We have previously shown that both endogenous auxin and ethylene promote adventitious root formation in the hypocotyls of derooted sunflower (Helianthus annuus) seedlings. Experiments here showed that promotive effects on rooting of the ethylene precursor, 1-aminocyclopropane-l-carboxylic acid (ACC) and the ethylene-releasing compound, ethephon (2-chloro-ethylphosphonic acid), depended on the existence of cotyledons and apical bud (major sources of auxin) or the presence of exogenously applied indole-3-acetic acid (IAA). Ethephon, ACC, aminoethoxyvinylglycine (an inhibitor of ethylene biosynthesis), and silver thiosulphate (STS, an inhibitor of ethylene action), applied for a length of time that significantly influenced adventitious rooting, showed no inhibitory effect on the basipetal transport of [3H]IAA. These regulators also had no effect on the metabolism of [3H]IAA and endogenous IAA levels measured by gas chromatography-mass spectrometry. ACC enhanced the rooting response of hypocotyls to exogenous IAA and decreased the inhibition of rooting by IAA transport inhibitor, N-1-naphthylphthalamic acid (NPA). STS reduced the rooting response of hypocotyls to exogenous IAA and increased the inhibition of rooting by NPA. Exogenous auxins promoted ethylene production in the rooting zone of the hypocotyls. Decapitation of the cuttings or application of NPA to the hypocotyl below the cotyledons did not alter ethylene production in the rooting zone, but greatly reduced the number of root primordia. We conclude that auxin is a primary controller of adventitious root formation in sunflower hypocotyls, while the effect of ethylene is mediated by auxin.
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Liu et al. (1992) studied this question.