Gibberellin-Dependent Root Elongation in Lactuca sativa: Recovery from Growth Retardant-Suppressed Elongation with Thickening by Low concentration of GA3

Higher concentrations of growth retardants, ancymidol and AMO-1718,were required to suppress root growth than hypocotyl growthin lettuce seedlings. Gibberellic acid (GA₃) counteracted theeffect of these growth retardants, but complete recovery ofroot growth was obtained only in a narrow range of growth retardantconcentrations. A much lower concentration of GA₃ (1 nM) wasneeded for recovery of root growth from ancymidol suppressionthan that for hypocotyl growth (100 (µM). GA₃ synergisticallypromoted root growth at moderate concentrations (10–100nM) with either ancymidol or AMO-1618. Ancymidol not only suppressed root elongation but also causedthickening of the elongation zone of the root, actions whichGA₃ completely canceled. Microscopic observation showed theseeffects were mainly due to the lateral expansion and shorteningof epidermal and cortical cells. Growth kinetics of roots recorded by a computer-regulated rhizometerindicated that the lag times of both growth suppression by ancymidoland growth recovery by GA₃ were about 4 h. -Naphthaleneacetic acid (NAA) did not counteract ancymidol suppressionof root and hypocotyl growth. These results support the concept that gibberellins play anindispensable role in root elongation at an extremely lowerconcentration than in hypocotyl elongation. (Received January 7, 1987; Accepted May 14, 1987)