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Higher concentrations of growth retardants, ancymidol and AMO-1718, were required to suppress root growth than hypocotyl growth in lettuce seedlings. Gibberellic acid (GA3) counteracted the effect of these growth retardants, but complete recovery of root growth was obtained only in a narrow range of growth retardant concentrations. A much lower concentration of GA3 (1 nM) was needed for recovery of root growth from ancymidol suppression than that for hypocotyl growth (100 (μM). GA3 synergistically promoted root growth at moderate concentrations (10–100 nM) with either ancymidol or AMO-1618. Ancymidol not only suppressed root elongation but also caused thickening of the elongation zone of the root, actions which GA3 completely canceled. Microscopic observation showed these effects were mainly due to the lateral expansion and shortening of epidermal and cortical cells. Growth kinetics of roots recorded by a computer-regulated rhizometer indicated that the lag times of both growth suppression by ancymidol and growth recovery by GA3 were about 4 h. α-Naphthaleneacetic acid (NAA) did not counteract ancymidol suppression of root and hypocotyl growth. These results support the concept that gibberellins play an indispensable role in root elongation at an extremely lower concentration than in hypocotyl elongation.
Eiichi Tanimoto (1987) studied this question.