capitalisation of soil P reserves.     

N2 fixation of pea hypernodulating mutants is more tolerant to root pruning than that of wild type

Background and aims

As a legume, pea plant has the ability to symbiotically fix N₂. However, symbiotic N₂ fixation is very sensitive to environmental stresses that affect plant growth, and there is little knowledge on the impact of root pruning on N₂ fixation and plant growth.

Methods

In this study, we removed half of the nodulated roots of pea wild-type Frisson and hypernodulating mutants P64, P118, and P121. Dinitrogen fixation was measured using ¹⁵N labeling and carbon assimilation and partitioning between plant organs using ¹³C labeling.

Results

Root pruning decreased N₂ fixation by ?46 to ?79 % in wild-type and mutants. Pea mutant P118 had a lower decrease of specific activity of N₂ fixation (?17 %) than both wild-type and other mutants (?36 to ?62 %). For all genotypes, root pruning increased root and nodule sinks strengths for carbon. For P118 and for P121, this was associated to higher nodule growth than for control plants, as measured 8 days after root pruning.

Conclusion

This is the first analysis of N₂-fixing plant response to root pruning. Importantly, we showed that some hypernodulating mutant pea lines (P118 and to a lesser extent P121) withstood this stress better than wild-type did.     

Gibberellin biosynthesis mutations and root development in pea

Dwarf mutants of pea (Pisum sativum), with impaired gibberellin (GA) biosynthesis in the shoot, were studied to determine whether the roots of these genotypes had altered elongation and GA levels. Mutations na, lh-2, and ls-1 reduced GA levels in root tips and taproot elongation, although in lh-2 and ls-1 roots the reduction in elongation was small (less than 15%). The na mutation reduced taproot length by about 50%. The roots of na plants elongated in response to applied GA(1) and recombining na with mutation sln (which blocks GA catabolism) increased GA(1) levels in root tips and completely restored normal root development. In shoots, Mendel's le-1 mutation impairs the 3beta-hydroxylation of GA(20) to the bioactive GA(1), resulting in dwarfism. However, GA(1) and GA(20) levels were normal in le-1 roots, as was root development. The null mutation le-2 also did not reduce root GA levels or elongation. The results support the theory that GAs are important for normal root elongation in pea, and indicate that a 3beta-hydroxylase gene other than LE operates in pea roots.     

Growth and curvature in seedlings of Pisum sativum and an ageotropic mutant

In an attempt to explain the influence of gravity on the behaviour of ageotropic plant organs, a pea mutant (Pisum sativum ageotropum) and normal pea (Pisum sativum cv. Sabel) were examined. The mutant has a significantly lower germination rate (large seeds: 25%, small seeds: 10%) than normal pea seeds (55%). Removal of testa increased germination dramatically, the values obtained were 63 and 89%, respectively. Immediately after imbibition the mutant from which the testa had been removed, developed more slowly than normal pea seeds; after 28 h the difference in elongation rate between the two types was reversed. When continuously stimulated geotropically in the horizontal position the elongation in the mutant is larger than in the normal pea roots kept in the same position. During a 24 h period starting 48 h after imbibition the mutant root elongated 45.0 mm while the value for the normal pea root was 11.5 mm. The course of the geotropic curvature in roots of the two types has been followed during a period of 24 h. Normal pea roots develop an asymmetry in the extreme root tip region after 30 min of horizontal stimulation. After prolonged stimulation (exceeding 2 h) the asymmetry has disappeared and the curvature distributed over the entire growth region. When roots of normal pea are stimulated continuously at various angles, the optimum angle of geotropic response is 90° with decreasing responses in the order 135° (i.e. the root tip is pointing obliquely upward) and 45°. The presumed ageotropic behaviour of the mutant has only to a certain extent been confirmed in the present study. When stimulated at 135° a slight positive curvature developed; stimulation at 90° and 45° gave a slight negative curvature.     

The effect of phosphorus availability on the carbon economy of contrasting common bean (Phaseolus vulgaris L.) genotypes

A common response to low phosphorus availability is increased relative biomass allocation to roots. The resulting increase in root:shoot ratio presumably enhances phosphorus acquisition, but may also reduce growth rates by diverting carbon to the production of heterotrophic rather than photosynthetic tissues. To assess the importance of increased carbon allocation to roots for the adaptation of plants to low P availability, carbon budgets were constructed for four common bean genotypes with contrasting adaptation to low phosphorus availability in the field ("phosphorus efficiency"). Solid-phase-buffered silica sand provided low (1 microM), medium (10 microM), and high (30 microM) phosphorus availability. Compared to the high phosphorus treatment, plant growth was reduced by 20% by medium phosphorus availability and by more than 90% by low phosphorus availability. Low phosphorus plants utilized a significantly larger fraction of their daytime net carbon assimilation on root respiration (c. 40%) compared to medium and high phosphorus plants (c. 20%). No significant difference was found among genotypes in this respect. Genotypes also had similar rates of P absorption per unit root weight and plant growth per unit of P absorbed. However, P-efficient genotypes allocated a larger fraction of their biomass to root growth, especially under low P conditions. Efficient genotypes had lower rates of root respiration than inefficient genotypes, which enabled them to maintain greater root biomass allocation than inefficient genotypes without increasing overall root carbon costs.     

Role of 2,4-diacetylphloroglucinol-producing fluorescent Pseudomonas spp. in the defense of plant roots

Plants have evolved strategies of stimulating and supporting specific groups of antagonistic microorganisms in the rhizosphere as a defense against diseases caused by soilborne plant pathogens owing to a lack of genetic resistance to some of the most common and widespread soilborne pathogens. Some of the best examples of natural microbial defense of plant roots occur in disease suppressive soils. Soil suppressiveness against many different diseases has been described. Take-all is an important root disease of wheat, and soils become suppressive to take-all when wheat or barley is grown continuously in a field following a disease outbreak; this phenomenon is known as take-all decline (TAD). In Washington State, USA and The Netherlands, TAD results from the enrichment during monoculture of populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas fluorescens to a density of 10 (5) CFU/g of root, the threshold required to suppress the take-all pathogen, Gaeumannomyces graminis var. tritici. 2,4-DAPG-producing P. fluorescens also are enriched by monoculture of other crops such as pea and flax, and evidence is accumulating that 2,4-DAPG producers contribute to the defense of plant roots in many different agroecosystems. At this time, 22 distinct genotypes of 2,4-DAPG producers (designated A - T, PfY and PfZ) have been defined by whole-cell repetitive sequence-based (rep)-PCR analysis, restriction fragment length polymorphism (RFLP) analysis of PHLD, and phylogenetic analysis of PHLD, but the number of genotypes is expected to increase. The genotype of an isolate is predictive of its rhizosphere competence on wheat and pea. Multiple genotypes often occur in a single soil and the crop species grown modulates the outcome of the competition among these genotypes in the rhizosphere. 2,4-DAPG producers are highly effective biocontrol agents against a variety of plant diseases and ideally suited for serving as vectors for expressing other biocontrol traits in the rhizosphere.     

Effect of nitrogen supply and defoliation on loss of organic compounds from roots of Festuca rubra

The aim of this study was to determine the effects of N-supply and defoliation on rhizodeposition from Festuca rubra, in the context of whole-plant C- partitioning and root morphology. Plants were grown for 36 d in axenic sand microcosms continuously percolated with nutrient solutions of either high or low N concentration (2 mM or 0.01 mM NH(4)NO(3), respectively). The effects of partial defoliation at weekly intervals were determined at high and low N. At low N, dry matter accumulation in roots and shoots was reduced significantly (P<0.001), with proportionately increased partitioning to roots, in comparison with the high N treatment. Root morphology was also affected by N-treatment; at low N, lower biomass production was offset by increased specific root length (P<0.001), reducing the magnitude of the significant (P=0.002) increase in total root length at high N. Cumulative release of organic C from roots of F: rubra over the experimental period was not altered significantly by N-treatment. However, as a proportion of net C-assimilation, rhizodeposition was significantly (P<0.001) greater at low N than at high N. Defoliation transiently (3-5 d) increased the release of soluble organic compounds from roots at each N-supply rate, and increased significantly (P<0.001) cumulative rhizodeposition over the experimental period. These effects of N-supply and defoliation on rhizodeposition are of importance in understanding interactions between plant and microbial productivity in grazed grasslands, and in interpretation of concurrent effects on microbially driven nutrient cycling processes in these systems.     

Plant growth and cation composition of two cultivars of spring wheat (Triticum aestivum L.) differing in P uptake efficiency.

Phosphorus (P)-zinc (Zn) interactions were investigated in two wheat cultivars (Brookton versus Krichauff) differing in P uptake efficiency. The experiment was done in a growth chamber. Rock phosphate (RP) or CaHPO4 (CaP) were used as P sources, and ammonium nitrate (AN) or nitrate only (NO) were used as nitrogen sources. Two Zn levels were used, 0.22 mg x kg(-1) (LZ) and 2.2 mg ZnSO4.7H2O x kg(-1) (HZ), respectively. P availability significantly affected plant biomass production, but Zn supply had little effect. Plants fed ammonium nitrate had significantly lower concentrations of cations than those fed nitrate only. Cultivar Brookton (with higher P uptake efficiency) consistently had lower concentrations of cations than cv. Krichauff (with low P uptake efficiency) under limited P supply. The differences in concentrations of cations increased with the decrease in P availability, but were not affected by Zn supply. The ratio of potassium in roots to shoots of cultivar Brookton was always higher than in cultivar Krichauff. Based on these findings, it is postulated that the lower concentrations of cations in cultivar Brookton are related to root exudation of organic anions, and a conceptual model is established to describe the regulation of root exudation of organic anions and concentrations of cations.     

The role of cell wall components from groundnut roots in solubilizing sparingly soluble phosphorus in low fertility soils

Groundnuts have a superior ability to take up P from soils with low P fertility compared to sorghum and soybean. Previous experiments showed that this ability was neither attributable to better root development nor to root exudates capable of solubilizing Fe- and Al-bound P, the sparingly soluble P forms in soils. Direct "contact reactions" between cell wall components from these 3 plant species (groundnut, soybean and sorghum) and P-fixing Fe and Al minerals were examined. Cell wall preparations from groundnut roots showed a superior P solubilizing ability than those of soybean and sorghum. Cell wall activity of groundnut roots may thus at least partly explain the superior growth of this crop under P-deficient conditions. To characterize the active site responsible for P solubilization, effects of pH, heat, addition of cations, and digestion with enzymes (pectinase and cellulase) or HCl on P solubilization were investigated. Conclusion are 1) Solubilizing ability is not related to root CEC because soybean with higher root CEC showed an inferior solubilizing ability compared to groundnut. 2) The reaction site of cell-walls of groundnut roots is stable against heating and digestion with cellulase and pectinase. 3) Solubilizing ability was severely reduced by digestion with HCl. 4) Pre-treating cell walls with either Al3+, Fe3+, or Ga3+ decreased solubilizing ability but cations with lower valency such as Na+, K+, Ca2+ or Mg2+ had no effect. Soaking roots of groundnuts grown in solution culture in 0.5 M NaOH for 30 seconds prior to cell wall preparation led to a 30% reduction in solubilization of P from FePO4 without permanently damaging plants. This suggests that 5) the active component of the cell walls was located on the root epidermal cell surfaces. Based on these results a phosphorus solubilizing mechanism is proposed.