Ecophysiological responses of five emergent-wetland plants to diminished water supply: an experimental microcosm study

Freshwater wetlands are fundamentally tied to hydrology as they are often found along the boundaries between terrestrial uplands and open waters. Although wetland systems are frequently prone to extended periods of flooding and exposure, the degree of water deprivation may intensify during periods of low precipitation or drought. Therefore, the purpose of this study was to evaluate plant–water relations in five emergent macrophytes (Carex alata, Juncus effusus, Peltandra virginica, Saururus cernuus, and Justicia americana) to simulated drought conditions. Weekly evaluations of tissue water content and xylem water potential (free energy of water in xylem tissues) were conducted on plants grown in experimental microcosms over a 9-week period. Plant performance was also evaluated in each species by monitoring the changes in plant biomass, leaf area, and survival. The results indicate that J. effusus and P. virginica performed better in both flooded and moderately dry conditions, and plants that maintained higher water content in water logged soils (i.e., J. americana) were less tolerant to drying conditions. This study also illustrates the importance of periodic water withdrawal on plant performance. In general, plants that were subjected to both flooded and dry conditions responded better physiologically than plants that were either continuously flooded or received extended droughts (≥4 weeks). Therefore, provided the duration of water deficit is not extensive, short periods of water withdrawal can enhance the performance and water relations in some emergent-wetland plant species.     

Latitudinal variation in plant size and relative growth rate in Arabidopsis thaliana

Latitude is an important determinant of local environmental conditions that affect plant growth. Forty ecotypes of Arabidopsis thaliana were selected from a wide range of latitudes (from 16°N to 63°N) to investigate genetic variation in plant size and relative growth rate (RGR) along a latitudinal gradient. Plants were grown in a greenhouse for 31 days, during which period three consecutive harvests were performed. Plants from high latitudes tended to have smaller plant size in terms of seed size, cotyledon width, rosette size, number of rosette leaves, size (leaf area) of the largest leaves, total leaf area, and total dry weight per plant than those from low latitudes. The mean (±SE) RGR across ecotypes was 0.229 (±0.0013) day⁻¹. There was, however, significant ecotypic variation, with RGR being negatively correlated with latitude. The two main components of RGR, leaf area ratio (LAR) and unit leaf rate (ULR), were also correlated with latitude: LAR increased with increasing latitude while ULR decreased with increasing latitude. It was also found that RGR tended to be negatively correlated with LAR, specific leaf area (SLA) and specific root length (SRL) but to be positively correlated with mean area per leaf (MAL) and ULR. The variation in RGR among ecotypes was relatively small compared with that in the other traits. RGR may be a conservative trait, whose variation is constrained by the trade-off between its physiological (i.e. ULR) and morphological (i.e. LAR) components. Received: 2 November 1997 / Accepted: 28 February 1998     

Responses to air temperature and soil moisture of growth of four dominant species on sand dunes of central Inner Mongolia

Little attention has been paid to how four dominant shrub species distributed in semi-arid areas respond to the combined effects of temperature and water supply. Seedlings of four species were grown in a glasshouse for eight weeks at air temperatures of 12.5/22.5, 15/25, 17.5/27.5, and 20/30°C (night/day) and with water supplies of 37.5, 75, 112.5, and 150 mm per month. When temperatures were 17.5/27.5 and 20/30°C relative growth rate (RGR) decreased for Artemisia ordosica, A. sphaerocephala, and Hedysarum laeve but not for Caragana korshinskii. RGR increased with increasing water availability for all four species and most treatments. In response to changing water availability, the RGR tended to correlate mainly with the physiological trait (net assimilation rate, NAR) and with dry matter allocation traits (below-ground to above-ground dry matter and leaf mass ratio). A higher ratio of below to above-ground dry matter for all four species under most treatments (0.3–1.7) and water-use efficiency (1.4–9.2 g kg⁻¹) may explain how all four species survive drought. Higher temperatures may be harmful to A. ordosica and A. sphaerocephala, under current precipitation levels (average 75 mm per month from mid-June to mid-August). These findings support the proposal that A. ordosica mixed with C. korshinskii will prove optimal for re-vegetation of degraded areas of the Ordos plateau. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Different combinations of morpho‐physiological traits are responsible for tolerance to drought in wild tomatoes Solanum chilense and Solanum peruvianum

Herbaceous species can modify leaf structure during the growing season in response to drought stress and water loss. Evolution can select combinations of traits in plants for efficient water use in restricted environments. We investigated plant traits that mediate adaptation and acclimation to water stress in two herbaceous drought‐tolerant species. Anatomical, morphological and physiological traits related to stems and leaves were examined under optimal watering (OW) and a long period of restricted watering (RW) in 11 accessions from three Solanaceae species (Solanum chilense, S. peruvianum and S. lycopersicum). The relationships between these traits were tested using linear regression and PCA. There were significant differences in anatomical traits between the species under both OW and RW, where leaf area correlated with stem diameter. Proline and total carbohydrates accumulated highly in S. chilense and S. peruvianum, respectively, and these osmolytes were strongly correlated with increased osmotic potential. Stomatal density varied between species but not between acclimation treatments, while stomatal rate was significantly higher in wild tomatoes. There was a strong positive relationship between stem growth rate and a group of traits together expressed as total stomatal number. Total stomata is described by integration of leaf area, stomatal density, height and internode length. It is proposed that constitutive adaptations and modifications through acclimation that mediate RW play an important role in tolerance to drought stress in herbaceous plants. The capacity for growth under drought stress was not associated with any single combination of traits in wild tomatoes, since the two species differed in relative levels of expression of various phenotypic traits.     

Physiological control of leaf methane emission from wetland plants

The transport of methane from the rhizosphere to the atmosphere takes place in the intercellular spaces and stomata of wetland plants, and foliar gas exchange is one of the critical steps of the transport process. The objectives of our research were to investigate: (i) variation in foliar gas exchange among four common wetland plant species (i.e., Peltandra virginica L., Orontium aquaticum L., Juncus effusus L., and Taxodium distichum L.), (ii) the role of key environmental factors (i.e., light, temperature, and carbon dioxide concentration) in controlling foliar methane emission, and (iii) physiological mechanisms underlying the variation in methane emission due to species and the environment. Experiments were conducted in an instantaneous, flow-through gas-exchange system that operated on a mass balance approach and concurrently measured foliar fluxes of methane, water vapor, and carbon dioxide. The chamber system allowed for the control of light, temperature, humidity, and carbon dioxide concentration. Diel patterns of methane emission varied among species, with daylight emissions from P. virginica and O. aquaticum 2–4 times those of J. effusus and T. distichum in saturating light. Foliar methane emission from P. virginica (1.80 μmol m⁻² s⁻¹) under ambient daylight conditions was an order of magnitude higher than that of the other three species (∼0.20 μmol m⁻² s⁻¹). As leaf temperature was increased by 10 °C, methane emission increased by a factor of 1.5–2.2, and the temperature effect was independent of stomatal conductance. When data were pooled among the four species, varying the light and carbon dioxide concentrations in a stepwise manner produced changes in foliar methane emission that were associated with stomatal conductance (r² = 0.52). To scale our observations to other wetland plant species, a stepwise multiple regression model is offered that incorporates stomatal conductance and net carbon dioxide assimilation to estimate instantaneous methane emission from foliar surfaces. The model indicates that changes in stomatal conductance affect methane emission three times more than equivalent changes in net carbon dioxide assimilation.     

Changes in growth and photosynthetic patterns of oil palm (<Emphasis Type="Italic">Elaeis guineensis</Emphasis> Jacq.) seedlings exposed to short-term CO<Subscript>2</Subscript> enrichment in a closed top chamber

Three varieties of oil palm seedlings (Deli Yangambi, Deli Urt, Deli AVROS) were exposed to three levels of CO₂ (400, 800, 1,200 μmol/mol) in split plot design to determine growth (net assimilation rate, NAR; relative growth rate, RGR) and photosynthetic patterns of the seedlings under short-term CO₂ exposure of 15 weeks. Increasing CO₂ from 400 to 800 and 1,200 μmol/mol significantly enhanced total biomass and leaf area, net photosynthesis (A) and water use efficiency (WUE) especially from weeks 9 to 15. By the end of week 15, total biomass increased by 113%, and A and WUE by one- and fivefold, respectively, while specific leaf area decreased by 37%. Both enhanced biomass and A under elevated CO₂ were effective in modifying NAR and RGR as shown by high correlation coefficient values (r ² = 0.68 and 0.72; r ² = 0.63 and 0.67, respectively), although WUE seemed to have more influence over the NAR (r ² = 0.97) and RGR (r ² = 0.93). Neither interspecific preference nor its interaction with CO₂ imposed any significant effect on parameters observed. Growth improvement with CO₂ seemed able to produce healthy, bigger and vigorous oil palm seedlings, and the technique may have potential to be developed for use to reduce nursery period.     

不同种植模式下重金属污染对牧草叶片光合特性和重金属含量的影响

为探索不同种植模式下重金属污染对牧草生理特性的影响,皇竹草(Pennisetum sinese)、黑麦草(Lolium perenne)和龙须草(Juncus effusus)采用单作和间作模式,研究了重金属污染对其叶片光合参数、光合色素和重金属含量的影响。结果表明,与单作相比,间作下皇竹草叶片的叶绿素(Chl)a、Chla+b和类胡萝卜素(Car)含量极显著上升,净光合作用速率(P_n)、气孔导度(G_s)、细胞间隙CO_2浓度(C_i)和Chl b含量极显著下降,但蒸腾速率(Tr)的差异不显著;黑麦草和龙须草叶片的C_i、Car含量极显著增加,P_n、T_r、G_s、Chl a、Chl b和Chl a+b显著或极显著下降(龙须草Chl a除外)。除龙须草的铜含量极显著降低外,3种牧草叶片和总生物量和重金属含量均极显著上升。与对照相比,低浓度污染的皇竹草叶片的光合参数、光合色素含量及叶片和总生物量极显著提高,而黑麦草和龙须草叶片的P_n、T_r、G_s、C_i、叶片和总生物量极显著减少,光合色素含量极显著增加,但黑麦草的T_r、C_i、叶片生物量及黑麦草和龙须草的Chl b差异不显著;重金属污染使3种牧草叶片的重金属含量均极显著上升。种植模式×重金属污染互作对牧草叶片的光合参数、光合色素含量、干质量及重金属含量的影响均极显著。因此,在复合污染土壤间作3种牧草有利于叶片干物质的积累和重金属的吸收,单作更有利于提高叶片的光合作用能力;且皇竹草对重金属污染的适应性强于黑麦草和龙须草。     

荒漠绿洲过渡带一年生草本植物对干旱胁迫的响应

选取河西走廊荒漠绿洲过渡带典型一年生草本植物雾冰藜（Bassia dasyphylla）、虎尾草（Chloris virgata）和狗尾草（Setaria viridis）为研究对象,设置5个水分梯度（正常水分（CK）,轻度干旱（5d）、中度干旱（10d）、重度干旱（15d）,重度干旱（15d）复水）,分析了3种一年生草本植物生理和形态等性状对干旱胁迫的响应。结果表明：一年生草本植物可以通过生理反应（渗透调节）适应轻度和中度干旱胁迫,而通过个体形态来适应重度干旱胁迫。在轻度和中度干旱处理下,一年生草本植物通过调控叶片渗透调节物质脯氨酸、可溶性蛋白和可溶性糖维持叶片渗透压,提高保水能力,叶绿素含量增加,使丙二醛含量维持在较低的水平,同时,根系活力增强,有效促进了根系水分吸收;而在重度胁迫下,渗透调节物质作用降低,丙二醛含量迅速增加,导致可溶性蛋白含量下降,叶绿素分解加速,植物生长受到抑制,在有限的生物量下,一年生草本植物主要通过根系伸长、根长与茎长的比增加和减小茎长来适应重度干旱胁迫,最终导致了种子百粒重和结种数量下降。     

Breaking dormancy during flood and drought: sublethal growth and physiological responses of three emergent wetland herbs used in bioretention basins

Bioretention basins are man-made topographic depressions designed to collect and retain surface water runoff. In most cases these basins are used to prevent flooding and/or remove environmentally harmful pollutants and sediments from entering natural aquatic systems. To maximize environmental benefits, these systems are often planted with flood-tolerant wetland hydrophytes that are capable of withstanding extended periods of drought. Unfortunately, little is known about how these plants respond to extreme hydrologies while breaking seasonal dormancy. The purpose of this study was to better understand the morphological and physiological responses of three wetland species (Pontederia cordata L., Saururus cernuus L., and Schoenoplectus tabernaemontani C.C. Gmel. Palla) often used in bioretention basins while emerging from dormancy in either flooded or drought conditions. Results indicate that only S. tabernaemontani was affected morphologically by drought with lower leaf area and aboveground biomass. While significant reductions in stomatal surface indices were also observed in drought-treated S. tabernaemontani, all three species had reductions in stomatal conductance (g) when grown in drier soils. Moreover, drought conditions promoted decreases in leaf water potential (Ψ _leaf) for all three species, and reductions in tissue water content (θ) for P. cordata and S. tabernaemontani. Based on the overall morphological and physiological responses, S. cernuus maintained the lowest productivity, and appeared to be the best suited for tolerating sustained soil water deficits. If high plant productivity is desired, however, S. tabernaemontani was able to maintain high plant growth while making necessary modifications that facilitated greater drought-resistance.     

Physiological and anatomical changes induced by drought in two olive cultivars (cv Zalmati and Chemlali)

Photosynthetic gas exchange, vegetative growth, water relations and fluorescence parameters as well as leaf anatomical characteristics were investigated on young plants of two Olea europaea L. cultivars (Chemlali and Zalmati), submitted to contrasting water availability regimes. Two-year-old olive trees, grown in pots in greenhouse, were not watered for 2 months. Relative growth rate (RGR), leaf water potential (Ψ_LW) and the leaf relative water content (LWC) of the two cultivars decreased with increasing water stress. Zalmati showed higher values of RGR and LWC and lower decreased values of Ψ_LW than Chemlali, in response to water deficit, particularly during severe drought stress. Water stress also caused a marked decline on photosynthetic capacity and chlorophyll fluorescence. The net photosynthetic rate, stomatal conductance, transpiration rate, the maximal photochemical efficiency of PSII (F _v/F _m) and the intrinsic efficiency of open PSII reaction centres (F′ _v/F′ _m) decreased as drought stress developed. In addition, drought conditions, reduced leaf chlorophyll and carotenoids contents especially at severe water stress. However, Zalmati plants were the less affected when compared with Chemlali. In both cultivars, stomatal control was the major factor affecting photosynthesis under moderate drought stress. At severe drought-stress levels, the non-stomatal component of photosynthesis is inhibited and inactivation of the photosystem II occurs. Leaf anatomical parameters show that drought stress resulted in an increase of the upper epidermis and palisade mesophyll thickness as well as an increase of the stomata and trichomes density. These changes were more characteristic in cv. ‘Zalmati’. Zalmati leaves also revealed lower specific leaf area and had higher density of foliar tissue. From the behaviour of Zalmati plants, with a smaller reduction in relative growth rate, net assimilation rate and chlorophyll fluorescence parameters, and with a thicker palisade parenchyma, and a higher stomatal and trichome density, we consider this cultivar more drought-tolerant than cv. Chemlali and therefore, very promising for cultivation in arid areas.     

An assessment of vegetation and environmental controls in the 1970s of the Mesopotamian wetlands of southern Iraq

A vast ecosystem of wetlands and lakes once covered the Mesopotamian Plain of southern Iraq. Widespread drainage in the 1990s nearly obliterated both components of the landscape. This paper reports the results of a study undertaken in 1972–1975 on the vegetation of the wetlands prior to drainage and provides a unique baseline for gauging future restoration of the wetland ecosystems in Mesopotamia. Five representative study sites were used to assess the flora, three of which were wetlands. A total of 371 plant species were recorded in the five sites, of which approximately 40% represent obligate or facultative wetland species. The wetland vegetation was classified into five major physiognomic forms (submerged, floating, herbaceous tall emergent, herbaceous low emergent and woody low emergent), which was further subdivided into 24 fresh and halophytic communities. Water levels greatly fluctuated across the different types of wetlands, and mean surface water depth ranged from below to greater than 2 m above the sediment surface, reflecting permanently, seasonally or intermittently wet habitats. Aboveground biomass was also highly variable among the communities. The Phragmites australis community, which was the most extensive community type, had the greatest biomass with an average value of approximately 5,000 g m⁻² in summer. Distribution and community composition were largely controlled by water levels and saline-freshwater gradients. Canonical correspondence analysis showed that salinity and water depth were the most important factors to explain species distribution. Environmental variables related to soil salinity separated halophytic species in woody low emergent and herbaceous low emergent forms (Tamarix galica, Cressa cretica, Alhagi mannifera, Aeluropus lagopoides, Juncus rigida, and Suaeda vermiculata) from other species. Their habitats were also the driest, and soil organic matter content was lower than those of other species. Habitats with deepest water were dominated by submerged aquatic and floating leaved species such as Nymphoides peltata, Ceratophyllum demersum, and Najas armata. Such diverse environmental conditions in the Mesopotamian wetland would be greatly affected by evapotranspiration, river water inputs from north, ground water inputs, local soil conditions, and a tide or seiche-controlled northward transgression of water from the Gulf. These environmental conditions should be considered in restoration plans if plant communities existed in the mid-1970s are to be part of the desired restoration goals. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cyclic electron flow,NPQ and photorespiration are crucial for the establishment of young plants of Ricinus communis and Jatropha curcas exposed to drought

• Although plant physiological responses to drought have been widely studied, the interaction between photoprotection, photorespiration and antioxidant metabolism in water‐stressed plants is scarcely addressed.
• This study aimed to evaluate the physiological adjustments preserving photosynthesis and growth in two plant species with different tolerance to drought: Jatropha curcas and Ricinus communis. We measured stress indicators, gas exchange, photochemistry of PSII and PSI, antioxidant enzymes, cyclic electron flow and photorespiration.
• Physiological stress indicators associated with reduction in growth confirmed R. communis as sensitive and J. curcas as tolerant to drought. Drought induced loss of photosynthesis in R. communis, whereas J. curcas maintained higher leaf gas exchange and photochemistry under drought. In addition, J. curcas showed higher dissipation of excess energy and presented higher cyclic electron flow when exposed to drought. Although none of these mechanisms have been triggered in R. communis, this species showed increases in photorespiration. R. communis displayed loss of Rubisco content while the Rubisco relative abundance did not change in J. curcas under drought. Accordingly, the in vivo maximum Rubisco carboxylation rate (V_cmax) and the maximum photosynthetic electron transport rate driving RuBP regeneration (J_max) were less affected in J. curcas. Both species displayed an efficient antioxidant mechanism by increasing activities of ascorbate peroxidase (APX) and superoxide dismutase (SOD).
• Overall, we suggest that the modulation of different photoprotective mechanisms is crucial to mitigate the effects caused by excess energy, maintaining photosynthetic apparatus efficiency and promoting the establishment of young plants of these two species under drought.

     

MORPHOMETRIC VARIATIONS OF FIVE TIDAL MARSH HALOPHYTES ALONG ENVIRONMENTAL GRADIENTS

Morphological characteristics of Deschampsia cespitosa, Grindelia integrifolia, Distichlis spicata, Jaumea carnosa, and Salicornia virginica varied significantly along transects between the upper and lower portions of an Oregon salt marsh. Plant height, height of the flowering shoot, leaf number and width, internode length, branching, and stem diameter varied with position in the marsh. Anatomical differences in stem structure were also obvious. Lignification of vascular bundles of D. spicata, the amount of secondary xylem in G. integrifolia, and vascular bundle size in J. carnosa were greatest in the upper marsh zone. The quantity of aerenchymatous tissue in S. virginica was greatest at the lower, wetter end of the transect. Reproductive effort was greatest at the upper distributional limit of G. integrifolia and D. spicata while greatest in J. carnosa at its lower limit. Soil moisture and soil chemical data were collected to relate morphometric variation to environmental parameters. Differences in plant structure may have application as an aid in determining wetland boundaries.     

Leaf optical properties during and after drought stress in triticale and maize genotypes differing in drought tolerance

The effect of a short (7 days) and prolonged (14 days) soil drought (D) on leaf optical properties (R reflectance, T transmittance and A absorbance) in PAR and NIR range of irradiation, and on changes in leaf water potential (ψ), leaf injury index (LI), leaf thickness (LT) and chlorophyll (a + b) content (Chl) was studied for maize and triticale genotypes differing in drought tolerance. Under control conditions (C) leaves of maize in comparison to triticale were better hydrated, were thicker and had higher content of chlorophyll (a + b). In non-stressed plants, small differences were observed in measurements of R, T and A. In the range from 500 to 600 nm, the differences between D-resistant and D-sensitive were observed only in transmittance (T) and in range from 700 to 1,100 nm in absorbance (A). In genotypes belonging to the group of D-sensitive T in PAR range and A in NIR range were two times higher than in D-resistant ones. However, in NIR range R for D-sensitive genotypes was lower than for D-resistant ones. The drought stress caused the decrease in ψ, Chl, LT and the increase in leaf injury index (LI). Soil drought applied within 14 days caused larger changes in these physiological characters in comparison to 7 days drought. The observed harmful influence of drought was more visible for maize than triticale. Moreover for genotypes belonging to D-sensitive ones, changes were larger than for D-resistant ones. Similar to changes in ψ, LT and LI drought stress caused changes in leaf optical properties parameters R, A and T. In the PAR range, the highest changes were observed in R, whereas changes in T and A, which were not considerable. Both in maize and triticale, increase in R was higher in plants subjected to 14 days drought than in plants exposed to drought for 7 days. In maize, increase in R was larger for D-sensitive genotype. For both species, changes in T and A of PAR range were small. In NIR range, an increase in R and A, and decrease in T were observed. After 7 days of recovery in plants subjected to shorter period of drought significant differences were still visible in most cases. The same was observed for ψ, LT, LI and Chl parameters. It shows that the period of 7 days rehydration is too short to remove the injuries caused by drought stress. This results indicate that measurements of R, T and A might be useful in practical application for the estimation of the drought tolerance level. Some limitations in the practical application for plant breeding may be caused by relatively high cost of necessary equipment.     

Enhanced drought tolerance in transgenic Leymus chinensis plants with constitutively expressed wheat TaLEA3

Leymus chinensis is an important grassland perennial grass. However, its drought tolerance requires to be improved. LEA (late embryogenesis abundant) genes are believed to confer resistance to drought and water deficiency. Using Agrobacterium-mediated transformation, a wheat LEA gene, TaLEA ₃ , was integrated into L. chinensis. The transgenic lines showed enhanced growth ability under drought stress during which transgenic lines had increased the relative water content, leaf water potential, relative average growth rate, but decreased the malondialdehyde content compared with the non-transgenic plant. Thus, transgenic breeding is an efficient approach to enhance drought tolerance in L. chinensis.     

Components of relative growth rate and sensitivity to nitrogen availability in annual and perennial species of Bromus

Summary Two grass species, the annual Bromus sterilis and the perennial Bromus erectus, were grown from seeds for 28 days in a hydroponic culture system at 1 and 100 M NO₃ ^- in the nutrient solution. At 100 M NO₃ ^-, the relative growth rate (RGR) of the perennial was 30% lower than that of the annual. This was only the consequence of the higher specific mass of its leaves, since its leaf mass ratio was higher than that of the annual and the unit leaf rates (ULR), calculated on an area basis, were similar for the two species. At 1 M, the RGR of the annual was 50% lower than at 100 M, while that of the perennial was not significantly lower. This was due mainly to a lower ULR for the annual. while for the perennial ULR was the same in both treatments. These differences between the two species were all the more striking in that the differences in total nitrogen concentrations and nitrate reductase activities between the two treatments were very similar for both species. These different responses together with differences in the nitrogen productivity of the two species suggest that the level of nutrient availability may play an important role in the distribution of these Bromus species in natural habitats. Scope: Components of growth and response to nitrate availability in annual and perennial grasses     

Ecological indicators of nutrient enrichment, freshwater wetlands, Midwestern United States (U.S.)

Vegetation and soil indicators of nutrient condition were evaluated in 30 wetlands, 10 each in 3 Nutrient Ecoregions (NE) (VI-Corn Belt and Northern Great Plains, VII-Mostly Glaciated Dairy Region, IX-Temperate Forested Plains and Hills) of the Midwestern United States (U.S.) to identify robust indicators for assessment of wetland nutrient enrichment and eutrophication. Nutrient condition was characterized by surface water inorganic N (NH₄-N, NO₃-N) and P (PO₄-P) concentrations measured seasonally for 1 year, plant available and total soil N and P, and aboveground biomass, leaf N and P and species composition of emergent vegetation measured at the end of the growing season. Aboveground biomass, nutrient uptake and species composition were positively related to surface water NH₄-N (N) but not to PO₄-P or NO₃-N. Aboveground biomass and biomass of aggressive species, Typha spp. plus Phalaris arundinacea, increased asymptotically with surface water N whereas leaf P, senesced leaf N and senesced leaf P increased linearly with N. And, species richness declined with surface water N. Soil total P was positively related to surface water PO₄-P but it was the only soil indicator related to wetland nutrient condition. Individual regressions for each NE generally were superior to a single regression for all NEs. In NE VI (Corn Belt), few indicators were related to surface water N because of the high degree of anthropogenic disturbance (85% of the landscape is cleared) as compared to NEs VII and IX (24–53% cleared). Of the indicators evaluated, stem height (r² = 0.42 for all NEs, r² = 0.56 for NE VII + IX) and percent biomass of aggressive species, Typha spp. plus Phalaris, (r² = 0.46 for all NEs, r² = 0.54 for NE VII + IX), were the best predictors of wetland nutrient enrichment. Vegetation-based indicators are a promising tool for assessment of wetland nutrient condition but they may not be effective in NEs where landscape disturbance is intense and widespread.     

Stress responses in Salix gracilistyla cuttings subjected to repetitive alternate flooding and drought

To determine the tolerance of Salix gracilistyla to repetitive alternate flooding and drought, we measured leaf stomatal conductance, pre-dawn water potential, osmotic adjustment, and biomass production under greenhouse conditions. We used a control and nine crossed treatments (F1-D1–F3-D3) in which we combined 1-, 2-, or 3-week floodings (F) and droughts (D). Leaf stomatal conductance was lowest in 3 weeks of flooding or drought when the preceding event (flood or drought) was also of a 3-week duration. Leaf pre-dawn water potential was reduced in 3 weeks of drought when preceded by 2 or 3 weeks of flooding. Cuttings had slight osmotic adjustments in repetitions of long floodings and droughts. During longer durations of drought in crossed experiments, plants had low root and shoot mass, few hypertrophic lenticels, and reduced leaf mass; when flooding duration increased in crossed experiments, root mass was reduced, there were more hypertrophic lenticels, and the leaf area was reduced. Cuttings achieved stress tolerance by inhibition of transpiration, osmotic adjustment, reduction of transpiration area, and development of hypertrophic lenticels. Stress tolerance was weak when repetitive 2- or 3-week floodings were combined with 3-week droughts. The duration of flooding and drought periods under which S. gracilistyla achieves stress tolerance may be critical in determining distributions along riverbanks.     

Plant distribution can be reflected by the different growth and morphological responses to water level and shade in two emergent macrophyte seedlings in the Sanjiang Plain

In the Sanjiang Plain (North East China), narrowleaf small reed (Deyeuxia angustifolia) usually distributes widely in typical meadow or marsh, while reed (Phragmites australis), the concomitant species, is distributed sparsely in the D. angustifolia communities or relative open sites. To date, the mechanisms responsible for their different distribution patterns are far from clear. Both water level and light are important factors determining plant distribution in wetland ecosystems and therefore, the aim of this paper was to identify the role of these two factors and their potential interaction on plant distribution in this plain. Growth responses and biomass allocation of the two macrophytes were investigated by growing them in three irradiances (300, 100, 20 μmol m⁻² s⁻¹) and two water levels (0 and 5 cm) under greenhouse conditions. Biomass accumulation, mean relative growth rate (RGR), height and mean relative elongation rate (RER) of both species significantly decreased with the reduction of light availability. Biomass accumulation, RGR, height and RER of P. australis were significantly inhibited by higher water level. However, water level had no effect on the growth of D. angustifolia. Stem mass fraction was higher at 0-cm water level in D. angustifolia, and was not affected by water level in P. australis. These data suggest that D. angustifolia has a higher adaptive ability to acclimate to flooding and shade stresses than does P. australis, which might be an important reason for their different distribution patterns.     

Decomposition of standing litter of the freshwater emergent macrophyte Juncus effusus

1. Standing dead plant litter of emergent macrophytes frequently constitutes a significant fraction of the detrital mass in many freshwater wetland and littoral habitats. Rates of leaf senescence and decomposition of the emergent macrophyte Juncus effusus were examined in a small freshwater wetland in central Alabama, U.S.A. Juncus effusus leaves in the initial stages of senescence were tagged in random plant tussocks and monitored periodically to determine in situ rates of leaf senescence and death. Fully senescent leaves were collected, placed in litter bags, and suspended above the sediments to simulate standing dead decay conditions. Litter bags were periodically retrieved over 2 years and analysed for weight loss, litter nutrient contents (N, P), associated fungal biomass and fungal taxa. 2. Senescence and death of J. effusus leaves proceeds from the leaf tip to the base at an exponential rate. The rate of senescence and death of leaf tissue increased with increasing temperatures. Plant litter decomposition was slow (k = 0.40 yr^–1), with 49% weight loss observed in 2 years. Both the nitrogen (N) and phosphorus (P) concentration (%) of litter increased during decomposition. However, the total amount of nitrogen (mg) in litter bags remained stable and phosphorus increased slightly during the study period. 3. Fungal biomass associated with plant litter, as measured by ergosterol concentrations, varied between 3 and 8% of the total detrital weight. Values were not significantly different among sampling dates (P > 0.05, ANOVA, Tukey). Fungi frequently identified on decaying litter were Drechslera sp., Conioscypha lignicola (Hyphomycetes), Phoma spp. (Coelomycetes), Panellus copelandii and Marasmiellus sp. (Basidiomycota). 4. These results support previous findings that plant litter of emergent macrophytes does not require submergence or collapse to the sediment surface to initiate microbial colonization and litter decomposition.