Tolerance of switchgrass to extreme soil moisture stress: Ecological implications

Switchgrass (Panicum virgatum L.), a native of eastern and central North America, is a leading candidate as a dedicated biofuel feedstock in the US due to its broad adaptability, rapid growth rate, and ability to grow in low production soils. To begin to characterize the important agronomic and ecological traits related to environmental tolerance of switchgrass, we evaluated fitness under stressful growing conditions. We assessed the germination, establishment, performance, and reproductive potential of four common accessions, both upland and lowland ecotypes, at various levels of soil moisture availability (moisture deficit to flooded) in the greenhouse. Seeds emerged and established (55–90% survival) under all soil moisture conditions (−0.3 MPa to flooded). Transplants of lowland ecotypes performed as well in flooded conditions as in field capacity controls, though flooding reduced performance of upland ecotypes. Drought treatments (−4.0 and −11.0 MPa) reduced tiller length and number, leaf area, and biomass production by up to 80%. However, once established, all plants survived at −4.0 MPa and had the same proportion of tillers in flower as at field capacity. The ability of switchgrass to germinate, establish, and flower in low moisture and flooded conditions, particularly lowland ecotypes, may increase the range of environments suitable for biofuel cultivation, and can serve as a baseline for further ecological studies and genetic improvement.     

Miscanthus × giganteus and Arundo donax shoot and rhizome tolerance of extreme moisture stress

Crops grown for bioenergy production are a mandated component of the United States energy portfolio. Giant miscanthus (Miscanthus × giganteus) is a leading bioenergy crop similar in habit to the invasive plant giant reed (Arundo donax). To characterize the environmental tolerance of giant miscanthus, we compared the soil moisture stress tolerance of giant miscanthus and giant reed under glasshouse conditions. We subjected both species to soil moisture conditions of severe drought (?4.2 MPa), mild drought (?0.5 MPa), field‐capacity (control), and flooded soils. These conditions were applied to two cohorts: one in which soil moisture conditions were imposed on newly planted rhizome fragments, and one in which conditions were imposed on established plants after 8 weeks of growth in field‐capacity soil. After 16 weeks, we harvested all plants, measured above‐ and belowground biomass, and evaluated the reproductive viability of rhizome fragments. The total biomass of each species under flooded conditions was not different from the field‐capacity control groups regardless of cohort. However, drought did affect the two cohorts differently. In the cohort treated after 8 weeks of growth, mild and severe drought conditions resulted in 56% and 66% reductions in biomass, averaged over both species, compared with the controls. In the cohort treated for the entire 16 weeks, mild and severe drought conditions resulted in 92% and 94% reductions in biomass. Rhizome fragments from both species and both cohorts showed 100% viability following flooded and control treatments; drought treatments reduced rhizome viability in both species, with a greater impact on giant miscanthus. Although giant miscanthus does not appear to have the potential to escape and establish in relatively dry upland ecosystems, it does show tolerance to flooded conditions similar to giant reed.     

Growth and allocation of the arctic sedges Eriohorum angustifolium and E. vaginatum: effects of variable soil oxygen and nutrient availability

In arctic tundra soil, oxygen depletion associated with soil flooding may control plant growth either directly through anoxia or indirectly through effects on nutrient availability. This study was designed to evaluate whether plant growth and physiology of two arctic sedge species are more strongly controlled by the direct or indirect effects of decreased soil aeration. Eriophorum angustifolium and E. vaginatum, which originate from flooded and well-drained habitats, respectively, were grown in an in situ transplant garden at two levels of soil oxygen, nitrogen, and phosphorus availability over two growing seasons. In both species, N addition had a stronger effect on growth and biomass allocation than P addition or soil oxygen depletion. Net photosynthesis and carbohydrate concentrations were relatively insensitive to changes in these factors. Biomass reallocated from shoots to below-ground parts in response to limited N supply was equally divided between roots (nutrient acquisition) and perennating rhizomes (storage tissue formation) in E. angustifolium. E. Vaginatum only increased its allocation to rhizomes. In the flood-tolerant E. angustifolium, growth was improved by soil anoxia and biomass allocation among plant parts was not significantly affected. Contrary to our initial hypothesis, whole-plant growth in E. vaginatum improved in flooded soils; however, it only did so when N availability was high. Under low N availability growth in flooded soils was reduced by 20% compared to growth in the aerobic environment. Reduced biomass allocation to rhizomes and thus to storage potential under anaerobic conditions may reduce long-term survival of E. vaginatum in flooded habitats.     

Arabidopsis thaliana as a new model host for plant-parasitic nematodes

We have established culture conditions for successful infection and development of several economically important cyst-forming and root-knot nematodes on Arabidopsis thaliana under monoxenic conditions. Complete life cycles were obtained with the sedentary cyst nematodes Heterodera schachtii, H. trifolii, H. cajani and the root-knot nematodes Meloidogyne incognita and M. arenariaas well as with the migratory nematode Pratylenchus penetrans. In contrast, H. goettingiana and Globodera rostochiensis were unable to develop on Arabidopsis roots. Tissue-culture quality agar and medium conditions optimized for hydroponic root culture were essential for successful infections. Detailed in-vivo observations were made inside Arabidopsis roots during the early infection stages of M. incognita and during complete development of H. schachtii. Seventy-four different ecotypes of Arabidopsis were screened for their susceptibility towards H. schachtii resulting in a range of infection rates. None of the ecotypes tested showed complete resistance in vitro. The use of Arabidopsis as a host for plant-parasitic nematodes will provide a new model system for the molecular genetic analysis of this interaction.     

Responses of three bottomland species with different flood tolerance capabilities to various flooding regimes

Seedlings of baldcypress (Taxodium distichum), nuttall oak (Quercus nuttalli), and cherrybark oak (Quercus falcata var.pagodaefolia) were subjected to four flooding treatments: control, continuously flooded, intermittently flooded, and partially flooded for 70 days in a greenhouse. The treatments imposed various durations and intensities of soil redox potential (Eh) conditions representing a range encountered by plants in their habitats. Morphological changes and gas exchange responses to the treatments differed among the study species. Rapid development of adventitious root and hypertrophied lenticels were observed in baldcypress and nuttall oak under all flooded treatments. Cherrybark oak had the highest percentage reduction in net photosynthesis ranging from 65–87%, whereas reductions in nuttall oak ranged between 35–68% and in baldcypress between 6–21% in response to various treatments. Recovery of gas exchange was noted in baldcypress but no significant recovery was found in oaks. The recovery in baldcypress contributed to the continued growth and biomass accumulation under various treatments. Little evidence of consistent changes in biomass allocation patterns in response to the treatments was found in baldcypress but total biomass decreased significantly under the continuously flooded treatment. In oaks, total biomass decreased significantly in all flooded treatments. The present findings demonstrated that physiological functions are adversely affected by low soil Eh conditions and the extent of such effects are dependent on the intensity and duration of soil reduction as well as the species' capability to respond to such conditions rapidly. Management plans concerned with regeneration of bottomland forested ecosystems should consider the species flood response capabilities at seedling stages as well as the timing, durations, and intensities of soil reduction at the specific site.     

With a little help from my friends: physiological integration facilitates invasion of wetland grass Elymus athericus into flooded soils

Tidal wetlands worldwide are undergoing rapid invasions by tall-growing clonal grasses. Prominent examples are invasions by species of the genera Spartina, Phragmites and Elymus. The responsible physiological and ecological drivers of these invasions are poorly understood. Physiological integration (PI) is a key trait of clonal plants, which enables the exchange of resources among ramets. We investigated PI in Elymus athericus, which has been rapidly spreading from high-marsh into low-marsh environments of European salt marshes during the last decades. We applied a nitrogen stable-isotope approach to trace nutrient translocation between ramets in a factorial mesocosm experiment. The experiment was set up to mimic an invasion pattern commonly found in tidal wetlands, i.e. from high-elevated and rarely flooded into low-elevated and frequently flooded microenvironments. We tested for intraspecific variability in PI by including two genotypes of Elymus that naturally occur at different elevations within the tidal frame, a high-marsh (HM) and a low-marsh (LM) genotype. PI strongly increased offspring ramet aboveground and belowground biomass by 62 and 81%, respectively. Offspring ramets under drained conditions had 95% greater belowground biomass than those under flooded conditions. LM genotype offspring ramets produced 27% more aboveground biomass than HM genotypes. Offspring ramets were clearly more enriched in ¹⁵N under flooded versus drained conditions; however, this positive effect of flooding on δ¹⁵N was only significant in the LM genotype. Our findings demonstrate the importance of PI for the growth of Elymus offspring ramets and thereby for the species' capacity for fast vegetative spread. We show that offspring ramets under stressful flooded conditions are more dependent on nutrient supply from parent ramets than those under drained conditions. Our data furthermore suggest a higher degree of adaptation to flooding via PI in the LM versus HM genotype. In conclusion, we highlight the importance of assessing PI and intraspecific trait variability to understand invasion processes within ecosystems.     

Responses of the saltmarsh rush Juncus kraussii to salinity and waterlogging

Juncus kraussii Hochst., an important saltmarsh macrophyte, is intensively harvested for many commercially orientated products and current populations are under threat of overexploitation. In saline, intertidal mud banks, this species occurs on higher ground, suggesting that it is adapted to lower salinities and less frequent inundation. The objectives of this study were to determine biomass accumulation, as well as morphological and physiological adaptations of J. kraussii to salinity and waterlogging stresses. Plants collected from the field were subjected to 0.2, 10, 30, 50 and 70% seawater under drained or flooded conditions for three months. Measurements were made of biomass accumulation, CO₂ exchange, chlorophyll fluorescence, ion and water relations. Furthermore, seed germination responses to a range of salinities were investigated. Total dry biomass accumulation, as well as the number and height of culms, decreased with increase in salinity under both flooded and drained conditions. Generally, CO₂ exchange, stomatal conductance, Photosystem II (PSII) quantum yield and electron transport rate (ETR) through PSII declined with increase in salinity in both the flooded and drained treatments. Predawn and midday ψ in culms decreased with increase in salinity, being lower under drained than flooded conditions. Inorganic solute concentrations in culms increased with increase in salinity, with Na⁺ and Cl⁻ being the predominant ions. Na⁺/K⁺ ratios in culms increased significantly with increase in salinity. Proline concentrations in roots and culms, which increased with salinity, were considerably higher under drained than flooded conditions. Germination was best at salinities less than 20% seawater and decreased significantly with further increase in salinity to 110% seawater. Transfer of ungerminated salt-treated seeds to distilled water stimulated germination. This study has demonstrated that J. kraussii is a highly salt and flood tolerant species, being able to grow and survive in salinities up to 70% seawater, under both drained and flooded conditions. Maximal growth occurred at low salinities (<10% seawater) under flooded condition.     

Variations in several morphological characteristics and Cd/Pb accumulation capacities among different ecotypes of torpedograss responding to Cd-Pb stresses

Torpedograss (Panicum repens) has been recognized as an useful plant species for phytoremediation of water-level-fluctuation zones, which is a worldwide challenge. In this study, 10 ecotypes collected from tropical zone and flooded habitats (Group A) and subtropical zone and drought habitats (Group B) were used to clarify their responses to Cd-Pb stresses and effects of long-term adaptation on their morphological features and Cd-Pb accumulation capacities. Branch capacity, shoot and root biomasses of Group A under control were smaller than those of Group B, while the opposite results were observed under Cd-Pb stresses. The average plant shoot Cd concentrations of Group A under L-Cd-Pb and H-Cd-Pb were 24.84 and 52.38 mg kg^?1, respectively, significantly lower than those of Group B (36.81 and 67.60 mg kg^?1), while the variation among each group was insignificant, suggesting that habitat isolation and long-term adaptation may have led to differentiation in morphological features and metal uptake capacity. Torpedograss possesses high tolerance to Cd-Pb toxicities, and those ecotypes with larger biomass had higher Cd-Pb accumulation capacities. Torpedograss is a potential plant species for Cd phytoremediation and approximately 16 years would be required to clean soil contained by Cd as high as 10 mg kg^?1 using the selected torpedograss ecotypes.     

Seed Banks and Species Richness Potential of Coal Slurry Ponds Reclaimed as Wetlands

Seed bank experiments are described to assess the species richness potential of coal slurry ponds reclaimed as wetlands (ranging from 6 to more than 40 years old). Experimental treatments test the drawdown and flooded conditions characteristic of the vegetation dynamics of emergent wetlands in the Upper Mississippi Valley. More seedlings, primarily annuals, emerged from exposed wet sediments (freely drained) than under continuous flooded sediments in cold ponds (339 versus 136 seedlings m^?2, respectively) and in natural ponds (163 versus 47, respectively). More seeds were produced by plants established in freely drained conditions than under flooded conditions from sediments in the coal ponds (26546 versus 1842 seeds m^?2, respectively) and the natural ponds (28430 versus 4526, respectively). Similarly, more biomass was also produced by these plants in freely drained than under flooded conditions in coal ponds (118 versus 47 g m^?2, respectively) and natural ponds (118 versus 52, respectively). Fertilization (NPK) did not affect germination for the most part, but it did affect seed set and biomass production, especially for C₄ annuals such as Echinochloa crusgalli and Panicum dichotomiflorum. I propose that lime (calcium carbonate) and fertilizer be applied during the first few scheduled drawdowns for these coal slurry ponds reclaimed as wetlands to increase the number of species and to allow their more rapid development as self-sustaining systems.     

长期淹水对河竹鞭根养分化学计量特征的影响

以长期淹水环境下能生长更新的河竹为材料,调查测定了人工喷灌供水(CK)、淹水6个月(TR)的河竹一年生竹鞭的根生物量和主要养分元素含量,分析长期淹水对河竹鞭根养分化学计量特征的影响,为河竹在水湿地和消落带植被恢复中的应用提供理论依据。结果显示:(1)与CK相比,TR处理下的河竹土中根的N、P、Mg和Ca含量显著降低,Fe含量显著升高,且N、K和Ca含量显著低于TR处理下水中根的含量,而Fe含量显著高于水中根。(2)TR处理的河竹土中根的C/N、C/P、C/K和P/K较CK显著升高,且C/K、N/K和P/K显著高于TR处理的水中根。(3)TR处理的河竹水中根的C-N、C-P、N-P均呈极显著正相关关系,土中根的C-P、C-K、P-K均呈极显著正相关关系;CK河竹土中根的C-P、C-K呈极显著正相关关系,且N-P显著相关;从相关系数看,TR处理下土中根的C-N、N-P和N-K相关性减弱,C-P、C-K和P-K相关性增强,而C-N、C-P、N-P和N-K相关性较水中根减弱,C-K和P-K相关性较水中根增强。(4)TR处理下鞭根生物量和C、N、P、K、Mg、Ca积累量较CK分别显著降低19.46%、42.04%、36.55%、41.39%、60.06%和38.46%,而Fe积累量显著升高,为CK的5.5倍;TR处理下土中根养分积累量显著高于水中根。研究表明,长期淹水虽阻碍了河竹鞭根的养分平衡吸收,但能够提高养分利用效率,并且土中根和水中根具有克隆分工特征,水中根主要起到氧气吸收应对缺氧环境胁迫的功能,是河竹适应长期淹水环境的重要生态对策。     

Flooding tolerance of Calophyllum brasiliense Camb. (Clusiaceae): morphological, physiological and growth responses

Calophyllum brasiliense Camb. (Clusiaceae) is a tree of swampy areas of the coastal “Restinga” in southeastern Brazil (a coastal sand-plain scrub and forest formation). To elucidate possible adaptive strategies that enable this species to occupy areas subjected to seasonal or perennial waterlogging, growth characters such as shoot height, biomass production, leaf expansion, new leaf development, stem diameter, carbon dioxide assimilation rate, stomatal conductance, chlorophyll concentration and fluorescence were studied in controls and plants flooded for up to 150 days. Although flooded plants kept incorporating carbon all through the experiment, their assimilation rate and growth rate were lower than control, non-flooded plants. Injuries such as leaf senescence and abscision were not observed but some flooded plants showed signs of leaf chlorosis. In view of its capacity to maintain carbon assimilation and growth during the treatment, C. brasiliense can be classified as flood-tolerant tree. Flooding induced hypertrophy of lenticels, increased stem diameter and development of adventitious roots. These characteristics of C. brasiliense are most probably responsible for its survival and success in naturally seasonally flooded areas, inhospitable environments for most tree species. Reduction in total chlorophyll concentration was probably the main cause of reduced carbon dioxide assimilation rate. Based on the results we recommend C. brasiliense for rehabilitation of native vegetation in flood-prone areas.     

Ecotypic differentiation of response to enhanced CO2 and temperature levels in Arabidopsis thaliana

Five ecotypes of Arabidopsis thaliana, from widely dispersed origins, were grown under combinations of ambient and elevated atmospheric CO₂ concentrations and ambient and elevated temperatures within solardomes. Total above-ground plant biomass was measured when the majority of plants across all ecotypes and treatments had formed seed pods. There were substantial differences in biomass between the ecotypes across all treatments. Temperature had no effect on biomass whilst CO₂ had a significant effect both alone and in interaction with ecotype. The CO₂ x ecotype interaction was mostly due to the enhancement of a single ecotype from the Cape Verde Islands.     

Ecophysiological adaptations of black spruce (<Emphasis Type="Italic"> Picea mariana</Emphasis>) and tamarack (<Emphasis Type="Italic"> Larix laricina</Emphasis>) seedlings to flooding

Black spruce [ Picea mariana (Mill.) B.S.P.] and tamarack [ Larix laricina (Du Roi) K. Koch] are the predominant tree species in boreal peatlands. The effects of 34 days of flooding on morphological and physiological responses were investigated in the greenhouse for black spruce and tamarack seedlings in their second growing season (18 months old). Flooding resulted in reduced root hydraulic conductance, net assimilation rate and stomatal conductance and increased needle electrolyte leakage in both species. Flooded tamarack seedlings maintained a higher net assimilation rate and stomatal conductance compared to flooded black spruce. Flooded tamarack seedlings were also able to maintain higher root hydraulic conductance compared to flooded black spruce seedlings at a comparable time period of flooding. Root respiration declined in both species under flooding. Sugar concentration increased in shoots while decreasing in roots in both species under flooding. Needles of flooded black spruce appeared necrotic and electrolyte leakage increased over time with flooding and remained significantly higher than in flooded tamarack seedlings. No visible damage symptoms were observed in flooded tamarack seedlings. Flooded tamarack seedlings developed adventitious roots beginning 16 days after the start of flooding treatment. Adventitious roots exhibited significantly higher root hydraulic conductivity than similarly sized flooded tamarack roots. Flooded black spruce lacked any such morphological adaptation. These results suggest that tamarack is better able to adjust both morphologically and physiologically to prolonged soil flooding than black spruce seedlings.     

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.     

Growth and water relations in a central North Carolina population of <Emphasis Type="Italic">Microstegium vimineum</Emphasis> (Trin.) A. Camus

The ability of an invasive plant to occupy new areas is often attributed to both morphological and physiological plasticities that allow them to remain viable over a wide range of environmental conditions. Studies addressing the ecological requirements of Microstegium vimineum often consider soil moisture or soil moisture along with other factors as important explanatory components for the establishment and persistence of this invasive monocot. However, controlled studies specifically targeting water relations in M. vimineum are needed. Therefore, the purpose of this study was to determine how different water availabilities influence the growth and physiological performance of M. vimineum. This study utilized experimental microcosms to achieve different water availabilities including low soil moisture (<15% water), moderate soil moisture (ca. 20–30%), and flooded conditions. While both flooded and low soil moisture resulted in diminished growth, M. vimineum still survived under these conditions. Physiological processes including C₄ metabolism, minimum stress under low water conditions, and the ability to increase tissue rigidity may confer some advantages to M. vimineum during periods of limiting water conditions. Similarly, the proportionally low root biomass, shallow root structure, and its ability to maintain stable water relations during flooding and/or soil waterlogging may facilitate M. vimineum’s ability to invade mesic habitats. It is likely, therefore, that the capacity to tolerate both low soil moistures and flooded conditions has enhanced the ability of M. vimineum to populate disturbed systems in central North Carolina.     

EFFECTS OF FLOODING AND NUTRIENT ENRICHMENT ON BIOMASS ALLOCATION IN ACER RUBRUM SEEDLINGS

A greenhouse experiment was conducted on Acer rubrum seedlings to evaluate the effects of flood frequency on production and allocation of biomass and to test the effects of N and P fertilization on production and allocation. Seedlings from the Dismal Swamp were subjected to three flood treatments (no flooding, intermittent flooding, and continuous flooding) and four enrichment treatments (no enrichment, N additions, P additions, and N + P additions). More continuous flooding resulted in less biomass production. Biomass increased during the study in all treatments except for root mass in the continuously flooded treatment. However, production of abundant adventitious roots compensated for the lack of normal root growth. Root/shoot ratios exhibited the greatest decreases in the continuously flooded plants. Plants with N + P added had significantly more leaf, stem, and total mass than the nonenriched plants four months into the study. The N + P additions had apparently compensated for the effects of flood stress in the continuously flooded plants by the end of the study. The fertilized seedlings accumulated higher concentrations of N and P, but their nutrient use efficiency (biomass production per unit nutrient absorbed) was lower than in the nonenriched plants. Acer rubrum seedlings survive flooded conditions through several adaptations; however, theirgrowth is slowed by continuous flooding.     

Targeting spatiotemporal dynamics of planktonic SAGMGC‐1 and segregation of ammonia‐oxidizing thaumarchaeota ecotypes by newly designed primers and quantitative polymerase chain reaction

The annual dynamics of three different ammonia‐oxidizing archaea (AOA) ecotypes (amoA gene) and of the SAGMGC‐1 (Nitrosotalea‐like aquatic Thaumarchaeota) group (16S rRNA gene) were studied by newly designed specific primers and quantitative polymerase chain reaction analysis in a deep oligotrophic high mountain lake (Lake Redon, Limnological Observatory of the Pyrenees, Spain). We observed segregated distributions of the main AOA populations, peaking separately in time and space, and under different ammonia concentrations and irradiance conditions. Strong positive correlation in gene abundances was found along the annual survey between 16S rRNA SAGMAGC‐1 and one of the amoA ecotypes suggesting the potential for ammonia oxidation in the freshwater SAGMAGC‐1 clade. We also observed dominance of Nitrosotalea‐like ecotypes over Nitrosopumilus‐like (Marine Group 1.1a) and not the same annual dynamics for the two thaumarchaeotal clades. The fine scale segregation in space and time of the different AOA ecotypes indicated the presence of phylogenetically close but ecologically segregated AOA species specifically adapted to specific environmental conditions. It remains to be elucidated what would be such environmental drivers.     

The mycorrhizal status of Phragmites australis in several polluted soils and sediments of an industrialised region of Northern Portugal

Roots of Phragmites australis from three polluted soils and sediments (a periodically flooded stream bank containing organic pollutants, a high-pH drying sedimentation pond and an acidic, periodically flooded sand polluted by industrial effluents) were sampled over a 1-year cycle of plant growth to assess the degree of colonisation by arbuscular mycorrhizal fungi (AMF). At the dry sedimentation pond, root samples of Juncus effusus and Salix atrocinerea were also taken to assess the presence of AMF throughout the year. Root colonisation was low (<5% root length colonised) but arbuscule presence peaked in P. australis during the spring and autumn prior to flowering. These changes in arbuscule abundance were also seen in a parallel greenhouse trial using seed taken from one of the sites. Roots of J. effusus contained mainly vesicular colonisation but arbuscule activity peaked during the winter months (December–March). S. atrocinerea roots were found to be ectomycorrhizal throughout the year but the fine feeder roots were colonised by AMF. The results confirm that semi-aquatics, like P. australis, can become arbuscular mycorrhizal but that this status changes during the year depending on soil moisture content and plant phenology. The influence of AMF in these polluted soils is uncertain but the potential exists to establish a more diverse plant ecosystem during the landscaping of these areas (phytostabilisation) by management of adapted plant and AMF ecotypes. Accepted: 6 November 2000     

Screening of giant reed (Arundo donax L.) ecotypes for biomass production under salt stress

Given its high biomass and plasticity, Arundo donax L. is a promising ligno-cellulosic crop for cultivation in marginal lands in temperate climates. In order to test for adaptation to salinity, growth parameters of several A. donax clones were evaluated under two salt regimes in hydroponics. Mild NaCl stress (50 mM NaCl, 5.6 mS cm^?1 EC, for 10 days) failed to discriminate between ecotypes, while a more severe NaCl treatment (150 mM, 18.8 mS cm^?1 EC, for 21 days) enabled the identification of ecotypes maintaining plant growth under high salinity. Among several biometric parameters, 4th leaf width, and shoot and root DW consistently highlighted differences between ecotypes. Gas-exchange parameters also responded to severe NaCl treatment, while the photosystem efficiency was good, regardless of treatment. The results confirm that A. donax can be considered moderately tolerant to NaCl stress, with variation between ecotypes. Our screening protocol identified ecotypes with higher biomass production under severe NaCl treatment and can be useful for preliminary evaluation of NaCl tolerant clones for increasing productivity under salinity. The detected inter-ecotype variability could also be investigated to identify suitable clones for different environments.     

ANATOMICAL AND METABOLIC RESPONSES TO WATERLOGGING AND SALINITY IN SPARTINA ALTERNIFLORA AND S. PATENS (POACEAE)

The effects of waterlogging and salinity (25 or 325 mol m ³ NaCl) stressors on the anatomy and metabolism of the marsh grasses 5. alterniflora Loisel. and S. patens Aiton (Muhl.) were investigated in a V factorial greenhouse experiment over 30 d. Waterlogging and salinity in combination resulted in anatomical and metabolic responses in both species. Waterlogging reduced soil redox potential and decreased root-specific gravity significantly in both species. The inadequacy of aerenchyma development under hypoxia to support aerobic root respiration in S. patens was indicated by significant increases in root alcohol dehydrogenase (ADH) activity of 1,752% and 420%, respectively, in the low and high salinity treatments. ADH activity was not increased significantly by flooding of S. alterniflora. Proline concentrations in roots and leaves were low at low salinities and increased significantly at high salinities in both species, but only under drained conditions. Decrease in leaf elongation by high salinity occurred in drained, but not flooded treatments in both species. Under flooded conditions, leaf elongation was significantly greater in S. alterniflora than S. patens. Greatest leaf elongation occurred in flooded low salinity S. alterniflora plants that had the least proline. Although both species are adapted to waterlogging and salinity, S. alterniflora appears to be more tolerant of reducing soil conditions and less responsive to higher salinity than S. patens.