EVIDENCE ON THE SITE OF ACTION OF GROWTH RETARDANTS

1. The ability of five growth retardants to inhibit the GA-inducedand endogenous growth of Avena leaf sections has been investigated.The retardants vary in effectiveness. The order, from most effectiveto least, is Phosfon D, Amo-1618, C011, CCC and B995. 2. The inhibition of growth caused by Phosfon D and Amo-1618is not reversed by GA. It is apparent that the retardants donot compete with GA at the site of GA-action. 3. Addition of IAA will partially reverse the inhibition inducedby Phosfon D or Amo-1618. It is concluded that the retardantsact in part in Avena leaf sections by interfering with the auxinmetabolism of the tisssue. ¹ Supported in part by grants G-14578 and GB-1950 from the NationalScience Foundation² Present address: Department of Botany, University of Washington,Seattle, Washington