EVIDENCE ON THE SITE OF ACTION OF GROWTH RETARDANTS1

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Abstract
1. The ability of five growth retardants to inhibit the GA-induced and endogenous growth of Avena leaf sections has been investigated. The retardants vary in effectiveness. The order, from most effective to least, is Phosfon D, Amo-1618, C011, CCC and B995. 2. The inhibition of growth caused by Phosfon D and Amo-1618 is not reversed by GA. It is apparent that the retardants do not compete with GA at the site of GA-action. 3. Addition of IAA will partially reverse the inhibition induced by Phosfon D or Amo-1618. It is concluded that the retardants act in part in Avena leaf sections by interfering with the auxin metabolism of the tisssue.