Phenotypic plasticity of introduced versus native purple loosestrife: univariate and multivariate reaction norm approaches

The plastic responses to environmental change by Lythrum salicaria (purple loosestrife) were compared between native plants derived from seeds collected in Europe and those introduced into North America. Plants from nine populations each were grown under two levels of water and nutrient conditions. At the end of the growing season, samples were evaluated for eight traits related to their life history, plant size/architecture, and reproduction. Genetic (G), environmental (E), and G × E interactions were assessed by restricted maximum likelihood (REML) analysis of covariance (ANCOVA) and multivariate analysis of covariance (MANCOVA). Both univariate and multivariate reaction norm analyses were used to test for differences in the magnitude and direction of phenotypic plasticity between introduced and native plants. Under high-nutrient conditions, introduced plants were taller and had more branches and greater aboveground biomass. They also exhibited significantly greater amounts of phenotypic plasticity for aboveground biomass than did the natives in response to changing nutrient levels in standing water. This difference in univariate plasticity contributed to the general contrast in multivariate plasticity between introduced and native plants. These results support the idea that introduced plants may successfully invade a habitat and grow better than native plants in response to increased resources.     

Evolution of phenotypic plasticity a comparative approach in the phylogenetic neighbourhood of Arabidopsis thaliana

The evolution of phenotypic plasticity has rarely been examined within an explicitly phylogenetic framework, making use of modern comparative techniques. Therefore, the purpose of this study was to determine phylogenetic patterns in the evolution of phenotypic plasticity in response to vegetation shade (the ‘shade avoidance’ syndrome) in the annual plant Arabidopsis thaliana and its close relatives. Specifically, we asked the following questions: (i) Do A. thaliana and related species differ within or among clades in the magnitude and/or pattern of plasticity to shade? (ii) Are the phenotypic variance–covariance matrices (phenotypic integration) of these taxa plastic to the changes in light quality induced by the presence of a canopy? (iii) To what extent does the variation in uni- and multivariate plasticity match the phylogeny of Arabidopsis? In order to address these questions we grew individuals from six taxa of known phylogenetic relationship in a greenhouse under full sun and under a grass canopy. Taxa differed in the magnitude, but not in the pattern, of plasticities for all traits. At the univariate level, the late flowering species, A. pumila and A. griffithiana, as well as the late flowering Moscow ecotype of A. thaliana, showed greater plasticity for allocation to vegetative and reproductive meristems. At the multivariate level, several taxa displayed a very low stability of their variance–covariance structures to environmental change, with only one taxon sharing as many as three principal components across environments. We conclude that both univariate and multivariate plasticities to vegetation shade can evolve rapidly within a genus of flowering plants, with little evidence of historical constraints (phylogenetic inertia).     

Effects of soil fertility and flooding regime on the growth of Ambrosia trifida

Understanding the effects of abiotic environmental factors on invasive plants species traits is of importance for practical prevention. To examine the effects of soil fertility and flooding regime on the growth of Ambrosia trifida L., a mesocosm experiment was conducted for 18 weeks. Two levels of soil fertility (high and low) and three types of flooding regime (non-flooded, flooded, and periodically flooded) were prepared. Shoot height and dry weight of each plant were measured. We found both individual and interactive effects of soil fertility and flooding regime on the overall growth performance of A. trifida (p?<?0.05). The highest shoot height (154.7?±?4.4 cm) and total dry weight (TDW, 13.0?±?1.4 g) were obtained under high fertility and non-flooded condition. Height and weight were relatively low under flooding conditions (flooded and periodically flooded). In particular, shoot height (102.3?±?3.2 cm) and TDW (3.2?±?0.3 g) were the lowest under low fertility and periodically flooded condition. On the other hand, the ratio of above- to below-ground dry weight was relatively high under flooded conditions, showing the adaptive phenotypic plasticity. Adventitious root formation and more biomass allocation to shoots were a flooding-adaptive mechanism of A. trifida, well developed under high fertility condition. We suggest maintaining appropriate water regime and avoiding eutrophication in wetlands would be necessary to prevent A. trifida from invading. These findings will contribute to the conservation of biodiversity in wetlands by effective management of A. trifida.

     

Ecological limits to plant phenotypic plasticity

Phenotypic plasticity is considered the major means by which plants cope with environmental heterogeneity. Although ubiquitous in nature, actual phenotypic plasticity is far from being maximal. This has been explained by the existence of internal limits to its expression. However, phenotypic plasticity takes place within an ecological context and plants are generally exposed to multifactor environments and to simultaneous interactions with many species. These external, ecological factors may limit phenotypic plasticity or curtail its adaptive value, but seldom have they been considered because limits to plasticity have typically addressed factors internal to the plant. We show that plastic responses to abiotic factors are reduced under situations of conservative resource use in stressful and unpredictable habitats, and that extreme levels in a given abiotic factor can negatively influence plastic responses to another factor. We illustrate how herbivory may limit plant phenotypic plasticity because damaged plants can only rarely attain the optimal phenotype in the challenging environment. Finally, it is examined how phenotypic changes involved in trait-mediated interactions can entail costs for the plant in further interactions with other species in the community. Ecological limits to plasticity must be included in any realistic approach to understand the evolution of plasticity in complex environments and to predict plant responses to global change.     

Patterns of univariate and multivariate plasticity to elevated carbon dioxide in six European populations of Arabidopsis thaliana

The impact of elevated carbon dioxide on plants is a growing concern in evolutionary ecology and global change biology. Characterizing patterns of phenotypic integration and multivariate plasticity to elevated carbon dioxide can provide insights into ecological and evolutionary dynamics in future human‐altered environments. Here, we examined univariate and multivariate responses to carbon enrichment in six functional traits among six European accessions of Arabidopsis thaliana. We detected phenotypic plasticity in both univariate and multivariate phenotypes, but did not find significant variation in plasticity (genotype by environment interactions) within or among accessions. Eigenvector, eigenvalue variance, and common principal components analyses showed that elevated carbon dioxide altered patterns of trait covariance, reduced the strength of phenotypic integration, and decreased population‐level differentiation in the multivariate phenotype. Our data suggest that future carbon dioxide conditions may influence evolutionary dynamics in natural populations of A. thaliana.     

Changes in reproductive investment with altitude in an alpine plant

Aims In perennial species, the allocation of resources to reproduction results in a reduction of allocation to vegetative growth and, therefore, impacts future reproductive success. As a consequence, variation in this trade-off is among the most important driving forces in the life-history evolution of perennial plants and can lead to locally adapted genotypes. In addition to genetic variation, phenotypic plasticity might also contribute to local adaptation of plants to local conditions by mediating changes in reproductive allocation. Knowledge on the importance of genetic and environmental effects on the trade-off between reproduction and vegetative growth is therefore essential to understand how plants may respond to environmental changes.Methods We conducted a transplant experiment along an altitudinal gradient from 425 to 1?921 m in the front range of the Western Alps of Switzerland to assess the influence of both altitudinal origin of populations and altitude of growing site on growth, reproductive investment and local adaptation in Poa alpina .Important findings In our study, the investment in reproduction increased with plant size. Plant growth and the relative importance of reproductive investment decreased in populations originating from higher altitudes compared to populations originating from lower altitudes. The changes in reproductive investment were mainly explained by differences in plant size. In contrast to genetic effects, phenotypic plasticity of all traits measured was low and not related to altitude. As a result, the population from the lowest altitude of origin performed best at all sites. Our results indicate that in P. alpina genetic differences in growth and reproductive investment are related to local conditions affecting growth, i.e. interspecific competition and soil moisture content.     

The plasticity of phenotypic integration in response to light and water availability in the pepper grass, <Emphasis Type="Italic">Lepidium bonariense</Emphasis>

Organisms that live in a heterogeneous environment face a number of important challenges. On one hand, they require the flexibility to respond to environmental conditions and change their phenotype accordingly. On the other, they are required to be robust in their overall body plan to ensure an integrated, functional organism. Here, we examine the relationship between phenotypic plasticity and integration in the common peppergrass, Lepidium bonariense, by examining the multivariate response of a series of functional traits to a combination of light and water treatments. Lepidium bonariense displayed considerable variation in phenotype in response to water and light availability with the extraction of the first two principal components retaining 85% of the total variation in our data set. Principal component 1 (PC1) largely reflects the negative genetic correlation between specific leaf area and overall plant size, whereas PC2 was typical of a shade-avoidance syndrome displayed by many species of vascular plants. Both PC1 and PC2 exhibited considerable variation among genotypes in phenotypic plasticity in response to the combined effect of light and water availability. Despite complex plasticity in this species, we demonstrate that variation in light and water availability did not significantly influence patterns of functional trait integration, with the genetic variance–covariance matrix remaining stable across environments.     

Population size and identity influence the reaction norm of the rare,endemic plant Cochlearia bavarica across a gradient of environmental stress

Habitat degradation and loss can result in population decline and genetic erosion, limiting the ability of organisms to cope with environmental change, whether this is through evolutionary genetic response (requiring genetic variation) or through phenotypic plasticity (i.e., the ability of a given genotype to express a variable phenotype across environments). Here we address the question whether plants from small populations are less plastic or more susceptible to environmental stress than plants from large populations. We collected seed families from small (<100) versus large natural populations (>1,000 flowering plants) of the rare, endemic plant Cochlearia bavarica (Brassicaceae). We exposed the seedlings to a range of environments, created by manipulating water supply and light intensity in a 2 x 2 factorial design in the greenhouse. We monitored plant growth and survival for 300 days. Significant effects of offspring environment on offspring characters demonstrated that there is phenotypic plasticity in the responses to environmental stress in this species. Significant effects of population size group, but mainly of population identity within the population size groups, and of maternal plant identity within populations indicated variation due to genetic (plus potentially maternal) variation for offspring traits. The environment x maternal plant identity interaction was rarely significant, providing little evidence for genetically- (plus potentially maternally-) based variation in plasticity within populations. However, significant environment x population-size-group and environment x population-identity interactions suggested that populations differed in the amount of plasticity, the mean amount being smaller in small populations than in large populations. Whereas on day 210 the differences between small and large populations were largest in the environment in which plants grew biggest (i.e., under benign conditions), on day 270 the difference was largest in stressful environments. These results show that population size and population identity can affect growth and survival differently across environmental stress gradients. Moreover, these effects can themselves be modified by time-dependent variation in the interaction between plants and their environment.     

Low genetic diversity contrasts with high phenotypic variability in heptaploid Spartina densiflora populations invading the Pacific coast of North America

Species can respond to environmental pressures through genetic and epigenetic changes and through phenotypic plasticity, but few studies have evaluated the relationships between genetic differentiation and phenotypic plasticity of plant species along changing environmental conditions throughout wide latitudinal ranges. We studied inter‐ and intrapopulation genetic diversity (using simple sequence repeats and chloroplast DNA sequencing) and inter‐ and intrapopulation phenotypic variability of 33 plant traits (using field and common‐garden measurements) for five populations of the invasive cordgrass Spartina densiflora Brongn. along the Pacific coast of North America from San Francisco Bay to Vancouver Island. Studied populations showed very low genetic diversity, high levels of phenotypic variability when growing in contrasted environments and high intrapopulation phenotypic variability for many plant traits. This intrapopulation phenotypic variability was especially high, irrespective of environmental conditions, for those traits showing also high phenotypic plasticity. Within‐population variation represented 84% of the total genetic variation coinciding with certain individual plants keeping consistent responses for three plant traits (chlorophyll b and carotenoid contents, and dead shoot biomass) in the field and in common‐garden conditions. These populations have most likely undergone genetic bottleneck since their introduction from South America; multiple introductions are unknown but possible as the population from Vancouver Island was the most recent and one of the most genetically diverse. S. densiflora appears as a species that would not be very affected itself by climate change and sea‐level rise as it can disperse, establish, and acclimate to contrasted environments along wide latitudinal ranges.     

The relative advantages of plasticity and fixity in different environments: when is it good for a plant to adjust?

Plant populations and species differ greatly in phenotypic plasticity. This could be because plasticity is advantageous under some conditions and disadvantageous or not advantageous under others. We distinguish adaptive from injurious and neutral plasticity and discuss when selection should favor adaptive plasticity over genetic differentiation or lack of phenotypic variation. It seems reasonable to hypothesize that selection is likely to favor plasticity when an environmental factor varies on the same spatial scale as the plant response unit, when the plant can respond to an environmental factor faster than the level of the factor changes, and when environmental variation is highly but not completely predictable. Phenotypic plasticity might also tend to be more advantageous when mean resource availability is high rather than low, when a response can occur late in development rather than early, and when a response is reversible rather than irreversible. There is substantial evidence for the hypothesis that predictability favors plasticity. However, available evidence does not support the hypothesis that high mean resource availability necessarily favors plasticity. Testing hypotheses about when it is good for a plant to adjust is central to understanding the diversity of plasticity in plants.     

Environmental Heterogeneity and Population Differentiation in Plasticity to Drought in <Emphasis Type="Italic">Convolvulus Chilensis</Emphasis> (Convolvulaceae)

Plant populations may show differentiation in phenotypic plasticity, and theory predicts that greater levels of environmental heterogeneity should select for higher magnitudes of phenotypic plasticity. We evaluated phenotypic responses to reduced soil moisture in plants of Convolvulus chilensis grown in a greenhouse from seeds collected in three natural populations that differ in environmental heterogeneity (precipitation regime). Among several morphological and ecophysiological traits evaluated, only four traits showed differentiation among populations in plasticity to soil moisture: leaf area, leaf shape, leaf area ratio (LAR), and foliar trichome density. In all of these traits plasticity to drought was greatest in plants from the population with the highest interannual variation in precipitation. We further tested the adaptive nature of these plastic responses by evaluating the relationship between phenotypic traits and total biomass, as a proxy for plant fitness, in the low water environment. Foliar trichome density appears to be the only trait that shows adaptive patterns of plasticity to drought. Plants from populations showing plasticity had higher trichome density when growing in soils with reduced moisture, and foliar trichome density was positively associated with total biomass. Co-ordinating editor: F. Stuefer     

Phenotypic plasticity and genetic differentiation of quantitative traits in genotypes of Leymus chinensis

Aims Adaptation mechanisms of plants to environment can be classified as genetic differentiation and phenotypic plasticity (environmental modification). The strategy and mechanism of plant adaptation is a hot topic in the field of evolutionary ecology. Leymus chinensis is one of constructive species in the Nei Mongol grassland. Particularly, Leymus chinensis is a rhizomatous and clonally reproductive grass, a genotype that can play an important role in the community. In this study, we aimed to (1) investigate the phenotypic plasticity of L. chinensis under different conditions, and (2) test the genetic differentiation and reaction norms (the relationship between the environment and the phenotype of an individual or a group of individuals) under four environmental conditions among different genotypes of L. chinensis. Methods Ten genotypes of L. chinensis were randomly selected. Under the control condition, we studied the effects of genotype, defoliation, drought and their interactions on 11 quantitative traits of growth (8 traits including photochemical efficiency of photosystem II, maximum net photosynthetic rate, transpiration rate, specific leaf area, relative growth rate, the number of tillers increased, aboveground and underground biomass growth), defense (total phenol concentration of leaf) and tolerance (non-structural carbohydrate content of root, root/shoot ratio) of L. chinensis. We studied the phenotypic plasticity, genetic differentiation and reaction norms mainly through tested the effect of environment and genotype on these traits. Important findings First, all 11 traits showed obvious phenotypic plasticity (i.e., significant effect of drought, defoliation and their interactions). The expression of 10 genotypes of L. chinensis was divergent under different environmental conditions. Interactions of genotype and environment significantly affected the maximum net photosynthetic rate, transpiration rate, specific leaf area, relative growth rate, total phenolic concentration of leaf, and total non-structural carbohydrate content of root. This indicated that the phenotypic plasticity of these five traits exhibited genetic differentiation. Second, the increase of number of tillers, belowground biomass and non-structural carbohydrate content of root did not show genetic differentiation under the same condition. The other eight traits showed significantly genetic differentiation, and the heritabilities (H²) of six traits related to growth were higher than 0.5. The leaf total phenol concentration and root/shoot ratio showed genetically differentiation only under the drought and defoliation condition, with the heritabilities being 0.145 and 0.201, respectively. These results explained why L. chinensis widely distributed in the Nei Mongol grassland, and provided genetic and environmental basis for related application and species conservation in this grassland ecosystem.     

Phenotypic plasticity to light competition and herbivory in Chenopodium album (Chenopodiaceae)

Competition and herbivory are ubiquitous environmental challenges that affect most plants. We examined the influence of phenotypic responses to either competition or herbivory on the subsequent response of the plants to the other factor. The stem-elongation response of Chenopodium album to light competition attenuated its resistance to caterpillar herbivory in terms of herbivore mortality, but not in terms of growth of the survivors. Plant responses to herbivory did not affect subsequent responses to light competition. Thus, plants were largely able to express phenotypic plasticity (a proportional increase in the phenotype) following previous exposure to a different environmental factor. Although plants were able to express sequential plasticity, the final phenotype expressed was limited by exposure to previous environmental factors: induced resistance reduced plant height and stem elongation made plants more palatable to herbivores. Phenotypic plasticity in response to competition and herbivory may thus limit the subsequent expression of adaptive phenotypes.     

Multiple simultaneous treatments change plant response from adaptive parental effects to within‐generation plasticity,in Arabidopsis thaliana

In general, studies on plant phenotypic plasticity concentrate on plant responses to different levels of a single environmental factor. Under natural conditions, however, multiple environmental factors often vary simultaneously. I studied the consequences for lifetime fitness caused by single treatments or treatment combinations by investigating patterns of phenotypic plasticity within and between generations. The parental plants (three genotypes of the annual plant Arabidopsis thaliana) received zero, one or two stress treatments at an early life‐stage. The treatments included wounding, shading, chilling, and their pairwise combinations. In the second generation, offspring of treated plants received either the parental or no treatment. Offspring of non‐treated plants were reared under all treatment conditions. Plants responded strongly to the treatments, especially through delayed reproduction, which positively affected lifetime fitness. Notably, treatment combinations triggered stronger plastic responses on average. Because the delay in reproduction was offset by a delay in senescence, the treatments resulted in a fitness gain instead of a loss. However, under adverse environmental conditions, this delay represents a potential fitness cost, especially when the time for reproduction is limited. The treatments ‘wounding’ and ‘shading’ triggered parental effects that increased fitness only in plants that themselves received the treatment. Untreated offspring of wounded or shaded parents performed like control plants. Also, these parental effects were not accompanied by potential fitness costs, such as delayed reproduction. Chilling triggered genotype‐specific parental effects that increased or reduced fitness. Of the treatment combinations only ‘wounding’ and ‘shading’ resulted in genotype‐specific parental effects that increased or reduced fitness independently of offspring treatment. These results suggest that the response of annual plants to treatment combinations triggers predominantly within‐generation plastic responses that include potential fitness costs, which cannot be inferred from studies that manipulate environmental factors individually. Therefore, single treatment studies likely underestimate the costs of plasticity in natural environments.     

Reaction norms of Arabidopsis. V. Flowering time controls phenotypic architecture in response to nutrient stress

The evolutionary and environmental stability of character correlations has increasingly been the focus of ecological and quantitative genetic studies. Although the genetic stability of character correlations is a central assumption of quantitative genetic models of phenotypic evolution, theoretical considerations suggest that both the genetic and the phenotypic architecture should change in response to selection and to environmental heterogeneity. We investigate genetic (population) differences and plasticity to nutrient availability of the phenotypic architecture describing the whole-plant phenotype of Arabidopsis thaliana (Brassicaceae). We found significant genetic differences among early and late flowering ecotypes in the relationships between several traits, when a path-analytical model was used to estimate character correlations. Furthermore, we found significant plasticity of several path coefficients when nutrient levels were altered. A whole-plant analysis considering all paths in the model simultaneously confirmed that populations of A. thaliana are characterized by distinct phenotypic architectures, and that these are altered in different ways by environmental changes. We discuss the implications of these findings for our understanding of selective pressure on and response by multivariate phenotypes.     

The fitness costs of developmental canalization and plasticity

Organisms are capable of an astonishing repertoire of phenotypic responses to the environment, and these often define important adaptive solutions to heterogeneous and unpredictable conditions. The terms ‘phenotypic plasticity’ and ‘canalization’ indicate whether environmental variation has a large or small effect on the phenotype. The evolution of canalization and plasticity is influenced by optimizing selection‐targeting traits within environments, but inherent fitness costs of plasticity may also be important. We present a meta‐analysis of 27 studies (of 16 species of plant and 7 animals) that have measured selection on the degree of plasticity independent of the characters expressed within environments. Costs of plasticity and canalization were equally frequent and usually mild; large costs were observed only in studies with low sample size. We tested the importance of several covariates, but only the degree of environmental stress was marginally positively related to the cost of plasticity. These findings suggest that costs of plasticity are often weak, and may influence phenotypic evolution only under stressful conditions.     

Fitness-related traits of allozyme genotypes in Bromus hordeaceus L. (Poaceae) associated with field habitat and experimental flooding

Genetic variation at alcohol dehydrogenase and phosphoglucose isomerase loci in Bromus hordeaceus has in an earlier study been found to show substantial microgeographic spatial structuring. The present study reports differences in fitness related characters between the enzyme genotypes, both from a field study and a greenhouse experiment. The field study showed overall differences in seed set between allozyme genotypes and also that Pgi-1b genotypes differed in number of seeds set at different levels of herb cover in their habitat. In the greenhouse, dry, normal or flooded conditions were applied. Seeds from individuals with the Adh-1b-11 genotype matured faster in the dry and slower in the flooded treatments than did seeds from individuals with the Adh-1b-22 genotype. Individuals containing Pgi-1b-1f1f alleles and Adh-1b-11 alleles are more plastic than individuals with other allele combinations, meaning that allozyme variation could partly explain what could be seen as adaptive phenotypic plasticity. Mean seed weight was different between dry and flooding treatments for Pgi-1b genotypes. There were also direct effects of allozyme genotype on the probability of survival, total plant weight, weight of reproductive parts, seed weight, days to seed maturation and the percentage of reproductive parts out of the total plant weight.     

Role of phenotypic plasticity and population differentiation in adaptation to novel environmental conditions

Species can adapt to new environmental conditions either through individual phenotypic plasticity, intraspecific genetic differentiation in adaptive traits, or both. Wild emmer wheat, Triticum dicoccoides, an annual grass with major distribution in Eastern Mediterranean region, is predicted to experience in the near future, as a result of global climate change, conditions more arid than in any part of the current species distribution. To understand the role of the above two means of adaptation, and the effect of population range position, we analyzed reaction norms, extent of plasticity, and phenotypic selection across two experimental environments of high and low water availability in two core and two peripheral populations of this species. We studied 12 quantitative traits, but focused primarily on the onset of reproduction and maternal investment, which are traits that are closely related to fitness and presumably involved in local adaptation in the studied species. We hypothesized that the population showing superior performance under novel environmental conditions will either be genetically differentiated in quantitative traits or exhibit higher phenotypic plasticity than the less successful populations. We found the core population K to be the most plastic in all three trait categories (phenology, reproductive traits, and fitness) and most successful among populations studied, in both experimental environments; at the same time, the core K population was clearly genetically differentiated from the two edge populations. Our results suggest that (1) two means of successful adaptation to new environmental conditions, phenotypic plasticity and adaptive genetic differentiation, are not mutually exclusive ways of achieving high adaptive ability; and (2) colonists from some core populations can be more successful in establishing beyond the current species range than colonists from the range extreme periphery with conditions seemingly closest to those in the new environment.     

Sesbania herbacea–Rhizobium huautlense Nodulation in Flooded Soils and Comparative Characterization of S. herbacea-Nodulating Rhizobia in Different Environments

Abstract The nodulation of S. herbacea was compared under flooded and non-flooded conditions in two different soils. One soil was from a flooded field in Sierra de Huautla, the native habitat of this legume, while the other soil was from a well-drained field in Cuernavaca, where rhizobia were found to nodulate the introduced S. herbacea plants. Nodulation of the plants was completely eliminated by flooding in the Cuernavaca soil, whereas nodules were obtained in the same soil under non-flooded conditions. In contrast, nodules were formed in Huautla soil under both flooded and non-flooded conditions. Most isolates, except isolate HS2, from Huautla soil and water were identified as R. huautlense by colony morphology, growth rate, PCR-RFLP of 16S rRNA genes, MLEE, cellular plasmid contents, and RFLP of nifH and nodDAB genes. Isolate HS2 was identified as Mesorhizobium sp. Isolates from Cuernavaca soil were different from R. huautlense in many aspects and were classified into five rDNA types within the genera Mesorhizobium, Rhizobium, and Sinorhizobium by PCR-RFLP of 16S rRNA genes. R. huautlense is a water Rhizobium species. Growth by denitrification under oxygen limitation or with ethanol was observed for R. huautlense bacteria but not for the isolates from Cuernavaca. In an interstrain nodulation competitive assay under both flooded and non-flooded conditions, R. huautlense strain S02 completely inhibited the nodulation of Mesorhizobium sp. Sn2, an isolate from Cuernavaca. From these results, we conclude that R. huautlense has the unique ability to nodulate S. herbacea not only in flooded soils, but in non-flooded soils as well. Received: 16 August 1999; Accepted: 28 December 1999; Online Publication: 13 June 2000     

不同坡向川西亚高山林木竞争与叶片表型可塑性的关系研究

表型可塑性是植物生长响应外界环境变化的重要表现形式,体现了植物个体在环境胁迫下的适合度。但是关于植物表型可塑性的驱动机制仍然存在很多争议。为了探讨植物表型可塑性的影响因素,以四川省阿坝藏族羌族自治州位于同一海拔梯度但坡向相反的天然次生林为研究对象,分析了不同坡向竞争强度与10种树木叶片功能性状表型可塑性的关系的差异。研究发现：(1)研究样地中阴坡水分和养分资源优于阳坡;(2)阴坡上林木平均种内和种间竞争强度高于阳坡,阴坡上林木种内竞争强度随着树木个体大小的增加而显著性减少,阳坡上林木种内竞争强度却随着个体大小的增加而增加;(3)阴坡上叶片表型可塑性高于阳坡,表型可塑性随着个体大小的增加而增加,在阳坡上却随着个体大小增加而降低。这些结果表明阴坡上水分等资源环境条件优于阳坡,林木生长受到环境资源限制较少。在林木生长过程中,较高的竞争强度引起的资源重叠加剧,尤其是种内竞争强度的变化,从而导致了阴坡上较高的叶片表型可塑性。因为较高的竞争强度,随着林木个体大小的增加,树木需要更高的可塑性赢得竞争优势来获取更多的资源支持生长。但是在阳坡上,资源相对缺乏,环境资源对树木生长的限制降低了叶片表型的可塑...