A 23-kDa, root exudate polypeptide co-segregates with aluminum resistance in Triticum aestivum

We previously reported that treatment with aluminum (Al) leads to the accumulation of several polypeptides (12-, 23-, and 43.5-kDa) in root exudates of an Al-resistant cultivar of Triticum aestivum. In this report, we examine the segregation of the 23-kDa, Al-induced polypeptide and the Al-resistant phenotype in single F₂ plants arising from a cross between Al-resistant and Al-sensitive doubled-haploid (DH) lines. Single plants and plant populations were screened for sensitivity/resistance to Al using synthesis of 1,3-β-glucans (callose) as a sensitive marker for Al injury. Callose production in the Al-sensitive cv. Katepwa was approximately 3-fold higher than observed in the Al-resistant cv. Maringa, or a near-isogenic line derived from Katepwa and Maringa (Alikat), over a broad range of Al concentrations (0–100 μM). Similar results were observed with DH lines developed from cv. Katepwa, which produced two–four times more callose than DH lines developed from cv. Alikat. When single plants from F₁ and F₂ populations derived from a cross between DH Katepwa and DH Alikat were scored for Al-induced callose production after 4 days exposure to 100 μM Al, all F₁ plants were Al-resistant and F₂ plants segregated approximately 3:1 for Al-resistance/sensitivity. A backcross population derived from crossing Al-resistant F₁ with Al-sensitive Katepwa, segregated 1:1 for Al-resistance/sensitivity. Thus, the Al-resistant phenotype is inherited in a monogenic, dominant fashion in our DH lines. Enhanced accumulation of the Al-induced, 23-kDa polypeptide in root exudates was a trait which co-segregated with the Al-resistant phenotype in F₂ populations. This polypeptide was strongly labeled with S-methionine after 3 days of Al exposure and 6 h labeling. When root exudate polypeptides were separated by immobilized metal ion affinity chromatography, the 23-kDa polypeptide demonstrated significant Al-binding capacity. This polypeptide has been purified to near-homogeneity, providing an opportunity to isolate the gene(s) encoding this polypeptide.     

Aluminum resistance in wheat involves maintenance of leaf Ca<Superscript>2+</Superscript> and Mg<Superscript>2+</Superscript> content,decreased lipid peroxidation and Al accumulation,and low photosystem II excitation pressure

The phytotoxic aluminum species (Al³⁺) is considered as the primary factor limiting crop productivity in over 40 % of world’s arable land that is acidic. We evaluated the responses of two wheat cultivars (Triticum aestivum L.) with differential Al resistance, cv. Yecora E (Al-resistant) and cv. Dio (Al-sensitive), exposed to 0, 37, 74 and 148 μM Al for 14 days in hydroponic culture at pH 4.5. With increasing Al concentration, leaf Ca²⁺ and Mg²⁺ content decreased, as well as the effective quantum yield of photosystem II (PSII) photochemistry (Φ _PSII ), while a gradual increase in leaf membrane lipid peroxidation, Al accumulation, photoinhibition (estimated as F _v /F _m ), and PSII excitation pressure (1 ? q _p ) occurred. However, the Al-resistant cultivar with lower Al accumulation, retained larger concentrations of Ca²⁺ and Mg²⁺ in the leaves and kept a larger fraction of the PSII reaction centres (RCs) in an open configuration, i.e. a higher ratio of oxidized to reduced quinone A (Q_A), than plants of the Al-sensitive cultivar. Four times higher Al concentration in the nutrient solution was required for Al-resistant plants (148 μM Al) than for Al-sensitive (37 μM Al), in order to establish the same closed RCs. Yet, the decline in photosynthetic efficiency in the cultivar Dio was not only due to closure of PSII RCs but also to a decrease in the quantum yield of the open RCs. We suggest that Al³⁺ toxicity may be mediated by nutrient deficiency and oxidative stress, and that Al-resistance of the wheat cultivar Yecora E, may be due at least partially, from the decreased Al accumulation that resulted to decreased reactive oxygen species (ROS) formation. However, under equal internal Al accumulation (exposure Al concentration: Dio 74 μM, Yecora E 148 μM) that resulted to the same oxidative stress, the reduced PSII excitation pressure and the better PSII functioning of the Al-resistant cultivar was probably due to the larger concentrations of Ca²⁺ and Mg²⁺ in the leaves. We propose that the different sensitivities of wheat cultivars to Al³⁺ toxicity can be correlated to differences in the redox state of Q_A. Thus, chlorophyll fluorescence measurements can be a promising tool for rapid screening of Al resistance in wheat cultivars.     

Aluminum resistance in maize cannot be solely explained by root organic acid exudation. A comparative physiological study

Root apical aluminum (Al) exclusion via Al-activated root citrate exudation is widely accepted as the main Al-resistance mechanism operating in maize (Zea mays) roots. Nonetheless, the correlation between Al resistance and this Al-exclusion mechanism has not been tested beyond a very small number of Al-resistant and Al-sensitive maize lines. In this study, we conducted a comparative study of the physiology of Al resistance using six different maize genotypes that capture the range of maize Al resistance and differ significantly in their genetic background (three Brazilian and three North American genotypes). In these maize lines, we were able to establish a clear correlation between root tip Al exclusion (based on root Al content) and Al resistance. Both Al-resistant genotypes and three of the four Al-sensitive lines exhibited a significant Al-activated citrate exudation, with no evidence for Al activation of root malate or phosphate release. There was a lack of correlation between differential Al resistance and root citrate exudation for the six maize genotypes; in fact, one of the Al-sensitive lines, Mo17, had the largest Al-activated citrate exudation of all of the maize lines. Our results indicate that although root organic acid release may play a role in maize Al resistance, it is clearly not the only or the main resistance mechanism operating in these maize roots. A number of other potential Al-resistance mechanisms were investigated, including release of other Al-chelating ligands, Al-induced alkalinization of rhizosphere pH, changes in internal levels of Al-chelating compounds in the root, and Al translocation to the shoot. However, we were unsuccessful in identifying additional Al-resistance mechanisms in maize. It is likely that a purely physiological approach may not be sufficient to identify these novel Al-resistance mechanisms in maize and this will require an interdisciplinary approach integrating genetic, molecular, and physiological investigations.     

Aluminum-induced citric acid secretion is not the sole mechanism of Al-resistance in maize

Aluminum-induced citric acid (CA) root secretion is a widely accepted mechanism to explain Al-resistance in maize. Nonetheless, several aspects of this mechanism remain controversial. In this study, we used paclobutrazol (PBZ), a plant growth retardant, to gain new insights into the relationship between Δ⁵-sterol composition, membrane permeability, (PM) H⁺-ATPase activity and CA secretion in an Al-sensitive (UFVM-100) and Al-resistant (UFVM-200) maize genotypes challenged with Al. The Al-sensitive genotype displayed greater concentrations of Al in the root tips and greater inhibition of root elongation (RE), which was accompanied by greater electrolyte leakage and greater reduction in the Δ⁵-sterols content after Al treatment. CA secretion by roots increased in both genotypes after Al treatment but to a greater extent in the Al-resistant genotype. The (PM) H⁺-ATPase activity was down-regulated in the sensitive cultivar and up-regulated in its resistant counterpart upon Al treatment. A significant correlation between (PM) H⁺-ATPase activity and CA secretion was observed, but only in the Al-resistant genotype. Upon adding PBZ to the Al-treated plants, differences in the RE and Δ⁵-sterol composition between the maize genotypes were fully abolished, whereas genotypic differences in CA secretion and (PM) H⁺-ATPase activity were reduced but not completely eliminated. Taken together, this information suggests the existence of other processes or mechanisms operating in the Al resistance in these two maize genotypes.     

High tolerance of aluminum in the grass species Cynodon aethiopicus

Concentrations of aluminum (Al) were determined in leaves of native terrestrial plants, macrophytes and fruit parts (watermelon and tomato) using inductively coupled plasma mass spectrometry. Al concentrations in water and soil were determined by inductively coupled plasma optical emission spectrometry. Potamogeton thunbergii (macrophyte) and Cynodon aethiopicus (terrestrial grass) had the highest leaf Al concentrations (2 and 1 g kg^?1 dw, respectively). Transfer factors (mg kg^?1 dw plants/mg kg^?1 dw soil) based on total Al concentrations in soil varied from 2 × 10^?3 to 0.05 and from 1.9 to 78 based on mobile Al concentrations determined after sequential extraction. Bioconcentration factors (mg kg^?1 dw plants/mg L^?1 water) varied from 19 to 9.5 × 10³ L kg^?1 dw. Plants can accumulate high concentrations of Al when growing in neutral pH soils and slightly alkaline lakes in the Ethiopian Rift Valley. Controlled experiments showed that C. aethiopicus can accumulate high levels of Al both in root and shoot. Compared to Arabidopsis thaliana, C. aethiopicus was more tolerant to Al exposure as ≥400 μM AlCl₃ was needed to inhibit root growth compared to 200 μM in A. thaliana. After exposing C. aethiopicus and A. thaliana in 800 μM AlCl_3, alkaline comet assay indicates significant DNA (deoxyribonucleic acid) damage in A. thaliana while C. aethiopicus was unaffected. No significant induction of reactive oxygen species (ROS), in terms of leaf H₂O₂ levels, could be observed in C. aethiopicus. C. aethiopicus has mechanisms to suppress both Al-induced ROS and DNA damage, thereby increasing tolerance of the species to high Al concentrations.     

Effect of aluminum on cytoplasmic Ca2+ homeostasis in root hairs of Arabidopsis thaliana (L.)

Aluminum inhibition of root growth is a major world agricultural problem where the cause of toxicity has been linked to changes in cellular calcium homeostasis. Therefore, the effect of aluminum ions (Al) on changes in cytoplasmic free calcium concentration ([Ca²⁺]_c) was followed in root hairs of wild-type, Al-sensitive and Al-resistant mutants of Arabidopsis thaliana (L.) Heynh. Generally, Al exposure resulted in prolonged elevations in tip-localized [Ca²⁺]_c in both wild-type and Al-sensitive root hairs. However, these Al-induced increases in [Ca²⁺]_c were not tightly correlated with growth inhibition, occurring up to 15 min after Al had induced growth to stop. Also, in 32% of root hairs examined growth stopped without a detectable change in [Ca²⁺]_c. In contrast, Al-resistant mutants showed little growth inhibition in response to AlCl₃ exposure and in no case was a change in [Ca²⁺]_c observed. Of the other externally applied stresses tested (oxidative and mechanical stress), both were found to inhibit root hair growth, but only oxidative stress (H₂O₂, 10 μM) caused a prolonged rise in [Ca²⁺]_c similar to that induced by Al. Again this increase occurred after growth had been inhibited. The lack of a tight correlation between Al exposure, growth inhibition and altered [Ca²⁺]_c dynamics suggests that although exposure of root hairs to toxic levels of Al causes an alteration in cellular Ca²⁺ homeostasis, this may not be a required event for Al toxicity. The elevation in [Ca²⁺]_c induced by Al also strongly suggests that the phytotoxic action of Al in root hairs is not through blockage of Ca²⁺-permeable channels required for Ca²⁺ influx into the cytoplasm. Received: 24 October 1997 / Accepted: 6 March 1998     

Root hair length and rhizosheath mass depend on soil porosity,strength and water content in barley genotypes

Selecting plants with improved root hair growth is a key strategy for improving phosphorus-uptake efficiency in agriculture. While significant inter- and intra-specific variation is reported for root hair length, it is not known whether these phenotypic differences are exhibited under conditions that are known to affect root hair elongation. This work investigates the effect of soil strength, soil water content (SWC) and soil particle size (SPS) on the root hair length of different root hair genotypes of barley. The root hair and rhizosheath development of five root hair genotypes of barley (Hordeum vulgare L.) was compared in soils with penetrometer resistances ranging from 0.03 to 4.45 MPa (dry bulk densities 1.2–1.7 g cm^?3). A “short” (SRH) and “long” root hair (LRH) genotype was selected to further investigate whether differentiation of these genotypes was related to SWC or SPS when grown in washed graded sand. In low-strength soil (<1.43 MPa), root hairs of the LRH genotype were on average 25 % longer than that of the SRH genotype. In high-strength soil, root hair length of the LRH genotype was shorter than that in low-strength soil and did not differ from that of the SRH genotype. Root hairs were shorter in wetter soils or soils with smaller particles, and again SRH and LRH did not differ in hair length. Longer root hairs were generally, but not always, associated with larger rhizosheaths, suggesting that mucilage adhesion was also important. The root hair growth of barley was found to be highly responsive to soil properties and this impacted on the expression of phenotypic differences in root hair length. While root hairs are an important trait for phosphorus acquisition in dense soils, the results highlight the importance of selecting multiple and potentially robust root traits to improve resource acquisition in agricultural systems.     

Aluminum resistance in wheat (Triticum aestivum) is associated with rapid, Al-induced changes in activities of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in root apices

We have investigated the effect of aluminum (Al) on the activity of glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) isolated from 5-mm root apices of 4-day-old wheat ( Triticum aestivum ) cultivars differing in resistance to Al. Rapid increases in G6PDH and 6PGDH activities were observed in Al-resistant cultivars (PT741 and Atlas 66) during the first 10 h of treatment with 100 μ M Al, while no change in the activity of either enzyme was observed in Al-sensitive cultivars (Katepwa and Neepawa) during a 24-h exposure to Al. The Al-induced increases in enzyme activities observed in the Al-resistant PT741 appear to reflect an induction of protein synthesis since the increases were completely abolished by 1 m M cycloheximide. No differences in G6PDH and 6PGDH activities were observed between the Al-sensitive and the Al-resistant genotypes when Al was supplied in vitro. Under these conditions, an increase in Al concentration from 0 to 1.4 m M caused a gradual decrease in activity of both enzymes, irrespective of the Al-resistance of whole seedlings. Aluminum-sensitive and aluminum-resistant cultivars also differed in the rate and extent of accumulation of slowly-exchanging Al in 5-mm root apices. During the first 6 h of Al treatment, Al accumulation was only 10% more rapid in Katepwa than in PT741. After 24-h exposure, accumulation in the Al-sensitive Katepwa, was two-fold higher. A decline in Al accumulation in a slowly-exchanging compartment as well as a decrease in activities of G6PDH and 6PGDH were found in the Al-resistant PT741, when seedlings were transferred to Al-free treatment solutions after 16-h exposure to 100 μ M Al. These results suggest that rapid induction of G6PDH and 6PGDH in the Al-resistant line PT741 by Al may play a role in the mechanism of Al resistance, possibly by regulation of the pentose phosphate pathway.     

Aluminum resistance in wheat (Triticum aestivum) is associated with rapid, Al-induced changes in activities of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in root apices

We have investigated the effect of aluminum (Al) on the activity of glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) isolated from 5-mm root apices of 4-day-old wheat ( Triticum aestivum ) cultivars differing in resistance to Al. Rapid increases in G6PDH and 6PGDH activities were observed in Al-resistant cultivars (PT741 and Atlas 66) during the first 10 h of treatment with 100 μ M Al, while no change in the activity of either enzyme was observed in Al-sensitive cultivars (Katepwa and Neepawa) during a 24-h exposure to Al. The Al-induced increases in enzyme activities observed in the Al-resistant PT741 appear to reflect an induction of protein synthesis since the increases were completely abolished by 1 m M cycloheximide. No differences in G6PDH and 6PGDH activities were observed between the Al-sensitive and the Al-resistant genotypes when Al was supplied in vitro. Under these conditions, an increase in Al concentration from 0 to 1.4 m M caused a gradual decrease in activity of both enzymes, irrespective of the Al-resistance of whole seedlings. Aluminum-sensitive and aluminum-resistant cultivars also differed in the rate and extent of accumulation of slowly-exchanging Al in 5-mm root apices. During the first 6 h of Al treatment, Al accumulation was only 10% more rapid in Katepwa than in PT741. After 24-h exposure, accumulation in the Al-sensitive Katepwa, was two-fold higher. A decline in Al accumulation in a slowly-exchanging compartment as well as a decrease in activities of G6PDH and 6PGDH were found in the Al-resistant PT741, when seedlings were transferred to Al-free treatment solutions after 16-h exposure to 100 μ M Al. These results suggest that rapid induction of G6PDH and 6PGDH in the Al-resistant line PT741 by Al may play a role in the mechanism of Al resistance, possibly by regulation of the pentose phosphate pathway.     

Calcium chloride and gibberellic acid protect linseed (Linum usitatissimum L.) from NaCl stress by inducing antioxidative defence system and osmoprotectant accumulation

Salinity stress affects many metabolic facets of plants and induces anatomical and morphological changes resulting in reduced growth and productivity. To overcome the damaging effects of salinity, different strategies of the application of nutrients with plant hormones are being adopted. The present study was carried out with an aim to find out whether application of calcium chloride (CaCl₂) and gibberellic acid (GA₃) could alleviate the detrimental effects of salinity stress on plant metabolism. Fifteen days old plants were supplied with (1) 0 mM NaCl + 0 mg CaCl₂ kg^?1 sand + 0 M GA₃ (control, T0); (2) 0 mM NaCl + 10 mg CaCl₂ kg^?1 sand + 0 M GA₃ (T1); (3) 0 mM NaCl + 0 mg CaCl₂ kg^?1 sand + 10^?6 M GA₃ (T2); (4) 150 mM NaCl + 0 mg CaCl₂ kg^?1 sand + 0 M GA₃ (T3); (5) 150 mM NaCl + 10 mg CaCl₂ kg^?1 sand + 0 M GA₃ (T4); (6) 150 mM NaCl + 0 mg CaCl₂ kg^?1 sand + 10^?6 M GA₃ (T5); (7) 150 mM NaCl + 10 mg CaCl₂ kg^?1 sand + 10^?6 M GA₃ (T6). To assess the response of the crop to NaCl, CaCl₂ and GA₃, plants were uprooted randomly at 60 days after sowing. The presence of NaCl in the growth medium decreased all the growth and physio-biochemical parameters, except electrolyte leakage, proline (Pro) and glycine betaine (GB) content, thiobarbituric acid reactive substances (TBARS), H₂O₂ content, activities of superoxide dismutase (SOD) and catalase (CAT) and leaf Na content, which exhibited an increase of 37.6, 29.3, 366.9, 107.5, 59.1, 17.1, 28.4 and 255.2%, respectively, compared to the control plants. However, application of CaCl₂ in combination with GA₃ appears to confer greater osmoprotection by the additive role with NaCl in Pro and GB accumulation. Although the activities of antioxidant enzymes (SOD, CAT and POX) were increased by salt stress, the combined application of CaCl₂ and GA₃ to salt-stressed plants further enhanced the activities of these enzymes by 25.1, 6.7 and 47.8%, respectively, compared to plants grown with NaCl alone. The present study showed that application of CaCl₂ and GA₃ alone as well as in combination mitigated the adverse effect of salinity, but combined application of these treatments proved more effective in alleviating the adverse effects of NaCl stress.     

The critical soil P levels for crop yield, soil fertility and environmental safety in different soil types

Background and aims

Sufficient soil phosphorus (P) is important for achieving optimal crop production, but excessive soil P levels may create a risk of P losses and associated eutrophication of surface waters. The aim of this study was to determine critical soil P levels for achieving optimal crop yields and minimal P losses in common soil types and dominant cropping systems in China.

Methods

Four long-term experiment sites were selected in China. The critical level of soil Olsen-P for crop yield was determined using the linear-plateau model. The relationships between the soil total P, Olsen-P and CaCl₂-P were evaluated using two-segment linear model to determine the soil P fertility rate and leaching change-point.

Results

The critical levels of soil Olsen-P for optimal crop yield ranged from 10.9 mg kg^?1 to 21.4 mg kg^?1, above which crop yield response less to the increasing of soil Olsen-P. The P leaching change-points of Olsen-P ranged from 39.9 mg kg^?1 to 90.2 mg kg^?1, above which soil CaCl₂-P greatly increasing with increasing soil Olsen-P. Similar change-point was found between soil total P and Olsen-P. Overall, the change-point ranged from 4.6 mg kg^?1 to 71.8 mg kg^?1 among all the four sites. These change-points were highly affected by crop specie, soil type, pH and soil organic matter content.

Conclusions

The three response curves could be used to access the soil Olsen-P status for crop yield, soil P fertility rate and soil P leaching risk for a sustainable soil P management in field.     

Quantitative trait loci for aluminum resistance in wheat

Quantitative trait loci (QTL) for wheat resistance to aluminum (Al) toxicity were analyzed using simple sequence repeats (SSRs) in a population of 192 F₆ recombinant inbred lines (RILs) derived from a cross between an Al-resistant cultivar, Atlas 66 and an Al-sensitive cultivar, Chisholm. Wheat reaction to Al was measured by relative root growth and root response to hematoxylin stain in nutrient-solution culture. After screening 1,028 SSR markers for polymorphisms between the parents and bulks, we identified two QTLs for Al resistance in Atlas 66. One major QTL was mapped on chromosome 4D that co-segregated with the Al-activated malate transporter gene (ALMT1). Another minor QTL was located on chromosome 3BL. Together, these two QTLs accounted for about 57% of the phenotypic variation in hematoxylin staining score and 50% of the variation in net root growth (NRG). Expression of the minor QTL on 3BL was suppressed by the major QTL on 4DL. The two QTLs for Al resistance in Atlas 66 were also verified in an additional RIL population derived from Atlas 66/Century. Several SSR markers closely linked to the QTLs were identified and have potential to be used for marker-assisted selection (MAS) to improve Al-resistance of wheat cultivars in breeding programs.     

Effect of anion channel antagonists and La3+ on citrate release, Al content and Al resistance in maize roots

The correlation between organic acid anion release and Al content was examined in two maize (Zea mays L.) inbred lines, Cat 100-6 (Al-resistant) and S 1587-17 (Al-sensitive) treated with anion channel antagonists and La3+, a cation channel blocker. In the intact roots of Al-resistant maize, the Al-induced excretion of citrate was inhibited by the anion channel antagonists niflumic acid, anthracene-9-carboxylic and ethacrinic acid. Citrate release in excised root apices was reduced by 60% in the presence of 15 microM niflumic acid, while the Al content increased by 42%. Nevertheless, Cat 100-6 accumulated less Al than S 1587-17 when the rate of citrate release was similar in both lines, indicating that other mechanisms of Al-resistance are operating in Cat 100-6. The presence of 60 microM La3+ did not change the rate of citrate release, but the Al content in excised root apices of Al-resistant plants was reduced by 70%. These results suggest that the Al distributed uniformly in the roots does not contribute to citrate release and possibly the activity of anion channels is correlated with the free activities of extracellular Al3+ close to anion channels.     

Effects of aluminium on root growth and apical root cells in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars. Reliability of screening tests to detect Al resistance at the seedling stage

The effects of Al₂(SO₄)₃·18H₂O on growth of root and apical root cells were studied in seedlings of rice cultivars differing in Al resistance including I Kong Pao and Aiwu (Al-sensitive) and IRAT 112 and IR6023-10-1-1 (Al-resistant). Inhibition of root growth was a typical effect of Al, and the extent of the inhibition depended on both cultivar and Al concentration. Al impaired the activity of the root meristem as indicated by reductions in its size, mitotic activity and the diameter of the meristematic cell nucleoli. Cell size in the elongation zone of the root was also reduced by Al. The reliability of the haematoxylin staining method to classify rice cultivars according to their Al-sensitivity failed to discriminate the Al-resistant IR6023-10-1-1 cultivar from the two sensitive cultivars. The results are discussed in relation to the Al resistance mechanisms operating in rice.     

Secretion of citrate from roots in response to aluminum and low phosphorus stresses in Stylosanthes

Excess aluminum (Al) ions and phosphorus (P) deficiency are the key factors that limit plant growth in acid soils. Secretion of organic acids (OA) from roots has been proposed as an Al-resistance mechanism. Nonetheless, the correlation between Al resistance and this mechanism has not been tested beyond a very small number of Al-resistant and Al-sensitive genotypes. To elucidate the mechanisms responsible for plant adaptability to acid soils, we studied the secretion of OA from roots of Stylosanthes in response to high-Al and low-P stresses using six different genotypes. Relative root inhibition by 50?µM Al ranged from 25–71% and differed significantly among six Stylosanthes genotypes. Al treatment induced the secretion of citrate from the roots of Stylosanthes seedling in a dose- and time-dependent manner. Moreover, the secretion rate was significantly higher in the Al-resistant genotype. On the other hand, inhibition of Al-induced citrate secretion by phenylisothiocyanate or 9-anthracenecarboxylic acid resulted in an increase in Al content in Stylosanthes root apices. P deficiency also induced citrate secretion from Stylosanthes seedling roots. Furthermore, citrate secretion was much more robust with exposure to both excess-Al and P-deficiency stresses than under either stress alone. Unlike Al-induced citrate secretion, which was rapid, low-P-induced secretion was a slow process, with significant increases in secretion only becoming evident after 6 d of treatment with free phosphate. The lag between treatment with Al and citrate secretion was approximately 4 h. These results suggest that the secretion of citrate is a mechanism for resistance to both excess-Al and low-P stresses in Stylosanthes.     

The effect of phosphate rock dissolution on soil chemical properties and wheat seedling root elongation

Soils of the Appalachian region of the United States are acidic and deficient in P. North Carolina phosphate rock (PR), a highly substituted fluoroapatite, should be quite reactive in these soils, allowing it to serve both as a source of P and a potential ameliorant of soil acidity. An experiment was conducted to evaluate the influence of PR dissolution on soil chemical properties and wheat (Triticum aestivum cv. Hart) seedling root elongation. Ten treatments including nine rates of PR (0, 12.5, 25, 50, 100, 200, 400, 800, and 1600 mg P kg^-1) and a CaCO₃ (1000 mg kg^-1) control were mixed with two acidic soils, moistened to a level corresponding to 33 kPa moisture tension and incubated for 30 days. Pregerminated wheat seedlings were grown for three days in the PR treated soils and the CaCO₃ control. Root length was significantly (P<0.05) increased both by PR treatments and CaCO₃, indicating that PR dissolution was ameliorating soil acidity. The PR treatments increased soil pH, exchangeable Ca, and soil solution Ca while lowering exchangeable Al and 0.01 M CaCl₂ extractable soil Al. Root growth in PR treatments was best described by an exponential equation (P<0.01) containing 0.01 M CaCl₂ extractable Al. The PR dissolution did not reduce total soil solution Al, but did release Al complexing anions into soil solution, which along with increased pH, shifted Al speciation from toxic to nontoxic forms. These results suggest that North Carolina PR should contribute to amelioration of soil acidity in acidic, low CEC soils of the Appalachian region.     

Biological amelioration of subsoil acidity through managing nitrate uptake by wheat crops

Subsoil acidity occurs in many agricultural lands in the world, and is considered to be an irreversible constraint due to amelioration difficulties. This field study aimed to develop a biological method to ameliorate subsoil acidity through the root-induced alkalisation resulting from nitrate uptake. Aluminium (Al)-tolerant wheat variety Diamondbird and Al-sensitive variety Janz (Triticum aestivum L.) were grown at two contrasting field sites with mild and severe subsurface acidity, respectively, and were supplied with either Ca(NO₃)₂ at the soil surface, Ca(NO₃)₂ at 10 cm depth or urea at 10 cm depth. Application of nitrate increased rhizosphere pH up to 0.5 units and bulk soil pH to 0.3 units, and to a depth >30 cm in the Kandosol. The placement of nitrate at 10 cm increased subsoil pH more than the surface application. Nitrate application increased nitrate concentration in soil profiles as expected, whereas urea application increased NH ₄ ⁺ concentration which in turn favored acidification processes. Diamondbird generally produced more tillers and shoot biomass at anthesis but the two varieties did not differ in grain yield or rhizosphere alkalisation. Similar grain yields were achieved under supply of nitrate and urea. The results suggest that biological amelioration through managing nitrate uptake is possible as part of an integrated approach to combat subsoil acidity in farming systems.     

The effect of aluminium on polypeptide pattern of cell wall proteins isolated from the roots of Al-sensitive and Al-resistant barley cultivars

Accumulation of some proteins isolated from the cell wall of roots of the Al-sensitive (Alfor) and the Al-resistant (Bavaria) barley cultivars were followed during treatment with different Al³⁺ concentrations, pH changes of the root medium, and several heavy metals (Cu²⁺, Cd²⁺, Co²⁺). SDS-PAGE analysis revealed an Al-induced accumulation of polypeptides with molecular mass of 14, and 16 kDa and a group of polypeptides around 27 kDa. The accumulation pattern of Al-induced polypeptides was very similar in both cultivars but in the Al-resistant Bavaria it was induced at lower Al concentration and earlier than it was in the Al-sensitive cultivar Alfor. Changes in pH values of root medium (pH 3.5–6.5) did not show any effect on the accumulation of Al-induced cell wall polypeptides either in Al-sensitive or in Al-tolerant barley cultivar. Heavy metals (Cu, Cd, and Co) at concentration of 10 μM resulted in similar accumulation of individual polypeptides as we found after Al treatment. In comparison to Al, quantitative differences in polypeptides accumulation induced by Cu, Cd and Co were less expressed that of Al treatment. More pronounced accumulation and earlier induction of individual cell wall polypeptides in roots of Al-resistant barley cultivar than in Al-sensitive, might indicate some possible role of these polypeptides in plant resistance to Al stress.     

Genetic and physiological analysis of doubled-haploid,aluminium-resistant lines of wheat provide evidence for the involvement of a 23 kD,root exudate polypeptide in mediating resistance

We have made use of a genetic approach to develop homozygous, near-isogenic germplasm for investigating aluminium (Al) resistance in Triticum aestivum L. A conventional backcross program was used to transfer Al resistance from the Al-resistant cultivar, Maringa, to a locally-adapted, Al-sensitive cultivar, Katepwa. At the third backcross stage, a single, resistant isoline (Alikat = Katepwa*3/Maringa) was chosen on the basis of superior root growth after 14 days of exposure to a broad range of Al concentrations (0 to 600 µM). Genetic analysis of doubled-haploid lines (DH) developed from this isoline suggested that resistance is controlled by a single dominant gene. Crosses between DH Alikat and DH Katepwa yielded an Al-resistant F1 population. Backcrossing this F1 population to DH Katepwa produced a population which segregated 1:1 for Al resistance, while selfing produced a population segregating 3 : 1 for Al resistance. Under conditions of Al stress, Al-resistant F2 plants released a suite of novel low molecular weight polypeptides into the rhizosphere. One of these polypeptides (23 kD) shows substantive Al-binding capacity and segregates with the resistant phenotype. While the precise mechanisms that mediate Al resistance are still unknown, this research has provided support for a possible role of the 23 kD exudate polypeptide in mediating resistance to Al. To more fully understand the role that this polypeptide plays in Al-resistance, we are attempting to clone this gene from microsequence data obtained from purified protein.     

Temporal patterns of net soil N mineralization and nitrification through secondary succession in the subtropical forests of eastern China

Linking temporal trends of soil nitrogen (N) transformation with shifting patterns of plants and consequently changes of litter quality during succession is important for understanding developmental patterns of ecosystem function. However, the successional direction of soil N mineralization and nitrification in relation to species shifts in the subtropical regions remains little studied. In this study, successional patterns of net soil N mineralization and nitrification rates, litter-fall, forest floor litter, fine root and soil properties were quantified through a successional sequence in the subtropical forests of eastern China. Net N mineralization rate was ‘U-shaped’ through succession: highest in climax evergreen broad-leaved forest (CE: 1.6?±?0.2 mg-N kg^?1 yr^?1) and secondary shrubs (SS: 1.4?±?0.2 mg-N kg^?1 yr^?1), lowest in conifer and evergreen broad-leaved mixed forest (MF: 1.1?±?0.1 mg-N kg^?1 yr^?1) and intermediate in conifer forest (CF: 1.2?±?0.1 mg-N kg^?1 yr^?1) and sub-climax forest (SE: 1.2?±?0.2 mg-N kg^?1 yr^?1). Soil nitrification increased with time (0.02?±?0.1, 0.2?±?0.1, 0.5?±?0.1, 0.2?±?0.1, and 0.6?±?0.1 mg-N kg^?1 yr^?1 in SS, CF, MF, SE and CE, respectively). Annual production of litter?fall increased through succession. Fine root stocks and total N concentration, soil total N, total carbon (C) and microbial biomass C also followed ‘U?shaped’ temporal trends in succession. Soil bulk density was highest in MF, lowest in CE, and intermediate in SS, CF and SE. Soil pH was significantly lowest in CE. Temporal patterns of soil N mineralization and nitrification were significant related to the growth of conifers (i.e. Pinus massoniana) and associated successional changes of litter-fall, forest floor, fine roots and soil properties. We concluded that, due to lower litter quality, the position of Pinus massoniana along the succession pathway played an important role in controlling temporal trends of soil N transformation.