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121.
Activities of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured in leaf extracts of field grown Amaranthus paniculatus L. (C4 ) during a natural diurnal irradiance and temperature pattern. Enzyme assays were run at both fixed (30°C) and the corresponding leaf temperature at the time of harvest. Light activation of PEP carboxylase (PEPCase) at fixed assay temperatures was expressed as a decrease in S0–5 (PEP) after a threshold (> 330 μmol m–2 s–1 ) photon fluence rate was surpassed at noon. Earlier in the morning, increase in apparent enzyme affinity for PEP was observed when the assay was run at leaf temperature, indicating a physiologically meaningfull effect of temperature on S0.5 (PEP). The 3.3-fold increase in PEPCase activity at low PEP and fixed assay temperature between the minimal and maximal irradiance and temperature hours of the day, became 12.8-, 11.5- and 7.4-fold when assays were run at the corresponding leaf temperature during three diurnal cycles with respective temperature differences (max minus min) of 9.0, 8.3 and 7.4°C. The extent of malate inhibition was the same for both day and night forms of PEPCase assayed at 35°C, but increased considerably with night enzyme at 25°C. The results indicate that light increases the apparent affinity of PEPCase for PEP and that at lower temperatures malate becomes more inhibitory. Pyruvate orthophosphate dikinase activity started to increase immediately after sunrise and the 10-fold increase at fixed temperature became 14.8-, 14.2- and 13.1-fold when assays were run at the above leaf temperatures. This indicates that the light effect predominates with pyruvate, orthophosphate dikinase, while with phosphoenolpyravate carboxylase, light and temperature co-operate to increase the day enzyme activities. 相似文献
122.
Janneke Hille Ris Lambers W. Stanley Harpole David Tilman Johannes Knops Peter B. Reich 《Ecology letters》2004,7(8):661-668
Species’ extinctions have spurred debate on whether interactions among few or among many species cause a positive diversity–productivity relationship in experimentally assembled grasslands. We addressed this question by quantifying the productivity of 14 species across an experimental diversity gradient in Minnesota. We found that interspecific interactions leading to coexistence and competitive displacement both determine which species overyield; i.e. are more productive at high diversity. Overyielding species were either superior N competitors (C4 grasses) or N fixers (legumes). Surprisingly, these species were not most productive in monoculture, thus, the ‘selection’ of productive species in diverse plots did not cause the positive diversity–productivity relationship. Both positive (with legumes) and negative interspecific interactions (with C4 grasses) determined whether individual species overyielded. Foliar pathogens did not cause overyielding, although other natural enemies may be responsible. Overyielding species are not displacing underyielding species over time, implying that other diversity‐promoting interactions also operate in this experiment. 相似文献
123.
D. Babendreier§ D. Schoch† S. Kuske S. Dorn† F. Bigler 《Agricultural and Forest Entomology》2003,5(3):199-208
Abstract 1 Trichogramma brassicae Bezdenko is inundatively released against the European corn borer in Switzerland. Because parasitoids dispersing from the release fields might pose a threat to native butterflies, the searching efficiency of T. brassicae was investigated in nontarget habitats. 2 In field studies, T. brassicae was released at rates of 120 000 females/ha. Parasitism of sentinel Ephestia kuehniella egg clusters was 1.6–3.6% in meadows and 2.0–4.0% in flower strips. The respective figures were 57.6–66.7% and 19.2–46.9% in maize, significantly higher than the parasitism rates in the nontarget habitats. Experiments carried out in small field cages confirmed these results: Again, significantly higher parasitism rates were found in maize compared to meadows and flower strips, and also compared to hedgerows (in sleeve cages). 3 To elucidate potential factors underlying the low searching efficiency in nontarget habitats, the behaviour of individual T. brassicae females was investigated on four meadow plants comparatively to maize and a filter paper control. Mean (±SE) walking speed on maize was 2.2 ± 0.2 mm/s, similar to three of the plants tested and filter paper but significantly higher than on Trifolium pratense (0.85 mm/s). A higher turning rate was found on T. pratense, Viola wittrockiana and Plantago lanceolata, in contrast to the longer leaved maize and Alopecurus pratensis. The number of wasps leaving the plant within the observation period differed significantly between plant species, and was twice as high for T. pratense (and the filter paper control) compared to the other plant species. 4 In a choice experiment carried out in a climate cabinet with all five host plant species in cages, we obtained the highest parasitism rates on maize and the lowest parasitism on T. pratense, thus confirming the behavioural observations. 5 In conclusion, there is evidence for a decreased searching efficiency on plants in nontarget habitats compared to maize. However, the data explain only part of the differences found between parasitism in maize compared to nontarget habitats. Other factors, such as the structural complexity of a habitat, may also play a role. We conclude that the risk for butterfly populations in the tested nontarget habitat due to mass released T. brassicae is low. 相似文献
124.
In this paper the NMR secondary chemical shifts, that are estimated from a set of 3D-structures, are compared with the observed ones to appraise the behaviour of a known x-ray diffraction structure (of the bovine pancreatic trypsin inhibitor protein) when various molecular dynamics are applied. The results of a 200 ps molecular dynamics under various conditions are analysed and different ways to modify the molecular dynamics are considered. With the purpose of avoiding the time-consuming explicit representation of the solvent (water) molecules, an attempt was made to understand the role of the solvent and to develop an implicit representation, which may be refined. A simulation of hydrophobic effects in an aqueous environment is also proposed which seems to provide a better approximation of the observed solution structure of the protein. 相似文献
125.
The synthesis of four β-lactam antibiotics (penicillin G, pivaloyloxymethyl ester of penicillin G, ampicillin and pivampicillin) catalyzed byEscherichia coli penicillin acylase has been investigated in water-methanol mixtures. The enzyme reactions were either thermodynamically or kinetically controlled at the same conditions using phenylacetic acid andd--phenylglycine methyl ester as acyl donors and 6-aminopenicillanic acid and pivaloyloxymethyl 6-aminopenicillanic acid as acyl acceptors. It has been found that the influences of the cosolvent content on the reaction rates and synthetic yields are significantly different depending on the substrates used in the experiments. On the other hand, within certain ranges of the methanol content (up to ca. 40% (v/v) the residual activities of the enzymes in water-methanol mixtures were only slightly lower than those in aqueous media. To analyze the factors that determine the reaction rate in water-cosolvent mixtures, the effect of methanol on the apparent pK values of the substrates has been investigated, and a mathematical model has been developed on the basis of the assumption that the enzyme binds non-ionized substrates. Model simulation results indicate that the solvent effect on reaction rates is mainly attributed to the kinetic effects of changes in apparent pK values. 相似文献
126.
127.
深圳福田红树林区底栖大型动物群落的空间分带及灌污的可能影响 总被引:4,自引:1,他引:3
本文自1991年4月至1993年1月对深圳福田红树林中底栖大型动物的空间分带及灌污的可能影响进行了研究。结果表明,该红树林湿地中主要出现的底栖动物为拟沼螺科,黑螺科,汇螺科,沙蟹科,方蟹科和弹涂鱼科种类。红树区内底栖动物从高潮位到低潮位可分为3个群落分布带:亮泽拟沼螺带;拟黑螺-褶痕相手蟹带;弧边招潮-印尼拟蟹守螺-刻纹拟沼螺带。群落的分带可能主要由潮位线,食物适应性及底质结构因素决定。林前泥滩底栖动物种类多样性最大,生物量最高。林内动物群落则表现出低种类多样性,高种群个体数的特点。群落总栖息密度的变化基本上由软体动物所主导。生活污水排灌对红树林中底栖大型动物的影响不明显,仅在排污口端引起少数污水动物种类的出现及群落总生物量轻微的升高。 相似文献
128.
129.
真菌诱导子对悬浮培养西洋参细胞的生理效应 总被引:14,自引:0,他引:14
报道了不同真菌诱导子对悬浮培养的西洋参(Panaxquinquefolium)细胞生长、皂甙和多糖合成,以及细胞内和培养液中过氧化物酶活性的生理效应。悬浮培养的西洋参细胞经刺盘孢菌(Colletotrichumnicoltianae)丝体诱导子处理后,总皂甙产率可由对照的296mg/L增加到679mg/L(约占细胞干重的(16.3%),比对照提高约1.3倍,而且总皂甙的85%排放在培养液中;经黑曲霉(Aspergillusnigran)诱导子处理后,细胞多糖含量可达到11.79%(细胞干重),比对照增加1倍多。初步纯化的刺盘孢菌丝体诱导子和尖孢镰刀菌(Fusuriumoxysporum)滤液诱导子在诱导处理前期能明显促进西洋参细胞生长,同时细胞内及培养液中过氧化物酶活性显著增加;随时间延长,细胞生长和酶活性逐步受到抑制。 相似文献
130.