甜瓜幼苗生长及光合特性与育苗基质pH相关性研究

以新疆厚皮甜瓜皇后为试材,在泥炭珍珠岩复合基质中,按一定比例加入CaCO3,构成pH梯度值分别为5.0、5.5、6.0、6.5、7.0、7.5和8.0的7种基质类型,研究了基质pH对甜瓜幼苗生长及其光合特性的影响。结果表明,基质酸碱性对甜瓜幼苗的光合特性、根系活力、单株叶面积、根系和地上部干物重都产生显著影响,酸性和微酸性基质(pH<6.0)时,幼苗叶片叶绿体超微结构发生降解,叶绿素堆积,叶片净光合速率下降,单株叶面积减小,根和地上部干重降低;pH为6.0～7.0的各处理在叶面积以及根和地上部干重指标上,F检验不显著;pH>7.0的微碱性和碱性基质虽然对幼苗产生不利影响,但与pH<6.0的处理比较,其影响要小些。鉴于此,甜瓜幼苗生长的基质pH范围为6.0～7.0,偏碱不会对幼苗产生严重的生理障碍。采用CaCO3调节基质pH时,最佳调节范围为pH6.0～6.5。     

Modelling the spatial configuration of refuges for a sustainable control of pests: a case study of Bt cotton

The 'high-dose-refuge' (HDR) strategy is widely recommended by the biotechnology industry and regulatory authorities to delay pest adaptation to transgenic crops that produce Bacillus thuringiensis (Bt) toxins. This involves cultivating nontoxic plants (refuges) in close proximity to crops producing a high dose of Bt toxin. The principal cost associated with this strategy is due to yield losses suffered by farmers growing unprotected, refuge plants. Using a population genetic model of selection in a spatially heterogeneous environment, we show the existence of an optimal spatial configuration of refuges that could prevent the evolution of resistance whilst reducing the use of costly refuges. In particular, the sustainable control of pests is achievable with the use of more aggregated distributions of nontransgenic plants and transgenic plants producing lower doses of toxin. The HDR strategy is thus suboptimal within the context of sustainable agricultural development.     

Comparison of three pyrethroid treatments of top-sheets for malaria control in emergencies: entomological and user acceptance studies in an Afghan refugee camp in Pakistan

Insecticide-treated bedding materials (sheets and blankets) could be protective against vectors of malaria and leishmaniasis--especially in complex emergencies, epidemics and natural disasters where people are more likely to sleep in exposed situations. Comparison of cotton top-sheets impregnated with different pyrethroids (permethrin 500 mg/m2, deltamethrin 25 mg/m2 or alphacypermethrin 25 mg/m2) for effectiveness against mosquitoes (Diptera: Culicidae) was undertaken in a refugee camp in Pakistan. Predominant species encountered were Anopheles stephensi Liston, An. pulcherrimus Theobald, An. nigerrimus Giles, Culex quinquefasciatus Say, Cx. tritaeniorhynchus Giles and other culicine mosquitoes. All three pyrethroid treatments performed significantly better than the untreated sheets in deterrence and killing of mosquitoes. No significant differences were found between the three insecticides tested in terms of entomological effect. Washing of the treated sheets greatly reduced their effectiveness. In a user acceptance study conducted among 88 families (divided into four groups), six families complained of irritation of the skin and mucous membranes. Of these reports, one was from the placebo group (using untreated sheets) and the other five (5/22=23%) from families using deltamethrin-treated sheets. All families allocated to permethrin and alphacypermethrin groups declared an appreciation for the intervention and reported no side-effects. Ten of the placebo group disliked the intervention, citing no prevention of mosquito biting as the reason. Side-effects associated with deltamethrin indicate that alphacypermethrin and permethrin are more appropriate first choice insecticides for treatment of sheets and blankets.     

Influence of litter and weather on seedling recruitment in a mixed oak-pine woodland

The effects of regular litter removal and annual variation in temperature and precipitation on seedling recruitment of species differing in their seed size and mode of dispersal were studied in a 16-year (1984-1999) experiment in a mixed oak-pine wood in southern Poland. Litter was the most important factor in determining spatial variability in seedling recruitment, whereas differences in climatic conditions among years, especially temperature fluctuations in late winter and early spring, determined the temporal variability in seedling recruitment. Compared with control plots, significantly more new individuals of bryophytes and seedlings as well as a number of new species of vascular plants were noted in the litter-removal plots over the 16-year study. Litter strongly impeded seedling emergence of small-seeded species. The negative effect of litter on seedling recruitment of large-seeded species and the recruitment of new shoots in species growing clonally was much weaker. There was a significant positive correlation between the numbers of seedlings in the litter-removal and control plots and temperatures in January to March. In the litter-removal plots this mainly affected small-seeded species. Seedling recruitment was less consistently related to variation in precipitation. Positive relationships were found only between the number of seedlings of large-seeded species in the litter-removal plots and precipitation in July of the current year and in September of the previous year, and between the number of seedlings in the control plots and precipitation in September and November of the previous year.     

Habitat and Population Effects on the Germination and Early Survival of the Invasive Weed, Lythrum salicaria L. (purple loosestrife)

In this study, we examined how environmental factors and differences among populations of Lythrum salicaria L. (Lythraceae; purple loosestrife) influenced the establishment of this species into both its preferred wetland habitat and secondary upland habitat. Seeds and seedlings from five parents from each of the three populations were planted in each wetland and upland of two sites in both disturbed and undisturbed plots. More seeds germinated in wetland than in upland sites (31% as compared to 11%) and, although a disturbance resulted in greater germination (23% as compared to 17% in undisturbed plots), there was no interaction with habitats. The results of a simultaneous greenhouse germination study in which seeds were planted under saturated and drained conditions suggest that the higher germination observed in wetland sites was due to the higher moisture present at such sites (79% vs. 45% in saturated and drained conditions, respectively). Disturbance enhanced both germination and growth of L. salicaria and this was especially so for seedlings. Dry weights of seedlings from disturbed plots were five times greater than those from undisturbed plots ( =9.2 vs. 1.8g in disturbed and undisturbed conditions, respectively). In addition to environmental factors, population differences are likely to play an important role in the spread of L. salicaria as we found differences between the populations in both life stages examined. One of the populations germinated equally well in both moisture treatments. In our seedling establishment experiment, one of the populations produced both the smallest plants and the most and earliest flowers, suggesting superior reproduction.     

The underground phase: a special challenge in studies of terrestrial orchid populations

Orchid seeds are minute and the first seedling stages of terrestrial species are underground, so there is little knowledge about the biology of the early life history and the size of seed/seedling populations relative to the number of plants that eventually emerge above ground. A recently developed field sowing technique makes it possible to explore a variety of topics such as the length of time that seeds spend as part of the soil seed bank, the phenology of seed germination, substrate requirements for germination, dependence of germination on fungal availability, substrate requirements and growth rate of seedlings, seedling phenology and development, and seedling-fungus specificity patterns in the soil. Several of these applications are relevant for conservation issues. Studies of Goodyera pubescens, Corallorhiza odontorhiza and Liparis lilifolia show considerable variation in germination strategies, patterns of seedling development and establishment, substrate requirements, and levels of specificity with the fungi.     

Genetic variation of stomatal conductance, blue light sensitivity and zeaxanthin content in guard cells of Pima cotton (Gossypium barbadense)

Pima S‐6 ( Gossypium barbadense L.) is a modern line with high stomatal conductance, while B368 is a primitive cotton with low conductance. The blue light sensitivity of adaxial guard cells, probed as the blue light‐dependent enhancement of the red light‐induced chlorophyll a fluorescence quenching, was investigated in these two cotton lines with contrasting stomatal conductance. Adaxial guard cells isolated from Pima S‐6 cotton plants had a significantly higher carotenoid content and a higher blue light sensitivity than those isolated from B368 plants. In a growth chamber‐grown F₂ population of a cross between these two lines, adaxial stomatal conductances of individual plants segregated over a range exceeding the average conductances of the parents. Carotenoid content and the blue light sensitivity of adaxial guard cells also segregated. The concentrations of xanthophylls and β‐carotene in the adaxial guard cells were poorly correlated with the blue light response, except for zeaxanthin. The co‐segregation of stomatal conductance and blue light sensitivity suggested that the stomatal response to blue light may play a role in the regulation of stomatal conductance in the intact leaf. Zeaxanthin content and blue light sensitivity also co‐segregated, suggesting that both parameters are under genetic control. The co‐segregation of zeaxanthin content, blue light sensitivity and stomatal conductance provides further evidence for a role of zeaxanthin in the blue light photoreception of guard cells.     

A simple Markov model for the assessment of host patch quality by foraging parasitoids

Insect parasitoids search for their hosts using a method that may be broken into three parts. First, they locate plants which may harbor their hosts, then they assess the quality of these plants to decide whether to search them further for hosts and, finally, if they decide to accept a plant for further search, they exploit the plant by searching for hosts and attacking them when they are found. We study the way that parasitoids assess plant quality by developing a mathematical model based on behavioral observations of foraging parasitoids that attack aphids which infest crucifers. Assessment of plants is based on the concentration of cues produced by hosts that inhabit them. Parasitoids are more likely to exploit plants on which more host cues are detected, and the willingness of a parasitoid to exploit a given plant depends on the quality of other plants that have been visited recently. Plants whose quality exceeds a certain threshold will be accepted for exploitation. The threshold for plant acceptance will change with the experience of the parasitoid, increasing when plants heavily infested with hosts are encountered, decreasing when uninfested plants are encountered. We analyze several rules that might describe how the acceptance threshold changes with parasitoid experience, and for each rule we show how the number of parasitoids willing to accept plants with various levels of infestation depends on the number of plants with various levels of infestation. We then consider different rules for exploitation of hosts on plants and find how the proportion of hosts attacked depends on host density. Received: 21 April 1998 / Accepted: 8 May 1998