A Relation between the Effects of Gibberellic Acid and Indolylacetic Acid on Plant Cell Extension

IF gibberellic acid, a metabolic product of the fungus Gibberella fujikuroi , is applied to growing plants, it commonly causes a marked increase in stem internode length, mainly attributable to cell extension. Some dwarf varieties of pea are particularly sensitive, visible increases in internode-length resulting from doses of 0.01 µgm. gibberellic acid per plant 1 . Auxins applied in a similar way have little or no effect ; but, since auxins do induce cell extension in some circumstances, comparisons of the physiological effects of auxins and gibberellic acid have been made. It has been shown 2 that gibberellic acid is inactive or of low activity in many conventional auxin assays, does not stimulate rooting of cuttings, does not inhibit development of lateral buds in decapitated plants, does not prevent leaf abscission and neither stimulates nor seriously inhibits root growth ; similar observations have been made 3 with the related material gibberellin A . Thus the spectrum of activity of gibberellic acid and gibberellin A is very different from that of the auxins ; Kato 3 has concluded that the mode of action is different from that of the auxins, but we 2 have preferred to leave the question undecided in the absence of more positive evidence. We have recently made some observations which throw light on the relation of auxin activity to that of gibberellic acid.