Growth and Survival Responses ofRumexSpecies to Flooded and Submerged Conditions: The Importance of Shoot Elongation, Underwater Photosynthesis and Reserve Carbohydrates

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Abstract
Plants of Rumex thyrsiflorus Fingerh., R. crispus L. and R. maritimus L., which are zoned along a gradient of elevation in a river foreland ecosystem, and differ in their flood-tolerance, were subjected to different flooding levels. Under conditions of soil flooding, the growth rates of the flood-tolerant R. crispus and R. maritimus were as high as under drained conditions, but that of the flood-intolerant R. thyrsiflorus was halved. Upon submergence, the low elevation species R. maritimus showed rapid shoot elongation; when elongation resulted in a protrusion of leaves above the water surface, the plants survived. Alternatively, underwater photosynthesis also led to a 100% survival of submerged R. maritimus plants, provided that enough inorganic carbon was made available in the water. This could be attributed in part to the use of photosynthetically-derived oxygen for root respiration; in a hydroculture experiment, with 5.0 mM CO2 in the water in the shoot environment, photosynthetically-derived oxygen contributed more than 50% to root oxygen consumption at low oxygen concentrations in the root environment. The intermediately elevated species R. crispus appeared to be much more tolerant towards conditions of prolonged total submergence: older plants survived eight weeks submergence in the dark. This response was explicable in terms of a dormancy-strategy, which is characterized by a slow consumption of carbohydrates stored in the tap-root. The differential responses of R. maritimus and R. crispus to total submergence reveal the limitations of flood-tolerance and reflect the different life-histories of the species.