Photolytic Decarboxylation of Carboxyl-14C-labelled Indol-3yl-acetic Acid in Leaves of the Apple Tree

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Abstract
The nature and rate of degradation of carboxyl-14C-labelled indol-3y1-acetic acid (IAA-[l-14C]) were studied in apple leaves. The labelled auxin was applied to the cut surface of the growing shoot after the apical part had been removed. The respiratory CO2 absorbed by chromatographic paper as Na2CO3 then freed by phosphoric acid was quantitatively measured by an internal gas counter. It was found that the concentration of 14CO2 evolved by leaves was 77 times higher in daylight than in darkness. The ratio of 14CO2/CO2 obtained from respiration from the upper surface of leaf blades was two and seven times higher than that from the lower surface after 15 and 30 h of daylight, respectively. No such differences were noticed in darkness. Similarly, the total radioactivity of leaf tissues tripled in daylight, presumably because of photosynthetic incorporation of radioactive CO2 evolved during decomposition of LAA. These facts demonstrate the photolytic character of auxin decarboxylation in apple leaves. Prolonged darkness seemed to provoke a large metabolite withdrawal from leaves and, to some extent, to protect auxin against decarboxylation.

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