C.W.P.M. Blom

Flooding: the survival strategies of plants

Floodplains and wetlands are highly suitable for plant ecological studies, whether for agricultural interests, nature conservation or basic science. Traditional work has entailed a descriptive approach at the community or individual plant level. Nowadays these studies are evolving into physiological research on relationships between flooding stress and vegetation zonation. Current experiments aim to unravel the adaptive mechanisms whereby terrestrial plants cope with the peculiar conditions of the floodplain, from the whole plant down to the cell.

Root morphology and aerenchyma formation as indicators for the flood-tolerance of Rumex species

(1) Rumex species are zoned along a gradient of elevation in the river ecosystem in The Netherlands. (2) Plants of R. thyrsiflorus, R. acetosa, R. obtusifolius, R. crispus, R. conglomeratus and R. maritimus were flooded to identify and quantify any relevant adaptive features and to test whether their distribution might be caused by a differential response to flooding in the growing season. (3) Most Rumex species have a tap-root from which the laterals originate. As a response to flooding, new laterals are formed. (4) The number, place of origin, growth direction and formation rate of new laterals differed between the species. (5) The number and formation rate of new roots were associated with the elevational distribution of the species: as a response to flooding, low-elevation species formed more new roots, and faster, than high-elevation species. (6) The high-elevation species had root porosity values lower than 10%; the intermediateand low-elevation species had values higher than 10%. (7) Schizogenous aerenchyma was constitutively formed by the low-elevation and floodtolerant R. maritimus, and not by the high-elevation and flood-intolerant species R. thyrsiflorus. In the intermediate-elevation species R. crispus it was induced in stagnant hypoxic solution cultures. (8) The results indicate that aerenchyma formation is closely connected with the growth rate of new roots. It appears that development of aerenchyma in the new roots is the main determinant in the flood-tolerance of Rumex species.

An Ethylene-Mediated Increase in Sensitivity to Auxin Induces Adventitious Root Formation in Flooded Rumex palustris Sm

The hormonal regulation of adventitious root formation induced by flooding of the root system was investigated in the wetland species Rumex palustris Sm. Adventitious root development at the base of the shoot is an important adaptation to flooded conditions and takes place soon after the onset of flooding. Decreases in either endogenous auxin or ethylene concentrations induced by application of inhibitors of either auxin transport or ethylene biosynthesis reduced the number of adventitious roots formed by flooded plants, suggesting an involvement of these hormones in the rooting process. The rooting response during flooding was preceded by increased endogenous ethylene concentrations in the root system. The endogenous auxin concentration did not change during flooding-induced rooting, but a continuous basipetal transport of auxin from the shoot to the rooting zone appeared to be essential in maintaining stable auxin concentrations. These results suggest that the higher ethylene concentration in soil-flooded plants increases the sensitivity of the root-forming tissues to endogenous indoleacetic acid, thus initiating the formation of adventitious roots.

Ethylene enhances gibberellin levels and petiole sensitivity in flooding-tolerant Rumex palustris but not in flooding-intolerant R. acetosa

Abstract. The role of gibberellin (GA) and ethylene in submergence-induced petiole elongation was studied in two species of the genus Rumex. Analysis of endogenous GAs in the flooding-tolerant Rumex palustris Sm. and the intolerant Rumex acetosa L. by gas chromatography-mass spectrometry showed for both species the presence of GA1, GA4, GA9, GA19, GA20 and GA53. Gas chromatography-mass spectrometry analysis of R. palustris petiole tissue of submerged plants showed an increase in levels of 13-OH GAs, especially GA1, compared with drained plants. This effect could be mimicked by application of 5 μL L−1 ethylene. In R. acetosa, no differences between levels of GAs in drained or submerged plants were found. In R. palustris, both submergence and ethylene treatment sensitized petioles to exogenous gibberellic acid (GA3). In R. acetosa the effect was opposite, i.e. submergence and ethylene de-sensitized petioles to GA3. Our results demonstrate the dual effect of ethylene in the submergence response related to flooding tolerance, i.e. in the flooding-tolerant R. palustris ethylene causes an increased concentration of and sensitivity to GA with respect to petiole elongation while in the intolerant R. acetosa ethylene reduces growth independent of GAs.

Survival tactics of Ranunculus species in river floodplains.

Abstract The flooding resistance of four Ranunculus species was studied under controlled conditions and related to the tactics used by these species to survive in their natural habitat in river floodplains. R. bulbosus, a species from seldom-flooded river levées, was relatively intolerant of both waterlogging and complete submergence, due to a constitutively low level of aerenchyma in the root system. This lack of gas spaces resulted in high mortality rates during flooding treatments and an inability to use photosynthetically derived oxygen for root respiration during complete submergence. The pioneer R. sceleratus, predominantly abundant in low lying mudflats, was very resistant to waterlogging and shallow floods. Due to its constitutively high root porosity and its ability to greatly increase the elongation rate of petioles under water this species can ameliorate flooding stress. However, when leaf blades of R. sceleratus were unable to reach the water surface, this species died as quickly as the flooding-intolerant R. bulbosus. This indicates that fast elongation of petioles under water competes for energy and respirable reserves with maintenance processes. R. repens, a species from lower, frequently inundated floodplains, was very tolerant of prolonged waterlogging and submergence. Its high resistance to complete submergence under continuous darkness indicates that this species tolerates hypoxic and/or anoxic tissue conditions via metabolic adjustments. Lysigenous aerenchyma was also induced in the primary root system and in newly developed laterals, and it was able to use oxygen generated by underwater photosynthesis, for root respiration. R. acris, a species from less frequently flooded areas, was as resistant to waterlogging and submergence in the light as R. repens. However, it has a lower resistance than R. repens to complete submergence in the dark. A submergence pre-treatment increased the maximum net underwater photosynthetic rate in R. bulbosus, whereas a significant decrease of light compensation points was observed in R. repens when it had previously been submerged. This study shows that Ranunculus species exhibit various strategies to cope with different flooding conditions. R. repens responds to flooding by its tolerance mechanism and R. sceleratus by avoidance. R. acris ameliorates submergence and R. bulbosus was not able to adapt high water tables.

An amalgamation between hormone physiology and plant ecology: A review on flooding resistance and ethylene

Both distribution of terrestrial plants and species composition in flood plain communities are strongly influenced by flooding (waterlogging, partial submergence, or submergence). The interaction between a plants flooding resistance and the seasonal timing, duration, depth, or frequency of flooding often determines plant distribution in flood plains. Flooding may be accompanied by marked physical changes in light, carbon availability, diffusion rate of gases, and density of the environment. Various physiological processes may be affected by these flooding-induced physical changes, including aerobic respiration, photosynthesis, and processes in which light acts as a source of information (e.g., phytochrome photoequilibrium). Certain plant species acclimatize and adapt to these physical changes to relieve the constraints imposed by the flooded environment. Underwater photosynthesis, enhanced shoot elongation, adventitious roots, and aerenchyma formation are typical adaptive responses which are believed to improve the oxygen status of submerged plants. Ethylene and other plant hormones play a central role in the initiation and regulation of most of these adaptive responses, which permit “escape” from anaerobiosis. Mechanisms of direct tolerance of anaerobic conditions, such as a vigorous fermentative respiratory pathway, are of particular importance when the plant is very deeply submerged, or during the night and when the water is sufficiently turbid to exclude light.Studies on the cosmopolitan genus Rumex, distributed in a flooding gradient on river flood plains, have integrated plant hormone physiology with plant ecology. Rumex species showed a high degree of interspecific variation in ethylene production rates, endogenous ethylene concentrations, ethylene sensitivity, and ethylene-mediated growth responses. The field distribution of Rumex species in flooding gradients is explained in terms of a balance between endogenous ethylene concentrations and sensitivity towards this growth regulator (“ethylene economy”). Much data has been gathered using a recently developed laser-driven photoacoustic detection technique capable of detecting six parts of ethylene in 1012 parts air flowing continuously over the plant.

Effects of sediment type and water level on biomass production of wetland plant species

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. [KEYWORDS: littoral vegetation; litter; organic matter; species diversity; zonation Emergent vegetation; growth; macrophytes; lake; decomposition; limitation; aeration; detritus; patterns; zonation]

Flooding regimes and life-history characteristics of short-lived species in river forelands

A large-scale outdoor experiment was carried out to study the effect of flooding on growth, survival and reproduction in relation to developmental stage in three short-lived species. Several cohorts of Chenopodium rubrum, Rumex maritimus and R. palustris were raised in accordance with a flooding regime which was based on the average flooding conditions of their natural habitats in river forelands. Survival of submergence in the pre-reproductive phase was high in both species of Rumex, but relatively low in C. rubrum. Biomass reduction following flooding depended on plant size before flooding and on the mean temperature of the flood-water. Both Rumex spp. were less reduced in size during flooding, and better capable of regeneration afterwards, than C. rubrum. Later-raised cohorts of the short-day species C. rubrum started to flower after a shorter time and at an ealier developmental stage than earlier cohorts (...)

Plants and Hormones: An ecophysiological view on timing and plasticity

1 This paper demonstrates the role played by plant hormones in linking environmental signals with plant responses. It concentrates on two strategies for a sessile organism as a plant to cope with changing environmental conditions: life cycle timing and phenotypic adjustment. 2 The significance of abscisic acid and gibberellins for dormancy and germination, respectively, is discussed in relation to life cycle timing. Cytokinins are presented in relation to a possible role in carbon allocation. The gaseous plant hormone ethylene is discussed in relation to its involvement in windand water-induced changes in shoot growth. 3 Evidence for the role played by plant hormones in developmental processes and plastic responses comes from only a very few plant species. It will be a task for ecologists to come to a more generalized understanding of the involvement of plant hormones in ecological processes by applying the existing knowledge to a much wider range of species.

The role of ethylene in shoot elongation with respect to survival and seed output of flooded Rumex maritimus L. plants

Rumex maritimus L. occurs in frequently flooded sites of lowland flood plains. Upon submergence this species exhibited rapid elongation of shoots, but the response depended upon the developmental stage when flooding was initiated. In the rosette stage, petioles showed a fast and large response; during early stem elongation the response of petioles was less, but the extension of the lower internodes considerable; during flowering stem development, high internodes extended and the contribution to final stem length diminished. Ethylene production by an intact plant before, during and after submergence was measured with a laser-driven photoacoustic technique. Internal ethylene concentrations increased within 12 h of submergence. Ethylene accumulated in the submerged plant due to increased synthesis as well as a reduced diffusion from the plant to the water (instead of air). The elongation response could in part be mimicked by exogenous ethylene. Directly after submergence a further increase of ethylene synthesis was observed, which may be of vital importance in causing shoots to continue their rapid elongation even after the water surface is reached. The responses of shoots were related to fitness in experimental field plots. Survival of submerged R. maritimus depended on its ability to emerge above the water surface, while seed production was positively correlated with shoot height above the