Availability of iron,manganese, zinc and phosphorus in submerged sodic soil as affected by amendments during the growth period of rice crop

Summary Effect of amendments, gypsum (12.5 tonnes/ha), farmyard manure (30 tonnes/ha), rice husk (30 tonnes/ha) and also no amendment (control) on the availability of native Fe, Mn and P and applied Zn in a highly sodic soil during the growth period of rice crop under submerged conditions was studied in a field experiment. Soil samples were collected at 0, 30, 60 and 90 days of crop growth. Results showed that extractable Fe (1N NH₄OAC pH 3) and Mn (1N NH₄OAC pH 7) increased with submergence upto 60 days of crop growth but thereafter remained either constant or declined slightly. Application of farmyard manure and rice husk resulted in marked improvement of these elements over gypsum and control. Increases in extractable Mn (water soluble plus exchangeable) as a result of submergence and crop growth under different amendments were accompanied by corresponding decreases in easily reducible Mn content of the soil. Application of 40 kg zinc sulphate per hectare to rice crop could substantially raise the available Zn status (DTPA extractable) of the soil in gypsum and farmyard manure treated plots while the increase was only marginal in rice husk and control plots indicating greater fixation of applied Zn. Available P (0.5M NaHCO₃ pH 8.5) behaved quite differently and decreased in the following order with crop growth: gypsum>rice husk>farmyard manure>control.     

A comparison of soil tests to predict the growth and nodulation of subterranean clover in aluminium-toxic topsoils

Total Al concentration or pH in 1∶5 10 mM CaCl₂ extracts and exchangeable Al in 100 mM BaCl₂ extracts cannot always distinguish between Al-toxic and Al-nontoxic topsoils. Our objectives were to compare the abilities of different measures of Al and pH in various extracts to predict the effects of acidity on growth and nodulation of subterranean clover. In a glasshouse experiment,Trifolium subterraneum L. cv. Mt Barker was grown in acidic soils from 3 sites in the Western Australian wheatbelt with different histories of phosphate fertilizer application. The pH was adjusted to give a range of 3.8–7 in the centrifuged soil solution (SS). Total (Al-tot), reactive Al (8-hydroxyquinoline-extractable, Al-HQ) and pH were measured in SS and 1∶5 extracts of KCl, CaCl₂ and LaCl₃. Another method of estimating reactive Al (Al which reacts with Chelex-100) was also measured in SS only. Other measurements included exchangeable Al and H, Ca in SS, and P in SS and the CaCl₂ extracts. Both plant growth and early nodulation decreased with increasing acidity. Plant growth in the acidified and unlimed treatments of all soils was best described by Al-HQ in SS, KCl or CaCl₂ (r²=0.68–0.70). Multiple regression of relative yield against Al or pH with the concentration of P in SS increased the percentage variation explained by 10% and 30%, respectively. Early nodulation was well correlated (r²=0.67–0.91) with pH or exch. H, Al-tot or exch. Al and Al-HQ. No improvement in the correlation was gained by including P using multiple regression. At constant ionic strength, increasing the valence of the extracting cation decreased the ability of soil tests to distinguish phytotoxic Al.     

Comparison of soil tests and leaf analysis as methods of diagnosing Zn deficiency in British Columbia apple orchards

Soil Zn extracted by 0.25M MgCl₂ or DTPA as an index of Zn availability was compared to Zn or P:Zn concentration in leaves for 40 commercial apple orchards in southern British Columbia. Sampled trees included a wide range of ages and cultivar/rootstock combinations. Leaves were sampled from the midportion of current season's shoots at four times; May–June 1984, July 1984, May 1985 and July 1985. At the same time the severity of the four Zn deficiency symptoms, rosetting, blind bud, little leaf and chlorosis was assessed. Little relationship was found between soil and leaf Zn measures, except in July 1984, when both terminal leaf Zn and P:Zn concentrations varied directly with 0.25M MgCl₂ extractable soil Zn. Soil Zn extracted by 0.25M MgCl₂ was more closely related to severity of deficiency symptoms on the trees at all four samples dates than was DTPA-soil Zn or leaf Zn concentration. Blindbud was the most useful indicator of deficiency in samples collected in May whereas chlorosis was the most useful one in July.     

Phosphate fertilization reduces zinc adsorption by calcareous soils

Summary Zinc adsorption isotherms were constructed for three calcereous soils which varied in carbonate contents, texture, and past history of phosphate fertilization. The equilibrium conditions were 25°C, 0.01 M CaCl₂ and 6 days.Higher phosphate fertilization of the soils reduced Zn adsorption. The effect of P was more in the soil with lower carbonate content which suggested that soil carbonates played a dominant role in the Zn adsorption characteristics of the soils.The adsorption data conformed to the Langmuir equation. Constants (k and b) calculated from the Langmuir isotherm showed that bonding energies (k) were inversely related to extractable P; i.e. higher Zn adsorption was associated with lower bonding energy. The Zn adsorption maxima (b) were higher for the soils with higher calcium carbonate equivalent.Adsorbed Zn was extracted with a single extraction of 0.005 M DTPA. The recovery was 91 percent for the Tandojam soil, 82 percent for the Tarnab soil, and 63 percent for the Kala shah Kaku soil, indicating that most of the adsorbed Zn is not irreversibly fixed by the soils and can be utilized by plant during growth.The results suggest that P-induced Zn deficiency could not be ascribed to precipitation of Zn as insoluble Zn-P compounds in soils. The increased Zn solubility with P fertilization is the evidence that P-Zn interaction does not reside in the growing medium external to plant.The work is part of Ph.D. thesis submitted to the University of Hawaii, Honolulu, U.S.A.     

Studies of ‘Available’ and isotopically exchangeable sulphur in some North Queensland soils

Summary Using ³⁵S-sulphate, the specific activity of various sulphur fractions in some diverse North Queensland soils has been followed for up to 185 days in a glasshouse experiment. The sulphur extracted with 0.01 M calcium phosphate was from the same pool as that used by the test plants, and since near full recovery of added ³⁵SO₄ was obtained initially, this fraction is comparable to the L-value. On the other hand, 0.5 M NaHCO₃ removed some soil sulphur that was not available to the plants.Liming caused an initial increase in the phosphate extractable fraction, the sulphur seemingly being released from the NaHCO₃ extractable fraction, but decreased sulphate sorption also contributed to the increase in S uptake by the plants upon liming. re]19750507     

Effects of soil acidification on the chemical extractability of Fe,Mn, Zn and Cu and the growth and micronutrient uptake of highbush blueberry plants

Summary The effects of soil acidification and micronutrient addition on levels of extractable Fe, Mn, Zn and Cu in a soil, and on the growth and micronutrient uptake of young highbush blueberry plants (Vaccinium corymbosum L. cv. Blueray) was investigated in a greenhouse study.Levels of 0.05M CaCl₂-extractable Fe, Mn, Zn and Cu increased as the pH was lowered from 7.0 to 3.8. However, the solubility (CaCl₂-extractability) of Fe and Cu was considerably less pH-dependent than that of Mn and Zn. With the exception of HCl-and DTPA-extractable Mn, micronutrients extractable with 0.1M HCl, 0.005M DTPA and 0.04M EDTA were unaffected or raised only slightly as the pH was lowered from 6.0 to 3.8. Quantities of Mn and Zn extractable with CaCl₂ were similar in magnitude to those extractable with HCl, DTPA and EDTA whilst, in contrast, the latter reagents extracted considerably more Cu and Fe than did CaCl₂. A fractionation of soil Zn and Cu revealed that soil acidification resulted in an increase in the CaCl₂- and pyrophosphate-extractable fractions and a smaller decrease in the oxalate-extractable fraction.Plant dry matter production increased consistently when the soil pH was lowered from 7.0 to 4.6 but there was a slight decline in dry matter as the pH was lowered to 3.8. Micronutrient additions had no influence on plant biomass although plant uptake was increased. As the pH was lowered, concentrations of plant Fe first decreased and then increased whilst those of Mn, and to a lesser extent Zn and Cu, increased markedly.     

The contents of risk elements,arsenic speciation,and possible interactions of elements and betalains in beetroot (<Emphasis Type="Italic">Beta vulgaris</Emphasis>, L.) growing in contaminated soil

The effect of enhanced soil risk element contents on the uptake of As, Cd, Pb, and Zn was determined in two pot experiments. Simultaneously, transformation of arsenic and its compounds in beetroot (Beta vulgaris L.) plants was investigated. The mobile fractions of elements were determined in 0.05 mol L⁻¹ (NH₄)₂SO₄ extracts and did not exceed 2% of total soil arsenic, 9% of total cadmium, 3% of total lead, and 8% of total zinc, respectively. Although the soils were extremely contaminated the mobile portions of the elements represented only a small fragment of the total element content. Arsenic contents in beet plants reached up to 25 mg As kg⁻¹ in roots and 48 mg As kg⁻¹ in leaves in the soil characterized by the highest mobile arsenic portion. Arsenic portions extractable with water and phosphate buffer from the beetroot samples did not show significant differences between the extraction agents but the extractability was affected by the arsenic concentration. Arsenic was almost quantitatively extractable from the samples with the lowest total arsenic concentration, whereas in the samples with the highest total arsenic concentration less than 25% was extractable. Arsenate was the dominant arsenic compound in the extracts (70% in phosphate buffer, 50% in water extracts). A small portion of dimethylarsinic acid, not exceeding 0.5%, was detected only in the sample growing in the soil with the highest arsenic concentration. The role of betalains (betanin, isobetanin, vulgaxanthin I and vulgaxanthin II) in transformation/detoxification of arsenic in plants was not confirmed in this experiment because the plants were able to grow in the contaminated soil without any symptoms of arsenic toxicity.     

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.     

Soil isotopically exchangeable zinc: A comparison between E and L values

The objectives of the present paper were: (i) to determine isotopically exchangeable zinc using two isotopic exchange methods (E and L values) in a series of polluted and non-polluted Swiss agricultural soils, and (ii) to evaluate the ability of chemical extraction methods to estimate plant-available soil Zn using isotopic techniques. The surface horizon (0–20 cm) of seven polluted and non-polluted soils representing a wide range in physico-chemical properties and Zn contents were sampled. An isotopic exchange kinetics (IEK) approach was used to assess, in a batch experiment, the isotopically exchangeable Zn content (E value). In order to determine the L values, a pot experiment was carried out with Lolium multiflorum (cv. Axis) in a growth chamber using a ⁶⁵Zn-isotope dilution technique. Total Zn uptake and the isotopic composition (⁶⁵Zn/^stableZn) were determined in Lolium multiflorum for five successive cuts. The amounts of zinc extracted by different chemicals were compared with L values and regression parameters were estimated. The isotopic composition in soil extracted by DTPA and EDTAAc at the end of the pot experiment was also determined. Results showed that the equation describing the increase of isotopically exchangeable Zn with time could be extrapolated to three months for polluted and non-polluted neutral and acidic soils, and that the results were not different from the amount of isotopically exchangeable Zn experimentally determined with Lolium multiflorum (L value). In alkaline soils however, results suggest that either ⁶⁵Zn sorption occurred in the batch experiment or that the concentration of Zn in the soil solution had been overestimated, leading to an overestimation of the E value compared to the L values. Furthermore, the specific activities measured in DTPA and EDTA extractions at the end of the pot experiment were significantly different compared to the specific activity of the plant, showing that both these chelating agents extract neither all the available soil Zn nor only the available soil Zn for plants. Abbreviations: C _Zn– concentration of Zn in a soil water extract (mg Zn L^?1); C _Zn?Plant– concentration of Zn in plant shoots (mg Zn kg^?1 DM); DTPA – diethylene triamine pentaacetic acid; E _1\min– amount of Zn isotopically exchangeable within one min (mg Zn kg^?1 soil); E _(t)\exp– amount of Zn isotopically exchangeable after t min derived from experimental results (mg Zn kg^?1 soil); E _(t)pred– amount of Zn isotopically exchangeable after t min predicted using kinetic parameters derived from a 100 min long isotope exchange kinetic experiment together with C _Zn, and Zn_HNO3 (mg Zn kg^?1 soil); EDTA – ethylene diamine tetraacetic acid; IC_P– isotopic composition of Zn in plant shoots; IC_DTPA– isotopic composition of Zn in the soil DTPA extract; IC_EDTA– isotopic composition of Zn in the soil EDTA extract; IC_SE– isotopic composition of Zn in the soil extracts; IEK – isotope exchange kinetics; L value – amount of plant available Zn (mg Zn kg^?1 soil); Lolium multiflorum; TEA – Triethanolamine; Zn_DTPA– Zn extractable by 0.005 M DTPA + 0.01 M CaCl2 + 0.1 M TEA; Zn_EDTA?NH4Ac– Zn extractable by 0.5 M NH₄Ac, 0.02 M EDTA; Zn_{EDTA?Ca(NO3)2}– Zn extractable by 0.005 M EDTA, 0.01 M Ca(NO₃)₂; Zn_KCl– Zn extractable by 1 M KCl; Zn_CaCl2– Zn extractable by 0.01 M CaCl₂; Zn_NaNO3– Zn extractable by 0.1 M NaNO₃; Zn_HNO3– Zn extractable by 2 M HNO₃.     

Evaluation of chemical methods for estimating available zinc and response of green gram (Phaseolus aureus roxb) to applied zinc in non-calcareous soils

Summary Studies were conducted in 22 non-calcareous soils (India) to evaluate various extractants,viz. (6N HCl, 0.1N HCl, EDTA (NH₄)₂CO₃, EDTA NH₄OAc, DTPA+CaCl₂ and 1M MgCl₂) to find critical levels of soil and plant Zn for green gram (Phaseolus aureus Roxb.). The order of extractability by the different extractants was 6N HCl>0.1N HCl>EDTA (NH₄)₂CO₃<EDTA NH₄OAc DTPA+CaCl₂>1M MgCl₂. Critical levels of 0.48 ppm DTPA × CaCl₂ extractable Zn, 0.80 ppm EDTA NH₄OAc extractable Zn, 0.70 ppm EDTA (NH₄)₂CO₃ extractable Zn, and 2.2 ppm 0.1N HCl extractable Zn were estimated for the soils tested. The critical Zn concentration in 6 weeks old plants was found to be 19 ppm. The 0.1N HCl method gave the best correlation (r=0.588^**) between extractable Zn and Bray's per cent yield, while with DTPA+CaCl₂, it was slightly low (r=0.542^**). The DTPA + CaCl₂ method gave significant (r=0.73^**) correlation with plant Zn concentration. The 0.1N HCl gave the higher correlation with Zn uptake (r=0.661^**) than DTPA (r=0.634^**) 6N HCl and 1M MgCl₂ method gave nonsignificant positive relationship with Bray's per cent yield. For noncalcareous soils apart from the common use of DTPA+CaCl₂, 0.1N HCl can also be used for predicting soil available Zn. The use of 0.1N HCl would be much cheaper than DTPA and other extractants used in the study.     

The effects of root-induced pH changes on the depletion of inorganic and organic phosphorus in the rhizosphere

A new method allowing control of rhizosphere pH and mineral nutrition was applied to study depletion of various organic and inorganic phosphorus fractions extractable sequentially with 0.5M KHCO₃ (pH 8.5), 0.1M NaOH and residual P extractable with 6M H₂SO₄ from the rhizosphere soil.Soil pH was affected about 2 mm from the root mat. Depletion zones of inorganic P (KHCO₃-P_i) extractable with 0.5M KHCO₃ extended up to about 4 mm but the depletion zones of all other P fractions were about 1 mm only. The root-induced decrease of soil pH from 6.7 to 5.5 increased the depletion of total P from all fractions by 20% and depletion of KHCO₃-P_i and residual P by 34% and 43%, respectively. Depletion of organic P (KHCO₃-P_o) extractable with 0.5M KHCO₃ was not affected by a change in rhizosphere pH. With constant or increased pH, depletion of inorganic P (NaOH-P_i) was 17% and organic P (NaOH-P_o) was 22% higher than with decreased pH. Only 54–60% of total P withdrawn from all fractions was from KHCO₃-P_i. Substantial amounts of KHCO₃-P_o and NaOH-P_o were mineralized and withdrawn from the rhizosphere within 1 mm from the root mat, as 11–15% of total P withdrawn originated from the organic P fractions. A remaining 11–16% was derived from NaOH-P_i, and 15–18% from residual P fractions likely to be rather immobile. Thus, 40–46% of the P withdrawn near the root mat of rape originated from non-mobile P fractions normally not included in 0.5M NaHCO₃ extraction used to obtain an index of plant-available soil P.     

Effect of different zinc sources and levels on inhibition of the apoptosis induced by glucocorticoid of thymocytes in vitro

This experiment was conducted to investigate the effects of zinc sulfate and zinc methionine (Zn-Met) and their levels on apoptosis induced by glucocorticoid of thymocytes and the possible mechanism. Dexamethasone was used to make the apoptosis model of thymocytes; zinc sulfate and zinc methionine were supplemented to the medium at levels of 0,50, 100, 500, and 1000 μM. The activity of cells,Cu,Zn superoxide dismutase (Cu,Zn-SOD), DNA ladder pattern, intracellular calcium concentration, and the percentage of apoptosis nuclei were determined. Both ZnSO₄ and Zn-Met could modulate apoptosis; they inhibited apoptosis and decreased DNA fragmentation. The regulation was concentration dependent. At levels of 50 and 100 μM, the effect of Zn-Met on inhibiting apoptosis was less efficient than that of ZnSO₄ (p<0.05), but the activity of the cells cultured with Zn-Met was higher than those cultured with ZnSO₄; they showed no difference in modulating apoptosis when added at levels of 500 and 1000 μM to the medium (p>0.05). Intracellular calcium concentrations of cells cultured with Zn-Met were higher than those cultured with ZnSO₄ at the same levels. Zinc supplementation decreased the concentration of intracellular calcium significantly (p<0.05) and increased the activity of Cu,Zn-SOD in the extract of the cells (p<0.05). Both zinc sulfate and Zn-Met could modulate apoptosis of thymocytes induced by glucocorticoid; the mechanism might involve the exchange of intracellular calcium, the redox of cells, and the two forms of zinc might go different ways in the regulations.     

Organogenesis,embryogenesis, and synthetic seed production in <Emphasis Type="Italic">Arnebia euchroma</Emphasis>—A critically endangered medicinal plant of the Himalaya

Summary This is the first report of simultaneous organogenesis and somatic embryogenesis in Arnebia euchroma, a highly valued, critically endangered medicinal plant of the Himalaya. Root-derived callus showed only rhizogenesis, whereas leaf-derived callus showed simutaneous organogenesis and somatic embryogenesis. Organogenesis was optimal (12.2 shoots per culture) in 1 μM indole-3-butyric acid combined with 2.5 μM 6-benzyladenine and induction of somatic embryogenesis (16.3 embryos per culture) occurred in 2.5 μM indole-3-butyric acid combined with 2.5 μM 6-benzyladenine. Shoots rooted (100%) best in half-strength Murashige and Skoog (MS) medium supplemented with 2.0 μM indole-3-butyric acid. Early cotyledonary-stage embryos encapsulated with 3% sodium alginate and calcium nitrate (100 mM for 25 min) showed 60.6% germination in MS medium. Rooted shoots transferred to a mixture of sterile soil, sand, and peat (1∶1∶1 by volume) showed 72% survival ex vitro. Application of these protocols would be helpful in reducing pressure in natural populations, in genetic transformation studies, and in long-term storage of elite genotypes through synthetic seed production.     

Computer simulation of Zn(II) speciation and effect of Gd(III) on Zn(II) speciation in human blood plasma

The speciation and distribution of Zn(II) and the effect of Gd(III) on Zn(II) speciation in human blood plasma were studied by computer simulation. The results show that, in normal blood plasma, the most predominant species of Zn(II) are [Zn(HSA)] (58.2%), [Zn(IgG)](20.1%), [Zn(Tf)] (10.4%), ternary complexes of [Zn(Cit)(Cys)] (6.6%) and of [Zn(Cys)(His)H] (1.6%), and the binary complex of [Zn(Cys)₂H] (1.2%). When zinc is deficient, the distribution of Zn(II) species is similar to that in normal blood plasma. Then, the distribution changes with increasing zinc(II) total concentration. Overloading Zn(II) is initially mainly bound to human serum albumin (HSA). As the available amount of HSA is exceeded, phosphate metal and carbonate metal species are established. Gd(III) entering human blood plasma predominantly competes for phosphate and carbonate to form precipitate species. However, Zn(II) complexes with phosphate and carbonate are negligible in normal blood plasma, so Gd(III) only have a little effect on zinc(II) species in human blood plasma at a concentration above 1.0×10⁻⁴ M.     

Lack of correlation between TBARS production and PUFA degradation during incubation of membrane erythrocytes in an OH⋅ (Fe2+/H2O2) generator system

We investigated the effects of an OH^⋅ (Fe²⁺/H₂O₂) generator system of erythrocyte membrane, particularly the time-course of lipid peroxidation as estimated by measurement of conjugated dienes, thiobarbituric reactive substances (TBARS), lipofuscin-like pigments, and α-tocopherol. Polyunsaturated fatty acids (PUFAs), especially arachidonic acid (20∶4 ω 6) and docosahexenoic acid (22∶6 ω 3), were also measured. Erythrocyte membranes were suspended in phosphate buffer containing Fe²⁺ (200 μM) and H₂O₂ (1.42 mM), and incubated in a shaking water bath at 37°C. Initially, there was an increase in TBARS and lipofuscin-like pigments, two well-known end products of PUFA oxidative degradation, whereas PUFAs remained unchanged (incubation time: 1 h). After two or more hours of incubation, marked lipid peroxidation was noted, with the appearance of conjugated dienes and a decrease of PUFAs, indicating that lipid peroxidation had occurred after a lag phase during which TBARS were not produced from PUFAs. This suggests that another OH^⋅ target was involved.     

Effect of zinc and cadmium on physiological and production characteristics in <Emphasis Type="Italic">Matricaria recutita</Emphasis>

Effects of zinc (12–180 μM) alone and in mixtures with 12 μM Cd on metal accumulation, dry masses of roots and shoots, root respiration rate, variable to maximum fluorescence ratio (F_V/F_M), and content of photosynthetic pigments were studied in hydroponically cultivated chamomile (Matricaria recutita) plants. The content of Zn in roots and shoots increased with the increasing external Zn concentration and its accumulation in the roots was higher than that in the shoots. While at lower Zn concentrations (12 and 60 μM) the presence of 12 μM Cd decreased Zn accumulation in the roots, treatment with 120 and 180 μM Zn together with 12 μM Cd caused enhancement of Zn content in the root. Presence of Zn (12–120 μM) decreased Cd accumulation in roots. On the other hand, Cd content in the shoots of plants treated with Zn + Cd exceeded that in the plants treated only with 12 μM Cd. Only higher Zn concentrations (120 and 180 μM) and Zn + Cd mixtures negatively influenced dry mass, chlorophyll (Chl) and carotenoid content, F_V/F_M and root respiration rate. Chl b was reduced to a higher extent than Chl a.     

Zinc deficiency-induced phytosiderophore release by the Triticaceae is not consistently expressed in solution culture

The effects of zinc (Zn) and iron (Fe) deficiencies on phytosiderophore (PS) exudation by three barley (Hordeum vulgare L.) cultivars differing in Zn efficiency were assessed using chelator-buffered nutrient solutions. A similar study was carried out with four wheat (Triticum aestivum L. and T. durum Desf.) cultivars, including the Zn-efficient Aroona and Zn-inefficient Durati. Despite severe Zn deficiency, none of the barley or wheat cultivars studied exhibited significantly elevated PS release rates, although there was significantly enhanced PS exudation under Fe deficiency. Aroona and Durati wheats were grown in a further study of the effects of phosphate (P) supply on PS release, using 100 μM KH₂PO₄ as high P, or solid hydroxyapatite as a supply of low-release P. Phytosiderophore exudation was not increased due to P treatment under control or Zn-deficient conditions, but was increased by Fe deficiency. Accumulation of P in shoots of Zn- and Fe-deficient plants was seen in both P treatments, somewhat more so under the KH₂PO₄ treatment. Zinc-efficient wheats and grasses have been previously shown to exude more PS under Zn deficiency than Zn-inefficient genotypes. We did not observe Zn-deficiency-induced PS release and were unable to replicate the results of previous researchers. The tendency for Zn deficiency to induce PS release seems to be method dependent, and we suggest that all reported instances may be explained by an induced physiological deficiency of Fe. Received: 25 October 1999 / Accepted: 3 December 1999     

Study on the Relationship between Soil Selenium and Plant Selenium Uptake

Various extraction methods have been used to determine selenium (Se) concentrations in soils and plants in the second seleniferous regions of China. Our results show tea Se contents in the study area range from 1.009 to 2.6 mg/kg, which reveal that the tea areas in Ziyang County are in seleniferous regions. The four extraction methods evaluated in this study provide different information concerning soil and plant Se levels. The quality control/quality assurance program for this project indicated there is excellent agreement between total soil Se and extractable Se. For example, phosphate extractable Se results from the field investigation and greenhouse study were found to be highly correlated (R ² > 0.91) by linear regression analyses. Results from rye seedling experiments further show phosphate extractable Se has significant correlations with plant Se uptake and that a 0.1 M solution of KH₂PO₄ can be used as the extractant of soil available Se. In the acid soil, the Brassica campestris yield could be significantly reduced when the content of Se⁶⁺–Se ≥ 0.5 mg/kg, and the influence on the yield was not as obvious when the content of Se⁶⁺–Se reached up to 2.0 mg/kg. The uptake by Brassica campestris of Se⁶⁺–Se is higher than that of Se⁴⁺–Se. The main factors influencing the biological availability of soil Se, in order of their importance are CaCO₃, the presence of silt grains, organic matter and the presence of clay grains. pH could affect KH₂PO₄ extractable Se through CaCO₃.     

Improving phosphorus availability in two upland soils of Vietnam using Tithonia diversifolia H.

Phosphorus was added to two acidic upland soils (a Cambisol and a Ferralsol) at two rates (9 mg P kg⁻¹ and 145 mg P kg⁻¹) either in an inorganic P form (KH₂PO₄) or as a green manure (Tithonia diversifolia H. at 2.5 g kg⁻¹ and 40 g kg⁻¹). The effect of P source on the chemical availability of P was assessed in an incubation experiment by measuring resin extractable P, soluble molybdate reactive (DMR-P) and unreactive P (DMU-P). Soil pH and extractable Al were monitored during the incubation period of 49 days. Green manure addition caused an immediate and sustained increase in soil pH and an immediate and sustained decrease in extractable Al. Labile P (resin P + DMR-P + DMU-P) was increased more by P added as a green manure than when added in inorganic form in one soil (Ferralsol), while it decreased or did not differ in the other one (Cambisol). In both soils, the concentrations of soluble DMU-P were frequently higher where Tithonia had been added. The effects of green manure amendment on physical factors governing the phosphorus supply through diffusive transport were also investigated. Aggregate size distribution was substantially changed by green manure amendment due to a shift in the percentage of microaggregates (<250 μm in diameter) to larger sizes. Changes in soil aggregation as a consequence of green manure amendment led to a reduction in specific surface area (SSA) of the whole soil. Coupled with the large increase in effective cation exchange capacity caused by green manure amendment in both soils, and the decrease in SSA, there was an increase in the net negative surface charge density in both soils. In summary, at a large addition rate – and in addition to the well-known effect derived from the extra supply in P, green manure amendment may improve the chemical availability and diffusive supply of P through the following mechanisms: (i) an increase in soil pH increasing the solubility of phosphate sources; (ii) a decrease in extractable Al reducing the fixation of added P; (iii) increased macro-aggregation and reduced specific surface area and porosity leading to fewer sorption sites for P and hence enhanced diffusion rates; and (iv) increased negative charges and reduced positive charges at the soil surface resulting in a net increase in repulsive force for P. The induced changes in most measured soil properties were smaller in the Ferralsol than in the Cambisol. This revised version was published online in June 2005 with a corrected article title.     

Effects of cadmium and copper on zinc transport kinetics by isolated renal proximal cells

Zinc, cadmium, and copper are known to interact in many transport processes, but the mechanism of inhibition is widely debated, being either competitive or noncompetitive according to the experimental model employed. We investigated the mechanisms of inhibition of zinc transport by cadmium and copper using renal proximal cells isolated from rabbit kidney. Initial rates of⁶⁵Zn uptake were assessed after 0.5 min of incubation. The kinetics parameters of zinc uptake obtained at 20°C were a J_max of 208.0±8.4 pmol· min⁻¹·(mg protein)⁻¹, aK _m of 15.0±1.5 μM and an unsaturable constant of 0.259±0.104 (n=8). Cadmium at 15 μM competitively inhibited zinc uptake. In the presence of 50 μM cadmium, or copper at both 15 and 50 μM, there was evidence of noncompetitive inhibition. These data suggest that zinc and cadmium enter renal proximal cells via a common, saturable, carrier-mediated process. The mechanisms of the noncompetitive inhibition observed at higher concentrations of cadmium or with copper require further investigation, but may involve a toxic effect on the cytoskeleton.