三峡库区9种植物种子萌发特性及其在植被恢复中的意义

三峡大坝蓄水后形成的库区消涨带面临植被消亡、生态退化等问题。为了筛选出适用于库区消涨带植被恢复的植物, 将9种1年生植物种子放置在库区消涨带不同海拔进行水淹(W 165-8 m, 121 d;W 155-18 m, 230 d;W 147-26 m, 271 d), 然后在实验室条件下进行萌发, 研究在消涨带淹水胁迫下这些种子的萌发特性。结果表明: (1) 除马唐(Digitaria sanguinalis)、小蓬草(Conyza canadensis)、金色狗尾草(Setaria glauca)种子在各条件下萌发率都较低外, 不同水淹条件对萌发率影响不同: 与新鲜种子相比, W 165水淹后, 旱稗(Echinochloa hispidula)和婆婆针(Bidens bipinnata)种子萌发率显著上升, 其余种子萌发率均显著下降; W 155水淹后, 所有种子的萌发率都显著下降且只有鱧肠(Eclipta prostrate)、黄花蒿(Artemisia annua)、合萌(Aeschynomene indica)3个物种有萌发, 萌发率分别为11.0%、7.3%和2.7%; W 147水淹后, 旱稗和婆婆针种子萌发率显著上升, 鱧肠种子无显著差异, 其余种子萌发率显著下降。(2) 鱧肠、黄花蒿、婆婆针和旱稗种子比其它物种更耐水淹。W 165水淹后, 鱧肠、黄花蒿、婆婆针、旱稗种子萌发率分别为44.7%、42%、20.7%和4.3%, W 147水淹后分别为76.3%、23%、15%和26.3%, 高于其他物种。(3) 水淹后种子萌发时间格局也受到影响, 大部分种子起始萌发时间推迟、萌发速度变慢。鱧肠、黄花蒿、婆婆针和旱稗的种子对三峡库区消涨带的水淹胁迫具有一定的适应能力, 可根据它们对水淹条件适应能力的差异在消涨带不同海拔高度进行植被恢复。     

Thellungilla halophila is more adaptive to salinity than Arabidopsis thaliana at stages of seed germination and seedling establishment

Thellungiella halophila is a salt tolerant relative of Arabidopsis thaliana with high genetic and morphological similarity. In the present study, effects of salinity on germination and seedling growth of T. halophila and A. thaliana were compared. The present results showed that the salinity inhibited seed germination in both species. Unexpectedly, percentages of seed germination in A. thaliana were higher than T. halophila in a range of 0?C200?mM NaCl. Seeds of both species could not germinate when the concentration of NaCl was over 200?mM. However, when compared with A. thaliana, seeds of T. halophila did not suffer ion toxicity, as evidenced by the higher final germination rate after ungerminated seeds pretreated with NaCl were transferred to distilled water. Seedlings of T. halophila were more salt tolerant than those of A. thaliana, e.g., seedlings of T. halophila had better plant growth (root length, fresh and dry mass), higher chlorophyll content, less MDA content and higher proline content and K⁺/Na⁺ ratio under salinity. These results indicate that T. halophila is more salt tolerant than A. thaliana during both seed germination and seedling stages and explain why A. thaliana is excluded from saline locations and T. halophila can survive in saline soils.     

Impact on Bacterial Community in Midguts of the Asian Corn Borer Larvae by Transgenic Trichoderma Strain Overexpressing a Heterologous chit42 Gene with Chitin-Binding Domain

This paper is the first report of the impact on the bacterial community in the midgut of the Asian corn borer (Ostrinia furnacalis) by the chitinase from the transgenic Trichoderma strain. In this study, we detected a change of the bacterial community in the midgut of the fourth instar larvae by using a culture-independent method. Results suggested that Proteobacteria and Firmicutes were the most highly represented phyla, being present in all the midgut bacterial communities. The observed species richness was simple, ranging from four to five of all the 16S rRNA clone libraries. When using Trichoderma fermentation liquids as additives, the percentages of the dominant flora in the total bacterial community in larval midgut changed significantly. The community of the genus Ochrobactrum in the midgut decreased significantly when the larvae were fed with the fermentation liquids of the transgenic Trichoderma strain Mc4. However, the Enterococcus community increased and then occupied the vacated niche of the Ochrobactrum members. Furthermore, the Shannon–Wiener (H) and the Simpson (1-D) indexes of the larval midgut bacterial library treated by feeding fermentation liquids of the transgenic Trichoderma strain Mc4 was the lowest compared with the culture medium, fermentation liquids of the wild type strain T30, and the sterile artificial diet. The Enterococcus sp. strain was isolated and characterized from the healthy larvae midgut of the Asian corn borer. An infection study of the Asian corn borer larvae using Enterococcus sp. ACB-1 revealed that a correlation existed between the increased Enterococcus community in the larval midgut and larval mortality. These results demonstrated that the transgenic Trichoderma strain could affect the composition of the midgut bacterial community. The change of the midgut bacterial community might be viewed as one of the factors resulting in the increased mortality of the Asian corn borer larvae.     

Effect of Light and Moisture Conditions and Seed Age on Germination of Three Closely Related Myosotis Species

Germination of three closely related species from the Myosotis palustris group (M. nemorosa, M. palustris subsp. laxiflora, M. caespitosa) differing in their habitats and capacity for clonal growth, was compared in two greenhouse experiments. To evaluate both inter- and intraspecific variation, each species was represented by seeds from several populations. Final germination percentage and germination rates T₅₀ were compared both between species and populations within species. In the first experiment, we studied the influence of two external factors, moisture and light. Four moisture levels (dry, wet, periodically flooded and permanently flooded soil) and three types of shading (without shading, shaded with green foil, shaded with solid paper sheet) were combined in a complete factorial design. In all three species, total germination percentage was the same in the three wettest treatments, and decreased in the dry treatment. Germination in the treatments shaded with green foil (simulating vegetation cover, which changed light quality) was significantly slower than in treatments without shading and treatments shaded with a solid paper sheet. There were significant differences among species, but we also found very pronounced differences among populations within a species. M. caespitosa had the uniformly highest germination percentage (reaching in some cases 100%) and also fastest germination. Germination of M. palustris subsp. laxiflora populations was slower and reached lower final proportions, and medium variability among populations. Inter-population variability in the final germination percentage was highest, and the final germination the lowest in M. nemorosa. In addition, M. nemorosa, a species typical for permanent meadow communities was delayed by permanent flooding. In the second experiment, we studied the effects of seed age and storage conditions. Three combinations of seed age and storage were used: younger seeds (half year old) with no-chilling, younger seeds with chilling and old seeds (three years old) with chilling. M. caespitosa had again the highest final germination percentage and fastest germination rates T₅₀. In addition, final germination percentage of this species slightly increased with seed age, whereas it decreased in the other two species. The germination behaviour corresponded well to expectation based on species life histories and habitat preferences. Remarkably stable and high germination percentages and fastest germination rates T₅₀ were ascertained in M. caespitosa, a species of disturbed habitats, with lowest capacity for clonal spread. M. palustris subsp. laxiflora (species with highest clonal capacity) and M. nemorosa (species with medium clonal potential) achieved lower, but still very high final germination percentage. In addition, M. nemorosa showed the highest inter-populations variability in our experiments.     

Conditional cold avoidance drives between-population variation in germination behaviour in Calluna vulgaris

Background and Aims

Across their range, widely distributed species are exposed to a variety of climatic and other environmental conditions, and accordingly may display variation in life history strategies. For seed germination in cold climates, two contrasting responses to variation in winter temperature have been documented: first, an increased ability to germinate at low temperatures (cold tolerance) as winter temperatures decrease, and secondly a reduced ability to germinate at low temperatures (cold avoidance) that concentrates germination towards the warmer parts of the season.

Methods

Germination responses were tested for Calluna vulgaris, the dominant species of European heathlands, from ten populations collected along broad-scale bioclimatic gradients (latitude, altitude) in Norway, covering a substantial fraction of the species'' climatic range. Incubation treatments varied from 10 to 25 °C, and germination performance across populations was analysed in relation to temperature conditions at the seed collection locations.

Key Results

Seeds from all populations germinated rapidly and to high final percentages under the warmer incubation temperatures. Under low incubation temperatures, cold-climate populations had significantly lower germination rates and percentages than warm-climate populations. While germination rates and percentages also increased with seed mass, seed mass did not vary along the climatic gradients, and therefore did not explain the variation in germination responses.

Conclusions

Variation in germination responses among Calluna populations was consistent with increased temperature requirements for germination towards colder climates, indicating a cold-avoidance germination strategy conditional on the temperature at the seeds'' origin. Along a gradient of increasing temperatures this suggests a shift in selection pressures on germination from climatic adversity (i.e. low temperatures and potential frost risk in early or late season) to competitive performance and better exploitation of the entire growing season.     

Germinating seeds or bulbils in 87 of 113 tested Arctic species indicate potential for ex situ seed bank storage

Arctic plant species are expected to lose range due to climate change. One approach to preserve the genetic and species diversity for the future is to store propagules in seed vaults. However, germinability of seeds is assumed to be low for Arctic species. We evaluated ex situ storage potential of 113 of the 161 native angiosperms of Svalbard by studying seed ripening and germination. Seeds or bulbils were collected, and germinability was tested after one winter of storage in the Svalbard Global Seed Vault. Twenty-six of the species did not produce ripe propagules, 8 produced bulbils, and 79 produced seeds. Bulbils sprouted to high percentages. Seeds of 10 species did not germinate, 22 had low germination (<20 %), 34 had germination of 21–70 %, and 13 had high germination percentages (>70 %). More than 70 % of the species belonging to Asteraceae, Brassicaceae, Caryophyllaceae, Juncaceae, Rosaceae, and Saxifragaceae germinated. Cold tolerant, common species had higher germination percentages than relatively thermophilous, rare species. Germination percentages were six times higher than observed in 1969 (n = 51) and 0.7 times that observed in 2008 (n = 22), indicating that recent climate warming improves germination in the Arctic. While in situ conservation is of vital importance, ex situ conservation in seed banks is a potential complementary conservation strategy for the majority of Arctic vascular plant species. For species that did not germinate, other methods for ex situ conservation should be sought, for example, growing in botanical gardens.     

Effects of a warmer climate on seed germination in the subarctic

Background and Aims

In a future warmer subarctic climate, the soil temperatures experienced by dispersed seeds are likely to increase during summer but may decrease during winter due to expected changes in snow depth, duration and quality. Because little is known about the dormancy-breaking and germination requirements of subarctic species, how warming may influence the timing and level of germination in these species was examined.

Methods

Under controlled conditions, how colder winter and warmer summer soil temperatures influenced germination was tested in 23 subarctic species. The cold stratification and warm incubation temperatures were derived from real soil temperature measurements in subarctic tundra and the temperatures were gradually changed over time to simulate different months of the year.

Key Results

Moderate summer warming (+2·5 °C) substantially accelerated germination in all but four species but did not affect germination percentages. Optimum germination temperatures (20/10°C) further decreased germination time and increased germination percentages in three species. Colder winter soil temperatures delayed the germination in ten species and decreased the germination percentage in four species, whereas the opposite was found in Silene acaulis. In most species, the combined effect of a reduced snow cover and summer warming resulted in earlier germination and thus a longer first growing season, which improves the chance of seedling survival. In particular the recruitment of (dwarf) shrubs (Vaccinium myrtillus, V. vitis-idaea, Betula nana), trees (Alnus incana, Betula pubescens) and grasses (Calamagrostis lapponica, C. purpurea) is likely to benefit from a warmer subarctic climate.

Conclusions

Seedling establishment is expected to improve in a future warmer subarctic climate, mainly by considerably earlier germination. The magnitudes of the responses are species-specific, which should be taken into account when modelling population growth and migration of subarctic species.Key words: Climate change, cold stratification, dwarf shrubs, germination percentage, incubation temperature, mean germination time, seedling establishment, seeds, Silene, subarctic species, Vaccinium, warming     

Predicting germination in semi-arid wildland seedbeds II. Field validation of wet thermal-time models

Accurate prediction of germination for species used for semi-arid land revegetation would support selection of plant materials for specific climatic conditions and sites. Wet thermal-time models predict germination time by summing progress toward germination subpopulation percentages as a function of temperature across intermittent wet periods or within singular wet periods. Wet periods may be defined by any reasonable seedbed water potential above which seeds are expected to imbibe sufficiently to germinate. These models may be especially applicable to the Artemisia steppe of the western U.S.A. where water availability limits germination in summer and early fall while cool temperatures limit germination in late fall, winter, and spring when soil water is available. To test accuracy of wet thermal-time models we placed seedbags with seeds of five species commonly used in wildland revegetation, as well as two collections of the invasive annual grass, Bromus tectorum L. into Artemisia tridentata Nutt. ssp. wyomingensis Beetle and Young zone seedbeds for 19 field incubation periods over four seasons. Hourly surface (1–3 cm) soil temperatures and soil water potentials were measured near the seedbags. These data were input into thermal-time models which predicted time to germination for each seedbag retrieval date. Binomial data representing agreement (1) or lack of agreement (0) of predicted and actual germination for each retrieval date were analyzed using logistic regression. Thermal summation method, season, water potential threshold, and species most affected accuracy of predictions (P < 0.0002). A model which defined a wet period as ≥−1.5 MPa soil water potential and summed progress toward germination across intermittent wet periods was most accurate in predicting actual germination by a retrieval date. Across all species, this model correctly predicted that germination would occur in seedbags 75–95% of the time over the latewinter to mid-spring seasons, but only 50–71% of the time for the fall-early winter season when time of soil water availability was least. Although the wet thermal-time model overestimated time to germination for some species and seasons, its accuracy should be high enough to evaluate germination potential by mid-spring for different species, sites, and climatic conditions.     

Effects of waterlogging on seed germination of three Mediterranean oak species: Ecological implications

Soil water saturation during prolonged periods of time generates a negative impact on nearly all terrestrial plants. In Mediterranean woodlands, precipitation can be very abundant during the wet season, inducing temporary soil waterlogging, coinciding with the seed dispersal and germination time of many species. We investigated the effects of waterlogging on seed germination and early root growth of three coexisting oak species (Quercus canariensis, Q. suber and Q. pyrenaica), by completely flooding of seeds for various periods of time. The three oak species showed a certain level of tolerance to waterlogging, only being affected those seeds subjected for long periods of submersion (over 30 days). Waterlogging during prolonged periods of time decreased the probability of seed germination in the three oak species, lengthened the time to germination, and hampered root development in two of the studied species. The main differences between oak species occurred in terms of root growth (Q. canariensis being the less affected, and Q. suber the most); these differential responses could be related to a species rank of waterlogging tolerance. Thus inter-specific differences in germination responses to waterlogging could contribute to explain, at least partially, species habitat and distribution patterns across landscapes. Seed mass also played an important role on different aspects of germination, though its relative importance varied as function of species and waterlogging treatment. The tolerance to stress induced by waterlogging increased with seed mass, but only in the case of Q. canariensis.     

Role of temperature in the germination ecology of three summer annual weeds

Summary Ambrosia artemisiifolia L., Chenopodium album L., and Amaranthus retroflexus L. are three summer annual weeds that occur in disturbed habitats. In nature, the peak germination season for A. artemisiifolia and C. album is in early to mid-spring, while in A. retroflexus the peak germination season is late spring to early summer. Furthermore, seeds of A. artemisiifolia germinate only in spring, while seeds of C. album and A. retroflexus germinate throughout the summer. In an attempt to explain the differential germination behavior of these three species in nature, changes in their germination responses to temperature during burial in a non-heated greenhouse from October 1974 to October 1975 were monitored. A high percentage of the seeds of all three species after-ripened during winter. Seeds of A. artemisiifolia and C. album germinated at temperatures characteristic of those in the field in early and mid-spring, but seeds of A. retroflexus required the higher temperatures of late spring and early summer for germination. Seeds of all three species germinated to higher percentages in light than in darkness. Non-dormant seeds of A. artemisiifolia that did not germinate in spring entered secondary dormancy. On the other hand, seeds of C. album and A. retroflexus that did not germinate when temperatures first became favorable for germination, did not enter secondary dormancy and, thus, retained the ability to germinate at summer field temperatures during summer. Thus, temporal differences in the germination behavior of these three species are caused by the differential reaction of the seeds to temperature during the annual temperature cycle.     

Effect of yak rumen content treatments on seed germination of 11 alpine meadow species on the Qinghai-Tibetan Plateau

The Qinghai-Tibetan Plateau is located in the ‘Third Pole’ of the world, characterized by a harsh environment. Despite this, the alpine meadow ecosystem have developed over a wide area but serious grassland degradation is threatening the ecological environment on the Plateau. Recruitment of new plants to the population, via germination and establishment, is vital to plant community survival. Previous work on the seeds in this area has mainly focused on community-wide germination strategies, seed germination characteristics and their correlations with seed size and seed mass. However, there have been no studies on the effects of soaking in rumen contents on the plant seed germination characteristics of alpine meadow species. The present study had two main objectives: (i) to determine the effect of fresh rumen content from yaks on seed germination characteristics and seedling growth of species common to the eastern Tibetan Plateau alpine meadow, and (ii) to develop an effective method to enhance seed germination. Seeds of 11 common species were collected together with fresh rumen content from three yaks that grazed there. Seed germination tests were conducted after they had been soaked in rumen content for one of six soaking periods (12, 24, 36, 48, 60 or 72 h). The seeds were incubated under natural light conditions of 8 h light at 25 °C and 16 h darkness at 15 °C, for the germination period of 34 days. The results showed that seed germination and seedling growth were affected by soaking time, seed coat completeness and seed type. After soaking in rumen content, the germination percentages of scarified (peeled or with the seed coat cut through) seeds of some species (true seeds Oxytropis ochrocephala and Medicago ruthenia var inschanica, nutlet Carex enervis, achenes Anemone rivularis and Polygonum sibiricum) and complete seeds of C. enervis, and A. rivularis were improved but the duration of soaking was also important. Seed germination of caryopsis Achnatherum inebrians (a toxic grass) was significantly inhibited by any exposure to rumen fluids. Scarified seeds generally had higher germination percentages than complete ones after treatment, but with the increase in soaking time, germination percentages declined and scarified seeds were more sensitive to the treatment than the complete seeds. After soaking in yak rumen content, the germination indices of scarified M. ruthenia at 12 h treatment, O. ochrocephala and achene Rumex acetosa at 12–24 h treatment, nutlet Kobresia humilis at 24 h treatment, P. sibiricum at 24–48 h treatment, C. enervis at 12–48 h treatment and A. rivularis at 12–60 h treatment were significantly higher than the control (P < 0.05), while the germination indices of complete C. enervis seeds at 12 h and 36 h treatment, and A. rivularis at 12–60 h treatment were significantly higher compared with the control. The germination indices of other species gradually decreased with the increase in soaking time. We concluded that yak rumen digestion could enhance, inhibit or not affect seed germination and seedling growth of the alpine meadow species, which might influence seedling recruitment, interspecific competition, and the plant community structure of the eastern Tibetan Plateau alpine meadow. Overall, yak digestion has a positive effect on alpine meadow seed germination and seed dispersal.     

Germination of the fully myco-heterotrophic orchid Cyrtosia septentrionalis is characterized by low fungal specificity and does not require direct seed-mycobiont contact

We examined the level of specificity exhibited by seeds of Cyrtosia septentrionalis for germination-inducing fungi. Three wood-decomposing Armillaria species (A. gallica, A. mellea subsp. nipponica and A. tabescens) known to colonize adult plants and an unidentified species of Polyporales isolated from naturally growing protocorms were tested. Xylobolus annosus, a free-living decomposer of Russulales, was included as a control. Seed germination occurred in a sawdust-based medium in sealed and unsealed containers in the presence of all fungi, indicating low mycorrhizal specificity in germination. Moreover, germination occurred even in modified containers in which the fungus was physically isolated from the orchid seeds, indicating that direct seed-fungus contact is not required. Higher germination percentages were observed in sealed containers in which a modified atmosphere, consisting of a lower O₂ and a higher CO₂ concentration, had been established in the air above the inoculated medium as a result of the saprophytic activity of the fungus. In nature, atmospheric conditions more effective for seed germination might be established by the action of mycobionts in decomposing wood. Seeds germinated in the presence of the unknown Polyporales species underwent further growth and development, as compared with the other fungi tested, when covered with soil.     

A novel sucrose hydrolase from the bombycoid silkworms Bombyx mori,Trilocha varians,and Samia cynthia ricini with a substrate specificity for sucrose

Although membrane-associated sucrase activity has been detected in the midgut of various lepidopteran species, it has not yet been identified and characterized at the molecular level. In the present study, we identified a novel sucrose hydrolase (SUH) gene from the following three bombycoid silkworms: Bombyx mori, Trilocha varians, and Samia cynthia ricini and named them BmSuh, TvSuh, and ScSuh, respectively. The EST dataset showed that BmSuh is one of the major glycoside hydrolase genes in the larval midgut of B. mori. These genes were almost exclusively expressed in the larval midgut in all three species, mainly at the feeding stage. SUHs are classified into the glycoside hydrolase family 13 and show significant homology to insect maltases. Enzymatic assays revealed that recombinant SUHs were distinct from conventional maltases and exhibited substrate specificity for sucrose. The recombinant BmSUH was less sensitive to sugar-mimic alkaloids than TvSUH and ScSUH, which may explain the reason why the sucrase activity in the B. mori midgut was less affected by the sugar-mimic alkaloids derived from mulberry.     

Germination ecology of Ambrosia artemisiifolia L. and Ambrosia trifida L. biotypes suspected of glyphosate resistance

The germination ecology of Ambrosia artemisiifolia and A. trifida glyphosate susceptible biotypes sampled in marginal areas, was compared with that of the same species but different biotypes suspected of glyphosate resistance, common and giant ragweed, respectively. The suspected resistant biotypes were sampled in Roundup Ready® soybean fields. Within each weed species, the seeds of the biotype sampled in marginal area were significantly bigger and heavier than those of the biotype sampled in the soybean fields. A. artemisiifolia biotypes exhibited a similar dormancy and germination, while differences between A. trifida biotypes were observed. A. artemisiifolia biotypes showed similar threshold temperature for germination, whereas, the threshold temperature of the susceptible A. trifida biotype was half as compared to that of the resistant A. trifida biotype. No significant differences in emergence as a function of sowing depth were observed between susceptible A. artemisiifolia and suspected resistant A. trifida biotype, while at a six-cm seedling depth the emergence of the A. artemisiifolia susceptible biotype was 2.5 times higher than that of the A. trifida suspected resistant biotype. This study identified important differences in seed germination between herbicide resistant and susceptible biotypes and relates this information to the ecology of species adapted to Roundup Ready® fields. Information obtained in this study supports sustainable management strategies, with continued use of glyphosate as a possibility.     

Interaction Effects of Seed Mass and Temperature on Germination in Australian Species of Frankenia (Frankeniaceae)

The seed size and number theories have been proposed to explain the advantages of having many small versus a few large seeds in plants. In particular, seed germination is predicted to be shaped by temperature, and may differ for small and large seeds. In this study, we experimentally test germination at different temperatures in 12 species of arid zone plants in the genus Frankenia L. that differ in seed mass. Seed mass was categorized as “smaller-seeded species” versus “larger-seeded species” for analysis (six species per category). Many of these species co-occur geographically and hence experience similar abiotic conditions (unpredictable rainfall, extremes in temperature, poor soil conditions). The results demonstrated differences in germination as a result of the temperature*seed mass(species) interaction effect. There were significant differences in germination rates across seed mass categories during the first eight days of germination. Germination rates were higher in the larger-seeded species than the smaller-seeded species. Smaller-seeded species had lower germination success but had higher germination rates at lower temperatures, and had a more stringent temperature as a germination cue. These findings are discussed in the context of life-history strategies in arid zone plants.     

Bacillus thuringiensis Cry1Ac Toxin-Binding and Pore-Forming Activity in Brush Border Membrane Vesicles Prepared from Anterior and Posterior Midgut Regions of Lepidopteran Larvae

It is generally accepted that Bacillus thuringiensis Cry toxins insert into the apical membrane of the larval midgut after binding to specific receptors, and there is evidence that the distribution of binding molecules along the midgut is not uniform. By use of the voltage-sensitive dye DiSC₃(5) and ¹²⁵I-labeled Cry1Ac, we have measured the effect of Cry1Ac in terms of permeabilization capacity and of binding parameters on brush border membrane vesicles (BBMV) prepared from the anterior and the posterior regions of the larval midgut from two insect species, Manduca sexta and Helicoverpa armigera. The permeabilizing activity was significantly higher with BBMV from the posterior region than with the one observed in the anterior region in both insect species. Instead, ¹²⁵I-Cry1Ac bound specifically to BBMV from the two midgut regions, with no significant differences in the binding parameters between the anterior and posterior regions within an insect species. N-acetylgalactosamine inhibition patterns on pore formation and binding differed between anterior and posterior midgut regions and between species, providing evidence of a multifaceted involvement of the sugar in the Cry1Ac mode of action. The analysis of binding and pore formation in different midgut regions could be an effective method to study differences in the mode of action of Cry1Ac toxin in different species.     

Midgut pH in different instars of three Aedes mosquito species and the relation between pH and susceptibility of larvae to a nuclear polyhedrosis virus

Midgut pH values were determined for second, third, and fourth instar larvae of Aedes epactius, A. atropalpus, and A. scutellaris. Larvae were fed mixtures of pH indicators and kaolin clay and observed with a stereoscope to determine midgut pH. No significant difference was found in midgut pH values among the three species when the same instar was compared. However, significant differences were found in the length of the high (9.0 to 10.0) pH region when comparing the three instars for each species. Increasing age of treated A. epactius larvae caused a decrease in susceptibility to a nuclear polyhedrosis virus—from 56% infection for 12-hr-old larvae declining to 8% infection for 72-hr larvae. The increased length of the high pH region may relate to decreasing susceptibility of A. epactius to a nuclear polyhedrosis virus as the larvae age.     

Seed germination conditions and implications for establishment of an epiphyte, <Emphasis Type="Italic">Aechmea bracteata</Emphasis> (Bromeliaceae)

Aechmea bracteata is a common epiphytic bromeliad found in symbiosis with many other species throughout tropical forests of Mexico and south through Panama. Given its importance and distribution in these forests, we asked how may A. bracteata be restored to areas where they have thrived in the past? We first investigated seed viability and response of seed germination to temperature, humidity, vapour pressure deficit (VPD), and light, under controlled growth chamber conditions. We recorded these environmental conditions within a seasonal tropical forest where this species is common and then conducted seed germination trials in various seral stages. In growth chambers, with constant water supply, highest germination percentages were at the highest temperature and lowest VPD levels. In the field, germination was less than 1%. Because of high temperatures and VPD within early seral stages, efforts to re-introduce A. bracteata into forests <20 years old are not likely to be successful.     

Germination in four low-várzea tree species of Central Amazonia

Trees of Central Amazonian white-water (várzea) forests are highly adapted to the annual inundations, which can last up to 7 months every year. Many trees synchronize fruit production to the period of highest water levels of the rivers, and hydrochory is especially common in species that colonize the low-lying flood-levels flooded for longer periods. The effect of the contact of diaspores with the river water is controversially discussed in literature. While many studies describe that flooding breaks the dormancy in seeds of many várzea tree species and is necessary for germination, other studies mention that seed buoyancy and/or submergence have negative effects on germination. Therefore, the present study was designed in order to test experimentally how seed buoyancy and seed submergence affect germination in four várzea tree species of the low-lying flood-levels. The tested species with buoyant seeds were Salix martiana and Pseudobombax munguba, those with submerged seeds Laetia corymbulosa and Vitex cymosa. 50 seeds from each species were (a) placed in water during a period of 15 days and afterwards moved to várzea substrate, thus simulating seed buoyancy and/or submergence in the natural environment, and (b) directly placed in várzea substrate, with four repetitions, respectively. Three species showed significantly higher percentages of germination in the flooded seeds than in the non-waterlogged seeds, while fruit-fibre involved seeds of P. munguba showed an opposite trend. In L. corymbulosa, germination initiated earlier in the submerged than in the control seeds, whereas there was no difference in the start of germination between waterlogged and non-waterlogged seeds of the other species. From buoyant seeds of P. munguba and S. martiana, seedlings with entirely formed cotyledons were developed while still in water. These seedlings were characterized by morphological differences in comparison to seedlings originating from non-waterlogged seeds and could not protrude the root into the soil (i.e. establish) when placed in the substrate. It is likely that the seed involving fruit-fibres contribute to long-distance dispersal in these species in the natural environment, and to stabilize seedlings when diaspores land on substrate. Concluding, contact with the river water did not disturb but on the contrary enhanced germination in the four studied species.