The Significance of Oxygen Transport and of Metabolic Adaptation in Flood‐Tolerance of Senecio Species

Experiments were designed to provide information about the physiological basis of flood‐tolerance in Senecio species. The oxygen concentration in roots of S. jacobaea L., S. viscosus L. and S. vulgaris L. became almost zero after transplantation to a solution of low oxygen concentration, and it was concluded that the flood‐sensitivity of these Senecio species could be due to insufficient oxygen transport from the shoots to the roots. The oxygen concentration in the roots of the flood‐tolerant S. congestus (R.Br.) DC., growing in a solution of low oxygen tension, was almost sufficient to maintain oxygen utilization at the rate observed in roots of plants, grown in an air‐saturated solution. Oxygen utilization by roots of the flood‐tolerant S. aquaticus Hill, growing in a solution of low oxygen tension, was inhibited 50%. However, the oxygen concentration in the roots of this species remained high enough to maintain cytochrome oxidase activity and oxidative phosphorylation at the rate observed in roots from an air‐saturated environment. The activity of a second (“alternative”) oxidase must have been drastically reduced. Alternative NADH‐oxidizing enzymes, like nitrate reductase which was induced by anaerobiosis in roots of S. aquaticus, might replace the regulatory function of the alternative oxidase. — Thus, in S. aquaticus root porosity and root length contributed to the maintenance of an oxygen concentration which was sufficient for uninhibited cytochrome oxidase activity and oxidative phosphorylation rate in roots growing in a solution of low oxygen tension.