A comparative ecophysiological study on the effects of waterlogging and submergence on dune slack plants: growth, survival and mineral nutrition in sand culture experiments

Coastal Wetland Species Rumex hydrolapathum: Tolerance against Flooding, Salinity, and Heavy Metals for Its Potential Use in Phytoremediation and Environmental Restoration Technologies

Plants with high biomass adapted to conditions of increased moisture and with significant salt tolerance appear to be particularly attractive candidates for phytoremediation studies. The aim of the present study was to examine the tolerance of Rumex hydrolapathum plants to freshwater, saltwater inundation, and soil contaminated with heavy metals, as well as its metal accumulation potential in controlled conditions. Six separate vegetation container experiments in controlled conditions were performed with R. hydrolapathum plants to study the effects of soil moisture, waterlogging with NaCl, soil Cd, soil Cr, soil Ni, and soil Pb in the form of a nitrate or acetate. Optimum plant growth occurred in waterlogged soil conditions. As the concentration of NaCl used for waterlogging increased, the mass of living leaves decreased, but that of dry leaves increased. As a result, the total biomass of leaves did not significantly change. R. hydrolapathum plants were extremely tolerant to Cd and Pb, moderately tolerant to Ni, and relatively sensitive to Cr. The plants had high capacity for metal accumulation in older and senescent leaves, especially for Na⁺, K⁺, Cd, and Ni. R. hydrolapathum plants can tolerate soil waterlogging with seawater-level salinity, which, together with the metal tolerance and potential for metal accumulation in leaves, make them excellently suited for use in a variety of wastewater treatment systems, including constructed wetlands.

Life-history trade-offs vary with resource availability across the geographic range of a widespread plant

Trade-offs between reproduction, growth and survival arise from limited resource availability in plants. Environmental stress is expected to exacerbate these negative correlations, but no studies have evaluated variation in life-history trade-offs throughout species geographic ranges. Here we analyse the costs of growth and reproduction across the latitudinal range of the widespread herb Plantago coronopus in Europe. We monitored the performance of thousands of individuals in 11 populations of P. coronopus, and tested whether the effects of growth and reproduction on a set of vital rates (growth, probability of survival, probability of reproduction and fecundity) varied with local precipitation and soil fertility. To account for variation in internal resources among individuals, we analysed trade-offs correcting for differences in size. Growth was negatively affected by previous growth and reproduction. We also found costs of growth and reproduction on survival, reproduction probability and fecundity, but only in populations with low soil fertility. Costs also increased with precipitation, possibly due to flooding-related stress. In contrast, growth was positively correlated with subsequent survival, and there was a positive covariation in reproduction between consecutive years under certain environments, a potential strategy to exploit temporary benign conditions. Overall, we found both negative and positive correlations among vital rates across P. coronopus geographic range. Trade-offs predominated under stressful conditions, and positive correlations arose particularly between related traits like reproduction investment across years. By analysing multiple and diverse fitness components along stress gradients, we can better understand life-history evolution across species' ranges, and their responses to environmental change.

Geographic variation in floral traits and the capacity of autonomous selfing across allopatric and sympatric populations of two closely related Centaurium species

Floral traits and the relative contribution of autonomous selfing to total seed set varies geographically and is often driven by the availability and abundance of suitable pollinators and/or the presence of co-flowering relatives. In the latter case, competition for pollinator services and costs of hybridization can select for floral traits that reduce interspecific gene flow and contribute to prezygotic isolation, potentially leading to geographic variation in floral divergence between allopatric and sympatric populations. In this study, we investigated variation in floral traits and its implications on the capacity of autonomous selfing in both allopatric and sympatric populations of two closely related Centaurium species(Gentianaceae) across two distinct geographic regions(UK and mainland Europe). Although the magnitude and direction of floral differentiation varied between regions, sympatric populations were always significantly more divergent in floral traits and the capacity to self autonomously than allopatric populations. These results indicate that mating systems can vary substantially within a species and that the joint occurrence of plant species can have a major impact on floral morphology and capacity of autonomous selfing, most likely as a way to reduce the probability of interspecific interference.

Recurrent Water Level Fluctuation Alleviates the Effects of Submergence Stress on the Invasive Riparian Plant Alternanthera philoxeroides

Recurrent water level fluctuation and submergence of plants are common in riparian zones. Our study objectives were to test the independent and interactive effects of submergence level and fluctuation frequency on a globally important riparian invasive plant, Alternanthera philoxeroides. To this end, we conducted a greenhouse experiment, in which ramets of the plants, obtained from a wetland in China, were treated with four fluctuation frequencies (0, 3, 6, and 12 cycles over a 96-day experimental period) under three water levels (0, 10, and 30 cm). We found that effects of fluctuation frequency were non-significant, negative, and positive under water levels of 0, 10 and 30 cm, respectively. As fluctuation frequency increased, the effects of increasing water level decreased significantly. When water levels were high, A. philoxeroides allocated greater biomass to shoot production probably in order to elongate and escape from submergence. However, as fluctuation frequency increased, biomass investment in roots and leaves also increased, probably in order to maximize nutrient absorption and photosynthesis, respectively. These results suggest that water level fluctuation may alleviate the effects of submergence on A. philoxeroides. In addition, A. philoxeroides showed significant phenotypic plasticity, adjusting its functional traits, such as number of nodes and leaves per stem, as well as stem diameter and pith cavity diameter, according to recurrent water level fluctuation. We conclude that A. philoxeroides may perform better in shallow water zones under conditions of disturbance that include recurrent water level fluctuation. This ability to adapt to disturbance likely promotes its growth and invasion in disturbed habitats.

The contribution of mating system variation to reproductive isolation in two closely related Centaurium species (Gentianaceae) with a generalized flower morphology

In closely related plant species that display strong similarities in phenology and pollinator communities, differences in breeding system and associated shifts in floral traits may have important effects on the magnitude and direction of heterospecific pollen flow and hybridization. Here, we quantified the strength of several pre- and postzygotic barriers acting between the facultatively outcrossing Centaurium erythraea and the predominantly selfing C. littorale via a suite of experiments, and estimated the frequency of hybridization in the field using molecular markers. The reproductive barriers primarily responsible for preventing hybridization were essentially prezygotic and these acted asymmetrically. Due to differences in floral display, pollen production, and pollen transfer rates, heterospecific pollen flow occurred predominantly from C. erythraea to C. littorale. In C. littorale, on the other hand, close anther-stigma positioning and resulting higher capacity for autonomous selfing functioned as an efficient barrier to counterbalance the higher risk for hybrid mating. In both species the action of all reproductive barriers resulted in a small opportunity for hybrid establishment, which was confirmed by the occurrence of only ∼1% putative hybrids in the field. Our findings confirm that differences in breeding system affect heterospecific pollen transfer patterns and that autonomous selfing may efficiently prevent hybridization.

The role of the tolerance-fecundity trade-off in maintaining intraspecific seed trait variation in a widespread dimorphic herb

Coexistence of species with different seed sizes is a long-standing issue in community ecology, and a trade-off between fecundity and stress tolerance has been proposed to explain co-occurrence in heterogeneous environments. Here we tested an intraspecific extension of this model: whether such trade-off also explains seed trait variation among populations of widespread plants under stress gradients. We collected seeds from 14 populations of Plantago coronopus along the Atlantic coast in North Africa and Europe. This herb presents seed dimorphism, producing large basal seeds with a mucilaginous coat that facilitates water absorption (more stress tolerant), and small apical seeds without coats (less stress tolerant). We analysed variation among populations in number, size and mucilage production of basal and apical seeds, and searched for relationships between local environment and plant size. Populations under higher stress (higher temperature, lower precipitation, lower soil organic matter) had fewer seeds per fruit, higher predominance of basal relative to apical seeds, and larger basal seeds with thicker mucilaginous coats. These results strongly suggest a trade-off between tolerance and fecundity at the fruit level underpins variation in seed traits among P. coronopus populations. However, seed production per plant showed the opposite pattern to seed production per fruit, and seemed related to plant size and other life-cycle components, as an additional strategy to cope with environmental variation across the range. The tolerance-fecundity model may constitute, under stress gradients, a broader ecological framework to explain trait variation than the classical seed size-number compromise, although several fecundity levels and traits should be considered to understand the diverse strategies of widespread plants to maximise fitness in each set of local conditions.

Eco-hydrological requirements of dune slack vegetation and the implications of climate change

Dune slacks are a seasonal coastal wetland habitat, whose plant assemblages and soil properties are strongly linked to a fluctuating water table. Climate change is predicted to cause major shifts in sand dune hydrological regimes, yet we know remarkably little about the tolerance of these communities to change, and their precise hydrological requirements are poorly quantified. Dune slack vegetation and soils were sampled within five vegetation types across four west coast UK sites. Relationships between vegetation assemblages, and parameters of soil development (moisture, loss on ignition, pH, KCl extractable ions) and groundwater hydrological regime (annual maximum and minimum water levels and range, duration of flooding) were established to define the environmental tolerances of different communities. In multivariate analysis of the vegetation, the dominant gradient was hydrological: dry to wet, followed by a secondary soil development gradient: young calcareous organic-poor soils to acidic/neutral soils with greater organic matter contents. Most measured hydrological and soil variables explained a significant proportion of observed variation in species composition when tested individually, with the exception of soil nitrate and soil calcium concentrations. Maximum water level was the key hydrological variable, and soil moisture and soil pH were the key soil variables. All hydrological and soil parameters together explained 22.5% of the total species variation. There were significant differences in hydrological and soil parameters between community types, with only 40 cm difference in mean annual minimum water levels (averaged over 4 years) separating the wettest and the driest dune slack communities. Therefore, predicted declines in water level exceeding 100 cm by 2080 are likely to have a major impact on the vegetation of these priority conservation habitats.

European dune slacks: Strong interactions of biology, pedogenesis and hydrology

Dune slacks are a unique type of wetland ecosystem, highly ranked on the international conservation agenda because of the occurrence of many rare and endangered plant species and their associated fauna. Ecologically they present some of the few examples of primary succession seres with a high degree of facilitation between functionally distinct groups of plants and a strong impact of the interannual variation of the water table. Recent research has focussed on the biological and environmental processes counteracting the rapid loss of diversity owing to human impacts along most north-west European coasts.

Mosses influence phosphorus cycling in rich fens by driving redox conditions in shallow soils

Mosses play an integral role in the hydrologic regimes of ecosystems where they cover the soil surface, and thus affect biogeochemical cycling of elements influenced by soil oxidation-reduction (redox) reactions, including the plant growth-limiting nutrients, nitrogen and phosphorus (P). In rich fens where P often limits plant growth, we hypothesized that feedbacks between mosses and redox conditions would determine P availability to shallow-rooted forb species that constitute much of these wetlands' unusually high plant species diversity. In a moss removal experiment in three fens, forb tissue P and microbial P were greater while anion exchange membrane (AEM) resin P was lower where mosses occurred than where they were removed, suggesting both higher availability and greater demand for P in moss-covered soils. Coupled physicochemical and biological mechanisms drove moss effects on P cycling, ultimately through effects on soil oxygenation or reduction: higher redox potential underlying mosses corresponded to greater microbial activity, phosphatase enzyme activity, and colonization by arbuscular mycorrhizal fungi (AMF), all of which can promote greater P availability to plants. These more oxidized soils stimulated: (1) greater microbial activity and root vigor; (2) correspondingly greater P demand via microbial uptake, forb uptake, and iron (Fe)-P reactions; and (3) greater P supply through soil and root phosphatase activity and AMF colonization. This work demonstrates that mosses improve vascular plant P acquisition by alleviating stresses caused by reducing conditions that would otherwise prevail in shallow underlying soils, thus providing a mechanism by which mosses facilitate plant species diversity in rich fens.

Positive interactions in communities

Current concepts of the role of interspecific interactions in communities have been shaped by a profusion of experimental studies of interspecific competition over the past few decades. Evidence for the importance of positive interactions - facilitations - in community organization and dynamics has accrued to the point where it warrants formal inclusion into community ecology theory, as it has been in evolutionary biology.

The role of biomarkers in environmental assessment (4). Terrestrial plants

The potential of metabolites, enzymatic processes and changes in plant performance as biomarkers in environmental assessment is reviewed. Biomarkers may be used as an early warning system of specific or general stress at each biological level, from molecules to ecosystems. The sensitivity of a species and, thus, the efficiency of a biomarker will depend on the degree of already present adaptation to environmental stress and on the homogeneity of the investigated population. Biomarkers for specific environmental stresses are scarce; better known are biomarkers for environmental stress complexes such as heavy metals, physiological drought and extreme temperature or biomarkers as a reaction on a full scale of environmental stresses. It is argued that a battery of biomarkers is necessary to evaluate chemical hazards to species.

Relationship between morphological and physiological responses to waterlogging and salinity in Sporobolus virginicus (L.) Kunth

The effects of waterlogging and salinity on morphological and physiological responses in the marsh grass Sporobolus virginicus (L.) Kunth were investigated in a 4×2 factorial experiment. Plants were subjected to four salinity levels (0, 100, 200 and 400 mol m^-3 NaCl) and two soil inundation conditions (drained and flooded) for 42 days. Flooding at 0 mol m^-3 NaCl caused initiation of adventitious surface roots, increased internal acration and plant height, induced alcohol dehydrogenase activity (ADH), and decreased belowground biomass and the number of culms per plant. Salinity increase from 0 to 400 mol m^-3 NaCl under drained conditions increased leaf and root proline concentrations and decreased photosynthesis, aboveground biomass, number of culms per plant and number of internodes per culm. Concurrent waterlogging and salinity induced ADH activity and adventitious surface roots but decreased plant height and aboveground biomass. Internal air space increased with waterlogging from 0 to 100 mol m^-3 NaCl but further increases in salinity to 400 mol m^-3 reduced air space. Combined waterlogging and salinity stresses, however, had no effect on photosynthesis or on the concentrations of proline in leaves or roots. These results are discussed in relation to the widespread colonization by S. virginicus of a wide range of coastal environments varying in soil salinity and in the frequency and intensity of waterlogging.

Waterlogging responses of Sporobolus virginicus (L.) Kunth

Flooding responses in Sporobolus virginicus (L.) Kunth., a perennial C₄ grass, propagated from plants collected on the fringes of a mangrove swamp, were examined in a glasshouse study over 42 days. Flooding significantly reduced soil redox potential, induced adventitious root development, shifted resource allocation from below- to above-ground components without affecting total biomass accumulation and significantly decreased below-ground/above-ground biomass ratios. Although soil waterlogging significantly increased alcohol dehydrogenase activity (ADH) after 30 h, significant increase in central air space by 45-50% of the cross-sectional stem area eliminated root hypoxia, and ADH activity decreased to levels equivalent to drained controls after 42 days. In addition, flooded plants exhibited significantly higher carbon dioxide assimilation rates but similar relative growth rates (RGR) to drained controls. The results indicate that S. virginicus responds to water-logging by a combination of metabolic, morphological and anatomical mechanisms, which may account for its widespread distribution in coastal lagoons, estuaries and marshes.