In river floodplains, vegetation zonation is strongly influenced by water-level fluctuations. Depending on elevation level and flood intensity, a natural as well as a human-influenced environmental gradient can be observed. The natural vegetation consists of softwood and hardwood forests. Grazed marsh and grassland are the characteristic communities of agriculturally used floodplains. Individual plants occurring in floodplains must develop adaptive mechanisms in order to survive transient floods. These mechanisms are discussed at the level of life history strategies and at the physiological level in relation to the position of the species in the floodplain gradient.Species from the higher sites favor rapid germination after seed release while species from lower sites postpone germination or survive in the vegetative stage as a response to flooding. Flood-tolerant species are able to form aerenchymatous roots and survive submergence by their ability to accelerate shoot extension which restores leaf-air contact. Phytohormones, especially ethylene, play an important role in the adaptive responses to flooding.The study of adaptive mechanisms of species occurring along the floodplain gradient will result in a better understanding of flooding-related processes acting at the community level.

Question: Along river floodplains lower distribution limits of plant species seem largely determined by their tolerance to rarely occurring floods in the growing season.Such distribution patterns remain fixed for many years suggesting additional effects of winter floods at lower positions.Our objective was to investigate the direct and indirect effects of winter floods on colonization of floodplains in a series of field experiments.Location: River Rhine, The Netherlands.Methods: We measured the direct effects of winter floods on seedling survival and seed removal and survival at low and high floodplain elevation.Indirect effects of winter flooding through changes in the soil were investigated by measuring seedling emergence on soil transplants that were exchanged between high and low floodplain elevation.To investigate indirect effects of floods on the germination environment through changes in the vegetation structure, we measured the effects of vegetation removal on recruitment of sown species.Results: Recruitment was seed limited at both floodplain elevations.An additional effect of vegetation removal on seedling emergence was also observed.Soil types from both zones did not differently affect seedling emergence.Seeds were not removed from the soil surface by a single winter flood.Moreover, seeds remained viable in the soil for at least two years, while the experimental plots were flooded several times during the experimental period.During one of those floods a thick sand layer was deposited at the low zone and subsequently no seedlings were observed anymore.Conclusions: Colonization of low floodplain zones in years between subsequent summer floods is prevented by seed limitation while the direct effects of winter floods are limited except for irregularly occurring sand depositions.

Field observations suggest that flooding events in the growing season are more detrimental than in winter. To clarify mechanisms producing these seasonal differences we analysed the role of plant acclimation, water temperature and oxygen concentration. We first tested the relative effects of seasonal acclimation and water temperature with three grassland species that differed in tolerance to summer floods (i.e. Rumex crispus, Rumex acetosa and Daucus carota). Our second experiment addressed the role of oxygen level relative to water temperature on biomass decay rate on a moderately intolerant species (i.e. R. acetosa).Irrespective of acclimation, biomass loss in warm water was considerably faster than in cold water. Given the concomitant decline in total non-structural carbohydrates, this was ascribed to the impact of water temperature on respiration rate. However, we only found a significant decline in carbohydrates for R. crispus and R. acetosa. D. carota seemed unable to access stored carbohydrates, which may explain its sensitivity for winter- and summer floods. Our second experiment provided no indication that the higher oxygen concentration may mitigate effects of flooding in cold water since a lower oxygen level of the water did not accelerate the rate of biomass loss.These findings indicate that temperature-driven respiration of carbohydrate reserves determines a species' response to winter flooding, whereas oxygen level or plant acclimation are unimportant. (c) 2005 Elsevier B.V All rights reserved.

It is generally assumed that floods during the growing season have a strong impact on the distribution of grassland plant species in river floodplains but this proposition has never been tested. We examined the survival and growth responses of twenty species, originating from mid‐ and high‐level floodplain grasslands along the River Rhine in the Netherlands, to total submergence for a maximum of two months in an outdoor flooding experiment. Plant survival and biomass reduction with flooding duration was determined as well as biomass recovery after de‐submergence.Our results indicate that species survival is the most prominent factor correlated with species distribution in floodplain areas. Relatively flood tolerant species occurred mainly at low elevations along the floodplain while more flood sensitive species were restricted to high parts of the floodplain gradient. Biomass reduction rates during submergence were only marginally significantly correlated with species lower distribution boundaries along the flooding gradient. Biomass recovery rate was significantly correlated with species distribution patterns in the field only after 2 weeks of complete submergence, but not after 4 and 8 weeks. Our results suggest that the more flood tolerant species can have various ways to survive and recover from flooding, ranging from low rates of biomass loss and low recovery to relatively high rates of biomass loss and quick recovery.Our results are consistent with the notion that disturbance by floods during the growing season is an important determinant of species lower distribution boundaries in river floodplains. They also suggest that high survival under flooding may be achieved by different physiological mechanisms. Such mechanisms are discussed in this paper.

Summary Summer floods whose severity is affected by flooding duration, submergence depth and underwater light availability, have a large impact on the zonation of riparian plant species. We analysed the range and variability of these flooding components in the River Rhine and quantified their effects on the ability of Arrhenatherum elatius, Achillea millefolium, Rumex acetosa and Rumex crispus, to survive periods of submergence under experimental conditions. Survival characteristics were used to model species’ lower distribution boundaries for extreme and average floods and were compared with the current field distribution. Different light conditions were simulated by implementing three scenarios of suspended load. Extreme deep Rhine floods are characterized by very low median light transmission levels (i.e. below 0.5%). The largest survival responses in the experiment were observed at such low levels (0.4–3.5 µmol m−2 s−1). Strong effects of light were found in R. crispus and A. millefolium, but responses were weaker in A. elatius and R. acetosa. Submergence depth also affected survival, but not as strongly as light. For the flood intolerant species (A. millefolium and A. elatius) the average flood was predicted to have little effect on field distributions under normal light conditions. However, their actual field distributions in 2000 corresponded to the predicted lower boundaries in the extreme years. This suggests that extreme years determine the distributions of these species for many years. The suspended load scenarios significantly modified the predicted lower boundaries in both extreme and average years, implying that plant lower distribution limits may be significantly shifted upwards or downwards depending on the suspended load of the river system. The predicted lower boundaries of the intermediately tolerant R. acetosa and the highly tolerant R. crispus for both extreme and average years were below the actual field distribution in 2000. This suggests that their current distribution is only partly influenced by major flood disturbances and that other factors, either proximate or historical, may play a prominent role.

In a study on the mechanism of stimulated petiole elongation in submerged plants, oxygen concentrations in petioles of the flood-tolerant plant Rumex palustris were measured with micro-electrodes. Short-term submergence lowered petiole partial oxygen pressure to c. 19 kPa whereas prolonged submergence under continuous illumination depressed oxygen levels to c. 8-12 kPa after 24 h. Oxygen levels in petioles depended on the presence of the lamina, even in submerged conditions, and on available light. In darkness, petiole oxygen levels in submerged plants dropped quickly to values as low as 0.5-4 kPa. It is hypothesized that prolonged submergence in the light is accompanied by a decrease in carbon dioxide in the petiole. Submergence-enhanced petiolar elongation rate was compared with emergent plants. Peak daily elongation rates occurred at the end of the dark period in emergent plants, but in the middle of the light period in submerged plants. We suggest that this shift in daily elongation pattern is induced by dependence of growth on photosynthetically derived oxygen in submerged plants. Implications of reduced oxygen for ethylene production are raised. Levels of 1- aminocyclopropane-1-carboxylic acid synthase and 1-aminocyclopropane-1-carboxylic acid oxidase and ethylene sensitivity are cited as potential factors in hypoxia-induced ethylene release.

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.

ABSTRACTGrowth in stagnant, oxygen‐deficient nutrient solution increased porosity in adventitious roots of two monocotyledonous (Carex acuta and Juncus effusus) and three dicotyledonous species (Caltha palustris, Ranunculus sceleratus and Rumex palustris) wetland species from 10 to 30% under aerated conditions to 20–45%. The spatial patterns of radial oxygen loss (ROL), determined with root‐sleeving oxygen electrodes, indicated a strong constitutive ‘barrier’ to ROL in the basal root zones of the two monocotyledonous species. In contrast, roots of the dicotyledonous species showed no significant ‘barrier’ to ROL when grown in aerated solution, and only a partial ‘barrier’ when grown in stagnant conditions. This partial ‘barrier’ was strongest in C. palustris, so that ROL from basal zones of roots of R. sceleratus and R. palustris was substantial when compared to the monocotyledonous species. ROL from the basal zones would decrease longitudinal diffusion of oxygen to the root apex, and therefore limit the maximum penetration depth of these roots into anaerobic soil. Further studies of a larger number of dicotyledonous wetland species from a range of substrates are required to elucidate the ecophysiological consequences of developing a partial, rather than a strong, ‘barrier’ to ROL.

ABSTRACTEthylene emission from wild‐type Agrobacterium tumefaciens (C58)‐induced stem tumours of Ricinus communis was continuously measured with two different methods, process gas chromatography and photo‐acoustic spectrometry. Ethylene production was as high as 700 pmol g FW–1 h–1, namely 140 times greater than emitted by non‐tumourized control stems. It was highest in 5‐week‐old tumours, independent of light, depressed by anoxia and, during water deficit it was stimulated by rewatering. A remarkable concomitant CO‐production was discovered. Accumulation of 1‐aminocyclopropane‐1‐carboxylic acid (ACC), the substrate of ACC‐oxidase, preceded ethylene emission with a maximum 2 weeks after tumour induction. Simultaneously, the xylem in the tumour‐adjacent host stem underwent drastic changes: it increased two to three times in thickness, vessel diameters decreased, the rays remained unlignified and became multiseriate. With increasing emission of ethylene aerenchyma developed in the non‐transformed, tumour‐surrounding tissue that formerly was stem cortex. Cotyledons reacted with epinastic symptoms indicating induction of senescence. The present results reveal an important role of ethylene, in addition to cytokinin and auxin, for the differentiation and physiology of A. tumefaciens‐induced tumours.

We investigated how water level and different sediment types affect the growth of wetland plant species. Twelve different species were grown in drained and waterlogged sediments, which represented types normally encountered in wetlands: a mineral sediment from exposed sites, a sediment from a sheltered site rich in labile organic matter and an organic sediment with decomposing litter of Phragmites australis (Cav.) Steudel. The tested species included both subordinate and dominant species inhabiting flooded or dry parts of the water-depth gradient.Due to nutrient limitation, biomass production of most species was lowest in the mineral sediment. In this substrate waterlogging only affected Cirsium arvense and Eupatorium cannabinum which were reduced to 30% and 16% of the production in the drained sediment. Most species performed best in the sediment with labile organic matter, even when waterlogged. Waterlogging in the reed litter sediment, when compared to the drained reed litter, decreased growth of six species: Iris pseudacorus by 40%, Myosotis scorpioides by 60%, Rorippa amphibia by 25%, Sium latifolium by 50%, Eupatorium cannabinum by 80%, and Epilobium hirsutum by 70%. The differences in plant performance between both organic sediments may be due to the presence of refractory organic matter. The specific responses in the reed litter sediment contrasted with the similar response to both other sediments. These results show that accumulation of litter, instead of accumulation of organic matter in,general, will be an important factor in determining species composition of littoral zones. They also indicate that, although litter does not favor subordinates above clonal dominants, litter accumulation may enhance species diversity on a large scale. (C) 1999 Elsevier Science B.V. All rights reserved.

ABSTRACT To characterize underwater photosynthetic performance in some terrestrial plants, we determined (i) underwater light acclimation (ii) underwater photosynthetic response to dissolved CO 2 , and (iii) underwater photosynthetic capacity during prolonged submergence in three species that differ in submergence tolerance: Phalaris arundinacea , Rumex crispus (both submergence‐tolerant) and Arrhenatherum elatius (submergence‐intolerant). None of the species had adjusted to low irradiance after 1 week of submergence. Under non‐submerged (control) conditions, only R. crispus displayed shade acclimation. Submergence increased the apparent quantum yield in this species, presumably because of the enhanced CO 2 affinity of the elongated leaves. In control plants of the grass species P. arundinacea and A. elatius , CO 2 affinities were higher than for R. crispus . The underwater photosynthetic capacity of R. crispus increased during 1 month of submergence. In P. arundinacea photosynthesis remained constant during 1 month of submergence at normal irradiance; at low irradiance a reduction in photosynthetic capacity was observed after 2 weeks, although there was no tissue degeneration. In contrast, underwater photosynthesis of the submergence‐intolerant species A. elatius collapsed rapidly under both irradiances, and this was accompanied by leaf decay. To describe photosynthesis versus irradiance curves, four models were evaluated. The hyperbolic tangent produced the best goodness‐of‐fit, whereas the rectangular hyperbola (Michaelis‐Menten model) gave relatively poor results.

Resistance to complete submergence was tested in three Rumex species that occur in the Dutch river forelands. The species differ in both habitat and life history characteristics. The annual or biennial R. maritimus and the biennial or short lived perennial R. palustris grow on frequently flooded mud flats of low elevation, while the perennial R. thyrsiflorus can be found on dykes and river dunes that are seldom flooded. The flooding characteristics of the habitats of the three species were determined. These data were used to design experiments to determine the survival and biomass development of the three species during submergence and the influence of plant size and light level on these parameters. It was shown in all three species that plants submerged during daytime were much more resistant to flooding than those submerged at night. This is most probably due to the generation of oxygen or carbohydrates by underwater photosynthesis. Mature plants of the three species showed higher survival after submergence than juvenile plants, which might be caused by higher carbohydrate levels in the taproots of mature plants. In addition, the three species clearly differed in survival and biomass development during submergence. Rumex thyrsiflorus, the species least subjected to flooding, is least tolerant to complete submergence. Rumex maritimus, which can avoid the floods by having a short life cycle, is less tolerant to submergence than R. palustris, which has to survive the floods as a vegetative plant. It was noted that some plants that survived the flooding period itself, still died in the following period of drained conditions, possibly due to post‐anoxic injury.

The population dynamics of the chemolithoautotrophic nitrifiers Nitrosomonas europaea and Nitrobacter winogradskyi were studied in gnotobiotic microcosms fed with ammonium in response to the presence or absence of the emergent macrophyte Glyceria maxima and the heterotrophic denitrifying bacterium Pseudomonas chlororaphis. By subjecting the plants to different day lengths, the effect of possibly limiting factors (i.e. oxygen and ammonium) on the interactions between the nitrifiers and denitrifying bacterium could be analysed. The presence of the plant had no effect on the growth of nitrifiers suggesting that, in addition to radial oxygen loss from the roots, other non-plant sources of oxygen (e.g. diffusion from the water layer) were important for nitrification. Potential nitrifying activities were suppressed by G. maxima due to ammonium uptake by the plants. Elongation of the day length in combination with the presence of G. maxima led to an increase in the number of P. chlororaphis. The presence of P. chlororaphis suppressed the growth of N. winogradskyi, but the growth of N. europaea and the potential nitrifying activities were not significantly affected. Potential denitrifying activities were stimulated by the plant, but showed no correlations with nitrifier activities or numbers. Apparently ammonium, and not oxygen, was the limiting factor for nitrification in the root zone of G. maxima. However, when the plant did not deplete the ammonium pool, P. chlororaphis could repress the nitrifiers indicating the latter's poor competitive status with respect to oxygen when the presence of root exudates allows for heterotrophic oxygen consumption.

Observations on floodplains of the Lower Rhine system in the Netherlands clearly indicate that summer flooding in particular has a large impact on flora and vegetation. A large group of floodplain species is very sensitive to flooding. The majority of these species are characteristic of species-rich grassland types and species-rich variants of hardwood forest, both occupying the highest levees in the floodplain. Observations also show that many aquatic and riparian species of well-isolated floodplain waters are extremely sensitive to incidental deep summer flooding. The serious threat of vast inundations during extremely high Rhine water levels, like those which occurred in the winters of 1993/1994 and 1995, has necessitated extensive efforts to secure future safety. This time, safety will be promoted by measures increasing the river's discharge capacity, rather than by raising dyke levels again. Artificial embankments will be removed, floodplain levels lowered and silted-up floodplain channels reopened. It has been stated that maintaining of the present range of flora elements and vegetation types will only be possible if sufficiently high levees and sufficiently isolated floodplain waters can successfully be incorporated in the rearranged floodplains.

Photoacoustic spectroscopy is a highly sensitive technique for measuring low molecular weight gases such as the plant hormone ethylene. Due to its high sensitivity (10 pi 1(-1) ethylene), photoacoustic spectroscopy can be combined with flow-through systems that avoid the need for enclosing excised plant parts in small volumes for head-space analysis. In this way, artifacts introduced by various accumulation techniques can be avoided and ethylene production monitored at short intervals in air or other gas mixtures as it flows out of a cuvette enclosing all or parr of an intact plant. The principles of this technique are described. Three case studies demonstrate the application of photoacoustic spectroscopy in flooding research. These studies concentrate oil accurate measurement of endogenous ethylene concentrations in submerged shoots and roots, root ethylene production under subambient oxygen pressures and the simultaneous measurement of ethylene production and leaf growth. In addition, the qualitative and quantitive methods previously used to measure the gaseous plant hormone ethylene are briefly reviewed. Finally, the future prospects of photoacoustic spectroscopy in flooding research are discussed. (C) 1997 Annals of Botany Company

Rumex palustris is a flooding‐resistant amphibious species from frequently flooded riversides, whereas Rumex acetosella is flooding‐sensitive and grows on dry sandy soils. Upon complete submergence, both species accumulate ethylene to similar levels. After more than four days, however, the ethylene concentration in R. acetosella plants strongly rises to an extremely high level, whereas it remains much lower in R. palustris plants. This latter species responds to ethylene with enhanced leaf elongation, whereas elongation in R. acetosella is insensitive to ethylene. Elongation rates of leaves were measured continuously during the first 8 h of submergence. A comparison of the elongation rates of R. palustris, R. acetosella and silver‐treated R. palustris plants demonstrated that R. palustris plants responded to ethylene within 1 h of submergence. In R. acetosella , clear symptoms of senescence and decay were observed within two weeks of submergence. In R. palustris plants, only the oldest leaf was senescent. To investigate the role of ethylene in the senescence process, the effects of silver ions on submerged plants, and the effects of prolonged exposure to an extremely high ethylene level on drained plants were studied in both Rumex species. The results demonstrated that although ethylene accelerated senescence of submerged R. acetosella plants, the process may have been caused by other factors. The slower senescence of R. palustris plants could not be explained by their lower ethylene concentration. Rather, it was caused by a much lower sensitivity of the senescence process to ethylene. Moreover, other factors may be less unfavourable in R. palustris than in R. acetosella plants under submerged conditions.

Rumex palustris, a flooding-tolerant plant, elongates its petioles in response to complete submergence. This response can be partly mimicked by enhanced ethylene levels and low O2 concentrations. High levels of CO2 do not markedly affect petiole elongation in R. palustris. Experiments with ethylene synthesis and action inhibitors demonstrate that treatment with low O2 concentrations enhances petiole extension by shifting sensitivity to ethylene without changing the rate of ethylene production. The expression level of the R. palustris gene coding for the putative ethylene receptor (RP-ERS1) is up-regulated by 3% O2 and increases after 20 min of exposure to a low concentration of O2, thus preceding the first significant increase in elongation observable after 40 to 50 min. In the flooding-sensitive species Rumex acetosa, submergence results in a different response pattern: petiole growth of the submerged plants is the same as for control plants. Exposure of R. acetosa to enhanced ethylene levels strongly inhibits petiole growth. This inhibitory effect of ethylene on R. acetosa can be reduced by both low levels of O2 and/or high concentrations of CO2.

Soil flooding results in unusually low oxygen concentrations and high ethylene concentrations in the roots of plants. This gas composition had a strongly negative effect on root elongation of two Rumex species. The effect of low oxygen concentrations was less severe when roots contained aerenchymatous tissues, such as in R. palustris Sm. R. thyrsiflorus Fingerh., which has little root porosity, was much more affected. Ethylene had an even stronger effect on root elongation than hypoxia, since very small concentrations (0.1 cm 3 m -3 ) reduced root extension in the two species, and higher concentrations inhibited elongation more severely than did anoxia in the culture medium. Thus, ethylene contributes strongly to the negative effects of flooding on root growth. An exception may be the highly aerenchymatous, adventitious roots of R. palustris. Aerenchyma in these roots provides a low-resistance diffusion pathway for both endogenously produced ethylene and shoot-derived oxygen. This paper shows that extension by roots of R. palustris in flooded soil depends almost completely on this shoot-derived oxygen, and that aerenchyma prevents accumulation of growth-inhibiting levels of ethylene in the root.

1. Adults of the emergent macrophytes Scirpus lacustris ssp. lacustris (S.l. lacustris), S. lacustris ssp. tabernaemontani (S.l. tabernaemontani) and S. maritimus occur along a gradient in water depth from deep to shallow water. This study examined whether seedlings of these taxa respond differently to changing hydrological conditions. 2. Seedlings of both S. lacustris subspecies showed the highest relative growth rate (RGR) under terrestrial growth conditions, whereas S. maritimus did so under submerged growth conditions. In all three taxa, shading reduced the mean RGR of terrestrial seedlings more strongly than that of submerged ones. 3. Scirpus maritimus and S.l. tabernaemontani maintained an erect growth form under water, whereas S.l. lacustris produced numerous long, floating leaves. 4. Under terrestrial growth conditions the specific leaf area (SLA) did not differ between taxa. Under submerged growth conditions the SLA differed as follows: S.l. lacustris > S.l. tabernae montani > S. maritimus. Irrespective of taxon and water level, the SLA was increased by shading. 5. Growth of all three taxa was reduced considerably after seedlings were transferred from terrestrial to submerged growth conditions. This effect was stronger with increasing age of seedlings. When transferred the other way round, seedlings of S.l. tabernaemontani and S. maritimus adapted quickly to the terrestrial growth conditions, whereas the thin leaves of S.l. lacustris partly dried out. 6. It was concluded that although seedling establishment of all three Scirpus taxa will be most successful under terrestrial conditions, subsequent fluctuating water levels may act as a strong selective force. This may determine the distribution of Scirpus taxa along a gradient in water depth during seedling establishment. [KEYWORDS: emergent macrophytes; heterophylly; plant size; relative growth rate; shading; submergence Aquatic macrophytes; amphibious plants; photosynthesis;submergence; responses; carbon; assimilation; strategies; vegetation; ethylene]

Accumulation of the gaseous plant hormone ethylene is very important for the induction of several responses of plants to flooding, However, little is known about the role of this gas in the formation of flooding-induced adventitious roots, Formation of adventitious roots in Rumex species is an adaptation of these plants to flooded soil conditions, The large air-spaces in these roots enables diffusion of gases between shoot and roots.Application of ethylene to non-flooded Rumex plants resulted in the formation of adventitious roots, In R. palustris Sm. shoot elongation and epinasty were also observed, The number of roots in R. thyrsiflorus Fingerh, was much lower than in R. palustris, which corresponds with the inherent difference in root forming capacity between these two species, Ethylene concentrations of 1.5-2 mu l l(-1) induced a maximum number of roots in both species.Quantification of ethylene escaping from root systems of Rumex plants that were de-submerged after a 24 h submergence period showed that average ethylene concentrations in submerged roots reached 1.8 and 9.1 mu l l(-1) in R, palustris and R. thyrsiflorus, respectively, Inhibition of ethylene production in R. palustris by L-alpha-(2-aminoethoxyvinyl)-glycine (AVG) or alpha-aminobutyric acid (AIB) decreased the number of adventitious roots induced by flooding, indicating that high ethylene concentrations may be a prerequisite for the flooding-induced formation of adventitious roots in Rumex species.