The relative roles of internal aeration, radial oxygen losses, iron exclusion and nutrient balances in flood-tolerance ofRumexspecies

Radial oxygen losses (ROL) from the roots of three Rumex species, that occur in the river ecosystem in The Netherlands and show a differential response towards flooding, were compared. Oxygen loss from whole root systems was demonstrated and the ROL of single roots was quantified. Radial oxygen losses were higher in the flood-tolerant R. maritimus and R. crispus than in the intolerant R. thyrsiflorus. In all species oxygen loss occurred over the whole root surface between the base and the apex, but the rates differed as well as root wall permeabilities to oxygen. High oxygen losses in R. maritimus and R. crispus were correlated with high internal oxygen pressures near the root apex, consistent with prolonged root growth under anaerobic conditions in these species. On a flooded clay soil, the more tolerant species showed soil penetration and iron oxidation to greater depths, but all species developed an iron plaque on the roots. Shoot iron content was highest in the flood-tolerant R. maritimus. Upon flooding of the flood-intolerant R. thyrsiflorus, however, there was a substantial decrease in shoot dry weight and tissue nutrient levels. This was attributed to restricted root development rather than to iron toxicity. toxicity.