Partial Submergence Research Articles

HYPERCRITICAL ACCRETION ONTO A NEWBORN NEUTRON STAR AND MAGNETIC FIELD SUBMERGENCE

We present magnetohydrodynamic numerical simulations of the late post-supernova hypercritical accretion to understand its effect on the magnetic field of the new-born neutron star. We consider as an example the case of a magnetic field loop protruding from the star's surface. The accreting matter is assumed to be non magnetized and, due to the high accretion rate, matter pressure dominates over magnetic pressure. We find that an accretion envelope develops very rapidly and once it becomes convectively stable the magnetic field is easily buried and pushed into the newly forming neutron star crust. However, for low enough accretion rates the accretion envelope remains convective for an extended period of time and only partial submergence of the magnetic field occurs due to a residual field that is maintained at the interface between the forming crust and the convective envelope. In this latter case, the outcome should be a weakly magnetized neutron star with a likely complicated field geometry. In our simulations we find the transition from total to partial submergence to occur around dotM ~ 10 M_sun/yr. Back-diffusion of the submerged magnetic field toward the surface, and the resulting growth of the dipolar component, may result in a delayed switch-on of a pulsar on time-scales of centuries to millenia.

Low-energy treatment of colourant wastes using sponge biofilters for the personal care product industry

Four trickling biofilter designs were assessed as low-energy alternatives to aerobic activated sludge (AS) for the treatment of personal care product industry wastes. The designs included partially submerged packed-media and sponge reactors with and without active aeration. Partial submergence was used to reduce active aeration needs. Simulated colourant wastes (up to COD=12,480mg/L, TN=128mg/L) were treated for 201days, including wastes with elevated oxidant levels. COD and TN removal efficiencies were always >79% and >30% (even without aeration). However, aerated sponge reactors consistently had the highest removal efficiencies, especially for TN (∼60%), and were most tolerant of elevated oxidants. This study shows sponge biofilters have great potential for treating colourant wastes because they achieve high treatment efficiencies and reduce energy use by >40% relative to AS systems.

The effects of submergence on anatomical, morphological and biomass allocation responses of tropical grasses Chloris gayana and Panicum coloratum at seedling stage

Submergence is a major factor affecting seedling recruitment in lowland grassland ecosystems. Our aim was to evaluate the tolerance to increasing flooding intensity of the seedlings of tropical grasses Chloris gayana K. and Panicum coloratum L., whose use as a forage species is increasing in humid grasslands. For this purpose, 2-week-old seedlings of C. gayana and P. coloratum were subjected to control, partial submergence (PS) and complete submergence (CS) in clear water for 14 days and allowed to grow for a subsequent 12-day period to assess their recovery. The following responses were assessed: generation of root aerenchyma, morphological changes and emergence from water, biomass allocation in relation to plant size, and biomass accumulation. Results showed that constitutive root aerenchyma was high in both species. Under PS and CS, root aerenchyma increased by up to 50–55% in C. gayana and up to 40–48% in P. coloratum. Under PS, the increase in seedling height for both species was the same as for controls. Under CS, C. gayana further increased its height and emerged more quickly from water; P. coloratum was not able to increase its height, and therefore the seedlings always remained underwater. The escape-from-water response of C. gayana was associated with preferential biomass allocation towards shoots and with a marked lengthening of leaf blades. By contrast, there was no change in allocation in P. coloratum, and its leaves were shorter under CS. The final biomass of C. gayana under CS was similar to that under PS, and equivalent to 54% of its controls. In P. coloratum, biomass under PS and CS were 64 and 21% of its controls (respectively), which indicates that injury caused by CS persisted during the post-submergence period. In conclusion, both species are tolerant to PS at the seedling stage. However, when flood depth increases by submerging the seedlings, C. gayana is able to escape from water while P. coloratum is not, thus strongly affecting its recovery. Therefore, C. gayana appears to be a more promising species for cultivation in lowland grasslands prone to flooding of unpredictable intensity.

Internode elongation pattern and differential response of rice genotypes to varying levels of flood water.

Rice plants are damaged during floods by complete or partial submergence. The pattern of expression of rice plant varies with different flood regimes. Accordingly, the morphological and physiological responses of rice genotypes were studied in field and pot experiments under different flood regimes. Wide genetic base lines of Oryza sativa and sub 1 introgressed submergence tolerant were compared in field experiments using principle component analysis. Further, based on internode elongation pattern, two genotypes - Bodikaburi and Pokkali - were selected for a pot experiment to find more on elongation pattern of internodes under different flood regimes. Short-term submergence in minimal water, elongation of blade, sheath, leaf area, DW, number of nodes in primary stem and survival present showed strong positive relationship with shoot length. Number of tillers, leaf area and DW were more affected and decreased during submergence than its non-submergence counterpart. Under different flooding regimes, cvv Bodikaburi and Pokkali exhibited different behaviour in the elongation pattern of the shoot. Partial submergence accelerated elongation of all internodes whereas complete submergence accelerated the top internode. Enhancement of shoot elongation during submergence in water is an escape strategy adopted by rice to resume aerobic metabolism and to improve carbon fixation. This escape strategy varies among genotypes with respect to different flooding regimes. Therefore, breeders should be aware of breeding of rice plants with suitable architecture for different flood- prone environments.

Colonization of substrates: Vendian sedentary benthos

Studies of fossils sampled from a single fossiliferous layer (monotopic series) and a summary of previous data on taphonomy and morphology of Vendian macroorganisms suggest that most strategies of attachment of sedentary benthos were formed in the Vendian. These included: (1) free living with possible organic gluing; (2) fixation on the substrate using sucker-like structures and various incrustations; (3) fixation of the basal parts by shallow submergence in the sediment; (4) anchoring into the sediment with discoidal and rhizoid-like holdfasts; and (5) partial submergence and infaunal lifestyle. The most widespread fixation in the Vendian was the attachment by discoid organs, which can be interpreted as symbiotrophic structures. Symbiotrophy in Vendian organisms submerged in the anoxic sediment can apparently be supported by indirect evidence: structural diversity of basal organs, their complex morphology, large area of contact with substrate, unlimited isometric growth, predominant fossilization of attachment discs; inconsistent systematization of organisms.

Plant adaptations and microbial processes in wetlands

Wetlands are terrestrial ecosystems characterized by high and fluctuating water tables. The spatial and temporal differences in the degree to which wetland soils are waterlogged create a very dynamic soil environment with, on average, lower oxygen concentrations than unsaturated soils. Wetland soils are characterized by gradients in redox conditions from totally oxidized to extremely reduced. These conditions require special adaptations for the plant and microbial species in the wetland. Wetland plants, particularly in wetlands with strongly fluctuating water tables, need adaptations to the shortage of oxygen in the root zone, but also to extended periods of dry conditions during low-water phases. Microbial communities are strongly coupled to redox cycles, in which alternative electron acceptors are used, for example nitrate, iron, manganese, sulphate and carbon dioxide. Plant–microbe interactions add to the complexity of the functioning of the wetland soil system. This Highlight section of Annals of Botany addresses the adaptations of plants and microbes in wetlands with fluctuating water levels, following a broad perspective. It contains three review papers and three primary scientific papers with experiments and field comparisons. The review by Parolin et al. gives an overview of adaptations of wetland plants in Amazonian floodplains to drought. The annual water cycle in these wetlands is quite predictable but shows an amplitude of 8 m, giving rise to a long flooded period as well as a long dry period each year. Previously, most emphasis in the evaluation of plant adaptations in these environments had been on the wet phase, focusing on the difficulty of overcoming long periods of waterlogged soil and (partial) submergence of the above-ground parts of plants. This paper for the first time evaluates plant adaptations to the dry period, in the context of the total annual cycle. The review paper by Laanbroek investigates how methane emission from wetland systems is controlled by microbial processes and influenced by wetland vegetation. The complex coupling of the various microbially mediated redox cycles, the leakage of oxygen from the internal plant gas system and the ‘escape’ of methane through this gas system to the atmosphere are evaluated. The review by Verhoeven and Setter deals with human control of water levels in wetlands to promote agricultural use. The emphasis is on the difference between sustainable and non-sustainable uses of wetlands for agriculture. Drainage of peatlands has led to severe soil subsidence and greenhouse gas emissions, whereas floodplains and rice fields are suitable for a more sustainable agricultural use. Developments in crop science, leading to increasingly waterlogging-tolerant crops are also discussed in this context. The paper by Sorrell and Hawes evaluates the importance of convective gas flow in the lacunae of helophytes for the occupation of relatively deep-water habitats. In their study, plant species lacking convective flow occurred in very shallow water only, whereas species with high rates of convective flow were able to grow in deep water, even in eutrophic habitats where the organic soil has a high oxygen demand. Li et al. carried out an experiment with Typha domingensis to test the ability of this species to grow in nutrient-rich, strongly reduced sediments. Growth was adversely affected by low redox potentials, but high availability of phosphates moderated this negative effect. The results are discussed in the context of competitive replacement of Cladium jamaicense by Typha domingensis in parts of the Everglades affected by high agricultural run-off. Finally, Yu and Ehrenfeld have studied the way in which the plant and microbe community structure was affected by water-level fluctuations and soil characteristics in New England forested wetlands. Both median level and variability in water tables turned out to affect plant and microbe communities; however, soil genesis, a result of both water-table position and geologic history, appeared to be even more important.

Escape from water or remain quiescent? Lotus tenuis changes its strategy depending on depth of submergence.

Two main strategies that allow plants to cope with soil waterlogging or deeper submergence are: (1) escaping by means of upward shoot elongation or (2) remaining quiescent underwater. This study investigates these strategies in Lotus tenuis, a forage legume of increasing importance in areas prone to soil waterlogging, shallow submergence or complete submergence. Plants of L. tenuis were subjected for 30 d to well-drained (control), waterlogged (water-saturated soil), partially submerged (6 cm water depth) and completely submerged conditions. Plant responses assessed were tissue porosity, shoot number and length, biomass and utilization of water-soluble carbohydrates (WSCs) and starch in the crown. Lotus tenuis adjusted its strategy depending on the depth of submergence. Root growth of partially submerged plants ceased and carbon allocation prioritized shoot lengthening (32 cm vs. 24.5 cm under other treatments), without depleting carbohydrate reserves to sustain the faster growth. These plants also developed more shoot and root porosity. In contrast, completely submerged plants became quiescent, with no associated biomass accumulation, new shoot production or shoot elongation. In addition, tissue porosity was not enhanced. The survival of completely submerged plants is attributed to consumption of WSCs and starch reserves from crowns (concentrations 50-75 % less than in other treatments). The forage legume L. tenuis has the flexibility either to escape from partial submergence by elongating its shoot more vigorously to avoid becoming totally submerged or to adopt a non-elongating quiescent strategy when completely immersed that is based on utilizing stored reserves. The possession of these alternative survival strategies helps to explain the success of L. tenuis in environments subjected to unpredictable flooding depths.

Hydrodynamic Performance of Single- and Double-Wave Screens

A comprehensive experimental program has been carried out in order to assess the characteristics like reflection and transmission coefficients of slotted wave screens formed with a series of horizontally placed circular elements. To assess the effect of partial submergence, four different relative depths of submergence (ds/h) 0.25, 0.50, 0.75, and 1.0, where h=water depth, and ds=depth of submergence of the wave screen were tested. The wave screens were subjected to regular waves of normal angle of incidence. Further, the pressures exerted on the individual elements of the wave screen and the chamber oscillations in the case of double screen model are discussed in this paper. The peak pressures on the screen are found to reduce with a decrease in its relative depth of submergence. Empirical equations for the prediction of reflection and transmission coefficients as a function of wave steepness, relative water depth, and depth of submergence of the screen for single and double screens are proposed. Such equations for the pressures on the screens are also reported in this paper.

Overland flow velocity and roughness properties in peatlands

Overland flow is an important component of peatland hydrology. Hydrological models of peatlands are being developed that require estimates of flow velocity and its controls. However, surprisingly little is known about overland flow velocities in peatlands. Some peatlands have also been drained using open ditches, and these need to be incorporated into flow models. This paper presents field data on the velocity of overland flow and drain flow in upland peatlands. The relationships between flow velocity, vegetation cover, slope, and water depth are explored. Sphagnum provided a significantly greater effective hydraulic roughness to overland flow than peatland grasses. In all cases, a significant break in process occurred for flows with water depths of around 1 cm so that there were two components of the roughness curve. This is consistent with partial submergence theory for very shallow flows where resistance increases with depth as the soil surface first becomes fully submerged. While each surface cover type should be considered separately, the results also suggest that a first‐order estimate of Darcy‐Weisbach roughness and mean velocity can be based on a single parameter for each surface cover. This paper presents an empirical overland flow velocity forecasting model that can be applied to peatlands. The model combines the partially submerged component for flows with water depths below 1 cm with the fully submerged component for flows with depths up to 5 cm, which are representative of the depths of flows that occur across peatlands.

SEPARATING THE EFFECTS OF PARTIAL SUBMERGENCE AND SOIL OXYGEN DEMAND ON PLANT PHYSIOLOGY

In wetlands, a distinct zonation of plant species composition occurs along moisture gradients, due to differential flooding tolerance of the species involved. However, "flooding" comprises two important, distinct stressors (soil oxygen demand [SOD] and partial submergence) that affect plant survival and growth. To investigate how these two flooding stressors affect plant performance, we executed a factorial experiment (water depth x SOD) for six plant species of nutrient-rich and nutrient-poor conditions, occurring along a moisture gradient in Dutch dune slacks. Physiological, growth, and biomass responses to changed oxygen availability were quantified for all species. The responses were consistent with field zonation, but the two stressors affected species differently. Increased SOD increased root oxygen deprivation, as indicated by either raised porosity or increased alcohol dehydrogenase (ADH) activity in roots of flood-intolerant species (Calamagrostis epigejos and Carex arenaria). While SOD affected root functioning, partial submergence tended more to reduce photosynthesis (as shown both by gas exchange and 13C assimilation), leaf dark respiration, 13C partitioning from shoots to roots, and growth of these species. These processes were especially affected if the root oxygen supply was depleted by a combination of flooding and increased SOD. In contrast, the most flood-tolerant species (Juncus subnodulosus and Typha latifolia) were unaffected by any treatment and maintained high internal oxygen concentrations at the shoot : root junction and low root ADH activity in all treatments. For these species, the internal oxygen transport capacity was well in excess of what was needed to maintain aerobic metabolism across all treatments, although there was some evidence for effects of SOD on their nitrogen partitioning (as indicated by 865N values) and photosynthesis. Two species intermediate in flooding tolerance (Carex nigra and Schoenus nigricans) responded more idiosyncratically, with different parameters responding to different treatments. These results show that partial submergence and soil flooding are two very different stressors to which species respond in different ways, and that their effects on physiology, survival, and growth are interactive. Understanding species zonation with water regimes can be improved by a better appreciation of how these factors affect plant metabolism independently and interactively.

Ethylene-induced stem growth of deepwater rice is correlated with expression of gibberellin- and abscisic acid-biosynthetic genes

Ethylene decreases the content of endogenous abscisic acid (ABA) and increases the level of bioactive gibberellin A1 (GA1) in the submerged internodes of deepwater rice. During partial submergence, internodes of deepwater rice undergo rapid elongation as a result of ethylene accumulation in the internodal lacunae. In anin vitro experiment using stem sections from deepwater rice, treatment with 5 μL L-1 ethylene promoted stem growth by up to 3.2-foId times over air treatment. Expression patterns were analyzed for genes that encode GA- and ABA-biosynthesis enzymes to determine any possible molecular basis for the changes observed in GA1 and ABA contents as a result of ethylene action. Expression of theOsGA20ox2 andOsGA20ox4 genes, which encode GA 20-oxidase, and of theOsGA3ox2 gene, which encodes the enzyme that converts GA20 to CA1, was up-regulated, whereas that of three ABA-biosynthetic genes —OsNCED1, OsNCED2, andOsNCEDS-was down-regulated in the presence of ethylene. These results indicate that GA and ABA contribute equally to the submergence-or ethylene-induced stem elongation of deepwater rice via the coordinated and opposite regulation of biosynthesis.

Nile delta margin: failed and fluidized deposits concentrated along distributary channels / La marge du delta du Nil : concentration de dépôts fracturés et fluidisés le long des paléo­chenaux fluviatiles

Abstract Previous sediment studies on the Holocene Nile delta margin have emphasized fluvial input and coastal processes, but have overlooked the failure of depositional sequences by syn- and post-depositional events. Herein, petrologic analysis of sediment sections in cores collected along the Nile delta margin indicates the presence of disturbed strata. These modified deposits are not randomly distributed, but are concentrated along former Holocene distributary channels, and generally absent between these Nile branches. Sediment failure involving fluidization, mass flow, and fault-offset prevailed from the mid- to late Holocene, especially since -4,500 years before present. Bedding disturbance was caused by deposition of sediment with high pore water pressure at and seaward of Nile distributary mouths, with flooding along channel mouths believed to be the major trigger of failure. These geohazards were responsible for destruction and partial submergence of the ancient cities of Herakleion and Eastern Canopus in western Abu Qir Bay from -100 B.C. to 741 A.D. Although floods no longer occur since the closure of the Aswan High Dam, some cities constructed on older water-saturated underconsolidated sediment along the modern delta coast remain prone to failure. Protection measures for expanding population centers on the Nile margin warrant thorough civil engineering surveys, including analyses of physical properties of the sediment substrate, with implementation of viable building codes.

Effects of submergence on the growth of Phragmites australis seedlings

Colonisation by reed seedlings, Phragmites australis (Cav.) Trin. ex Steud. is rare and usually occurs after drawdown and when shallow water prevails. P. australis seeds have high rates of germination but successful colonisation is dependent upon subsequent water depths. We investigated the capacity of young reed plants to resist a 4 weeks submergence stress within a 5 months period, and their subsequent recovery. A pond experiment examined the interactions between submergence depth and the age of the seedlings at submergence. Four submergence treatments were used. In two partial submergence treatments, 50 and 80% of the initial leaf area was submerged. In two total submergence treatments, plants were either submerged at 125% of their initial height with possible subsequent development of emerged leaves, or the water was deepened as they grew to maintain total submergence for 4 weeks. The ages at submergence were 40, 60 and 80 days. Plants were harvested at 5 months. Shoot elongation, biomass allocations to aerial biomass, roots and rhizomes, and photosynthetic activity of aerial leaves were measured. Redox potential was measured for a subsample. Mortality (18.7%) occurred only in the permanent submergence treatment for 40-day-old seedlings. In all treatments, submerged leaves senesced, except the terminal (youngest) leaves of permanently submerged plants. Submergence differentially affected shoot length and biomass, depending upon the intensity of the treatment and the seedling age. The major differences were found between the two partial and two total submergence treatments. Partial submergence (50 and 80%) significantly enhanced biomass accumulation and growth, whereas total submergence largely decreased biomass production and growth in length, with less effect on shoot numbers. The 80-day-old seedlings tolerated submergence better but growth was poorest in medium-aged plants (60-day-old). Increased elongation of the growing internodes of up to 140% was caused by submergence, and photosynthetic activity was enhanced by 85% in emergent leaves of plants initially submerged but allowed to produce emerged leaves during the treatment period. Young P. australis plants require shallow water levels without long lasting submergence to grow and survive. Tolerance to submergence increases with age. These processes contribute to define the conditions for colonisation via seeds in P. australis.

Flooding tolerance of Carex species in relation to field distribution and aerenchyma formation.

The flooding tolerance of Carex species was studied in relation to their field distribution and their capacity to form root aerenchyma under controlled conditions. In an alpine meadow, six Carex species were selected which were distributed in a clear zonation correlating with water content of the soil. Carex sempervirens and C. ferruginea were only found on nonflooded soil, the latter species preferring moister conditions. Carex davalliana and C. nigra were both associated with water-saturated soil, whereas C. limosa and C. rostrata preferred partially submerged conditions. Carex davalliana and C. limosa were bound to flooded soils with a relatively high redox potential and horizontally flowing groundwater. Carex rostrata and C. nigra grew in stagnant soil-flooded conditions with low soil redox potentials. The amount of aerenchyma in the roots of all species increased when grown in oxygen-deficient stagnant agar. This increase in root porosity, combined with increased root diameter, presumably improved internal aeration of the roots. Although all species survived experimental soil flooding, partial submergence was lethal to C. sempervirens and, surprisingly, also to the wetland species C. davalliana. Carex ferruginea showed a reduced growth rate during partial submergence. The three other species, all wetland plants, reached highest biomass production under soil-flooded and partially submerged conditions, with slower growth on free-draining soil. It is concluded that aerenchyma is not constitutive in the Carex species under study, and is best developed in Carex species from wetlands. Species with less aerenchyma perform poorly when soil-flooded, but conditions of partial submergence could even affect species with a considerable amount of root aerenchyma.

The utility of aggregate processing fines in the rehabilitation of dolomite quarries

It is economically and environmentally desirable to use aggregate processing fines (pond fines), a by-product of dolomite crushing/washing operations, in the rehabilitation plans of the aggregate quarry where they are produced. A study was undertaken to determine if the physical and mechanical properties of these dominantly silt-sized materials also made them suitable as embankment material from a geotechnical perspective. The after-use proposed for the dolomite quarry investigated in this study (Ontario, Canada) was to be a naturalized area surrounding a large water reservoir, and pond fines were being used to construct sloped embankments at the base of the vertical perimeter quarry walls for this purpose. The specific objectives were (1) to measure and analyse the physical and mechanical properties of the pond fines that are relevant to their usage in site rehabilitation (earthen embankment), and (2) to predict the long-term stability of the embankments given current emplacement practices and a scenario of partial submergence by a reservoir. Shear strength, measured by direct shear and drop cone penetrometer methods, increased with higher clay-sized particle content and higher dry bulk density, with lower sand content, and it peaked at soil water contents (4–9 per cent kg kg−1) well below the measured field capacity of about 16 per cent kg kg−1. Factors of safety calculated for slope stability (modified Bishop method of analysis) decreased with decreasing shear strength (cohesion, angle of shearing resistance), with increasing slope gradient, and with the introduction of a water table. Nevertheless, the factors of safety were acceptably high for all scenarios tested (≥1·8), including scenarios using the maximum measured slope gradient of 37 per cent. Increasing the dry bulk density by 0·1 g cm−3 over the mean density measured in situ (i.e. to about 87 per cent of the Standard Proctor maximum dry density) did not improve the calculated factors of safety for slope stability enough to warrant increased mechanical compaction of the pond fines during embankment construction. Copyright © 2000 John Wiley & Sons, Ltd.

Responses to flooding intensity in Leontodon taraxacoides

Natural flooding is one of the major factors affecting vegetation dynamics in many regions of the world. The Flooding Pampa Grasslands (Argentina) are frequently exposed to flooding events of diverse intensity and duration, some of which Leontodon taraxacoides, an exotic dicot. frequent in these grasslands, seems to survive. Its responses to four different water depths (0, 1, 7 and 13 cm) were studied. The results indicate that plants in conditions of total submergence (depth of 13 cm) did not survive. In less severe flood conditions, increases in the leaf insertion angle resulted in the maintenance of a large proportion of the total leaf area above the water. Differences in leaf length and a decrease in the width and the proportion of lobes per leaf were also found under partial submergence conditions (depth of 7 cm). Root and leaf aerenchyma, present in unflooded plants, showed a significant increase in flood conditions. In spite of the anatomical and morphological responses, total biomass and leaf area were severely affected by water depth. Control plants allocated more biomass to reproductive organs, while partly submerged plants allocated more to leaves and less to reproductive organs. Mature L. taraxacoides plants presented a wide range of plastic adjustment as a survival strategy in soil anaerobiosis, and appear to be able to survive short spring floods in a vegetative state; in contrast, they might not tolerate total submergence conditions imposed by more intense and long‐lasting floods.

Deepwater rice: A model plant to study stem elongation

Semiaquatic plants grow mostly in flood plains and along river beds and are adapted to survive partial submergence during periods of flooding ([Blom and Voesenek, 1996][1]). Among their adaptive features are the development of internal air channels (aerenchyma) that facilitate aeration of submerged

Phosphorus uptake by bean nodules

Phosphate (P) uptake experiments were conducted to determine the routes of P entry into nitrogen-fixing common bean root nodules and the availability of this P to the bacteroids within the nodules. Nodulated common bean plants were cultured for 30 days in a hydroponic system that contained either a complete nutrient solution (+P) or a nutrient solution where P was not added (−P). The plant roots were then exposed to [32P]-phosphate-labeled +P nutrient solution and P accumulation in bacteroids and nodules was measured. For labeling, plants were suspended in the nutrient solution so that the root system was either fully or partially submerged. Because bean nodules are heavily clustered in the root crown, most of the nodules in the partial submergence treatments were not in contact with the nutrient solution and we could study nodules that had not been directly exposed to 32P. Regardless of P treatment of the host plant, P accumulation in bacteroids was significantly greater and more rapid when the nodule surface was in contact with the labeled nutrient solution, suggesting that nodules can import P from the solution. P entry through the nodule surface was verified in experiments where 32P-labeled nutrient solution was applied to the nodule surface only. In full submergence treatments, 32P accumulation in bacteroids of −P plants was significantly greater than in bacteroids of +P plants. As measured by total leaf label, uptake and translocation of P to the shoot was greater in −P plants and still greater in partial submergence treatments where most of the nodules were not in contact with the labeled nutrient solution. Results of these experiments suggest that, in addition to P acquisition via the vascular system, P flow to symbiotic tissues and bacteroids can originate from the soil solution immediately surrounding the nodule. This was particularly evident in nodules of plants that had been limited for P.

The effect of flooding on the recruitment of reed marsh and tall forb plant species

Recruitment of plant species in wetlands dominated by Phragmites australis often results in a zonation of two vegetation types. A development of reed marshes takes place in the shallow flooded parts where the dominant P. australis becomes accompanied by interstitial marsh species. The vegetation on higher elevations becomes dominated by tall forbs. We investigated whether this zonation is related to the effects of flooding on the recruitment of component species of both vegetation types. All but one species had maximum germination at field capacity and germination was strongly reduced in drier soils and flooded soils. Nearly all seedlings of reed marsh species, irrespective of whether they initially supported two or six true leaves, survived eight weeks of total inundation. Survival of tall forbs was however significantly decreased by inundation. At both seedling stages a considerable number of Cirsium arvense lost their leaves when submerged, but all plants were able to recover when drained. Eupatorium cannabinum suffered extra mortality when transferred from flooded to drained conditions, whereas only young seedlings of Epilobium hirsutum died. Of all investigated species E. hirsutum was the only one with a significantly increased stem length after one week of submergence. Root porosities did not differ between reed marsh species and tall forbs but stem porosities of the reed marsh species Mentha aquatica and Lycopus europaeus were significantly higher than those of the tall forbs C. arvense and E. cannabinum. It was concluded that both reed marsh species and tall forbs require lowered water levels for germination. Flooding during the seedling stage will assign species to their position on the elevation gradient. Reed marsh species establish at lower elevations because they are best able to survive flooding and because their higher stem porosities allows them to withstand higher levels of partial submergence.

Expression characteristics of OS-ACS1 and OS-ACS2, two members of the 1-aminocyclopropane-1-carboxylate synthase gene family in rice (Oryza sativa L. cv. Habiganj Aman II) during partial submergence.

Deepwater rice can grow in the regions of Southeast Asia that are flooded during the monsoon season because it has several adaptations allowing it to survive under flooded conditions. One such adaptation is the ability for rapid internode elongation upon partial submergence to maintain its foliage above the rising flood water levels. Ethylene is considered to be the trigger of this growth response because deepwater conditions not only trap ethylene in submerged organs, but also enhance the activity of 1-aminocyclopropane-1-carboxylate (ACC) synthase. Herein we have studied the expression characteristics of two members of the five-member multigene family encoding ACC synthase in rice OS-ACS1 and OS-ACS2 and show that partial submergence induces expression of OS-ACS1 and suppresses expression of OS-ACS2. The induction of OS-ACS1 occurs within 12 h of partial submergence and at low oxygen concentrations. The data also suggest that deepwater conditions posttranscriptionally regulate ACC synthase activity. OS-ACS1 gene expression may contribute to longer-term ethylene production, but not to the initial, growth-promoting increase in ethylene synthesis.