Endogenous Hormonal Profiles in Hop Development

Changes in gibberellins (GAs), indole-3-acetic acid (IAA), and cytokinins associated with the transition from vegetative growth to reproductive growth in Humulus lupulus L. buds and leaves harvested at fortnight intervals were studied. During vegetative growth, GA₁ increased gradually and the lowest content was observed during flower development. Both GA₃ and GA₄ showed a dramatic increase in the samples taken from the apical part of axillary branches from plants 4–5 m high, which corresponds to the maximum vegetative development prior to macroscopically visible inflorescences. Notable increases in the cytokinins trans-zeatin (t-Z), isopentenyladenine (iP), and the riboside and ribotide forms of iP were also obtained. The auxin, indole-3-acetic acid, was the most abundant plant hormone, and its content was highest during vegetative growth. These results show for the first time a relationship between endogenous hormone profiles and both vegetative and reproductive development in hop plants, which may be relevant for future research on the control of the flowering by exogenous hormone applications.