Changes in Paramagnetic Manganese (Mn2+) and Related Enzyme Activities in Isolated Cucumber Cotyledons During Growth

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Abstract
Changes in the activities of IAA oxidase, peroxidase, ascorbic acid utilization (AAU), and in the level of paramagnetic manganese (Mn2+) have been studied during kinetin-induced growth of the isolated cucumber cotyledons in light or in dark. In kinetin-treated cotyledons exposed to light, inhibition in the level of paramagnetic manganese corresponds with an enhancement in IAA oxidase activity. The level of paramagnetic manganese shows an inverse correlation with IAA oxidase activity. In darkness the level of Mn2+ does not show the same correlation with IAA oxidase activity as in the light. Kinetin stimulates peroxidase activity both in the light and in darkness. Enhancement of IAA oxidase activity and no corresponding change in the level of paramagnetic manganese indicates that the oxidation of IAA in dark-grown, kinetin-treated cotyledons is brought about by peroxidase. It appears that the phenolic cofactors required for the oxidation of manganese and IAA may be limiting in kinetin-treated cotyledons in darkness. Thus in the light, IAA oxidation seems to be brought about by peroxidase as well as manganese, whereas in darkness it is mediated by peroxidase alone. Increase in IAA oxidase activity during kinetin-induced growth of the isolated cotyledons is incompatible with the idea that increased IAA oxidase activity would limit the availability of auxin for growth. Kinetin does not mimic the action of light on IAA oxidase activity; on the contrary, it removes the inhibitory effect of light on IAA oxidase activity probably through the synthesis of IAA oxidase activators.