Rumex thyrsiflorus, Rumex crispus andRumex maritimus show a differential flood-tolerance in the river ecosystem in the Netherlands.R. thyrsiflorus occurs at high-elevated habitats and is flood-intolerant, the other two species occur at lower-elevated habitats and are flood-tolerant. We compared their respiratory activity under aerobic and anaerobic conditions in the root environment and quantified the internal gas transport. The results indicate that aerial oxygen can be used for root respiration in both aerobically and anaerobically grown plants. The amount of oxygen used via internal aeration increased with decreasing oxygen concentration in the root environment. Aerobically grown plants ofR. maritimus andR. crispus already showed a high internal aeration, but there was a significant increase in internal oxygen transport in anaerobic plants, where new, aerenchymatous roots had formed. This indicates the functional significance of new root formation for respiration in these species upon hypoxia. After two weeks of anaerobiosis, more than 50% of the total respiration of the roots of young plants ofR. maritumus and 40% of roots of young plants ofR. crispus was due to internal aeration at low oxygen concentrations in the root environment.
InR. maritimus both young and old plants performed in this way, inR. crispus only young plants, whileR. thyrsiflorus showed some internal aeration, but this was hardly detectable. These differences can be explained on the basis of a different morphology and concomitant diffusive resistance of both root and shoot system.
In experiments with different submergence levels of the shoot, the amount of internal aeration was positively correlated to the total leaf area protruding above the water surface inR. maritimus. This indicates a functional significance of the petiole and leaf elongation response upon total submergence of this species.