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

Plants of Rumex thyrsiflorus Fingerh., R. crispus L. and R.maritimus L., which are zoned along a gradient of elevationin a river foreland ecosystem, and differ in their flood-tolerance,were subjected to different flooding levels. Under conditionsof 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 showedrapid shoot elongation; when elongation resulted in a protrusionof leaves above the water surface, the plants survived. Alternatively,underwater photosynthesis also led to a 100% survival of submergedR. maritimus plants, provided that enough inorganic carbon wasmade available in the water. This could be attributed in partto the use of photosynthetically-derived oxygen for root respiration;in a hydroculture experiment, with 5.0 mM CO₂ in the water inthe shoot environment, photosynthetically-derived oxygen contributedmore than 50% to root oxygen consumption at low oxygen concentrationsin the root environment. The intermediately elevated species R. crispus appeared to bemuch more tolerant towards conditions of prolonged total submergence:older plants survived eight weeks submergence in the dark. Thisresponse was explicable in terms of a dormancy-strategy, whichis characterized by a slow consumption of carbohydrates storedin the tap-root. The differential responses of R. maritimusand R. crispus to total submergence reveal the limitations offlood-tolerance and reflect the different life-histories ofthe species. Key words: Photosynthesis, Rumex, submergence, carbohydrates, growth rate, shoot elongation     

Oxygen uptake by roots of Rumex species at different temperatures: the relative importance of diffusive resistance and enzyme kinetics

Abstract. Oxygen uptake characteristics of the roots of three Rumex species were compared, and related to kinetics of the respiratory system and to root anatomy. The observed differences could not be explained by differences in fundamental characteristics of the oxygen uptake system: with all three species, cytochrome-mediated respiration contributed 70% and cyanide-insensitive (alternative) respiration 30% of the total respiration rate, and apparent K_m values of cytochrome oxidase were lower than those obtained for the alternative oxidase in all cases. However, differences in critical oxygen pressure for respiration (COPR) and in apparent K_m for oxygen, were strongly correlated with differences in root porosity and root diameter. K_m(O₂) values at high and low temperatures were determined, and from Arrhenius plots of oxygen uptake rates between 11 and 32°C, the role of diffusional impedance could be estimated. Root respiration of Rumex maritimus and R. crispus , both with high root porosity, but differing in root diameter, had a low K_m for oxygen (3–7 mmol m⁻³). In contrast with this were the responses of R. thvrsiflorus , which has thin roots but low root porosity: a high K_m (10-20 mmol m⁻³) was found at all temperatures. The role of diffusional impedance as a function of temperature in oxygen uptake rate by the three species is discussed and related to the differential resistance of the species towards flooding.