Ca2+ effects on ethylene, carbon dioxide and 1-aminocyclopropane-1-carboxylic acid synthase activity

The response of pericarp disks from ripening tomato (Lycopersicon esculentum Mill. cv. Traveler‘76) to CaCl₂, additions was studied to determine the effect of Ca²⁺ on ethylene and CO₂ production. Application of 5 mM CaCl₂ resulted in a 2, 20, 33, 39, and 50% increase in ethylene production in disks obtained from preclimacteric minimum, climacteric rise, climacteric peak, one, and two days postclimacteric fruit, respectively. CaCl₂ concentrations of 10 and 50 mM gave no additional stimulation of ethylene production; CO₂ production at 5 mM CaCl₂ was not different from controls, but is increased at 10 and 50mM CaCl₂. CaCl₂ also increased ethylene production in disks treated with 1-aminocyclopropane-1-carboxylic acid (ACC) or aminoethoxy-vinylglycine. Chloride salts of K⁺, Na⁺, Mg²⁺, Sr²⁺ and La³⁺ did not stimulate ethylene production. SrCl₂ stimulated ethylene production to a lesser degree than CaCl₂. Disks from potato (Solanum tuberosum L. cv. Katahdin) tubers produced greater quantities of ethylene and ACC when 5 mM CaCl₂ was included in the incubation medium (K. B. Evensen, 1983. Physiol. Plant. 60:125–128). Ca²⁺-treated disks had more than three times as much ACC synthase activity as control disks after 18 to 24 h incubation, when ethylene and ACC were maximal. The apparent K_m for S-adenosylmethionine was 13 μM at 29°C, pH 8.0 in extracts from both Ca²⁺-treated and control disks. Inclusion of 1 to 50 mM CaCl₂ in the assay medium did not significantly affect enzyme activity. ACC synthase extracted from control and Ca²⁺-treated disks had a pH optimum of 8.5 and an apparent molecular weight of 72 kdalton, estimated by gel filtration. It is likely that the presence of Ca²⁺ in the buffer allows greater synthesis of ACC synthase as part of the wound-healing response in potato, while in tomato the predominant effect is on membrane stabilization.     

Phosphate potential and phosphate capacity of soils

Summary The relationship between the phosphate potential (I) and the amount of phosphate (Q), added to the soil has been examined by equilibrating soil samples with 0.001M or 0.01M CaCl₂ solutions containing various amounts of phosphate. For one neutral and two alkaline soils the Q/I relationship depends on the CaCl₂ concentration and the pH in such a way that the apparent values of I decrease when the CaCl₂ concentration increases from 0.001 M to 0.01M. The difference between the two values increases when the pH increases. When correction is made for the formation of the soluble calcium phosphate complex, CaHPO₄, the Q/I relationship becomes independent of the CaCl₂ concentration. The initial phosphate potential (I₀) determined by interpolation, is also found to be independent of the CaCl₂ concentration. The necessary amount of phosphate to be added or removed per gram of soil in order to obtain a certain alteration of the phosphate potential is designated the differential phosphate potential buffering capacity, DPBC. For ten soils DPBC-values are determined on the basis of the Q/I relationships, (ΔQ/ΔI)_Io, and found to be independent of the CaCl₂ concentration. The content of colloids and of inorganic phosphate accounts for a significant part of the variation in the DPBC for different soils. The importance of the DPBC for characterization of the phosphate status of soils in respect to phosphate supply to plants is briefly discussed. The author is indebted to professor, Dr. H. C. Aslyng, head of the department for his suggestions and helpful criticism during the progress of this work.     

Increasing NaCl and CaCl2 concentrations in the growth medium of quince leaves: II. Effects on shoot regeneration

Summary The effects of NaCl and CaCl₂ on shoot regeneration from quince (Cydonia oblonga BA L29 clone) leaves were investigated. Caulogenesis was induced on in vitro-grown leaves treated for 2d in liquid Murashige and Skoog (MS) medium with 11.3 μM 2,4-dichlorophenoxyacetic acid and cultured on MS gelled medium supplemented with 4.5 μM thidiazuron and 0.5 μM naphthaleneacetic acid. Three experiments were performed: in the first, we compared the effects of NaCl at 0, 25, 50, 100, and 200 mM in factorial combination with 3, 9, and 27 mM CaCl₂. In the second, NaCl was tested at 0, 5, 10, 20, 40, and 80 mM with CaCl₂ at 0.3, 1.0, and 3.0 mM. The third experiment was carried out with the same experimental design as the second one but replacing NaCl with Na₂SO₄. Shoot regeneration was evaluated after 50 d of culturing: 25 in darkness and 25 in white light. In the first experiment, shoot regeneration was very poor and was observed only at the lower salt concentrations. In the second experiment, the percentages of caulogenic leaves were much higher, but decreased with increasing NaCl concentration. The more pronounced negative effect of the highest NaCl concentrations appeared to be partly mitigated by CaCl₂ at 1 and 3 mM. The presence of 3 mM CaCl₂, in the experiment with Na₂SO₄, appeared to be even more effective in reducing the adverse effect of sodium stress on caulogenesis. This result was attributed to the lower Cl⁻ concentration in the growth medium, which resulted from replacing NaCl with Na₂SO₄. NaCl applied at low concentrations (5 and 10 mM) in combination with 3 mM CaCl₂ exerted a favorable effect on adventitious shoot regeneration. As regards the Na⁺ and Ca²⁺ interaction, when the Na⁺/Ca²⁺ ratio was below roughly 35 and 20, with NaCl and Na₂SO₄, respectively, at least 60% of leaves showed regenerating capacity, but optimal values of this ratio were not derived.     

The effects of soluble-Al on root growth and radicle elongation

Summary In order to determine the effects of concentration on plant growth, aluminium (Al) was extracted (10^–3 M CaCl₂) from 4 acid brown hill soils which had been treated with superphosphate at rates equivalent to 0 to 300 kg P ha^–1. The soils ranged in pH (CaCl₂) from 3.5 to 4.9, and Al concentration from 0 to 0.6 mM. The effects of Al on ryegrass growth in the 4 soils in a glasshouse was compared with its effect on radicle elongation of seeds germinated in contact with CaCl₂ extracts from the same soils.Ryegrass root growth in the glasshouse, and radicle elongation in the bioassay test were both unaffected by Al concentrations below 0.1 mM. Root growth was substantially reduced when Al concentration exceeded 0.1 mM and above 0.2 mM growth was almost completely inhibited. Radicle elongation rate was also reduced when the concentration of Al was greater than 0.2 mM agreeing well with the observation from the pot experiment.It is concluded that because of its speed and convenience the bioassay method offers a useful method of establishing critical levels of Al for crop plants.     

Increasing NaCl and CaCl<Subscript>2</Subscript> concentrations in the growth medium of quince leaves: I. Effects on somatic embryo and root regeneration

Summary The effects of increasing concentrations of NaCl and CaCl₂ on quince (Cydonia oblonga Mill. BA 29 clone) somatic embryogenesis and adventitious root regeneration were investigated. Leaves collected from in vitro-grown shoots were used as explants and induced for 2d in liquid Murashige and Skoog medium containing 11.3 μM 2,4-dichlorophenoxyacetic acid. Explants were then cultured on semisolid Murashige and Skoog medium enriched with 4.7 μM kinetin and 0.5 μM naphthaleneacetic acid under red light for 25 d and under white light for another 25 d. Two experiments were performed: in the first, NaCl was used at 0,25, 50, 100, and 200 mM in factorial combination with CaCl₂ at 3, 9, and 27 mM; in the second, NaCl was applied at 0, 5, 10, 20, 40, and 80 mM in combination with CaCl₂ at 0.3, 1.0, and 3.0 mM. Quince leaves revealed the capacity to regenerate somatic embryos and/or adventitious roots. Quantitative and qualitative regeneration from leaves was affected by NaCl treatments: increasing NaCl concentrations, in combination with CaCl₂ at 1 mM, led to an increase in the proportion of leaves producing somatic embryos only, and to a decrease of both leaves regenerating roots only and leaves simultaneously producing somatic embryos and adventitious roots. This suggests a beneficial effect of salt stress on the embryogenic process. The regeneration response decreased with increasing salt concentrations and was almost totally inhibited above 50 mM NaCl and 9 mM CaCl₂. The presence of CaCl₂ in the culture medium apparently mitigated the effects of salt stress, but only when NaCl was applied at 40 mM. NaCl at 5 mM, in the presence of 0.3 or 1 mM CaCl₂, was favorable both to somatic embryo and root production. No value of the ratio Na⁺/Ca²⁺ was found to be optimal for the regeneration processes.     

Phosphate supplying power of rock phosphates in an oxisol

Summary The P-supplying power of triple superphosphate, three apatitic rock phospates and a calcined aluminum rock phosphate were tested by measuring the quantities of fertilizer P recovered in soybean and in four chemical extractants, after 3-day and 75-day periods of contact between soil and fertilizer.The triple superphosphate supplied the highest amounts of P, but it lost efficiency during the longer incubation period. The rock phosphates maintained their original efficiencies, probably as a result of a balance between P released from the fertilizer and P converted into non-labile forms.The following coefficients of correlation between P uptake by soybean from an acid oxisol and P extracted by chemical extractants, after the two incubation periods, were found: 0.902^** for 0.01M CaCl₂; 0.823^** for anion-exchange resin; 0.720^** for 0.03N NH₄F+0.025N HCl; –0.037 (n.s.) for 0.025N H₂SO₄+0.050N HCl.The acid NH₄F solubilized residual calcined aluminum phosphate particles, and double acid extracted P from residual apatite particles, thus accounting for their poorer performances in predicting availability of fertilizer P.The relative efficiencies of the rock phosphates could largely be predicted after an incubation period of only three days. This finding attests to the presence in these rock phosphates of an easily soluble fraction of P which is not indicative of the degree of reactiveness of the phosphate as a whole.on leave at the Agricultural University during 1977.     

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.     

Influence of pH,exchangeable aluminium and 0.02M CaCl2-extractable aluminium on the growth and nitrogen-fixing activity of white clover (Trifolium repens) in some New Zealand soils

The effects of soil acidity on the growth and N₂-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H₂PO₄)₂) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl₂-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO₃) resulted in increased plant growth on most soils. N₂[C₂H₂]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl₂-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl₂-extractable Al levels of less than about 3.3 g g^–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl₂-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl₂ extracts.     

Chickpea and white lupin rhizosphere carboxylates vary with soil properties and enhance phosphorus uptake

Chickpea and white lupin roots are able to exude large amounts of carboxylates, but the resulting concentrations in the rhizosphere vary widely. We grew chickpea in pots in eleven different Western Australian soils, all with low phosphorus concentrations. While final plant mass varied more than two-fold and phosphorus content almost five-fold, there were only minor changes in root morphological traits that potentially enhance phosphorus uptake (e.g., the proportion of plant mass allocated to roots, or the length of roots per unit root mass). In contrast, the concentration of carboxylates (mainly malonate, citrate and malate, extracted using a 0.2 mM CaCl₂ solution) varied ten-fold (averaging 2.3 mol g^–1 dry rhizosphere soil, approximately equivalent to a soil solution concentration of 23 mM). Plant phosphorus uptake was positively correlated with the concentration of carboxylates in the rhizosphere, and it was consistently higher in soils with a smaller capacity to sorb phosphorus. Phosphorus content was not correlated with bicarbonate-extractable phosphorus or any other single soil trait. These results suggest that exuded carboxylates increased the availability of phosphorus to the plant, however, the factors that affected root exudation rates are not known. When grown in the same six soils, three commonly used Western Australian chickpea cultivars had very similar rhizosphere carboxylate concentrations (extracted using a 0.2 mM CaCl₂ solution), suggesting that there is little genetic variation for this trait in chickpea. Variation in the concentration of carboxylates in the rhizosphere of white lupin did not parallel that of chickpea across the six soils. However, in both species the proportion of citrate decreased and that of malate increased at lower soil pH. We conclude that patterns of variation in root exudates need to be understood to optimise the use of this trait in enhancing crop phosphorus uptake.     

The growth of WI-38 cells in a serum-free,growth factor-free,medium with elevated calcium concentrations

Summary The growth of WI-38 cells in serum-free growth medium with and without hormone supplementation in the presence of elevated Ca²⁺ concentrations was investigated. At 5 mM CaCl₂, WI-38 cells seeded at low density without serum or hormone supplementation showed up to a 12-fold increased in cell number at saturation density over that obtained at day 1. Saturation densities were comparable when either 5 mM CaCl₂ or epidermal growth factor (1 mM CaCl₂) was used in the presence of insulin, dexamethasone and transferrin. Combining suboptimal doses of epidermal growth factor and CaCl₂ resulted in an additive effect on saturation density. Thus, nornal human diploid cells are capable of substantial growth in serum-free, hormone-free growth medium. In contrast, confluent cultures refed with the same medium are not responsive to elevated Ca²⁺ concentrations. In fact, elevated Ca²⁺ concentrations inhibited the proliferative response of confluent cultures to epidermal growth factor, but enhanced their response to the combined treatment of insulin, transferrin and dexamethasone. This work was supported by the United States Public Health Society grants T-32, CA09171 and AG-00378. Editor's Statement This paper rigorously dissects the interplay among external Ca²⁺ concentration, cell density and specific growth factors on fibroblast growth in defined medium. Wallace L. McKeehan     

In vitro modulation of filament bundling in f-actin and keratins by annexin II and calcium

Summary In our preliminary subcellular localization experiment we demonstrated that annexin II co-localized with submembranous actin in subpopulations of both cultured fibroblasts and keratinocytes. To investigate the physical interaction between annexin II and actin at the cell periphery, in vitro reconstitution experiments were carried out with keratins used as a control. Annexin II, isolated by immunoaffinity column chromatography, was found to exist as globular structures measuring 10 to 25 nm in diameter by rotary shadowing, similar to a previous report. We believe that these structures represent its polymeric forms. By negative staining, monomeric annexin II was detectable as tapered rods, measuring 6 nm in length and 1 to 2 nm in diameter. When annexin II was mixed with actin in 3 mM piperazine-N, N-bis-2-ethanesulfonic acid (PIPES) buffer with 10 mM NaCl₂, 2 mM MgCl₂ and 0.1 mM CaCl₂, thick twisting actin bundles formed, confirming previous reports. This bundling was much reduced when calcium was removed. In the presence of 5 mM ethylenediamine tetra-acetic acid (EDTA) in 5 mM tris, pH 7.2, keratins were found to form a network of filaments, which began to disassemble when the chelator was removed and became fragmented when 0.1 mM CaCl₂ was added. Keratins under the same conditions did not fragment when annexin II was present. These results suggest that annexin II, in conjunction with Ca²⁺, may be involved in a flexible system accommodating changes in the membrane cytoskeletal framework at the cell periphery in keratinocytes.     

Characterization of soil phosphorus by anion exchange resin adsorption and P32-equilibration

Summary Adsorption of phosphate by the anion-exchange resin Dowex-2 was investigated. The resin adsorbed small quantities of P from solution quantitatively. The rate of P-adsorption by resin agitated in solution was proportional to the P-concentration in solution, and was independent of the rate of diffusion of adsorbed P in the resin. When 1 g of soil was shaken continuously with 1 g of resin in 100 ml of water, the rate of P-adsorption by the resin was controlled by the rate of P-release from the soil. Quantities of P adsorbed from soil by resin after different lengths of time were less than those equilibrated with P³² during the same time intervals. The curves showing quantity of P adsorbed vs. time could be satisfactorily described by the hypothesis that there were three simultaneous reactions differing in rate, each reaction being first-order with respect to P. The same was true of the P³²-equilibration data, except that the rate of the slowest reaction was apparently independent of time. In a group of 16 soils, the correlation between P adsorbed by the resin in 2 hours and P-availability to plants in the greenhouse, measured by the isotope-dilution method of Fried and Dean, was 0.95. The corresponding correlation between P extracted by the 0.25N HCl — 0.03N NH₄F extractant of Bray and Kurtz was 0.91.Joint contribution from the Iowa Agricultural Experiment Station and the Eastern Soil and Water Management Section, U.S. Department of Agriculture. Journal Paper No. J-2639 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project No. 1183.Graduate Assistants, Professor of Soils, and Associate Professor of Chemistry, respectively.     

Effects of temperature and prior flooding on intensity and sorption of phosphorus in soil

Summary Effects of temperature and flooded-drained soil conditions on 0.01M CaCl₂ extractable phosphorus (soluble P) were investigated in four soils over the period of 42 days after fertilizer-P application. These soils show severe induced P deficiency problem in crops following flooded rice culture. The effects of temperature on the reaction rate constants were determined and activation energy was calculated. Increasing soil temperature as well as prior flooding of soil decreased soluble P concentration but the effect of the latter was dominant. The decrease in soluble P concentration in these soils with time followed a first order kinetics and the rate constant (K₁) increased as the temperature increased from 10°C to 30°C. The activation energy (Ea) for the kinetics of soluble P concentration in soil, as affected by temperature, was found to be 8.9 and 34.5 KJ mol⁻¹ for Meyers and Willows clay, respectively, over the temperature range studied.     

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.     

Response of corn to micronutrients (Zn and Cu) on a saline soil. I. Growth and ionic relationships

Summary Response of corn to Zn and Cu on a salinized soil in pots has been studied. Zinc increased the dry weights of tops and roots at all levels of NaCl+CaCl₂. Increasing Zn level increased the weights considerably at 10 mM NaCl+CaCl₂. Copper increased the weight of tops at 10 mM NaCl+CaCl₂: it had little effects on plant weights at 50–125 mM NaCl+CaCl₂. The growth response of plant to low Cu was somewhat similar to that of higher Zinc.NaCl+CaCl₂ treatments, in general, increased Zn concentrations in tops as well as roots. At low Zn application, Zn concentrations in the tops were higher than those in the roots but at high Zn application, the concentrations of Zn in tops were similar to those in the roots.NaCl+CaCl₂ treatments increased Cu concentrations in the tops to a slight extent but had a depressive effect on those in the roots. Copper concentrations in the tops were, however, much below those in the roots. The greater retention of Cu in the roots remains to be explained.     

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.     

Transformations of fertilizer nitrogen in soil

Summary Five crops of oats were grown over a 14-month period on a Chester silt loam soil fertilized with N¹⁵-labelled (NH₄)₂SO₄. All plant material from the first four crops was returned to the soil. Following a fifth crop, oat tops and roots were harvested, and the soil was subjected to repeated extractions by autoclaving in 0.01M CaCl₂. The distribution of N¹⁵ and of indigenous soil N among chemical fractions of the extracts, and in the acid-soluble and acid-soluble and acid-insoluble portions of the soil residues following 0.01M CaCl₂ extraction, was remarkably similar. Since appreciable equilibrations between added N¹⁵ and the more resistant forms of soil organic N is unlikely, it is postulated that fertilizer N became incorporated in newly-formed complexes, similar to those already present in the soil. This view is in harmony with the finding that percentage removals of total and N¹⁵-labelled N remained almost the same, even with recovery of approximately 55 per cent of the amounts originally present. N mineralization capacity of the soil was reduced appreciably as a result of extraction.     

川西亚高山三种森林恢复途径对土壤生物有效磷的影响

土壤生物有效磷在提高森林生产力和生物地球化学循环中起着至关重要的作用,研究不同森林恢复途径对土壤生物有效磷的影响对于退化森林的适应性恢复和可持续经营具有重要意义。选取川西亚高山不同恢复途径下形成的3种森林类型,即粗枝云杉人工林（人工种植,PF）、岷江冷杉-红桦天然次生林（自然更新,NF）和粗枝云杉阔叶混交林（人工种植后自然更新,MF）,采用基于生物有效性的土壤磷分级方法测定土壤生物有效磷（CaCl₂-P、Citrate-P、Enzyme-P和HCl-P）,探究不同森林恢复途径对土壤生物有效磷的影响。结果表明：不同森林恢复途径对土壤生物有效磷影响显著（P<0.05）,NF和MF的土壤Citrate-P和Enzyme-P显著高于PF（P<0.05）,而PF的土壤HCl-P显著高于NF（P<0.05）。自然更新是3种森林恢复途径中最能提高土壤生物有效磷的方式。3种森林恢复途径下的土壤生物有效磷组分与速效磷均呈现显著的正相关关系,且NF的土壤速效磷与生物有效磷的相关性更强（CaCl₂-P除外）。显著影响NF土壤生物有效磷的土壤理化性质有全钾、铵态氮含量和pH值,且全钾对NF的土壤生物有效磷变异的解释程度最高（r²=0.63,P=0.001）。土壤pH值、钙和可溶性有机碳含量是显著影响MF土壤生物有效磷的主要土壤理化性质（P<0.05）。对PF的土壤生物有效磷具有显著影响的土壤理化性质是土壤有机碳、铁和可溶性有机碳含量。土壤理化性质对3种恢复途径下森林土壤生物有效磷的解释率均超过了80%,森林恢复途径对土壤生物有效磷的影响与土壤理化性质有关。     

Assessment of phosphorus leaching losses from arable land

Phosphorus (P) losses from soil to water by erosion and surface runoff have been much studied and quantified. However, P losses by leaching have received much less attention, mainly because, until recently, the quantities involved were not considered to be of environmental significance. Furthermore, P leaching losses, unlike P losses from erosion or surface runoff were not believed to be related to rates of P addition, as inorganic fertilizer or manures. Here we report results from a number of field and laboratory experiments, designed to assess the significance of P leaching losses from soil to water. Annual cumulative total P losses in drainage waters from four UK field sites ranged from about 0.03 to 5 kg P ha⁻¹ during 2001–2002. Molybdate reactive P ranged from 45–57%, soluble organic P from 10–13% and particulate P from 29–45% of total P on the two sites (Broadbalk and Woburn) where they could be accurately measured. The proportions of these different P forms were comparable in all treatments, including drainage waters from the unfertilised soils and soils receiving long-term applications of farmyard manure or inorganic fertilizer. In all soils, there was indication of an Olsen- (0.5 M NaHC0₃-, pH 8.5) extractable P concentration, (termed the Change-Point), where P measured in field drainage waters or in laboratory soil extracts of 0.01 M CaCl₂ began to increase linearly as Olsen-P increased. There was also some agreement between drainage water-P or CaCl₂-P and the Olsen-P concentration where the Change-Point occurred. This suggests that CaCl₂-extractable P may provide an approximate indicator of soil P concentrations above which significant quantities of P may be lost by leaching under field conditions. There were positive linear relationships between soil dithionate-extractable Al and soil organic C with the Change-Point: [Change Point = [(0.049)[Al3+] minus (9.2)(% organic C)] accounting for 93% of the variance in the data. If this relationship holds under further testing it could well be a useful predictor of Change-Points in different soils. Phosphate sorption isotherms were used to study the soil P concentrations above which P was at risk of moving from soil to water. They showed that soil solution P concentrations were significantly lower between pH 6.9–7.2 than between pH 7.7–8.1, with implications for P loss from soil to water.     

Calcium-cadmium interaction on sugar absorption across the rabbit jejunum

The element Cd is considered to have no biological function and is highly toxic to humans and animals. Toxic effects of this metal upon cell membrane structure and function have been shown. On the other hand, Ca is an essential element in a wide variety of cellular activities. The present study was initiated to research whether the interaction between Ca and Cd could affect D-galactose absorption across the rabbit jejunum in vitro. In media with Ca²⁺, when CdCl₂ was present at 0.5 or 1 mM, Cd was found to significantly reduce the sugar absorption. In Ca²⁺-free media, where CaCl₂, was omitted and replaced isotonically with choline chloride, the sugar transport was not modified by Cd, but when CaCl₂ was replaced isotonically with MgCl₂, the inhibition is observed. Verapamil at 10⁻⁶ M (blocking mainly Ca²⁺ transport) did not modify the inhibitory effect of cadmium on D-galactose transport. When 10⁻⁶ M of A 23187 (Ca²⁺ specific ionophore) was added in media with/without Ca²⁺; CdCl₂ produced no change in D-galactose transport. These results suggest that Ca and Cd could have affinity for the same chemical groups of enterocyte membrane, which would be related with the intestinal absorption of D-galactose.