growth substances of plants. Twenty-seven of them are known; they are all tetracyclic diterpenes and carboxylic acids. The basic structural unit of the gibberellins is gibberellin A₉ (I); the other gibberellins are considered to be derivatives of it. The gibberellins are unstable and decompose rapidly in acid or alkaline mediums. The greatest biological activity is more often shown by gibberellic acid (A₃), which differs from A⁹ by the presence of hydroxyls at the carbon atoms (shown by the arrows) and by a double bond (II); the molecular weight is 346.39, melting point 233°-235° C.

The gibberellins were discovered by the Japanese scientist E. Kurosawa in 1926 while studying a rice disease (excessive growth) caused by the fungus Gibberella fujikuroi Sow. In 1935 the Japanese scientist T. Yabuta isolated crystalline gibberellins from this fungus and gave them their present name. In higher plants the fast-growing tissues are richest in gibberellins; they are present in unripe seeds and fruits, shoots, and developing cotyledons and leaves. The gibberellins are components of the system that regulates plant growth. They accelerate cell separation in the zone immediately adjacent to the tip of the stem and growth in the expansion phase. Gibberellins stimulate growth (mainly of stems and stalks) more strongly than auxins, and under some conditions they can accelerate the growth of leaves, flowers, and fruits. They stimulate the development of plants (depending on the temperature and photoperiod) and, under certain conditions, flowering and setting of fruits. Light promotes the formation of gibberellins in plants. An absence or excess of gibberellins causes some pathological symptoms—dwarfism or excessive growth.

In practice the gibberellins are used in plant growing to increase the yield of hemp and flax fibers, to increase the fruit size of seedless grapes, to accelerate the fruit-bearing of tomatoes, to raise yields of grasses, and to stimulate seed germination (treatment with gibberellins destroys the dormant state of the tissues and has a stratifying action on the seeds; with natural passage of seeds from a state of dormancy the content of endogenic gibberellins increases). Because gibberellins markedly accelerate the growth of plant verdure, their use should be accompanied by an increase in feeding of plants. Gibberellins are produced mainly by a microbiological method from the products of vital activity of molds of the genus Fusarium.