Experimentally reduced root–microbe interactions reveal limited plasticity in functional root traits in Acer and Quercus

Background and Aims

Interactions between roots and soil microbes are critical components of below-ground ecology. It is essential to quantify the magnitude of root trait variation both among and within species, including variation due to plasticity. In addition to contextualizing the magnitude of plasticity relative to differences between species, studies of plasticity can ascertain if plasticity is predictable and whether an environmental factor elicits changes in traits that are functionally advantageous.

Methods

To compare functional traits and trait plasticities in fine root tissues with natural and reduced levels of colonization by microbial symbionts, trimmed and surface-sterilized root segments of 2-year-old Acer rubrum and Quercus rubra seedlings were manipulated. Segments were then replanted into satellite pots filled with control or heat-treated soil, both originally derived from a natural forest. Mycorrhizal colonization was near zero in roots grown in heat-treated soil; roots grown in control soil matched the higher colonization levels observed in unmanipulated root samples collected from field locations.

Key Results

Between-treatment comparisons revealed negligible plasticity for root diameter, branching intensity and nitrogen concentration across both species. Roots from treated soils had decreased tissue density (approx. 10–20 %) and increased specific root length (approx. 10–30 %). In contrast, species differences were significant and greater than treatment effects in traits other than tissue density. Interspecific trait differences were also significant in field samples, which generally resembled greenhouse samples.

Conclusions

The combination of experimental and field approaches was useful for contextualizing trait plasticity in comparison with inter- and intra-specific trait variation. Findings that root traits are largely species dependent, with the exception of root tissue density, are discussed in the context of current literature on root trait variation, interactions with symbionts and recent progress in standardization of methods for quantifying root traits.     

Vulnerability to cavitation,hydraulic efficiency,growth and survival in an insular pine (Pinus canariensis)

Background and Aims

It is widely accepted that hydraulic failure due to xylem embolism is a key factor contributing to drought-induced mortality in trees. In the present study, an attempt is made to disentangle phenotypic plasticity from genetic variation in hydraulic traits across the entire distribution area of a tree species to detect adaptation to local environments.

Methods

A series of traits related to hydraulics (vulnerability to cavitation and hydraulic conductivity in branches), growth performance and leaf mass per area were assessed in eight Pinus canariensis populations growing in two common gardens under contrasting environments. In addition, the neutral genetic variability (F_ST) and the genetic differentiation of phenotypic variation (Q_ST) were compared in order to identify the evolutionary forces acting on these traits.

Key Results

The variability for hydraulic traits was largely due to phenotypic plasticity. Nevertheless, the vulnerability to cavitation displayed a significant genetic variability (approx. 5 % of the explained variation), and a significant genetic × environment interaction (between 5 and 19 % of the explained variation). The strong correlation between vulnerability to cavitation and survival in the xeric common garden (r = –0·81; P < 0·05) suggests a role for the former in the adaptation to xeric environments. Populations from drier sites and higher temperature seasonality were less vulnerable to cavitation than those growing at mesic sites. No trade-off between xylem safety and efficiency was detected. Q_ST of parameters of the vulnerability curve (0·365 for P₅₀ and the slope of the vulnerability curve and 0·452 for P₈₈) differed substantially from F_ST (0·091), indicating divergent selection. In contrast, genetic drift alone was found to be sufficient to explain patterns of differentiation for xylem efficiency and growth.

Conclusions

The ability of P. canariensis to inhabit a wide range of ecosystems seemed to be associated with high phenotypic plasticity and some degree of local adaptations of xylem and leaf traits. Resistance to cavitation conferred adaptive potential for this species to adapt successfully to xeric conditions.     

Highly local environmental variability promotes intrapopulation divergence of quantitative traits: an example from tropical rain forest trees

Background and Aims

In habitat mosaics, plant populations face environmental heterogeneity over short geographical distances. Such steep environmental gradients can induce ecological divergence. Lowland rainforests of the Guiana Shield are characterized by sharp, short-distance environmental variations related to topography and soil characteristics (from waterlogged bottomlands on hydromorphic soils to well-drained terra firme on ferralitic soils). Continuous plant populations distributed along such gradients are an interesting system to study intrapopulation divergence at highly local scales. This study tested (1) whether conspecific populations growing in different habitats diverge at functional traits, and (2) whether they diverge in the same way as congeneric species having different habitat preferences.

Methods

Phenotypic differentiation was studied within continuous populations occupying different habitats for two congeneric, sympatric, and ecologically divergent tree species (Eperua falcata and E. grandiflora, Fabaceae). Over 3000 seeds collected from three habitats were germinated and grown in a common garden experiment, and 23 morphological, biomass, resource allocation and physiological traits were measured.

Key Results

In both species, seedling populations native of different habitats displayed phenotypic divergence for several traits (including seedling growth, biomass allocation, leaf chemistry, photosynthesis and carbon isotope composition). This may occur through heritable genetic variation or other maternally inherited effects. For a sub-set of traits, the intraspecific divergence associated with environmental variation coincided with interspecific divergence.

Conclusions

The results indicate that mother trees from different habitats transmit divergent trait values to their progeny, and suggest that local environmental variation selects for different trait optima even at a very local spatial scale. Traits for which differentiation within species follows the same pattern as differentiation between species indicate that the same ecological processes underlie intra- and interspecific variation.     

Leaf trait co-ordination in relation to construction cost, carbon gain and resource-use efficiency in exotic invasive and native woody vine species

Background and Aims

Success of invasive plant species is thought to be linked with their higher leaf carbon fixation strategy, enabling them to capture and utilize resources better than native species, and thus pre-empt and maintain space. However, these traits are not well-defined for invasive woody vines.

Methods

In a glass house setting, experiments were conducted to examine how leaf carbon gain strategies differ between non-indigenous invasive and native woody vines of south-eastern Australia, by investigating their biomass gain, leaf structural, nutrient and physiological traits under changing light and moisture regimes.

Key Results

Leaf construction cost (CC), calorific value and carbon : nitrogen (C : N) ratio were lower in the invasive group, while ash content, N, maximum photosynthesis, light-use efficiency, photosynthetic energy-use efficiency (PEUE) and specific leaf area (SLA) were higher in this group relative to the native group. Trait plasticity, relative growth rate (RGR), photosynthetic nitrogen-use efficiency and water-use efficiency did not differ significantly between the groups. However, across light resource, regression analyses indicated that at a common (same) leaf CC and PEUE, a higher biomass RGR resulted for the invasive group; also at a common SLA, a lower CC but higher N resulted for the invasive group. Overall, trait co-ordination (using pair-wise correlation analyses) was better in the invasive group. Ordination using 16 leaf traits indicated that the major axis of invasive-native dichotomy is primarily driven by SLA and CC (including its components and/or derivative of PEUE) and was significantly linked with RGR.

Conclusions

These results demonstrated that while not all measures of leaf resource traits may differ between the two groups, the higher level of trait correlation and higher revenue returned (RGR) per unit of major resource need (CC) and use (PEUE) in the invasive group is in line with their rapid spread where introduced.     

Long-term functional plasticity in plant hydraulic architecture in response to supplemental moisture

Background and Aims

Plasticity in structural and functional traits related to water balance may determine plant performance and survival in ecosystems characterized by water limitation or high levels of rainfall variability, particularly in perennial herbaceous species with long generation cycles. This paper addresses whether and the extent to which several such seasonal to long-term traits respond to changes in moisture availability.

Methods

Using a novel approach that integrates ecology, physiology and anatomy, a comparison was made of lifetime functional traits in the root xylem of a long-lived perennial herb (Potentilla diversifolia, Rosaceae) growing in dry habitats with those of nearby individuals growing where soil moisture had been supplemented for 14 years. Traditional parameters such as specific leaf area (SLA) and above-ground growth were also assessed.

Key Results

Individuals from the site receiving supplemental moisture consistently showed significant responses in all considered traits related to water balance: SLA was greater by 24 %; roots developed 19 % less starch storing tissue, an indicator for drought-stress tolerance; and vessel size distributions shifted towards wider elements that collectively conducted water 54 % more efficiently – but only during the years for which moisture was supplemented. In contrast, above-ground growth parameters showed insignificant or inconsistent responses.

Conclusions

The phenotypic changes documented represent consistent, dynamic responses to increased moisture availability that should increase plant competitive ability. The functional plasticity of xylem anatomy quantified in this study constitutes a mechanistic basis for anticipating the differential success of plant species in response to climate variability and change, particularly where water limitation occurs.     

Nitrogen and water addition reduce leaf longevity of steppe species

Background and aims

Changes in supplies of resources will modify plant functional traits. However, few experimental studies have addressed the effects of nitrogen and water variations, either singly or in combination, on functional traits.

Methods

A 2-year field experiment was conducted to test the effects of nitrogen and water addition on leaf longevity and other functional traits of the two dominant (Agropyron cristatum and Stipa krylovii) and three most common species (Cleistogenes squarrosa, Melilotoides ruthenica and Potentilla tanacetifolia) in a temperate steppe in northern China.

Key Results

Additional nitrogen and water increased leaf nitrogen content and net photosynthetic rate, and changed other measured functional traits. Leaf longevity decreased significantly with both nitrogen addition (–6 days in 2007 and –5·4 days in 2008; both P < 0·001) and watering (–13 days in 2007 and –9·9 days in 2008; both P < 0·001), and significant differences in leaf longevity were also found among species. Nitrogen and water interacted to affect leaf longevity and other functional traits. Soil water content explained approx. 70 % of the shifts in leaf longevity. Biomass at both species and community level increased under water and nitrogen addition because of the increase in leaf biomass production per individual plant.

Conclusions

The results suggest that additional nitrogen and water supplies reduce plant leaf longevity. Soil water availability might play a fundamental role in determining leaf longevity and other leaf functional traits, and its effects can be modified by soil nitrogen availability in semi-arid areas. The different responses of species to resource alterations may cause different global change ramifications under future climate change scenarios.     

Phenotypic plasticity as an index of drought tolerance in three Patagonian steppe grasses

Background and Aims

Despite general agreement regarding the adaptive importance of plasticity, evidence for the role of environmental resource availability in plants is scarce. In arid and semi-arid environments, the persistence and dominance of perennial species depends on their capacity to tolerate drought: tolerance could be given on one extreme by fixed traits and, on the other, by plastic traits. To understand drought tolerance of species it is necessary to know the plasticity of their water economy-related traits, i.e. the position in the fixed–plastic continuum.

Methods

Three conspicuous co-existing perennial grasses from a Patagonian steppe were grown under controlled conditions with four levels of steady-state water availability. Evaluated traits were divided into two groups. The first was associated with potential plant performance and correlated with fitness, and included above-ground biomass, total biomass, tillering and tiller density at harvest. The second group consisted of traits associated with mechanisms of plant adjustment to environmental changes and included root biomass, shoot/root ratio, tiller biomass, length of total elongated leaf, length of yellow tissue divided by time and final length divided by the time taken to reach final length.

Key Results and Conclusions

The most plastic species along this drought gradient was the most sensitive to drought, whereas the least plastic and slowest growing was the most tolerant. This negative relationship between tolerance and plasticity was true for fitness-related traits but was trait-dependent for underlying traits. Remarkably, the most tolerant species had the highest positive plasticity (i.e. opposite to the default response to stress) in an underlying trait, directly explaining its drought resistance: it increased absolute root biomass. The niche differentiation axis that allows the coexistence of species in this group of perennial dryland grasses, all limited by soil surface moisture, would be a functional one of fixed versus plastic responses.     

Modification of flower architecture during early stages in the evolution of self-fertilization

Background and Aims

The evolution of selfing from outcrossing is characterized by a series of morphological changes to flowers culminating in the selfing syndrome. However, which morphological traits initiate increased self-pollination and which are accumulated after self-fertilization establishes is poorly understood. Because the expression of floral traits may depend on the conditions experienced by an individual during flower development, investigation of changes in mating system should also account for environmental and developmental factors. Here, early stages in the evolution of self-pollination are investigated by comparing floral traits among Brazilian populations of Eichhornia paniculata (Pontederiaceae), an annual aquatic that displays variation in selfing rates associated with the breakdown of tristyly to semi-homostyly.

Methods

Thirty-one Brazilian populations under uniform glasshouse conditions were compared to investigate genetic and environmental influences on flower size and stigma–anther separation (herkogamy), two traits that commonly vary in association with transitions to selfing. Within-plant variation in herkogamy was also examined and plants grown under contrasting environmental conditions were compared to examine to what extent this trait exhibits phenotypic plasticity.

Key Results

In E. paniculata a reduction in herkogamy is the principal modification initiating the evolution of selfing. Significantly, reduced herkogamy was restricted to the mid-styled morph and occurred independently of flower size. Significant genetic variation for herkogamy was detected among populations and families, including genotypes exhibiting developmental instability of stamen position with bimodal distributions of herkogamy values. Cloned genets exposed to contrasting growth conditions demonstrated environmental control of herkogamy and genotypic differences in plasticity of this trait.

Conclusions

The ability to modify herkogamy independently of other floral traits, genetic variation in the environmental sensitivity of herkogamy, and the production of modified and unmodified flowers within some individuals, reveal the potential for dynamic control of the mating system in a species that commonly confronts heterogeneous aquatic environments.Key words: Eichhornia paniculata, expressivity, flower morphology, herkogamy, phenotypic plasticity, pleiotropy, population variation, self-fertilization, stigma–anther separation, outcrossing, tristyly     

Linking ethylene to nitrogen-dependent leaf longevity of grass species in a temperate steppe

Background and Aims

Leaf longevity is an important plant functional trait that often varies with soil nitrogen supply. Ethylene is a classical plant hormone involved in the control of senescence and abscission, but its role in nitrogen-dependent leaf longevity is largely unknown.

Methods

Pot and field experiments were performed to examine the effects of nitrogen addition on leaf longevity and ethylene production in two dominant plant species, Agropyron cristatum and Stipa krylovii, in a temperate steppe in northern China.

Key Results

Nitrogen addition increased leaf ethylene production and nitrogen concentration but shortened leaf longevity; the addition of cobalt chloride, an ethylene biosynthesis inhibitor, reduced leaf nitrogen concentration and increased leaf longevity. Path analysis indicated that nitrogen addition reduced leaf longevity mainly through altering leaf ethylene production.

Conclusions

These findings provide the first experimental evidence in support of the involvement of ethylene in nitrogen-induced decrease in leaf longevity.     

Genotype,development and tissue-derived variation of cell-wall properties in the lignocellulosic energy crop Miscanthus

Background and Aims

Species and hybrids of the genus Miscanthus contain attributes that make them front-runners among current selections of dedicated bioenergy crops. A key trait for plant biomass conversion to biofuels and biomaterials is cell-wall quality; however, knowledge of cell-wall composition and biology in Miscanthus species is limited. This study presents data on cell-wall compositional changes as a function of development and tissue type across selected genotypes, and considers implications for the development of miscanthus as a sustainable and renewable bioenergy feedstock.

Methods

Cell-wall biomass was analysed for 25 genotypes, considering different developmental stages and stem vs. leaf compositional variability, by Fourier transform mid-infrared spectroscopy and lignin determination. In addition, a Clostridium phytofermentans bioassay was used to assess cell-wall digestibility and conversion to ethanol.

Key Results

Important cell-wall compositional differences between miscanthus stem and leaf samples were found to be predominantly associated with structural carbohydrates. Lignin content increased as plants matured and was higher in stem tissues. Although stem lignin concentration correlated inversely with ethanol production, no such correlation was observed for leaves. Leaf tissue contributed significantly to total above-ground biomass at all stages, although the extent of this contribution was genotype-dependent.

Conclusions

It is hypothesized that divergent carbohydrate compositions and modifications in stem and leaf tissues are major determinants for observed differences in cell-wall quality. The findings indicate that improvement of lignocellulosic feedstocks should encompass tissue-dependent variation as it affects amenability to biological conversion. For gene–trait associations relating to cell-wall quality, the data support the separate examination of leaf and stem composition, as tissue-specific traits may be masked by considering only total above-ground biomass samples, and sample variability could be mostly due to varying tissue contributions to total biomass.     

A comparative analysis of leaf shape of wheat, barley and maize using an empirical shape model

Background and Aims

The phenotypes of grasses show differences depending on growth conditions and ontogenetic stage. Understanding these responses and finding suitable mathematical formalizations are an essential part of the development of plant and crop models. Usually, a marked change in architecture between juvenile and adult plants is observed, where dimension and shape of leaves are likely to change. In this paper, the plasticity of leaf shape is analysed according to growth conditions and ontogeny.

Methods

Leaf shape of Triticum aestivum, Hordeum vulgare and Zea mays cultivars grown under varying conditions was measured using digital image processing. An empirical leaf shape model was fitted to measured shape data of single leaves. Obtained values of model parameters were used to analyse the patterns in leaf shape.

Key Results

The model was able to delineate leaf shape of all studied species. The model error was small. Differences in leaf shape between juvenile and adult leaves in T. aestivum and H. vulgare were observed. Varying growth conditions impacted leaf dimensions but did not impact leaf shape of the respective species.

Conclusions

Leaf shape of the studied T. aestivum and H. vulgare cultivars was remarkably stable for a comparable ontogenetic stage (leaf rank), but differed between stages. Along with other aspects of grass architecture, leaf shape changed during the transition from juvenile to adult growth phase. Model-based analysis of leaf shape is a method to investigate these differences. Presented results can be integrated into architectural models of plant development to delineate leaf shape for different species, cultivars and environmental conditions.     

Invasion strategies in clonal aquatic plants: are phenotypic differences caused by phenotypic plasticity or local adaptation?

Background and Aims

The successful spread of invasive plants in new environments is often linked to multiple introductions and a diverse gene pool that facilitates local adaptation to variable environmental conditions. For clonal plants, however, phenotypic plasticity may be equally important. Here the primary adaptive strategy in three non-native, clonally reproducing macrophytes (Egeria densa, Elodea canadensis and Lagarosiphon major) in New Zealand freshwaters were examined and an attempt was made to link observed differences in plant morphology to local variation in habitat conditions.

Methods

Field populations with a large phenotypic variety were sampled in a range of lakes and streams with different chemical and physical properties. The phenotypic plasticity of the species before and after cultivation was studied in a common garden growth experiment, and the genetic diversity of these same populations was also quantified.

Key Results

For all three species, greater variation in plant characteristics was found before they were grown in standardized conditions. Moreover, field populations displayed remarkably little genetic variation and there was little interaction between habitat conditions and plant morphological characteristics.

Conclusions

The results indicate that at the current stage of spread into New Zealand, the primary adaptive strategy of these three invasive macrophytes is phenotypic plasticity. However, while limited, the possibility that genetic diversity between populations may facilitate ecotypic differentiation in the future cannot be excluded. These results thus indicate that invasive clonal aquatic plants adapt to new introduced areas by phenotypic plasticity. Inorganic carbon, nitrogen and phosphorous were important in controlling plant size of E. canadensis and L. major, but no other relationships between plant characteristics and habitat conditions were apparent. This implies that within-species differences in plant size can be explained by local nutrient conditions. All together this strongly suggests that invasive clonal aquatic plants adapt to a wide range of habitats in introduced areas by phenotypic plasticity rather than local adaptation.     

Intraspecific variation in the magnitude and pattern of flooding-induced shoot elongation in Rumex palustris

Background and Aims

Intraspecific variation in flooding tolerance is the basic pre-condition for adaptive flooding tolerance to evolve, and flooding-induced shoot elongation is an important trait that enables plants to survive shallow, prolonged flooding. Here an investigation was conducted to determine to what extent variation in flooding-induced leaf elongation exists among and within populations of the wetland species Rumex palustris, and whether the magnitude of elongation can be linked to habitat characteristics.

Methods

Offspring of eight genotypes collected in each of 12 populations from different sites (ranging from river mudflats with dynamic flooding regimes to areas with stagnant water) were submerged, and petioles, laminas and roots were harvested separately to measure traits related to elongation and plant growth.

Key Results

We found strong elongation of petioles upon submergence, and both among- and within-population variation in this trait, not only in final length, but also in the timing of the elongation response. However, the variation in elongation responses could not be linked to habitat type.

Conclusions

Spatio-temporal variation in the duration and depth of flooding in combination with a presumably weak selection against flooding-induced elongation may have contributed to the maintenance of large genetic variation in flooding-related traits among and within populations.     

Tests for the joint evolution of mating system and drought escape in Mimulus

Background and Aims

Self-fertilizing taxa are often found at the range margins of their progenitors, where sub-optimal habitats may select for alternative physiological strategies. The extent to which self-fertilization is favoured directly vs. arising indirectly through correlations with other adaptive life history traits is unclear. Trait responses to selection depend on genetic variation and covariation, as well as phenotypic and genetic responses to altered environmental conditions. We tested predictions of the hypothesis that self-fertilization in Mimulus arises through direct selection on physiological and developmental traits that allow seasonal drought escape.

Methods

Phenotypic selection on mating system and drought escape traits was estimated in field populations of M. guttatus. In addition, trait phenotype and phenotypic selection were compared between experimental wet and dry soil in two greenhouse populations each of M. guttatus and M. nasutus. Finally, genetic variation and covariation for traits were compared between wet and dry soil treatments in a greenhouse population of M. guttatus.

Key Results

Consistent with predictions, selection for early flowering was generally stronger than for mating system traits, and selection for early flowering was stronger in dry soil. Inconsistent with predictions, selection for water-use efficiency was largely absent; selection for large flowers was stronger than for drought escape in the field; and most drought escape and mating system traits were not genetically correlated. A positive genetic correlation between flowering time and flower size, which opposed the adaptive contour, emerged only in wet soil, suggesting that variation in water availability may maintain variation in these traits. Plastic responses to soil moisture treatments supported the idea that taxonomic divergence could have been facilitated by plasticity in flowering time and selfing.

Conclusions

The hypothesis that plant mating systems may evolve indirectly via selection on correlated life history characteristics is plausible and warrants increased attention.     

Geographical differentiation in floral traits across the distribution range of the Patagonian oil-secreting Calceolaria polyrhiza: do pollinators matter?

Background and Aims

The underlying evolutionary processes of pollinator-driven floral diversification are still poorly understood. According to the Grant–Stebbins model speciation begins with adaptive local differentiation in the response to spatial heterogeneity in pollinators. Although this crucial process links the micro- and macroevolution of floral adaptation, it has received little attention. In this study geographical phenotypic variation was investigated in Patagonian Calceolaria polyrhiza and its pollinators, two oil-collecting bee species that differ in body size and geographical distribution.

Methods

Patterns of phenotypic variation were examined together with their relationships with pollinators and abiotic factors. Six floral and seven vegetative traits were measured in 45 populations distributed across the entire species range. Climatic and edaphic parameters were determined for 25 selected sites, 2–16 bees per site of the most frequent pollinator species were captured, and a critical flower–bee mechanical fitting trait involved in effective pollination was measured. Geographical patterns of phenotypic and environmental variation were examined using uni- and multivariate analyses. Decoupled geographical variation between corolla area and floral traits related to the mechanical fit of pollinators was explored using a Mantel test.

Key Results

The body length of pollinators and the floral traits related to mechanical fit were strongly correlated with each other. Geographical variation of the mechanical-fit-related traits was decoupled from variation in corolla size; the latter had a geographical pattern consistent with that of the vegetative traits and was mainly affected by climatic gradients.

Conclusions

The results are consistent with pollinators playing a key role in shaping floral phenotype at a geographical scale and promoting the differentiation of two floral ecotypes. The relationship between the critical floral-fit-related trait and bee length remained significant even in models that included various environmental variables and an allometric predictor (corolla area). The abiotic environment also has an important role, mainly affecting floral size. Decoupled geographical variation between floral mechanical-fit-related traits and floral size would represent a strategy to maintain plant–pollinator phenotypic matching in this environmentally heterogeneous area.     

Molecular and quantitative trait variation within and among small fragmented populations of the endangered plant species Psilopeganum sinense

Background and Aims

Natural selection and genetic drift are important evolutionary forces in determining genetic and phenotypic differentiation in plant populations. The extent to which these two distinct evolutionary forces affect locally adaptive quantitative traits has been well studied in common plant and animal species. However, we know less about how quantitative traits respond to selection pressures and drift in endangered species that have small population sizes and fragmented distributions. To address this question, this study assessed the relative strengths of selection and genetic drift in shaping population differentiation of phenotypic traits in Psilopeganum sinense, a naturally rare and recently endangered plant species.

Methods

Population differentiation at five quantitative traits (Q_ST) obtained from a common garden experiment was compared with differentiation at putatively neutral microsatellite markers (F_ST) in seven populations of P. sinense. Q_ST estimates were derived using a Bayesian hierarchical variance component method.

Key Results

Trait-specific Q_ST values were equal to or lower than F_ST. Neutral genetic diversity was not correlated with quantitative genetic variation within the populations of P. sinense.

Conclusions

Despite the prevalent empirical evidence for Q_ST > F_ST, the results instead suggest a definitive role of stabilizing selection and drift leading to phenotypic differentiation among small populations. Three traits exhibited a significantly lower Q_ST relative to F_ST, suggesting that populations of P. sinense might have experienced stabilizing selection for the same optimal phenotypes despite large geographical distances between populations and habitat fragmentation. For the other two traits, Q_ST estimates were of the same magnitude as F_ST, indicating that divergence in these traits could have been achieved by genetic drift alone. The lack of correlation between molecular marker and quantitative genetic variation suggests that sophisticated considerations are required for the inference of conservation measures of P. sinense from neutral genetic markers.     

Variation in inbreeding depression and plasticity across native and non-native field environments

Background and Aims

Since the early 1990s, research on genetic variation of phenotypic plasticity has expanded and empirical research has emphasized the role of the environment on the expression of inbreeding depression. An emerging question is how these two evolutionary ecology mechanisms interact in novel environments. Interest in this area has grown with the need to understand the establishment of populations in response to climate change, and to human-assisted transport to novel environments.

Methods

We compare performance in the field of outcrossed (O) and inbred lines (S1, S2) from 20 maternal families from each of two native populations of Mimulus guttatus. The experiment was planted in California in each population''s home site, in the other populations''s home site, in a novel site within the native range of M. guttatus, and in a novel site within the non-native range in North America. The experiment included nearly 6500 individuals. Survival, sexual reproduction and above-ground biomass were examined in order to evaluate inbreeding depression, and stem diameter and plant height were examined in order to evaluate phenotypic plasticity.

Key Results

Across all field sites, approx. 36 % of plants survived to flowering. Inbreeding depression differed among sites and outcrossed offspring generally outperformed selfed offspring. However, in the native-novel site, self-progeny performed better or equally well as outcross progeny. Significant phenotypic plasticity and genetic variation in plasticity was detected in the two architectural traits measured. The absolute value of plasticity showed the most marked difference between home and non-native novel site or non-native-novel site. Evidence was detected for an interaction between inbreeding and plasticity for stem diameter.

Conclusions

The results demonstrate that during initial population establishment, both inbreeding depression and phenotypic plasticity vary among field sites, and may be an important response to environments outside a species'' currently occupied range. However, the interaction between inbreeding and plasticity may be limited and environment-dependent.     

Nitrogen stress response of a hybrid species: a gene expression study

Background and Aims

Low soil fertility limits growth and productivity in many natural and agricultural systems, where the ability to sense and respond to nutrient limitation is important for success. Helianthus anomalus is an annual sunflower of hybrid origin that is adapted to desert sand-dune substrates with lower fertility than its parental species, H. annuus and H. petiolaris. Previous studies have shown that H. anomalus has traits generally associated with adaptation to low-fertility habitats, including a lower inherent relative growth rate and longer leaf lifetime.

Methods

Here, a cDNA microarray is used to identify gene expression differences that potentially contribute to increased tolerance of low fertility of the hybrid species by comparing the nitrogen stress response of all three species with high- and low-nutrient treatments.

Key Results

Relative to the set of genes on the microarray, the genes showing differential expression in the hybrid species compared with its parents are enriched in stress-response genes, developmental genes, and genes involved in responses to biotic or abiotic stimuli. After a correction for multiple comparisons, five unique genes show a significantly different response to nitrogen limitation in H. anomalus compared with H. petiolaris and H. annuus. The Arabidopsis thaliana homologue of one of the five genes, catalase 1, has been shown to affect the timing of leaf senescence, and thus leaf lifespan.

Conclusions

The five genes identified in this analysis will be examined further as candidate genes for the adaptive stress response in H. anomalus. Genes that improve growth and productivity under nutrient stress could be used to improve crops for lower soil fertility which is common in marginal agricultural settings.     

Kinship rivalry does not trigger specific allocation strategies in Lupinus angustifolius

Background and Aims

Research on the ability of plants to recognize kin and modify plant development to ameliorate competition with coexisting relatives is an area of very active current exploration. Empirical evidence, however, is insufficient to provide a sound picture of this phenomenon.

Methods

An experiment was designed to assess multi-trait phenotypic expression in response to competition with conspecifics of varied degrees of genealogical relatedness. Groups of siblings, cousins and strangers of Lupinus angustifolius were set in competition in a pots assay. Several whole-plant and organ-level traits, directly related to competition for above- and below-ground resources, were measured. In addition, group-level root proliferation was measured as a key response trait to relatedness to neighbours, as identified in previous work.

Key Results

No major significant phenotypic differences were found between individuals and groups that could be assigned to the gradient of relatedness used here. This occurred in univariate models, and also when multi-trait interactions were evaluated through multi-group comparisons of Structural Equation Models. Root proliferation was higher in phenotypically more heterogeneous groups, but phenotypic heterogeneity was independent of the relatedness treatments of the experiment, and root proliferation was alike in the neighbourhoods of siblings, cousins and strangers.

Conclusions

In contrast to recent findings in other species, genealogical relatedness to competing neighbours has a negligible impact on the phenotypic expression of individuals and groups of L. angustifolius. This suggests that kin recognition needs further exploration to assess its generality, the ecological scenarios where it might have been favoured or penalized by natural selection, and its preponderance in different plant lineages.