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941.
942.
Renzhong Wang Liang Chen Yuguang Bai Chunwang Xiao 《Journal of Plant Growth Regulation》2008,27(1):39-48
A natural grassland in northeastern China dominated by Leymus chinensis (Trin.) was subject to drought treatments to determine the seasonal dynamics in resource partitioning to shoot and storage
organs in response to drought. The growing season was divided into six stages according to the phenology of L. chinensis. Plant samples of L. chinensis were collected at each stage to determine biomass, gross calorific value, relative water content, and key mineral contents
of plant parts, including rhizomes, roots, leaves, and stems. Resource partitioning to shoot and storage organs as measured
by biomass, gross calorific value, and N, K+, and Na+ contents varied significantly among phenologic stages. Drought treatment (natural precipitation, 50–60 % of field capacity)
significantly reduced biomass, gross calorific value, relative water content of shoot, and N and K+ contents in both shoot and storage organs, but it enhanced rhizome : shoot ratio and Na+ content. A negative correlation in biomass, gross calorific value, and K+ and Na+ contents between shoot and storage organs was found throughout the growing season, which may have been accentuated by drought
when soil moisture was limited. Our results indicate that resource partitioning to storage plays an important role in regulating
plant growth of L. chinensis, especially under drought conditions. 相似文献
943.
Zhen-Yu Wang Feng-Min Li You-Cai Xiong Bing-Cheng Xu 《Journal of Plant Growth Regulation》2008,27(4):309-319
The soil-water threshold range of chemical signals and reactive oxygen species (ROS) homeostasis could have a profound impact
on drought tolerance in wheat. A pot experiment was used to investigate the homeostasis between ROS and antioxidant defense
at five harvest dates, and its role in the correlation between soil-water threshold range of chemical signals and drought
tolerance in three wheat (Triticum aestivum) cultivars during progressive soil drying. The cultivars were bred at different periods, cv. BM1 (old), cv. Xiaoyan6 (recent),
and cv. Shan229 (modern). They were treated with progressive soil drying. Shoot biomass was affected by drought imposed by
two water treatments (90% and 55% field water capacity). The modern wheat cultivar had a lower ROS content and higher ROS-scavenging
antioxidant capacity with greater soil drying (68–25% soil water content) compared with the older cultivar. The modern cultivar
also had excellent adaptation to drought, with a longer survival of 22.7 days and less reduction in shoot biomass of 20.9%
due to early chemical signals and better balance between ROS production and antioxidants. The older cultivar had survival
of 15.3 days and 37.3% reduction of shoot biomass. A wider soil-water threshold range of chemical signals was positively correlated
with improved drought tolerance and better ROS homeostasis. These results suggest that ROS homeostasis acts as a regulator
in relationships between the soil-water threshold range of chemical signals and drought tolerance. 相似文献
944.
It is often stated that patterns of nonsynonymous rate variation among mammalian lineages are more irregular than expected or overdispersed under the neutral model, whereas synonymous sites conform to the neutral model. Here we reexamined genome-wide patterns of the variance to mean ratio, or index of dispersion (R), of substitutions in proteins from human, mouse, and dog. Contrary to the prevailing notion, we found that the mean index of dispersion for nonsynonymous sites of mammalian proteins is not significantly different from 1. We propose that earlier analyses were biased because the data included disproportionately more protein hormones, which tend to be more dispersed than genes in other functional categories. Synonymous sites exhibit greater degree of dispersion than nonsynonymous sites, although similar to earlier estimates and potentially due to errors associated with correction for multiple hits. Overall, our analysis identifies strong genome-wide generation-time effect and natural selection as important determinants of among-lineage variation of protein evolutionary rates. Furthermore, patterns of lineage-specific selective constraint are consistent with the nearly neutral model of molecular evolution. 相似文献
945.
During the extremely dry period between 2000 and 2003, the water level of Lake Balaton decreased by 82 cm and 80% of the stony littoral, an important habitat for the zebra mussel (Dreissena polymorpha), became dry. A recovery period started in 2004 due to intense precipitation, which increased water levels in the lake. Seasonal and spatial variations of the relative abundance, population density, population structure and biomass of the zebra mussel and the relative abundance of the amphipod Chelicorophium curvispinum were monitored in the period of 2003–2005 at four different shoreline sections and in two different portions (on the bottom and near the surface portion of the rip-rap) of Lake Balaton. Along with these studies, a quantitative survey of mussel larvae found in the plankton and of the abundance of mussel feeding diving ducks were made. As a consequence of the water level fall, on the dried part of the stony littoral, numerous zebra mussel druses perished. Following the dry period in early 2004, the relative abundance of the mussel on the bottom stones was smaller than in 2003 and the bottom community was dominated by C. curvispinum. By the end of 2004 and during 2005, the water level returned to normal and the surfaces of the reinundated stones were conducive to the successful colonization of zebra mussels. Hence, they returned as the dominant fauna in 2005. The stones near the surface might provide a new substrate for the recruitment of zebra mussels, probably offering more suitable substrata for the settlement in 2005 than in 2003. Therefore, the new substrata available in 2005 may have encouraged better and more rapid zebra mussel colonization than before. Zebra mussels may be better competitors for new space than C. curvispinum. A minor change of water-level fluctuation in 2005 and the reduction in population size of the mussel feeding waterfowl could have contributed to the intensive spread of zebra mussel by 2005. 相似文献
946.
Aquatic predators and habitat permanence can jointly affect benthic invertebrate biomass and community composition. In 2006
I sampled fish and invertebrates in ten ponds embedded in a seasonal wetland before and after a natural drought. Drought reduced
fish biomass and density leaving some ponds in a fishless condition when rains returned in July. In July, large aquatic insects
and crayfish colonized and reproduced in the ponds, but did not colonize all of the ponds equally. Using measurements of fish
abundance and other environmental parameters of the ponds, I conducted linear regression analyses to explore potential drivers
of variable invertebrate biomass in July. Fish biomass had a negative effect on invertebrate biomass and it explained more
of the variation in total invertebrate biomass and total non-shrimp biomass than fish abundance (number of fish caught). Dissolved
oxygen and pond depth were both correlated with fish biomass, but were poorer predictors of invertebrate biomass. Ponds with
few or no fish had 20× greater total biomass and 200× more non-shrimp biomass than ponds with high fish biomass. Shrimp dominated
the invertebrate composition, and were only found in the two deepest ponds with the highest fish biomass; predatory insects
and crayfish dominated the other eight ponds. When taxa were analyzed separately, fish biomass explained a large portion of
the variation for predatory insects (Coleoptera, Hemiptera, and Odonata) and crayfish (Procambarus alleni), but dissolved oxygen was the best predictor of larval stratiomyid (order Diptera) biomass. These results are generally
consistent with studies demonstrating negative effects of fish on large predatory invertebrates, but also suggest that more
severe local droughts can seasonally enhance insect and crayfish populations by generating fishless or nearly fishless conditions.
Handling editor: J. Trexler 相似文献
947.
Southeastern Australia is presently experiencing one of the worst droughts observed in the region in the last 200 years. The
consequences of drought have been far reaching both for human consumptive uses and for aquatic ecosystems, and serve to highlight
several important aspects of the nature of droughts, their ecological impacts, and how humans respond to them. Running water
ecosystems are the dominant form of freshwater ecosystem in Australia, yet, despite the high frequency of drought we lack
a basic understanding of the consequences of long-term droughts (as distinct from seasonal droughts) as an ecosystem disturbance,
and more is known about drought effects on flowing than on standing waters. Drought is well defined and characterised meteorologically,
but hydrologically its characterisation is equivocal. While drought severely impacts natural aquatic ecosystems, its effects
have been and are exacerbated by direct and indirect anthropogenic modifications to streams and their catchments. In streams
the major impacts are the loss of water and habitat availability, and the reduction, if not severing, of connectivity (lateral,
longitudinal and vertical). Despite the relative frequency of drought in Australia we have failed to develop long-term management
strategies capable of contending with droughts and their impacts, particularly in catchments where human disturbances have
reduced the natural resistance and resilience of aquatic ecosystems, and where the demand for consumptive water use is high
and rising. Here, we provide a commentary on drought and its implications for the management of freshwater ecosystems. We
begin with a general discussion of drought and its impacts on streams and rivers before discussing some of the more specific
management issues and response strategies that have arisen in response to the current drought in Australia. Throughout we
consider global as well as local examples. We conclude by highlighting important knowledge gaps and by providing some general
principles for better incorporating droughts and their impacts into river management strategies.
Handling editor: K. Martens 相似文献
948.
Phytoplankton as a monitoring tool in a tropical urban shallow reservoir (Garças Pond): the assemblage index application 总被引:3,自引:3,他引:0
This study aimed at evaluating phytoplankton as a monitoring tool for water quality assessment in an urban shallow eutrophic reservoir considering temporal and vertical scales. Garças Reservoir is located in the Parque Estadual das Fontes do Ipiranga Biological Reserve (23°38′08″S and 23°40′18″S; 46°36′48″W and 46°38′00″W) that lies in the southeastern part of the Municipality of São Paulo, southeast Brazil. Samplings were carried out monthly during 8 consecutive years (1997–2004) following the water column vertical profile (5 depths: subsurface, 1, 2, 3 m and ~20 cm from the bottom). Abiotic variables analyzed were: water temperature, electric conductivity, DO, pH, total alkalinity, free CO2, dissolved inorganic carbon, N series, P series and SiO4H4. Biological variables studied were: total density, total biomass and chlorophyll a, which were integrated arithmetically. At the beginning of the 8 year series, Garças Reservoir was an eutrophic ecosystem with 20% of its surface covered by Eichhornia crassipes (phase I: January 1997–March 1998). Water hyacinth reached 70% of pond surface coverage (phase II: April 1998–August 1999), and then it was mechanically removed (phase III: September 1999–December 2004). After this intervention, drastic alteration in the limnological features was detected, leading to the conclusion that removal of the aquatic macrophyte modified nutrient dynamics drastically reduced water transparency and led to photosynthetic productivity and phytoplankton biomass increase, the latter becoming a physical barrier to light penetration. Twenty one functional groups ‘sensu’ Reynolds were identified. Cyanobacteria contribution played the main role during the drastic alterations that occurred after water hyacinth removal. Results of ecological status of reservoir using Q index showed statistical difference among the 3 limnological phases (one way ANOVA; F = 119.4; P = 0.000). Regarding Q index classification, Garças Reservoir limnological phases were characterized as follows: (1) phase I: 0 ≥ Q ≤ 2.9, medium to bad; (2) phase II: 1.4 ≥ Q ≤ 3, tolerable to medium; and (3) phase III: 0 ≥ Q ≤ 1.5, bad to tolerable ecological states. 相似文献
949.
Climate is a key resource for many types of tourism and as such can be measured and evaluated. An index approach is required
for this task because of the multifaceted nature of weather and the complex ways that weather variables come together to give
meaning to climate for tourism. Here we address the deficiencies of past indices by devising a theoretically sound and empirically
tested method that integrates the various facets of climate and weather into a single index called the Climate Index for Tourism
(CIT). CIT rates the climate resource for activities that are highly climate/weather sensitive, specifically, beach “sun,
sea and sand” (3S) holidays. CIT integrates thermal (T), aesthetic (A) and physical (P) facets of weather, which are combined
in a weather typology matrix to determine a climate satisfaction rating that ranges from very poor (1 = unacceptable) to very
good (7 = optimal). Parameter A refers to sky condition and P to rain or high wind. T is the body-atmosphere energy balance
that integrates the environmental and physiological thermal variables, such as solar heat load, heat loss by convection (wind)
and by evaporation (sweating), longwave radiation exchange and metabolic heat (activity level). Rather than use T as a net
energy (calorific) value, CIT requires that it be expressed as thermal sensation using the standard nine-point ASHRAE scale
(“very hot” to “very cold”). In this way, any of the several body-atmosphere energy balance schemes available may be used,
maximizing the flexibility of the index. A survey (N = 331) was used to validate the initial CIT. Respondents were asked to rate nine thermal states (T) with different sky conditions
(A). They were also asked to assess the impact of high winds or prolonged rain on the perceived quality of the overall weather
condition. The data was analysed statistically to complete the weather typology matrix, which covered every possible combination
of T, A and P. Conditions considered to be optimal (CIT class 6–7) for 3S tourism were those that were “slightly warm” with
clear skies or scattered cloud (≤25% cloud). Acceptable conditions (CIT = 4–5) fell within the thermal range “indifferent”
to “hot” even when the sky was overcast. Wind equal to or in excess of 6 m/s (22 km/h) or rain resulted in the CIT rating
dropping to 1 or 2 (unacceptable) and was thus an override of pleasant thermal conditions. Further cross-cultural research
is underway to examine whether climate preferences vary with different social and cultural tourist segments internationally. 相似文献
950.
Genotypic variation in seedling root architectural traits and implications for drought adaptation in wheat (Triticum aestivum L.) 总被引:1,自引:0,他引:1
Ahmad M. Manschadi Graeme L. Hammer John T. Christopher Peter deVoil 《Plant and Soil》2008,303(1-2):115-129
Root system characteristics are of fundamental importance to soil exploration and below-ground resource acquisition. Root
architectural traits determine the in situ space-filling properties of a root system or root architecture. The growth angle
of root axes is a principal component of root system architecture that has been strongly associated with acquisition efficiency
in many crop species. The aims of this study were to examine the extent of genotypic variability for the growth angle and
number of seminal roots in 27 current Australian and 3 CIMMYT wheat (Triticum aestivum L.) genotypes, and to quantify using fractal analysis the root system architecture of a subset of wheat genotypes contrasting
in drought tolerance and seminal root characteristics. The growth angle and number of seminal roots showed significant genotypic
variation among the wheat genotypes with values ranging from 36 to 56 (degrees) and 3 to 5 (plant−1), respectively. Cluster analysis of wheat genotypes based on similarity in their seminal root characteristics resulted in
four groups. The group composition reflected to some extent the genetic background and environmental adaptation of genotypes.
Wheat cultivars grown widely in the Mediterranean environments of southern and western Australia generally had wider growth
angle and lower number of seminal axes. In contrast, cultivars with superior performance on deep clay soils in the northern
cropping region, such as SeriM82, Baxter, Babax, and Dharwar Dry exhibited a narrower angle of seminal axes. The wheat genotypes
also showed significant variation in fractal dimension (D). The D values calculated for the individual segments of each root system suggested that, compared to the standard cultivar Hartog,
the drought-tolerant genotypes adapted to the northern region tended to distribute relatively more roots in the soil volume
directly underneath the plant. These findings suggest that wheat root system architecture is closely linked to the angle of
seminal root axes at the seedling stage. The implications of genotypic variation in the seminal root characteristics and fractal
dimension for specific adaptation to drought environment types are discussed with emphasis on the possible exploitation of
root architectural traits in breeding for improved wheat cultivars for water-limited environments. 相似文献