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There are two classes of gibberellins: the 19-carbon gibberellins and the 20-carbon gibberellins. The 19-carbon gibberellins,formed from 20-carbon gibberellins,are the bio-logically active forms. Gibberellins also vary according to the position and number of hydroxyl groups linked to the carbon atoms of the ent-gibberellane skeleton. Hydroxylation has a profound influence on biological activity. Probably the best-defined role for gibberellins in regulating the developmental processes in higher plants is stem growth. The cellular basis for gibberellin-induced stem growth can be either an increase in the length of pith cells in the stem or primarily the production of a greater number of cells. Applied gibberellins can often promote germination of dormant seeds,a capability suggesting that gibberellins are involved in the process of breaking dormancy. Gibberellins are intimately involved in other aspects of seed germination as well. Applied gibberellins promote or induce flowering in plants that require either cold or long days for flower induction. Gibberellin is probably not the flowering hormone or floral stimulus,because the floral stimulus appears to be identical or similar in all response types. The application of gibberellins often modifies sex expression,usually causing an increase in the number of male flowers. See also Dormancy; Flower; Plant growth; Seed. Although gibberellins have limited use in agriculture compared with other agricultural chemicals such as herbicides,several important applications have been developed,including the production of seedless grapes. Application of gibberellin at bloom results in increased berry size and reduced berry rotting. Gibberellins are also used to increase barley malt yields for brewing and to reduce the time necessary for the malting process to reach completion. Gibberellins have found significant applications in plant breeding. Other uses for gibberellin in agriculture include reduction of rind discoloration in citrus fruits,increased yield in sugarcane,stimulation of fruit set in fruit trees,and increased petiole growth in celery. See also Plant hormones
赤霉素最突出的生理效应是促进茎的伸长和诱导长日植物在短日条件下抽薹开花。各种植物对赤霉素的敏感程度不同。遗传上矮生的植物如矮生的玉米和豌豆对赤霉素最敏感，经赤霉素处理后株型与非矮生的相似；非矮生植物则只有轻微的反应。有些植物遗传上矮生性的原因就是缺乏内源赤霉素（另一些则不然）。赤霉素在种子发芽中起调节作用。许多禾谷类植物例如大麦的种子中的淀粉，在发芽时迅速水解；如果把胚去掉，淀粉就不水解。用赤霉素处理无胚的种子，淀粉就又能水解，证明了赤霉素可以代替胚引起淀粉水解。赤霉素能代替红光促进光敏感植物莴苣种子的发芽和代替胡萝卜开花所需要的春化作用。赤霉素还能引起某些植物单性果实的形成。对某些植物，特别是无籽葡萄品种，在开花时用赤霉素处理，可促进无籽果实的发育。但对某些生理现象有时有抑制作用。
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