Effects of Growth Retardants on Biosynthesis of Gibberellin Precursors in Root Tips of Peas, Pisum sativum L.1

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Abstract
Pea root tips were pre-incubated for 2 hr in the presence of Alar, Amo–1618, or Cycocel after which a quantity of 2-14C-mevalonic acid was added to the medium. The incorporation of mevalonic acid subsequent to decarboxylation into 3 gibberellic acid precursors, trans-geranylgeraniol, kauren-19-ol, and kauren-19-al was ascertained by paper and thin layer chromatography. The amount incorporated into a fourth acidic fraction containing kaurenoic acid was also estimated. In the presence of Alar, the amount of radioactivity in trans-geranylgeraniol increased while that in kauren-19-al decreased simultaneously; the radioactivity in the acidic and kauren-19-ol fractions remained unchanged. Amo-1618 at 125 ppm induced an accumulation of radioactive acidic materials and concurrent decreases in the other 3 fractions. With Cycocel, the incorporation of 14C in kauren-19-ol and kauren-19-al remained unchanged while that in the acidic fraction increased and trans-geranylageraniol decreased. Amo-1618 was 100-fold more effective than Cycocel in altering the levels of GA precursors. The Alar concentration required to affect the biosynthesis of GA precursors was the same as that which inhibited epicotyl elongation of intact pea seedlings. The findings support the hypothesis that these growth retardants disrupt GA biosynthesis which in turn affects subapical meristematic activity, thereby inhibiting internodal elongation.

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