The IAA-oxidase system of citrus roots

The activity of indoleacetic acid oxidase in citrus plants (roots of mature trees and roots and stems of seedlings) was demonstrated. The interrelationships between 2,4-dichlorophenol, Mn⁺⁺ and the enzymatic activity were examined. The presence of Mn⁺⁺ was not found necessary, even in dialyzed enzyme reaction mixtures. However, Mn⁺⁺ promoted the enzymatic activity at the very high concentration of 10^-1M. The inhibitory effect induced by excessive amounts of 2,4-dichlorophenol (10^-3 M) was removed in the presence of Mn⁺⁺. The enzyme showed a specificity to IAA as compared with other synthetic auxins in a wheat coleoptile bioassay. Moreover, the enzyme acted selectively on some endogenous auxins found in citrus tissues. The existence of endogenous inhibitors in stems and cofactors in roots was also demonstrated.