Respiration of the Roots of Flood‐Tolerant and Flood‐Intolerant Senecio Species: Affinity for Oxygen and Resistance to Cyanide

The affinity of respiration for oxygen in the roots of six Senecio species studied was low compared with the affinity of cytochrome oxidase for oxygen. Half saturation values of approximately 22 μM oxygen were measured. Root respiration was to a large extent insensitive to cyanide in flood‐tolerant as well as in flood‐sensitive species. The evidence presented suggests that high activity of salicylhydroxamic acid (SHAM)‐sensitive oxidase in Senecio roots was the basis for the low oxygen affinity and for the high cyanide‐insensitivity of root respiration in the Senecio species. Methods are described to determine the in vivo activity of the SHAM‐sensitive oxidase. It was estimated that it contributed 70% to the total root respiration. The presence of SHAM‐sensitive oxidase activity could explain a higher efficiency of root growth respiration under a low oxygen tension if this alternate oxidase was inhibited at a low oxygen concentration in the root medium. However, the SHAM‐sensitive oxidase was not specifically involved in either growth respiration or maintenance respiration. Its significance in regulation of the redox state of the cells is discussed.