Showing papers by "Wageningen University and Research Centre published in 1998"

Interactions between the atmosphere and terrestrial ecosystems: influence on weather and climate

Abstract: This paper overviews the short-term (biophysical) and long-term (out to around 100 year timescales; biogeochemical and biogeographical) influences of the land surface on weather and climate. From our review of the literature, the evidence is convincing that terrestrial ecosystem dynamics on these timescales significantly influence atmospheric processes. In studies of past and possible future climate change, terrestrial ecosystem dynamics are as important as changes in atmospheric dynamics and composition, ocean circulation, ice sheet extent, and orbit perturbations.

Grazing and Conservation Management

Abstract: Theoretical background the development of grassland communities in north-western Europe evaluation - changes in plant species richness changes in plant communities structural diversity and boundary effects effects on the fauna effects on soil characteristics (nutrients, decomposition, accumulation of litter) changes in Dutch landscapes in relation to mangement options limiting factors - animal performance and carrying capacity grazing in relation to other environmental factors (hydrology, acid rain, plant geography, time) Conclusion and perspectives.

Physiological activity of Campylobacter jejuni far below the minimal growth temperature.

Abstract: The behavior of Campylobacter jejuni at environmental temperatures was examined by determining the physiological activities of this human pathogen. The minimal growth temperatures were found to be 32 and 31°C for strains 104 and ATCC 33560, respectively. Both strains exhibited a sudden decrease in growth rate from the maximum to zero within a few degrees not only near the maximal growth temperature but also near the minimal growth temperature. This could be an indication that a temperature-dependent transition in the structure of a key enzyme(s) or regulatory compound(s) determines the minimal growth temperature. Oxygen consumption, catalase activity, ATP generation, and protein synthesis were observed at temperatures as low as 4°C, indicating that vital cellular processes were still functioning. PCR analysis showed that cold shock protein genes, which play a role in low-temperature adaptation in many bacteria, are not present in C. jejuni. The fact that chemotaxis and aerotaxis could be observed at all temperatures shows that the pathogen is able to move to favorable places at environmental temperatures, which may have significant implications for the survival of C. jejuni in the environment.

Root morphological plasticity and nutrient acquisition of perennial grass species from habitats of different nutrient availability.

Abstract: We studied the root foraging ability and its consequences for the nutrient acquisition of five grass species that differ in relative growth rate and that occur in habitats that differ widely in nutrient availability. Foraging responses were quantified, based on the performance of the plants in homogeneous and heterogeneous soil environments of the same overall nutrient availability. Although all species tended to produce a significantly higher root length density in a nutrient-rich patch, this response was significant only for the faster-growing species. The increased root length density resulted from small, though not significant, changes in root biomass and specific root length. The effectiveness of root proliferation was determined by quantifying the total amount of nutrients (N and P) accumulated by the plants over the course of the experiment. Plants acquired more N in a heterogeneous environment than in a homogeneous environment, although the total nutrient availability was the same. The ability to acquire nutrients (N or P) in the heterogeneous environment was not related to the ability of species to increase root length density in response to local nutrient enrichment. In contrast to other studies, our results suggest that the role of morphological plasticity of roots in acquiring patchily distributed resources is limited. Possible reasons for this discrepancy are discussed.

Modeling and field evidence of finger formation and finger recurrence in a water repellent sandy soil

Abstract: With prolonged rainfall, infiltrating wetting fronts in water repellent soils may become unstable, leading to the formation of high-velocity flow paths, the so-called fingers. Finger formation is generally regarded as a potential cause for the rapid transport of water and contaminants through the unsaturated zone of soils. For the first time, field evidence of the process of finger formation and finger recurrence is given for a water repellent sandy soil. Theoretical analysis and model simulations indicate that finger formation results from hysteresis in the water retention function, and the character of the formation depends on the shape of the main wetting and main drainage branches of that function. Once fingers are established, hysteresis causes fingers to recur along the same pathways during following rain events. Leaching of hydrophobic substances from these fingered pathways makes the soil within the pathways more wettable than the surrounding soil. Thus, in the long-term, instability-driven fingers might become heterogeneity-driven fingers.

The importance of herbivore interactions for the dynamics of African savanna woodlands : an hypothesis

Abstract: Current hypotheses to explain dynamic transitions between savanna grasslands and woodlands in Africa focus on grazing by elephant or the influence of fire. Using a simple mathematical model, this paper argues that interactions between small herbivores such as impala or buffalo and large herbivores such as elephant or giraffe may provide a plausible alternative hypothesis. The interplay of competition and facilitation between these types of herbivores could explain transitions between grassland and woodland and vice versa. A review of the literat- ure is presented in support of this hypothesis.

Application of the redox potential for controling a sulfide oxidizing bioreactor

Abstract: The investigations described show that the formation of elemental sulfur from the biological oxidation of sulfide can be optimized by controling the redox state of the solution. The nonsoluble sulfur can be removed by gravity sedimentation and re-used as a raw material, i.e., in bioleaching processes. It was shown that, by supplying an almost stoichiometrical amount of oxygen to the recirculated gas phase, the formation of sulfate is minimized. The redox potential is mainly determined by the sulfide concentration because this compound has a high standard exchange current density with the platinum electrode surface. By maintaining a particular redox setpoint value, in fact, the reactor becomes a “sulfide-stat.” It was shown that in a sulfide-oxidizing bioreactor the measured redox potential, using a polished redox electrode, is kinetically determined rather than thermodynamically. The optimal redox value for sulfur formation is between −147 and −137 mV (H2 reference electrode, 30°C, pH 8). The presented results are currently used for controling several full-scale installations, which desulfurize biogas and high-pressure natural gas. © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 60: 147–155, 1998.

Learning: Shifting paradigms in education and extension studies

Abstract: (1998). Learning: Shifting paradigms in education and extension studies. The Journal of Agricultural Education and Extension: Vol. 5, No. 3, pp. 143-161.

Hexose transport in growing petunia pollen tubes and characterization of a pollen-specific, putative monosaccharide transporter.

Abstract: We investigated the molecular and physiological processes of sugar uptake and metabolism during pollen tube growth and plant fertilization. In vitro germination assays showed that petunia (Petunia hybrida) pollen can germinate and grow not only in medium containing sucrose (Suc) as a carbon source, but also in medium containing the monosaccharides glucose (Glc) or fructose (Fru). Furthermore, high-performance liquid chromatography analysis demonstrated a rapid and complete conversion of Suc into equimolar amounts of Glc and Fru when pollen was cultured in a medium containing 2% Suc. This indicates the presence of wall-bound invertase activity and uptake of sugars in the form of monosaccharides by the growing pollen tube. A cDNA designated pmt1 (petunia monosaccharide transporter 1), which is highly homologous to plant monosaccharide transporters, was isolated from petunia. Pmt1 belongs to a small gene family and is expressed specifically in the male gametophyte, but not in any other vegetative or floral tissues. Pmt1 is activated after the first pollen mitosis, and high levels of mRNA accumulate in mature and germinating pollen. A model describing the transport of sugars to the style, the conversion of Suc into Glc and Fru, and the active uptake by a monosaccharide transporter into the pollen tube is presented.

Fruit and fish consumption: a possible explanation for population differences in COPD mortality (The Seven Countries Study).

Abstract: Objective To investigate whether average intake of antioxidants, fruits, vegetables and fish may help to explain international differences in chronic obstructive pulmonary disease (COPD) mortality. Design Ecological analysis using information on baseline diet and the 25-year COPD mortality rate in the 16 cohorts of the Seven Countries Study. Setting Population-based cohorts. Subjects Men aged 40-59 years at baseline. Methods Dietary information was collected at baseline in small random samples of each cohort. In 1987 the reported foods were bought locally and analysed chemically. After 25 years of follow-up the underlying cause of death of those who died was established centrally. COPD mortality rate ratios were calculated, for a change equivalent to 10% of the overall mean consumption of a dietary factor. Results We observed independent inverse associations between 25-year COPD mortality and baseline consumption of fruits (rate ratio 0.49; 95% confidence interval 0.36-0.67) and fish (rate ratio, 0.97; 95% confidence interval 0.93-1.00), after adjustment for potential confounders. COPD mortality showed no statistically significant association with intake of antioxidants or vegetables. Fruit and fish consumption together explained about 67% of the variance in the COPD mortality rates of the cohorts. Conclusions Fruit and fish consumption may partly explain population differences in COPD mortality. This is in accordance with suggestions for a relationship between fruit and fish consumption and COPD observed in studies in individuals.

Clustered organization and transcriptional analysis of a family of five csp genes of Lactococcus lactis MG1363

Abstract: SUMMARY: A family of genes encoding cold-shock proteins, named cspA, cspB, cspC, cspD and cspE, was cloned and sequenced from Lactococcus lactis MG1363. The genes cspA and cspB and the genes cspC and cspD are located in tandem repeats, an organization of csp genes that has never been encountered before. The five genes encode small (7.1-706 kDa) proteins with high mutual sequence identities (up to 85 O/O) and high identities (about 45-65 %) with the major cold- shock proteins from Escherichia coli (CspA) and Bacillus subtilis (CspB)., Northern-blot analysis revealed single transcripts of about 300 nucleotides for each csp gene and showed that cspA, cspB, cspC and cspD mRNA levels were strongly increased upon cold shock to 10 "C (about lo-, 40-, 10- and 30-fold compared to 30 "C, respectively), whereas the cspE mRNA level was not increased. The expression of the cold-induced csp genes was highest in the. 6-8 h lag phase after cold shock. A differential expression in time, in which cspA and cspC were maximally expressed at 2 h and cspB and cspD at 4 h after cold shock, was observed. The -35 and -10 regions of the five promoters were identified and transcriptional start sites were mapped in each case by primer extension at different temperatures which confirmed that regulation takes place at the transcriptional level. Significant differences were observed between the 5′-untranslated leader regions of the four cold-induced csp genes and the corresponding region of the non-cold-induced cspE gene.

Mixture model analysis of complex samples

Abstract: We investigate the effects of a complex sampling design on the estimation of mixture models. An approximate or pseudo likelihood approach is proposed to obtain consistent estimates of class-specific parameters when the sample arises from such a complex design. The effects of ignoring the sample design are demonstrated empirically in the context of an international value segmentation study in which a multinomial mixture model is applied to identify segment-level value rankings. The analysis reveals that ignoring the sample design results in both an incorrect number of segments as identified by information criteria and biased estimates of segment-level parameters.

Multiplicative Models for Interaction in Three-Way ANOVA, with Applications to Plant Breeding

Abstract: SUMMARY In plant breeding, multiplicative models for two-way analysis of variance interaction have become a general means of describing genotype-by-environment interaction. These models offer parsimonious descriptions and facilitate interpretations in biological terms. A disadvantage of the prevailing dominance of two-way multiplicative models is that data with more complicated environmental structure are often forced to fit the two-way framework. As a partial solution to this problem, three-way multiplicative models are presented that can be used in addition to the more familiar two-way multiplicative models. Most importantly, a three-way generalization is given of the twoway singular-value decomposition that can be applied for closer inspection of three-way analysis of variance interaction, much in the same way as its two-way counterpart is used for two-way interaction. Two real data sets are analyzed to illustrate how two- and three-way multiplicative models for interaction jointly provide parsimonious descriptions of various types of genotype-byenvironment interaction. These parsimonious descriptions are often open to meaningful biological interpretations as well.

Diurnal and seasonal variation of carbon dioxide exchange from a former true raised bog

Abstract: Carbon dioxide exchange was measured, using the eddy covariance technique, during a one and a half year period in 1994 and 1995. The measurements took place over a former true raised bog, characterized by a shallow peat layer and a vegetation dominated by Molinia caerulea. The growing season extended from May until late October, with a maximum LAI in August of 1.7. The carbon balance shows a net release of 97 g C m-2 y-1 (265 kg C ha-1 y-1) from the peat bog ecosystem to the atmosphere. During June, July and August there is net consumption of CO2, while during the rest of the year there is net production of CO2. The average daytime assimilation rates ranged between - 0.2 and - 0.5 mg CO2 m-2 s-1 (-45 and -11.3 μmol CO2 m-2 s-1), in a period where the LAI ranged between 1 and 1.7. A high vapour pressure deficit (> 15 hPa) corresponding with high temperatures was found to reduce the assimilation rate by on average 50%. Apart from these factors, LAI and the soil temperature codetermine the net exchange of CO2. The total nocturnal respiration during the growing season lies within the same order as the average daytime net assimilation rate. Temperature was found to be the main factor controlling soil respiration, with a Q10 of 4.8.

Regio- and Stereospecific Conversion of 4-Alkylphenols by the Covalent Flavoprotein Vanillyl-Alcohol Oxidase

Abstract: The regio- and stereospecific conversion of prochiral 4-alkylphenols by the covalent flavoprotein vanillylalcohol oxidase was investigated. The enzyme was active, with 4-alkylphenols bearing aliphatic side chains of up to seven carbon atoms. Optimal catalytic efficiency occurred with 4-ethylphenol and 4-n-propylphenols. These short-chain 4-alkylphenols are stereoselectively hydroxylated to the corresponding (R)-1-(4*-hydroxyphenyl)alcohols (F. P. Drijfhout, M. W. Fraaije, H. Jongejan, W. J. H. van Berkel, and M. C. R. Franssen, Biotechnol. Bioeng. 59:171‐177, 1998). (S)-1-(4*-Hydroxyphenyl)ethanol was found to be a far better substrate than (R)-1-(4*-hydroxyphenyl)ethanol, explaining why during the enzymatic conversion of 4-ethylphenol nearly no 4-hydroxyacetophenone is formed. Medium-chain 4-alkylphenols were exclusively converted by vanillylalcohol oxidase to the corresponding 1-(4*-hydroxyphenyl)alkenes. The relative cis-trans stereochemistry of these reactions was strongly dependent on the nature of the alkyl side chain. The enzymatic conversion of 4-sec-butylphenol resulted in two (4*-hydroxyphenyl)-sec-butene isomers with identical masses but different fragmentation patterns. We conclude that the water accessibility of the enzyme active site and the orientation of the hydrophobic alkyl side chain of the substrate are of major importance in determining the regiospecific and stereochemical outcome of vanillyl-alcohol oxidase-mediated conversions of 4-alkylphenols. Vanillyl-alcohol oxidase (EC 1.1.3.38) is a flavoprotein from Penicillium simplicissimum that originally was shown to catalyze the oxidation of vanillyl alcohol to vanillin with the simultaneous reduction of molecular oxygen to hydrogen peroxide (2). The biological function of vanillyl-alcohol oxidase is unknown, but recent studies have indicated that the enzyme is involved in the biodegradation of 4-(methoxymethyl)phenol (6). Vanillyl-alcohol oxidase is a homooctamer, with each 64kDa subunit containing an 8a-(N 3 -histidyl)-flavin adenine

Major histocompatibility genes in cyprinid fishes: Theory and practice

Abstract: Summary: The first teleostean MHC sequences were described for carp. Subsequent studies in a number of cyprinid fishes showed that the class I sequences of these fishes are of particular interest. Two distinct lineages (Cyca-Z and Cyca-U) are found in the common and ginbuna crucian carp, but only the U lineage is present in zebrafish and other non-cyprinid species. The presence of the Z lineage is hypothesised to be the result of an allotetraploidisation event. Both phylogenetic analyses and amino acid sequence comparisons suggest that Cyca-Z sequences are non-classical class I sequences, probably similar to CD I. The comprehensive phylogenetic analyses of these sequences revealed different phylogenetic histories of the exons encoding the extracellular domains. The MHC genes were studied in laboratory and natural models. The natural model addressed the evolution of MHC genes in a Barbus species flock. Sequence analysis of class I and class II supported the species designation of the morphotypes present in the lake, and as a consequence the trans-species hypothesis of MHC polymorphism. The laboratory model involves the generation of gynogenetic clones, which can be divergently selected for traits such as high and low antibody response. The role of MHC molecules can be investigated further by producing a panel of isogenic lines.