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Please note: most of the diagrams that are referred to in this page
are copyrighted and found in print in: L. Taiz and E. Zeiger. 2002. Plant Physiology. 3ded. Sinauer Associates, Inc. Sunderland, MA. additional figures come from: Buchannan, Gruissem & Jones. 2000. Biochemistry & Molecular Biology of Plants American Society of Plant Physiologists, Rockville, MD. The diagrams will NOT load unless you enter an assigned password. Passwords are assigned only to students at Eastern Connecticut State University who are enrolled in the Plant Physiology course. Passwords cannot be legally assigned to any other individuals. |
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As with many of the hormones, there are various sensitivities in various tissues that change over time. Because of this, GA elicits pleiotropic responses. Your book lists several of these.
GA stimulates growth of dwarf varieties, and dwarfing agents that block GA biosynthesis can be used to control height of tall forms. Virtually all Easter lilies, poinsettias, and chrysanthemums that you buy at a florist have been treated with "dwarfing agents" (GA biosynthesis inhibitors).
GA stimulates the development of flowers or pine cones, increases floral size, but also makes dicot flowers male-only in certain "plastic" hermaphroditic species. In monocots, gibberellins can induce female expression in the tassels (usually male flowers). In GA defective mutant corn, the ears produce anthers:
GA stimulates the expansion of fruits. Probably the most famous example is in grapes. The seedless green and red table grapes you enjoy lack ovules to produce GAs, so the growers must spray (or dip) the senescing flower clusters with GA to get the fruits to enlarge. So every table grape you buy at the store is thus treated with GA at least 3 times in the growing season. Fortunately GA is not known to cause and human problem in studies.
GA stimulates seed germination in species that are kept dormant by Abscisic Acid (remember your project on this idea?). One organism has been extensively studied by plant physiologists in this regard: Barley (Hordeum vulgare). This seed is germinated (malted) to produce beer. The sprouting process involves in part the conversion of starch in the endosperm of the seed into maltose (a disaccharide). This maltose is converted to alcohol and carbon dioxide by fermentation of the seed extract by yeast (fungi) during beer making.
Gibberellins are known to stimulate the de-novo synthesis (think central dogma) of α-amylase in the aleurone cells that surround the starchy endosperm. This response is summarized graphically below.
This and much more has been condensed into a diagram depicting the total response.
As we leave this chapter, let me take a parting shot at your textbook. I think it has shamefully made attributions and statements that are misleading and false in some sections. I believe the book has hidden or ignored several lines of evidence and introduced errors by these omissions. The work of Richard Pharis and Peter Kaufman, in particular, should have been elucidated and included. I have not dignified this book's treatment of stem growth by discussion here for this reason. I understand that some researchers are more socially accepted than others, that the "American Plant Physiology Club" often excludes Canadians, and so on...but to hide or exclude their work in a general textbook is not acceptable. That is particularly true when that work so clearly addresses mechanisms that have not been found elsewhere, but which has been verified independently.