Growth-regulator Interactions in the Growth of the Shoot System ofAvena sativaSeedlings

Abstract

1. Segments, 3.5 mm. long, cut from the first internode of Avena sativa seedlings grown in complete darkness respond to both auxins and gibberellic acid by accelerated extension. 2. The optimum concentration of indole-3-acetic acid (IAA) is 10 p.p.m. and of gibberellic acid (GA) is 0.1 p.p.m. 3. The degree of stimulation relative to the growth of control segments is affected by the inclusion in the segement of the node between the internode and coleoptile. Thus the gibberellin effect is greatly increased while the IAA effect is decreased. The optimal concentrations are not affected by inclusion of the node. 4. These results can best be explained in terms of the supply by the node tissue of an endogenous auxin which is necessary for the expression of GA action. 5. Numerous factorial experiments demonstrated that there is no detectable interaction between applied IAA and GA in the promotion of first-internode extension. This implies that the postulated endogenous auxin which synergized GAA action in (4) is either an active form of IAA produced only in the node tissue or is a completely different auxin. 6. No synergism of growth-promotive action can be detected between GA and the two synthetic auxins I-naphthylacetic acid and 2,4-dichlorophenoxyacetic acid. 7. p-chlorophenoxy-iso-butyric acid (PCIB) anc 2,4,6-trichlorophenoxyacetic acid (2,4,6-T) act as weak auxins and thus antagonize competitively the promotive action of GA. 8. The anti-auxin α-(I-naphythyl-methyl-sulphide)propionic acid (NMSP) antagonizes competitively the promotive action of both IAA and GA. 9. The facts under (5)–(8) suggest that auxins and GA are acting at the same growth-promotion centres and may compete for them. 10. Growth inhibitions are induced by high concentrations of PCIB, 2,4,6-T and NMSP. The inhibitions produced by PCIB and 2,4,6-T are both synergized by supra-optimal concentrations of IAA while that of NMSP is synergized by supra-optimal concentrations of both IAA and GA. This similarity of the effects of IAA and GA suggests that their inhibition actions also are of a closely similar nature.