Depolymerization of the Glutenin Macropolymer During Dough Mixing: I. Changes in Levels,Molecular Weight Distribution,and Overall Composition

Changes in the amounts, molecular weight distributions, and levels of major groups of subunits in the glutenin macropolymer (GMP) of doughs during mixing were investigated. The GMP (gel protein) is the unreduced fraction of gluten protein that remains as a layer on top of the starch after extraction of SDS-soluble proteins and centrifugation. Experiments involved doughs prepared from flours derived from one weak and one strong cultivar and lines derived from cv. Olympic that were null for specific high molecular weight glutenin subunits (HMW-GS). During mixing, the amount of GMP decreased; the major changes occurred before peak mixing time (MT, achievement of peak resistance). In addition, the average apparent molecular weight of GMP (determined by both size-exclusion HPLC and multilayer gel electrophoresis) decreased during mixing, but in this case, the major changes were seen later in the mixing process, during dough breakdown. Even after extensive mixing, polymers and oligomers were released, not free glutenin subunits. During dough breakdown, the composition of GMP also changed, such that the proportion of HMW-GS decreased but β-amylases/D low molecular weight glutenin subunits (LMW-GS) increased. Changes in the total amounts of other LMW-GS typically were smaller with a decrease in the proportion of B subunits and an increase in the proportion of C subunits. The major changes in GMP composition were observed after peak MT (peak resistance) occurring earlier and to a greater extent in the weaker dough. Our results suggest that dough breakdown during mixing may be triggered by loss of HMW-GS, leading to changes in the molecular weight distribution and composition of the disulfide-bonded GMP.     

Changes in SDS Solubility of Glutenin Polymers During Dough Mixing and Resting

An online coupling of high‐performance size‐exclusion chromatography (HPSEC) combined with multiangle laser‐light scattering (MALLS) and a reverse‐phase HPLC procedure were used to characterize and reveal the polydispersity of the glutenin polymers of doughs during mixing and resting. Experiments involved doughs prepared from several samples of a common French wheat cultivar (Soissons) differing in total amount of SDS‐unextractable glutenin polymers. During dough mixing, the amounts, size distribution of protein, and glutenin subunit composition within the SDS‐unextractable polymers changed. However, the major changes in SDS‐unextractable glutenin content and size distribution occurred before the peak mixing time (MT) was reached, whereas detectable changes in subunit composition also occurred after the peak MT. Even if sonication, which was used to solubilize the total wheat glutenin, can narrow the glutenin size distribution, HPSEC‐MALLS revealed a close relationship between the SDS solubility of the glutenin polymers and size distribution, confirming a depolymerization and repolymerization hypothesis. During the depolymerization of the SDS‐unextractable polymers, glutenin subunits were released in nonrandom order, which indicated that the polymers have a hierarchical structure. Some HMW glutenin subunits (HMW‐GS), especially 1D×5, were particularly resistant to the depolymerization mechanism. This suggested that the subunit plays a major role in forming the backbone of the SDS‐unextractable polymers, consistent with the potential to form branched structure. These studies suggest that the SDS‐unextrac‐table polymers in flours have a well‐ordered structure that can be modified by dough mixing and resting.     

In Vitro Polymerization of Wheat Glutenin Subunits with Inorganic Oxidizing Agents. I. Comparison of Single‐Step and Stepwise Oxidations of High Molecular Weight Glutenin Subunits

High molecular weight glutenin subunits (HMW‐GS) were isolated from wheat flour and polymerized in vitro at pH 3.0 with different oxidizing agents (KBrO₃, KIO₃, H₂O₂). An oxidation protocol with single addition of oxidant (single‐step oxidation) was compared with a set‐up in which the oxidant was added in multiple steps (stepwise oxidation). Changes in size distribution were evaluated with size‐exclusion HPLC, multilayer SDS‐PAGE, and flow‐field flow fractionation (flow‐FFF). Flow‐FFF is particularly suitable for measuring changes in glutenin size in the very high size ranges. In order of increasing sizes of the resulting polymers, the different oxidizing agents could be ranked as KBrO₃ < KIO₃ < H₂O₂. However, none of the oxidation conditions allowed for a complete polymerization of HMW‐GS. Interestingly, it was found that high concentrations of KIO₃ negatively affect the degree of polymerization. A similar observation was not made with KBrO₃ or H₂O₂. SDS‐PAGE showed that y‐type HMW‐GS particularly failed to incorporate in glutenin polymers. Simultaneously, these HMW‐GS displayed higher mobilities on SDS‐PAGE that can be ascribed to the formation of intrachain SS bonds. Possible explanations for the incomplete polymerization of HMW‐GS are given.     

Development of a Panel of Specific Monoclonal Antibodies to High Molecular Weight Glutenin Subunits and Their Application in Genetic Screening

High molecular weight glutenin subunits (HMW-GS) encoded by different chromosomal loci and alleles (1, 2, 5, 7, 10, and 12) were purified using reversed-phase HPLC from reduced, aqueous propanol extracts of flour from aneuploid or null wheat lines. Unlike previous libraries of monoclonal antibodies developed in our laboratory to SDS-extracted or alkylated HMW-GS, several of the monoclonal antibodies (mAb) developed in this study had a range of specificity patterns for HMW-GS in enzyme-linked immunosorbent assay (ELISA) and on immunoblots. A subset of the mAb bound either x- or y-type HMW-GS but not other gluten proteins, while a few antibodies bound one (mAb 110622, 110421, 140820), or two (mAb 101319, 110804, 140705, 1410460) HMW-GS expressed in each cultivar tested. In most cases, antibodies bound equally to the subunits encoded by different HMW-GS alleles. The more specific antibodies should be useful in research on the quantitative variation of HMW-GS expression and in studies of the role of particular HMW-GS in dough structure. The mAb 101319, which was prepared to subunit 1, bound to HMW-GS 1Bx subunits in ELISA and on immunoblots. This antibody also provided a higher absorbance value in ELISA with extracts of wheat lines expressing the Glu-Ble allele (HMW-GS 20) compared with the Glu-Bli allele (HMW-GS 17+18). Another mAb (110622) detected subunit 2 more strongly than subunit 5 in ELISA and produced a higher signal in immunoblots with subunit 2 even though these subunits are >98.7% homologous in amino acid sequence. An ELISA assay using this antibody was optimized for discrimination of wheat lines with the allelic pairs of subunits 1Dx5-1Dy10 from those with 1Dx2-1Dy12, with the former lines providing stronger dough properties and superior breadmaking quality. The performance of this assay was unaffected by other variations at HMW-GS loci and was demonstrated in sets of biotypes, doubled haploid, and cross-bred breeder's lines.     

The Combination of Rhizopus chinensis Lipase and Transglutaminase Affects the Rheology and Glutenin Macropolymer Properties of Frozen Dough

The combination of Rhizopus chinensis lipase (RCL) and transglutaminase (TG) was previously reported to improve the quality of frozen dough bread. In this study, the effects of RCL, TG, and their combination on the modification of glutenin macropolymer (GMP) and rheological properties of dough during frozen storage were investigated. Frozen storage changed both GMP and rheology properties of dough. TG treatment significantly decreased the ratio of high‐molecular‐weight glutenin subunits to low‐molecular‐weight glutenin subunits and GMP content in fresh dough, and GMP particle size increased. The effect of RCL on GMP properties was not significant, but its combination with TG dramatically increased the proportion of the larger particles and weighted average volume (D_4.3) in GMP. The treatment with the enzyme combination could have inhibited the depolymerization of GMP, which slowed down the decrease rate of some parameters such as GMP content, proportion of larger particles, D_4.3, and release of free amino and thiol groups during frozen storage. The modification of GMP properties by enzyme treatment weakened the effect of the freezing process on rheological properties of dough, especially TG treatment and its combination with RCL. Correlation between GMP particle size and dough properties (dough tensile force and elastic modulus) after freezing and enzyme treatment were confirmed.     

卫星搭载小麦SP3代高分子麦谷蛋白亚基的SDS-PAGE分析

利用十二烷基磺酸钠-聚丙烯胺凝胶电泳（SDS-PAGE）技术对返回式卫星搭载小麦两个品种SP3代籽粒的高分子麦谷蛋白亚基（HMW-GS）进行了分析,并按照相关评分方法计算了高分子量麦谷蛋白Glu-1位点的品质得分。结果表明,经卫星搭载可产生较高频率的HMW-GS基因变异,陕253和西农1043 两个品种SP3代HMW-GS组成的变异频率分别为27.08%和27.45%。陕253和西农1043 SP3代的小麦品质得分分别为7分和6分,陕253 SP3代变异株为优质小麦。     

Measurement of Dynamic Dough Density and Effect of Surfactants and Flour Type on Aeration During Mixing and Gas Retention During Proofing

A new method for measuring dough densities is presented, based on weighing small dough samples in air and immersed in xylene. The method can be used to evaluate the air content of low‐density doughs and to follow the changing density of a proofing dough sample. The method is applied to evaluate the effect of flour strength and surfactant addition on dough aeration and subsequent proofing. Doughs were mixed in a high‐speed mixer from two flours, a strong breadmaking flour and a weak flour. Surfactants sodium stearoyl lactylate (SSL) and diacetyl tartrate esters of monoglyceride (DATEM) were added at three levels, and the air content, proofing dynamics, and baked loaf quality were evaluated. The air content of dough was proportional to headspace pressure in the mixer, while the strong flour occluded less air than the weak flour. Surfactants greatly improved the volume of baked loaves but appeared to have no significant effect on air incorporation during mixing. The addition of surfactants appeared to increase the rate of growth of the dough piece during proofing, possibly due to increased bubble breakup during mixing or to increased rates of mass transfer of CO₂ into bubbles during proofing.     

Quantitative Variation of HMW Glutenin Subunits from Hard Red Spring Wheats Grown in Different Environments

The objective of this study was to investigate the quantitative variation of HMW glutenin subunits in relation to glutenin polymers and hence breadmaking quality across different environments. Six genotypes of hard red spring (HRS) wheat were grown at seven locations in North Dakota in 1998 in a randomized complete‐block experimental design with three replicates at each location. Unreduced SDS‐soluble glutenins of flour were fractionated by multistacking SDS‐PAGE into different sized glutenin polymers, followed by SDS‐PAGE and imaging densitometry to determine the quantitative variation of HMW glutenin subunits. SDS‐insoluble glutenin polymers also were examined for their quantitative composition of HMW glutenin subunits. The results showed that the percentage of HMW glutenin subunits was significantly affected by growing locations. The quantity of HMW glutenin subunits in SDS‐insoluble glutenins was significantly and positively correlated with loaf volume. SDS‐insoluble glutenin polymers had a higher percentage of HMW glutenin subunits than did SDS‐soluble glutenins. SDS‐insoluble glutenin polymers in flour were positively and significantly correlated in proportions of both total and individual HMW glutenin subunits in total SDS glutenins. SDS‐insoluble glutenin polymers also were positively and significantly correlated with the combined proportion of HMW glutenin subunits 2* + 5. The results of this study indicated that either subunit 2* or 5 might be more important in forming a greater quantity of larger SDS‐insoluble glutenin polymers than other subunits. SDS‐insoluble glutenin polymers from different cultivars or locations could have different quantities of HMW glutenin subunits in their composition. SDS‐insoluble glutenin polymers with more HMW glutenin subunits might be larger sized than those with less HMW glutenin subunits. Environment significantly influenced the quantitative variation of HMW glutenin subunits, which in turn affected the size distribution of glutenin polymers, and hence breadmaking quality.     

Purification and Characterization of the Glutenin Subunits of Triticum tauschii,Progenitor of the D Genome in Hexaploid Bread Wheat

N-terminal amino acid sequences and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) molecular weights have been determined for high-performance liquid chromatography (HPLC)-purified high molecular weight (HMW) and low molecular weight (LMW) glutenin subunits (GS) of Triticum tauschii ssp. strangulata, contributor of the D genome to hexaploid bread wheat. The use of three different extraction procedures resulted in similar glutenin preparations. On the basis of N-terminal sequences, the same types of glutenin subunits that have been reported in bread and durum wheats (HMW-GS of both the x and y types and LMW-GS of the LMW-s, LMW-m, α-, and γ-types) were found in T. tauschii. However, the HMW-GS in T. tauschii were in greater proportion relative to LMW-GS when compared to reported values for a bread and durum wheat. Our results support the likelihood that differences in the proportions of the various subunits contributed by the A, B, and D genomes, rather than qualitative differences in the types of subunits, are responsible for the major differences in quality characteristics between bread wheat and durum wheat.     

Molecular Modeling of the N-terminal Regions of High Molecular Weight Glutenin Subunits 7 and 5 in Relation to Intramolecular Disulfide Bond Formation

Analyses of cystine peptides derived from the high molecular weight glutenin subunits (HMW-GS) 5 and 7 indicate that, in spite of a distinct sequence homology between the two subunits in the N-terminal region, different disulfide linkages of cysteine residues are present in these regions. To investigate the structural basis for these experimental results, the conformational structures of the polypeptide chains corresponding to the N-terminal regions (first 50 amino acids) of the wheat HMW-GS 5 and 7 were modeled by computer methods. Secondary structures were predicted by the method of Rost and Sander (1993) and, to the extent appropriate, applied to the constructed polypeptide chains. The resulting structures were energy-minimized and subjected to simulated heating and dynamic equilibration. In the final structure of subunit 5, the first two cysteines were located in a region of continuous α-helix. If folding to the helical form occurs rapidly during biosynthesis as expected, the distance between the sulfhydryl groups of these two cysteines would be great enough (≈2.2 nm) to make intramolecular disulfide bond formation unlikely. Although a somewhat similar region of α-helix was predicted for the subunit 7, in some predictions the helix was interrupted between the first two cysteines, and this break was assigned either extended structure or arbitrarily modeled as an inverse γ-turn. In the final structure of subunit 7 with the assigned inverseγ-turn, after energy minimization, heating, and dynamics, the two cysteines approached one another closely (≈0.4 nm). Formation of an intramolecular disulfide bond appeared a likely possibility. This model is in accord with experimental evidence for this latter intramolecular bond (Köhler et al 1993). In agreement with the modeling, an equivalent intramolecular disulfide bond of subunit 5 has not been found and experimental evidence for a different arrangement is presented.     

Genetic Diversity of Glutenin Protein Subunits Composition in Durum Wheat Landraces [Triticum turgidum ssp. turgidum Convar. durum (Desf.) MacKey] from the Mediterranean Basin

The genetic diversity of high and low molecular weight glutenin subunits of 63 durum wheat landraces from different geographical regions in the Mediterranean Basin was studied using SDS-PAGE. Great variability in glutenin composition was found, with 42 high and low molecular weight glutenin haplotypes, 20 allele combinations at the HMW-GS loci, and 18 at the LMW-GS. All five possible LMW models were detected in all Mediterranean regions. Rare alleles were found at Glu-B1 locus in high frequencies and a priori related alleles to grain quality were also observed. Global genetic diversity index was relatively high (0.67); it ranged from 0.33 to 0.66. Cluster analysis on the frequency patterns of origins grouped genotypes following a geographical structure. Rogers’ distance coefficient on frequency pattern for each region of origin showed two germplasm pools with distinct quality profiles, where South West Asian landraces were very different from the landraces of other Mediterranean areas. The relationship between different regions of origin is discussed and two possible ways of introduction of wheat in the Iberian Peninsula (N Africa and SE Europe) are hypothesized. The use of Mediterranean durum wheat landraces as source of genetic variability for grain quality improvement is highly recommended.     

Quantitative Determination of High Molecular Weight Glutenin Subunits of Hard Red Spring Wheat by SDS-PAGE. I. Quantitative Effects of Total Amounts on Breadmaking Quality Characteristics

Thirteen hard red spring wheat genotypes in which seven genotypes had the same high molecular weight (HMW) glutenin subunits (2*, 7+9, 5+10) were compared for their physical-chemical and breadmaking properties. These samples were categorized into three groups based on their dough mixing and baking performances as follows: the strong dough (SD) group (six genotypes), characterized by the strongest dough mixing (average stability, 35 min); the good loaf (GL) group (four genotypes), characterized by the largest loaf volume; and the poor loaf (PL) group (three genotypes), characterized by the smallest loaf volume. Total flour proteins were fractionated into 0.5M salt-soluble proteins, 2% SDS-soluble proteins, and residue proteins (insoluble in SDS buffer). SDS-soluble proteins, residue proteins, and total flour proteins were analyzed by SDS-PAGE and densitometry procedures to determine the proportions of HMW glutenin subunits, medium molecular weight proteins, and low molecular weight proteins in relation to the total amount of proteins. No differences in the amount of salt-soluble proteins were found among the different groups of samples. Solubilities of gluten proteins (total proteins minus salt-soluble proteins) in SDS buffer were related to the differences in dough strength and baking quality among the three groups. The SD group had the lowest solubility and the PL group had the highest. SDS-PAGE analysis showed that SDS-soluble proteins of the SD group contained a smaller amount of HMW glutenin subunits than those of the GL and PL groups. The highest proportions of HMW glutenin subunits in total flour proteins were found in the SD group, while the PL group had the lowest percentage of HMW glutenin subunits in their total flour proteins. These results showed that the total quantities of HMW glutenin subunits played an important role in determining the dough mixing strength and breadmaking performance of hard red spring wheats.     

Accumulation of High‐Molecular‐Weight Glutenin Subunits in Superior and Inferior Grains of a Winter Wheat,Yangmai 158

The difference in accumulation of high‐molecular‐weight glutenin subunits (HMW‐GS) in superior (basal) and inferior (distal) grains results in the nonuniformity of grain quality in a winter wheat (Triticum aestivum L. ‘Yangmai 158’). The HMW‐GS accumulation and glutenin macropolymer (GMP) content were studied in superior and inferior grains during the grain‐filling period. Compared with inferior grains, HMW‐GS was formed earlier and total accumulation amount was higher in superior grains. The total HMW‐GS content was higher in superior grain than inferior grain, except at maturity. For individual HMW‐GS types, the accumulation and content of subunit 7 were the highest, followed by subunit 12, and those of subunit 8 were the lowest, followed by subunit 2 in superior grain. In contrast, the accumulation and content of subunit 7 at maturity were significantly higher than subunit 8 but similar between subunit 2 and subunit 12 in inferior grain. Moreover, the accumulation of subunit 7 and 12 in superior grain was significantly higher than in inferior grain. However, compared with the inferior grain, the GMP accumulation was higher but content was lower in superior grain at maturity.     

Wheat Intercultivar Differences in Susceptibility of Glutenin Protein to Effects of Bug (Eurygaster integriceps) Protease

Preharvest bug damage to wheat can cause significant losses in bread‐making quality. One of the most prevalent forms of bug damage which frequently occurs in most countries of the Middle East, Eastern Europe and North Africa can be attributed to Heteropterous insects, particularly Eurygaster spp. Intercultivar differences in the susceptibility of glutenin to proteolytic degradation by the bug Eurygaster integriceps were investigated using six breadwheat cultivars of Turkish origin. Crude enzyme extract was prepared with distilled water from bug‐damaged wheat. The freeze‐dried extract was blended with sound samples of ground wheat, and the mixture was incubated in distilled water for 30 and 60 min at 37°C and subsequently freeze‐dried. The proteolytic effects of bug damage were determined on large polymeric glutenin. The latter was measured as 50% 1‐propanol insoluble (50PI) glutenin extractable with 50% 1‐propanol in reductant dithiothreitol. The decreases in the amount of 50PI glutenin and the high and low molecular weight subunits were quantified using reversed‐phase HPLC. There was a substantial and progressive decrease in the quantity of 50PI glutenin and its subunits with increasing incubation time. Intercultivar differences were observed that were unrelated to intrinsic levels of proteolytic activity. After 60 min of incubation, the relative decrease in 50PI glutenin compared with control samples ranged from 43% (cv. Ankara) to 65 % (cv. Kirkpinar). Some cultivars (Lancer, Ankara and Gün) with similar levels of intrinsic proteolytic activity showed significantly different responses to bug protease. One cultivar (cv. Kirkpinar) with the lowest proteolytic activity was the most susceptible. High quality breadwheats (cvs. Bezostaya, Lancer, Kiraç and Gün) were generally more resistant to the bug protease, although Ankara, with both intermediate protease activity and breadmaking quality, was the most resistant cultivar. While the 50PI glutenin test was very effective in quantifying the damaging effects of bug protease on wheat protein quality, the nature of the intercultivar differences was unclear.     

Allelic Variation at Glu-D1 Locus for High Molecular Weight (HMW) Glutenin Subunits: Quantification by Multistacking SDS-PAGE of Wheat Grown Under Nitrogen Fertilization

Two biotypes of an Australian wheat cultivar, Warigal, differing only in the Glu-D1 high molecular weight (HMW) glutenin subunits 5+10 and 2+12 were used in this study. The objective was to examine the effects of nitrogen fertilization and allelic variation at the Glu-D1 locus on the characteristics of glutenin polymers. Unreduced proteins containing the SDS-soluble glutenins and the other protein classes were analyzed by multistacking SDS-PAGE which separates the glutenin into six distinctly different-sized aggregates. The results showed that nitrogen fertilization significantly increased protein quantity, ratio of polymers to monomeric proteins, and sizes of SDS-soluble glutenins. Nitrogen fertilization affected the proportions of HMW subunits in both SDS-soluble and SDS-insoluble glutenin polymers and the ratio of x to y subunits in SDS-insoluble glutenin polymers. Nitrogen fertilization, however, did not cause a significant change in ratio of SDS-soluble to SDS-insoluble glutenins. SDS-insoluble glutenins had a greater ratio of HMW to LMW and x to y subunits, especially with a higher increase of 1Dx subunits, than SDS-soluble glutenins. The HMW/LMW subunit ratio and the x/y subunit ratio may be used to predict sizes of glutenin polymers. The biotype with 5+10 subunits had a greater x/y subunit ratio in the SDS-insoluble glutenins than the 2+12 type. A greater proportion of subunit 5 was formed than subunit 2 in the SDS-insoluble glutenin polymers. Both nitrogen fertilization and allelic variation at Glu-D1 loci could affect the characteristics of glutenin polymers.     

Structural Modifications of Gluten Proteins in Strong and Weak Wheat Dough During Mixing

The network‐forming attributes of gluten have been investigated for decades, but no study has comprehensively addressed the differences in gluten network evolution between strong and weak wheat types (hard and soft wheat). This study monitored changes in SDS protein extractability, SDS‐accessible thiols, protein surface hydrophobicity, molecular weight distribution, and secondary structural features of proteins during mixing to bring out the molecular determinants of protein network formation in hard and soft wheat dough. Soft wheat flour and dough exhibited greater protein extractability and more accessible thiols than hard wheat flour and dough. The addition of the thiol‐blocking agent N‐ethylmaleimide (NEM) resulted in similar results for protein extractability and accessible thiols in hard and soft wheat samples. Soft wheat dough had greater protein surface hydrophobicity than hard wheat and exhibited a larger decrease in surface hydrophobicity in the presence of NEM. Formation of high‐molecular‐weight (HMW) protein in soft wheat dough was primarily because of formation of disulfides among low‐molecular‐weight (LMW) proteins, as indicated by the absence of changes in protein distribution when NEM was present, whereas in hard wheat dough the LMW fraction formed disulfide interaction with the HMW fraction. Fourier transform infrared spectroscopy indicated formation of β‐sheets in dough from either wheat type at peak mixing torque. Formation of β‐sheets in soft wheat dough appears to be driven by hydrophobic interactions, whereas disulfide linkages stabilize secondary structure elements in hard wheat dough.     

Effect of Fermentation and Oil Incorporation on the Retention of E Vitamers During Breadmaking

All forms of baking and processing cause a loss of nutrients, including vitamin E, but little is known about these occurrences or if they could be avoided. The objective of this research was to study the incorporation of palm oil and the stability of vitamin E in palm oil during breadmaking. Wheat meal and rye breads were baked with and without the addition of 0, 2, 5, or 8% palm oil. The eight E group vitamers (tocopherols and tocotrienols) were extracted by using accelerated solvent extraction, freeze dried, and then analyzed with normal‐phase HPLC. Compared with the controls, the inclusion of palm oil was found to increase the quantity of all forms of vitamin E in the final baked products. It is concluded that palm oil is effective in increasing the vitamin E content of whole grain bread.     

Mineral Concentrations of Cool Season Pasture Forages in North Florida During the Winter-Spring Grazing Season: II. Trace Minerals

Trace mineral concentrations of annual cool season pasture forages grazed by growing beef cattle during late fall-winter-spring grazing season were evaluated during two experimental cool season grazing studies, each lasting two years at the North Florida Research and Education Center (NFREC), Marianna, Florida. Eight 1.32 ha fenced pastures or paddocks were divided into two groups of pasture land preparation/planting methods, four pastures for the sod seeding treatments (SS) and four for the prepared seedbed treatments (PS). Two different pasture forages, small grains, (rye/oats mix) with or without ryegrass for the first two years (Study 1); and oats with ryegrass or ryegrass only for the last two years (Study 2) were planted in these pasture lands. Each of the four forage, type, and cultivation combination treatments was assigned to two pastures each year, thereby giving two replicates per pasture treatment per year. Forage samples were collected at the start of pasture grazing and twice monthly thereafter until the end of grazing season, pooled by month, and analyzed for copper (Cu), iron (Fe), zinc (Zn), manganese (Mn), cobalt (Co), molybdenum (Mo), and selenium (Se). Liver biopsies and blood plasma samples were collected from the tester cattle only during the spring of year two of Study 2. Liver was analyzed for Cu, Fe, Mn, Co, Mo, and Se and plasma for Cu, Fe, Zn, and Se. Forage trace mineral concentrations were found to differ by month in Cu (P < 0.01), Fe and Zn (P < 0.0001) in both studies, and with Mn (P < 0.0001) in Study 2 only. Pasture forage type effects on Cu (P < 0.05), Fe and Zn (P < 0.01), and Se (P < 0.05) and forage type by month interactions on Cu and Mn (P < 0.0001), and Zn (P < 0.05) were observed in Study 2. Forage concentrations of Cu, Zn, Mn, and Mo in Study 1 and Mn, Mo, and Se in Study 2 were affected (P < 0.05) by pasture land preparation/planting methods in that these minerals were found to be lower from forages of sod-seeded treatments than from those of prepared seedbed treatments. Forage Cu concentrations were lower than the minimum requirements (10 ppm, DM) for beef cattle among months in both studies. Oats-ryegrass pastures of Study 2 had surprisingly low Fe concentrations (P < 0.01) in all months of the winter-spring grazing season. Cobalt, Mn, Mo, and Se did not vary much month to month during the winter-spring grazing months. All mean forage Se concentrations were lower than the requirements (0.10 ppm, DM) for grazing beef cattle. There were no differences (P > 0.05) in mean Se values between the two studies. Liver Cu, Fe, Co, and Se concentrations were sufficient to indicate adequate status of these minerals in tester animals from both forage types. Liver concentrations of Mn and Mo were slightly low, indicating a low status or these minerals. Plasma concentrations of Cu, Fe, Se, and Zn were all above the recommended concentrations for beef cattle. In conclusion, trace minerals deficient in North Florida during the cool season were Cu, Co, and Se, and a special consideration should be given to include adequate amounts while supplementing the mineral mixtures to growing beef cattle since forage samples reflected deficient concentrations of these minerals.     

Contributory factors to soil spatial variability in an ultisol. II. Retention of living trees in situ following land clearing

Abstract

The effect of retaining living trees in situ following manual land clearing on soil physical properties was studied in a Typic Kandiudult in southern Cameroon. Soil compaction in the surface 100 mm was greatest with complete clearing and least under forest, with retention of living trees resulting in soil compaction levels which were intermediate to both the former. Soil compaction in the surface 100 mm also increased with increasing distance from the tree trunk (or tree stump with complete clearing). Both the above observations were attributed to a combination of high root density, high macrofaunal activity, high ground cover, high organic matter content and low traffic under forest and at the base of trees or tree stumps. Increasing soil compaction also occurred with increasing depth, and was attributed to the existence of few biopores in the subsoil horizons. Absence of biopores was thought to be due to low root densities in the subsoil caused by a combination of low macroporosity, low air porosities during the wet season and low pH. In comparison to sub‐humid and semi‐arid ecologies, therefore, the beneficial effects of retaining living trees in situ following land clearing at this site were less.     

黄土高原植被演替过程中相对土壤酶活性的变化特征

明确土壤相对酶活性在植被恢复过程中的变化特征及影响因素,对客观揭示植被次生演替过程中微生物活性变化特征具有重要意义.利用时空互代法,选取黄土丘陵区植被次生演替过程中农田、草地、灌木、先锋林和顶级群落5个阶段为研究对象,探讨了β-1,4-葡萄糖苷酶(BG)、纤维二糖水解酶(CBH)、β-木糖苷酶(BX)、N-乙酰-β-D...