Plant uptake of sulphur as related to changes in the HI-reducible and total sulphur fractions in soil

Although considerable progress has been made in relating extractable soil S to plant S availability, most of these studies determined the extractable soil S at the beginning of the experiment to use as an index of soil S status. This bears little or no relationship to the S taken up by plants during the entire growing season. The present study investigates the changes in extractable soil S with time and relates these to changes in plant S uptake. Six soils with different long-term fertiliser histories (0, 21, 42 kg of S as superphosphate ha^–1 applied since 1952) and animal camping treatments (camp and non-camp) were used in two pot systems (with and without plants). Carrier-free ³⁵SO₄–S was added to the soils, to provide the information on the transformations of recently added S between the different extractable S forms in soils and whether these transformations could predict plant-available S. The soils were pre-conditioned and then transferred to the glasshouse, where one set of pots were planted with perennial ryegrass (Lolium perenne L.) while the other set was left uncropped. Periodic plant harvests and soil samplings at four weekly intervals were conducted over a period of 20 weeks to determine plant S uptake and amounts of extractable soil S and ³⁵S forms using five extractants. Same extractions of soil S and ³⁵S were conducted for the initial soils. Results showed that HI-reducible and total soil S extracted by CaCl₂, KH₂PO₄ and by KCl at 40°C were utilised significantly by plants but not those extracted by NaHCO₃ and NaOH extractants. However, after the 8th week, plants continued to take up S even though levels of S extracted from the soil by CaCl₂, KH₂PO₄ and by KCl at 40°C remained low and unchanged. These results suggest that soil S taken up by plants after the 8th week period originated directly from the mineralisation of soil organic S from S pools other than those present in the extractable soil S forms. Similar results were shown by ³⁵S data, thereby confirming the complexity of determining plant S availability based on soil S extraction methods.     

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     

Chemical characterization of two extracts used in the determination of available soil nitrogen

Summary Two soil extracts used for chemical indexes for N availability, 0.01M NaHCO₃ and boiling 0.01M CaCl₂, were analyzed in effort to learn more about the nature of the extracted organic matter (O.M.). The two extracts appeared to remove different fractions of the soil O.M. A study of five soils showed that the C/N value of the NaHCO₃ extract (following decarbonation) was significantly higher than that of the total soil O.M.; while the C/N value in the boiling CaCl₂ extract was not significantly different from that in the soil O.M. There was also significant variation in C/N values among soils for the boiling CaCl₂ extract. The extracts of three soils were analyzed for apparent molecular weight distribution using gel filtration and the results compared to those for base-extracted humic substances. Almost all the molecules in the extracts had apparent molecular weights less than 21,000 daltons while 21 to 47% of the humic substances from the same soils (extracted with 0.5M NaOH) had molecular weights greater than 21,000 daltons. In the boiling CaCl₂ extract, 78 to 87% of the humic substances had apparent molecular weights less than 1,000 daltons, whereas with the NaHCO₃ extract, 42 to 83% of the humic substances were in the 1,000 to 21,000 dalton range. Forty-three to 92% of the N extracted by the NaHCO₃ was in protein form, and 8 to 30% was ninhydrin-detectable. In the boiling CaCl₂ extract 25 to 30% of the extracted N was ninhydrin-detectable. For the same 10 soils, ninhydrin-detectable N values of the boiling CaCl₂ extract appeared closely related to greenhouse and field relative N uptake, while the ninhydrin-detectable N values of the NaHCO₃ extract appeared unrelated to both. The protein N and protein in plus ninhydrin-detectable N values of the NaHCO₃ extract were closely related to greenhouse relative N uptake only. The results of this study indicated that specific fractions of the soil O.M. were being extracted by the two solutions and that significant differences existed in the chemical nature of the two extracts. Paper No. 6175 of the J. Ser. of the Pennsylvania Agric. Exp. Stn. Authorized for publication Jan. 26, 1981.     

Studies on sulphur in vertisols

Summary Some soil and plant test methods were evaluated for predicting response of soybean crop (Glycine max (L.) Merr.) to S application in vertisols. Morgan's reagent, 500 ppm P containing Ca(H₂PO₄)₂.H₂O and KH₂PO₄ solutions, 0.5N NH₄OAc+0.25N HOAc and 0.15% CaCl₂ were found to be suitable extractants for measuring available soil S. The critical limits of extractable S were 9.0 ppm by Morgan's reagent, 10.0 ppm by phosphate solutions, 8.0 ppm by 0.5N NH₄OAc +0.25N HOAc and 14.0 ppm by 0.15% CaCl₂. Morgan's reagent was regarded as superior to other soil test methods in view of its high relationship with S uptake by plants, A values and relative yield. Critical S concentration in soybean plants varied with age. It was 0.15% and 0.185% for 36 and 60 days old plants, respectively. The critical N/S ratio on the other hand appeared to be constant at about 16.5 during vegetative growth period. Constancy of critical N/S ratio in plants was attributed to the near constancy of N/S ratio in plant proteins. There was highly significant relationship between response of soybean to S and to N, supporting the conclusion of some earlier workers that any soil showing large responses to N may not be supplying adequate S from the mineralization of soil organic matter.     

Sulphur fractionation in calcareous soils and bioavailability to plants

Sulphur fractionation and availability to plants are poorly understood in calcareous soils. Sixty-four calcareous soils containing varying amounts of CaCO₃ were collected from ten provinces in China and their S fractions determined. Organic S was the predominant fraction of S, accounting for on average 77% of the soil total S. The amounts of adsorbed sulphate were found to be negligible. 1 M HCl extracted substantially more sulphate than either 0.01 M CaCl₂ or 0.016 M KH₂PO₄, indicating the existence of water-insoluble but acid-soluble sulphate, probably in the form of sulphate co-precipitated with CaCO₃. The concentrations of water-insoluble sulphate correlated positively with the contents of CaCO₃ and accounted for 0.03–40.3% (mean 11.7%) of soil total S. To test the bioavailability of water-insoluble sulphate, a sulphate-CaCO₃ co-precipitate labelled with ³⁵S was prepared and added to a calcareous soil in a pot experiment with either NH₄⁺ or NO₃^– as the N source. In 29 days, wheat plants took up 10.6% and 3.0% of the ³⁵S added to the soil in the NH₄⁺ and NO₃^– treatments, respectively. At the end of the pot experiment, the decrease of water-insoluble, acid-soluble, sulphate was more apparent in the NH₄⁺ than in the NO₃^– treatment. The results indicate that sulphate co-precipitated with CaCO₃ in calcareous soils may become partly available for plant uptake, depending on rhizosphere pH, if the field precipitate is similar to the laboratory prepared sample studied.     

The effect of sesquioxides on the availability of molybdenum toTrifolium subterraneum

Summary Freshly precipitated ferric and aluminium oxides were added separately to soil and their effects on the availability of both native and added molybdenum were determined using subterranean clover (Trifolium subterraneum L.) as the experimental plant. Both oxides lowered the molybdenum content of the clover and, although it had a smaller specific surface area, ferric oxide had a greater effect than aluminium oxide. In a parallel study with similarly treated soil, the amounts of molybdenum extracted by 0.01M CaCl₂, 0.82M NaHCO₃ andM NH₄OAc were also lowered by both oxides, and ferric oxide again had the greater effect. Extraction with 0.275M NaHC₂O₄, as proposed elsewhere for assessing available molybdenum, did not show these differences.     

Optimization of submerged culture conditions for exopolysaccharide production in <Emphasis Type="Italic">Sarcodon aspratus</Emphasis> (Berk) S.lto TG-3

The effect of medium components (carbon, nitrogen, and mineral sources) and environmental factors (initial pH and temperature) for mycelial growth and exopolysaccharide (EPS) production in Sarcodon aspratus(Berk) S.lto TG-3 was investigated. The optimal temperature (25°C) and initial pH (5.0) for the EPS production in shake flask cultures of S. aspratus were determined using the two-dimensional contour plot. The most suitable carbon, nitrogen, and mineral sources for EPS production were glucose, yeast extract, CaCl₂ and KH₂PO₄, respectively. Notably, the EPS production was significantly enhanced by supplementation of calcium ion. Subsequently, the optimum concentration of glucose (30gl^–1), yeast extract (15gl^–1), CaCl₂ (1.1gl^–1), and KH₂PO₄ (1.2gl^–1) were determined using the orthogonal matrix method. The effects of nutritional requirement on the mycelial growth of S.aspratuswere in regular sequence of glucose>KH₂PO₄>yeast extract>CaCl₂, and those on EPS production were in the order of glucose>yeast extract>CaCl₂>KH₂PO₄. Under the optimal culture conditions, the maximum EPS concentration in a 5-l stirred-tank reactor was 2.68gl^–1 after 4days of fermentation, which was 6-fold higher than that at a basal medium. The two-dimensional contour plot and orthogonal matrix method allowed us to find the relationship between environmental factors and nutritional requirement by determining optimal operating conditions for maximum EPS production in S.asparatus. The statistical experiments used in this work can be useful strategies for optimization of submerged culture processes for other mushrooms.     

Effects of organic sulfur compounds on extraction and determination of inorganic sulfate

The common methods for determining inorganic soil sulfate may be affected by the extraction of sulfate from organic sulfur compounds such as ester sulfates. In order to test this, various synthetic organic sulfur compounds (ranging from ester sulfates to sulfonates and C-bonded sulfur) were extracted with deionized water or with two common sulfate extractors (0.5 M NaHCO₃ and 0.02 M NaH₂PO₄). Similar amounts of dissolved sulfate were detected in all extracts of the aromatic ester sulfate hydroxyquinoline sulfate. Sulfate was not released from aliphatic ester sulfates or C-bonded sulfur. Ion chromatography was compared to a turbidimetric method for the determination of sulfate. The latter method, based on BaSO₄-precipitation, appeared to be unsuitable for determining sulfate in organically influenced solutions. Barium precipitated sulfate as well as ester sulfates. Furthermore, the photometry of BaSO₄ was influenced by specific absorption of dissolved organic compounds, leading to a misinterpretation of the sulfate concentration in the solution.     

An attempt to measure phosphate potential in the saturation extract of certain Alfisols of Greece

Summary Dihydrogen phosphate ion activity (H₂PO₄) and phosphate potential were measured in the saturation extracts of 14 representative Red Mediterranean soils (Alfisols) of Greece, estimating the ionic strength from a) measured ionic concentrations b) actual ionic concentrations and c) electrical conductivity measurements. The values of (H₂PO₄) and phosphate potential thus obtained were correlated with P-uptake by rye-grass in a pot experiment. The correlation coefficients did not differ significantly (p≤0.001) from one another and from those of (H₂PO₄) and phosphate potential measured in 0.01M CaCl₂ extracts.     

Histochemical demonstration of carbonic anhydrase activity

Summary Freeze-dried frozen sections are floated on the surface of the freshly prepared incubation mixture (CoSO₄ 1.75 × 10^–3 M, H₂SO₄ 5.3 × 10^–2 M, NaHCO₃ 1.57 × 10^–2 M and KH₂PO₄ 1.17 to 11.7 × 10^–3 M; demonstration of weak activity requires high phosphate). A compound containing cobalt and phosphorous precipitates at carbonic anhydrase sites and is converted to CoS. Adequate staining requires only 2–10 minutes of incubation. Actazolamide inhibits the staining reaction in specific concentrations. Actazolamidein vivo, 20 mg/kgi.v. to mice 30 minutes before sacrifice also inhibited the staining. The proportion phosphorous in the specific precipitate increases with KH₂PO₄ of the medium (shown by the addition of⁶⁰Co and³²P). An explanation of the reaction mechanism is given, based on the catalyzed loss of CO₂ in the surface layer. The inclusion of phosphate in the medium makes this modification ofHäusler's method so sensitive that it shows carbonic anhydrase activity in for instance stratum spinosum of the skin.This investigation was supported by grants from the Medical Faculty, University of Uppsala and from the U.S. National Institutes of Health (Grant NB 3060 to E.Bárány).     

Modeling some mineral nutrient requirements for micropropagated wild apricot shoot cultures

A response surface methodology (RSM) experimental design was applied for improving micropropagation of a wild apricot, Prunus armeniaca Lam., from the mountains of Kazakhstan. In an initial study, woody plant medium (WPM) mineral nutrients [calcium nitrate, ammonium nitrate, mesos (calcium chloride, potassium phosphate and magnesium sulfate) potassium sulfate and minor nutrients] were tested in a response surface methodology (RSM) experiment. Shoot quality was the best when nitrogen and mesos (CaCl₂, MgSO₄, K₂SO₄, KH₂PO₄) compounds were altered. In this study an expanded mesos optimization experiment was run. Data taken included a subjective quality rating, shoot length, shoot number, leaf color and size, callus and physiological disorders. Data were analyzed by Classification and Regression Tree Analysis (CART), a data mining technique that provides specific cutoff values for data and easy to interpret data trees. The CART analysis indicated that the best quality would be with ≤2.4× WPM levels of KH₂PO₄ and ≤0.75× MgSO₄. Shoot length was affected by K₂SO₄, but most shoots were of good size at any concentration. Shoot multiplication was affected by KH₂PO₄, but there were >5 shoots at any concentration. Leaf color was best with ≤2.41× KH₂PO₄ and ≤1.22× K₂SO₄. Based on the CART analysis, a recommendation for improved mesos compounds was developed. Each of the individual trees was analyzed and the cutoff points determined so that all the growth characteristics could be considered in the final concentrations chosen. Using the combined results from the CART analysis, the suggested medium would include WPM with CaCl₂ 2.7×, MgSO₄ 2.7×, K₂SO₄ 0.8×, KH₂PO₄ 0.75×.     

Nature of the calcium effect in phosphate uptake by barley roots

Summary The rates of phosphate uptake of young intact plants were measured in solution at concentrations between 3.2×10^–7 M and 3.2×10^–5 M KH₂PO₄. The addition of 0.005 to 0.01M CaCl₂ increased the uptake rates to values of up to 4 times those of controls. The increases in uptake relative to the controls were most marked at the lowest phosphate concentrations. The addition of KCl also increased the uptake rates, but to a much smaller extent. The results are analyzed by the enzyme kinetic theory, and it is concluded that the uptake of phosphate at low ionic strengths is impeded by negative potentials at the root surface.The experimental results in this letter have appeared in a thesis.     

Simultaneous extraction of soil phytoavailable cadmium,copper, and lead by chelating resin membrane

Cadmium, copper, and lead were extracted from suspensions of contaminated soils using metal chelating exchange resin membranes. Nine soils with widely varying properties and Cd, Cu and Pb levels were tested. Soil suspensions made up with 4 g in 40 mL deionized water were equilibrated with 5 cm² Ca-saturated Chelex exchange resin membrane which was retained inside a polypropylene bag and shaken at 150 rpm for 24 hrs. Resin membrane extractable Cd, Cu and Pb of the soils were correlated with Cd, Cu, and Pb uptake by young wheat seedlings grown in these soils and compared with soil Cd, Cu, and Pb extracted by 0.1 M HCl, 0.01 M CaCl₂, and 0.005 M Diethylenetriamine pentaacetic acid (DTPA). The amounts of Cd, Cu and Pb extracted by the Ca-saturated Chelex membrane from all tested soils correlated well with those absorbed by young wheat seedlings. The Ca-saturated Chelex membrane extractable Cd, Cu and Pb of the soil had the strongest correlation with plant uptake Cd, Cu and Pb among the extraction methods we tested. It was demonstrated that the Ca-saturated Chelex membrane extraction is an appropriate method in simultaneously estimating Cd, Cu and Pb phytoavailability of soil and is applicable to a wide range of soils.     

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.     

The Influence of Phosphate Uptake on Cation Uptake in Fusarium oxysporum f. sp. vasinfectum

Fusarium oxysporum grown in a low phosphate medium was found to take up several times as much K from KH₂PO₄ as from KCI solutions. Large amounts of phosphate also were taken up from KH₂PO₄. Similar large uptakes of Na and phosphate took place from solutions of NaH₂PO₄. Substantial quanties of phosphate were taken up from solutions of Ca(H₂PO₄)₂ in the absence of any appreciable Ca uptake. When the fungus was grown in a medium containing high phosphate, little or no uptake of phosphate from KH₂PO₄ solutions occured and the K Uptake was at the same level as from KCI solutions. During large phosphate uptake sizable reductions in the organic acid content of the fungal cells were observed. Much, but not all, of the data could be explained on the basis of maintenance of charge balance within the cells. – The respiratory rate of fungus, grown in a low P medium, was markedly increased in KH₂PO₄ solution. Fungus, grown in a medium with high phosphate, had a higher respiratory rate which showed only a slight response to KH₂PO₄ solution. Fungus, grown in a medium with high phosphate, had a higher respiratory rate which showed only a slight response to KH₂PO₄.     

Buffering capacity of the chief components of nutritive media for algae

The buffering capacity of solutions of KH₂PO₄ and NaHCO₃ increases with their concentration, the behaviour being describable by mathematical expressions. Solutions of KH₂PO₄ prepared from tap water exhibit a buffering capacity higher by an order of magnitude than those prepared from distilled water. However, there is no difference between the buffering capacities of solutions of NaHCO₃ prepared from tap and distilled water. Urea and NH₄NO₃ have almost no effect on the buffering capacities of sol utions.     

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.     

Growth and physiological characteristics of wasabi plantlets cultured by photoautotrophic micropropagation at different temperatures

Iranian seedless barberry is a very recalcitrant species in in vitro culture which does not show appropriate growth on standard culture media. Response surface methodology was employed to evaluate the effects of changing macronutrients concentrations on establishment and proliferation phases. KNO₃ and NH₄NO₃ macronutrients at 0.3 to 1.5?×?MS medium levels and CaCl₂, MgSO₄ and KH₂PO₄ macronutrients in a range of 0.5 to 1.5?×?MS medium concentrations were tested in a response surface design with 30 treatments. Many significant interactions were found among the macronutrients. High concentrations of KNO₃, NH₄NO₃ and CaCl₂ improved the growth rate in the establishment phase. The growth rate in media containing high KNO₃ and low CaCl₂ was high. Reduced concentrations of CaCl₂ and KNO₃ decreased hyperhydricity. The greatest hyperhydricity was induced when both NH₄NO₃ and CaCl₂ were used at 1.5?×?MS level. The number of hooked leaves decreased as KH₂PO₄ increased and MgSO₄ reduced. In the proliferation phase, there were many significant interactions among the macronutrients. Increased concentration of NH₄NO₃ and reduced concentration of KH₂PO₄ improved the growth rate. Proliferation rate increased in media containing high concentration of KNO₃ and low to moderate concentrations of NH₄NO₃. The greatest production of new tissues and organs was seen in media with high KNO₃ and moderate to high CaCl₂. High concentration of NH₄NO₃ and low concentration of KH₂PO₄ also increased production of new tissues and organs. No shoot apical meristem was seen when CaCl₂ level was high and KNO₃ level was low. Formation of shoot apical meristem required high KH₂PO₄ concentration and low CaCl₂ concentration. Finally, low concentration of KNO₃ and low to moderate concentrations of NH₄NO₃ increased phenol exudation.

     

Beneficial effects of combination of P and Zn for green gram (Phaseolus aureus L.)

Summary Dicalcium phosphate dihydrate and potassium dihydrogen phosphate were applied to calcareous soil in combination with zinc sulphate in the molar ratio of 100∶0, 100∶1, 100∶2 and 100∶4. Increasing addition of zinc sulphate into P sources gradually increased KCl (0.01M) soluble P and Zn, NaHCO₃ (0.5M, pH 8.5) extractable P and DTPA extractable Zn from soil and grain and straw yields of green gram.     

Effects of liming on the extractability of Fe,Mn, Zn and Cu from a peat medium and the growth and micronutrient uptake of highbush blueberry plants

Summary Levels of extractable micronutrients in a peat and the growth and nutrient uptake of young highbush blueberry plants (Vaccinium corymbosum L cv. Blueray) were studied in a greenhouse experiment in response to liming and two rates of addition of Fe, Mn, Zn and Cu.Levels of extractable micronutrients showed different trends with liming depending upon the extractant used and the element being considered. Levels of 0.05M CaCl₂-extractable Fe, Mn and Zn decreased as the pH was raised whilst those of Cu first decreased and then increased again. There was a general decline in 0.1M HCl-extractable Fe, Mn and Cu with increasing pH but levels of Zn were not greatly affected. Levels of 0.005M DTPA extractable Fe, Mn Zn and Cu generally declined but those extractable with 0.04M EDTA were either unaffected or increased as the pH was raised. Levels of CaCl₂-extractable Mn and Zn were the same order of magnitude as those extractable with HCl, DTPA and EDTA. In contrast, the latter reagents extracted considerably more Fe and Cu than did CaCl₂.Dry matter yields of plants were increased as the pH was raised from 3.9 to 4.3 but then decreased markedly as the pH was raised further to 6.7. With increasing pH, concentrations of plant Fe generally increased those of Mn were decreased and those of Zn and Cu were not greatly affected except for a marked decline in plant Cu at pH 6.7.