Gibberellin movement: Review and new evidence

The movement of the plant growth regulators known as gibberellins through excised organ sections has been investigated using, first, direct bioassays, then radioactively labelled gibberellins, and more recently gas chromatography-mass spectrometry to identify definitively the compounds in the receiver agar. Of the dozens of gibberellins currently known, the movement of only the biologically very active gibberellins A₁ and A₃ have been studied in detail. In comparison with the well-known polar movement of indolyl-3-acetic acid, gibberellins A₁ and A₃ show little if any polarity of movement in shoot tissues, through which they move at the rate of 1-2 mm/hr, like the cytokinins, abscisic acid and 2,4-dichlorophenoxyacetic acid - all noticeably slower than the 5-7 mm/hr at which indolyl-3-acetic acid moves basipetally through similar shoot sections. However, through root sections these gibberellins do move with polarity, the predominant movement being from the root tip towards the root base. This is the opposite of indolyl-3-acetic acid's predominant polarity through such root sections. The interpretation, based on results with excised transport sections, of gibberellins A₁ and A₃ as mobile and thereby "hormonal" substances is supported by comparison of movement of gibberellins A₁ and A₅ in a species in which both gibberellins are endogenous, and by reports of gibberellin movement when added to intact or grafted plants.