Comparative Potency of Nine Gibberellins

Abstract

Gibberellins A₁ to A₉ have been compared, each at several dose levels, in bioassays based on extension of stems of dwarf garden pea (Pisum sativum), dwarf bean (Phaseolus vulgaris) and Lunaria annua, of hypocotyls of cucumber (Cucumis sativus) and lettuce (Lactuca sativa), and of leaf sheaths of three dwarf mutants (d–1, d–3, d–5) of maize (Zea mays). Gibberellins A₃ (gibberellic acid) and A₇ are of high potency in most bioassays. A₈ is of negligible potency in all and is probably not a functional hormone. The other gibberellins show a more or less marked tendency to specificity. The plants used as bioassay material also differ in the specificity of their response. Some, for example, maize dwarfs d–3 and d–5 and lettuce, respond well to most gibberellins; others, for example, cucumber, respond only to a few; extreme specificity is shown by Lunaria annua which, in the unvernalized condition, responds by stem elongation only to gibberellin A₇. Dose/response curves of the various gibberellins are usually parallel, but certain exceptions to this have been found. Possible explanations of specificity are discussed in relation to the results obtained, and it is concluded that insufficient evidence is available to make it possible to draw any valid conclusions. Current definitions of gibberellins, whether based on chemical structure or biological activity, are unsatisfactory.