Physiological significance of indoleacetic acid and factors determining its level in cotyledons of Lupinus albus during germination and growth

The results demonstrate the profile of the endogenous indole-3-acetic acid (IAA) in the cotyledons of Lupinus albus L. ( L. termis Forssk.) during germination and seedling growth. The auxin level increases markedly after seed hydration, especially during the time of radicle emergence 24 h after the onset of imbibition. This rise is accompanied by a minimal IAA-oxidase activity, formation of indoleacetylaspartic acid (IAAsp) and an increase in the endogenous tryptophan and tryptophan-carboxyl-¹⁴C degradation, though the latter cannot account for the high IAA level detected during early stages of germination. It is believed that cotyledons are a source of IAA to the developing embryonic axis. – The auxin level drops in the cotyledons during seedling growth, 2–18 days after sowing. This is true also for IAAsp and tryptophan-degrading activity of enzyme extracts. Conversely, endogenous tryptophan is increasingly liberated up to day 14, and IAA-oxidase activity climbs to a peak detected on day 12, prior to the appearance of senescence in the cotyledons. – The physiological significance of IAA and the factors regulating its level in the cotyledons during germination and growth are discussed.