| 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 |
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