Blom, C.W.P.M., Bögemann, G.M., Laan, P., van der Sman, A.J.M., van Flooding frequencies and intensities determine both species composition and the behaviour of in dividual plants along many rivers in the world. In this context, this paper describes the vegetation zonation associated with the fluctuating water levels of the river Rhine in the eastern part of The Netherlands. To obtain insight into the morphological and physiological processes of plants that have been shown to possess contrasting tolerances to Hooding, certain species were chosen as being repre sentative of the vegetation types from the river foreland and subsequently used in experimental stud ies. These species, of the genera Rume.x and Chenopodium, were subjected to various flooding regimes

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Abstract. Rum ex m aritim us L. occurs in fre quently 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 elonga tion 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. maritim us depended on its ability to emerge above the water surface, while seed production was positively correlated with shoot height above the water.

The influence of different waterlogging treatments on above-gro und development and subsequent flowering and seed production of Chenopodium rubrum L. and Rumex maritimus L. was studied in a greenhou se experiment.C. rubrum, a typical occupant of low sa ndy beaches at tim es when floods have subsided, appeared to be suppressed by a ll waterloggin g regimes. R. maritimus, an inh abitor of wet mud flats of old river beds, was found to be quite tolerant to waterloggi ng conditions; early waterlogging, during the rose tte stage, led to an increase in dry weight and seed production in some ca es. In genera l, seed production cha nged more than biomass as a result of waterlogging; responses greatly depend on regimes. Intermittent waterlogging genera ll y caused more dama ge than did continuou s waterlogging, especia ll y in the case of C. rubrum . Even R. maritimus produced less seeds und er conditions of intermittent waterlogging. The res ults demon strate that adaptations to , and dama ge from , flooding grea tly depend on floodin g regim es which vary substantially in the field situation .

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