Publikasjoner
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2010
Forfattere
Marcel Van Oijen Ad Schapendonk Mats HøglindSammendrag
Background and Aims The carbon balance of vegetation is dominated by the two large fluxes of photosynthesis (P) and respiration (R). Mechanistic models have attempted to simulate the two fluxes separately, each with their own set of internal and external controls. This has led to model predictions where environmental change causes R to exceed P, with consequent dieback of vegetation. However, empirical evidence suggests that the R : P ratio is constrained to a narrow range of about 0·4-0·5. Physiological explanations for the narrow range are not conclusive. The aim of this work is to introduce a novel perspective by theoretical study of the quantitative relationship between the four carbon fluxes of P, R, growth and storage (or its inverse, remobilization). Methods Starting from the law of conservation of mass - in this case carbon - equations are derived for the relative magnitudes of all carbon fluxes, which depend on only two parameters: the R : P ratio and the relative rate of storage of carbon in remobilizable reserves. The equations are used to explain observed flux ratios and to analyse incomplete data sets of carbon fluxes. Key Results The storage rate is shown to be a freely varying parameter, whereas R : P is narrowly constrained. This explains the constancy of the ratio reported in the literature. With the information thus gained, a data set of R and P in grassland was analysed, and flux estimates could be derived for the periods after cuts in which plant growth is dominated by remobilization before photosynthesis takes over. Conclusions It is concluded that the relative magnitudes of photosynthesis, respiration, growth and substrate storage are indeed tightly constrained, but because of mass conservation rather than for physiological reasons. This facilitates analysis of incomplete data sets. Mechanistic models, as the embodiment of physiological mechanisms, need to show consistency with the constraints.
Sammendrag
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Forfattere
Vitalis Wafula Wekesa S. Vidal R.A. Silva E.M.M. Ortega Ingeborg Klingen Italo Delalibera Jr.Sammendrag
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Sammendrag
Organic fields are often assumed to have fewer pests and more beneficials than conventionally managed fields. We monitored 12 Norwegian strawberry fields, 6 organic and 6 conventional fields, by sampling leaves twice a year in 2002 and 2003. Young folded leaflets were visually inspected for eggs and adult females of strawberry mite (Phytonemus pallidus fragariae), and mature leaves were used for extraction of mobile stages of two-spotted spider mite (Tetranychus urticae). The spider mites were examined for infection of the mite-pathogenic fungus Neozygites floridana. Predatory mites (Phytoseiidae) were recorded on both leaf types, and the females mounted and identified. We also sampled leaves from selected plants in the boundary vegetation of most fields to look for sources of T. urticae, phytoseiids and N. floridana. Soil was sampled from each field, to study the natural occurrence of entomopathogenic nematodes. All samples were taken 0-13 metres from the border vegetation. Both pest mites tended to be more abundant in conventional than in organic fields, while the number of phytoseiid mites was very low in both growing systems. Nevertheless, three phytoseiid species were recorded for the first time in Norway: Amblyseius rademacheri Dosse, Neoseiulus kodryensis (Kolodochka) and Neoseiulus reductus (Wainstein). N. floridana infection found in T. urticae females varied from 0-19%, and was higher in 2002 than in 2003. The fungus was recorded at least once in all 12 fields, and there was no consistent difference between the two growing systems. In 2002 there was a significant negative correlation between % T. urticae with N. floridana hyphal bodies found in the first sampling and the number of T. urticae present in the second sampling about 4 weeks later. Beneficial nematodes (mostly Steinernema) tended to occur in more of the organic than of the conventional soil samples. To sum up, both pests (T. urticae and P. pallidus) and one of the beneficial groups (entomopathogenic nematodes) seemed to conform to the expected difference between organic and conventional fields. There are many possible mechanisms related to the differences in pesticide and fertilization regimes that could lead to such a pattern. For the two remaining beneficials (N. floridana and Phytoseiidae) we could not find a consistent correlation between abundance and growing system.
Forfattere
Aslaf Belachew Arne Stensvand Nina Trandem Vitalis Wafula Wekesa Gilberto de Moraes Ingeborg KlingenSammendrag
The co-occurrence of powdery mildew, Podosphaera aphanis, and the two-spotted spider mite, Tetranychus urticae, on strawberry plants requires a strategic use of fungicides to control powdery mildew without harming predatory mites and other beneficial organisms. Sulfur has been used for two centuries to manage powdery mildew and is the only fungicide approved for use in organic strawberry production in Norway. However, there are contrasting reports in the literature about the effect of sulfur on predatory mites and two-spotted spider mite from field studies. Controlled laboratory experiments were therefore conducted on strawberry leaf disks to study the main as well as the interacting effects of sulfur on P. aphanis, T. urticae and the predatory mite, Phytoseiulus persimilis. The following seven treatment combinations: 1) Sulfur + P. aphanis 2) Sulfur + T. urticae 3) Sulfur + P. persimilis 4) Sulfur + P.aphanis + P.persimilis 5) Sulfur + P.aphanis + T.urticae 6) Sulfur + P. Persimilis + T.urticae 7) Sulfur + P.aphanis + P.persimilis + T.urticae and seven parallel treatment combinations with water instead of sulfur (control) were used. Leaf disks were dipped in a sulfur (Thiovit Jet) solution or water and inoculated with P. aphanis after the sulfur/ water had dried on the leaf surface. In treatments with mites, five female T. urticae and one female P. persimilis were added per leaf disk. Preliminary analysis of the results showed that only the predatory mite and sulfur significantly (P ≤ 0.05) affected egg production and mortality of T. urticae. In the treatment that combined sulfur + P. aphanis + P. persimilis + T. urticae a significantly reduction in T. urticae egg production was seen. However, there was no significant effect on egg production and mortality of T. urticae in the three-way or two-way interactions of sulfur, powdery mildew and predatory mite. There was no significant effect of sulfur on mortality of P. persimilis, and sulfur did not seem to affect the efficiency of this predatory mite. Powdery mildew did not affect T. urticae probably because the mildew was not well developed due to inoculation at the same time as the mites were added. In another experiment, however, mites were released on leaf disks that had well developed powdery mildew, and here a negative effect of the mildew on T. urticae egg production was clearly seen.
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