The effects of gibberellins on elongation are seen in
—Bolting of biennials, to produce flowers during the first growing season
—Reversal of genetic dwarfism
Gibberellins are important to breaking dormancy of gibberellins to seeds will counter the normal environmental cues, such as exposure to low temperatures. They stimulate RNA to promote synthesis of enzymes that convert stored nutrients (starches) to sugars needed for rapid cell respiration during germination. Gibberethns can, as can auxins, promote parthenocarpy in many plants, which has commercial applications, although the most significant use in the grape industry, where grapes grow larger, and with longer internodes.
Abscisic acid (ABA) is a hormone that functions by inhibiting growth activities in times Of environmental stress rather than by promoting growth. It often serves as an antagonist to the other growth promoting hormones in plants. Abscisic acid, which is also synthesised from mevalonic acid, got its name from the erroneous beliefthat it promoted the formation of abscission layers in leaves and fruits. It does not, although leaf abscission accompanies dormancy in many plants.
ABA promotes seed dormancy activities. ABA levels are high when seeds mature, promoting lowered metabolism and synthesis of proteins needed to withstand the dehydration associated with dormancy. Seeds germinate when ABA is degraded by some environmental action. Desert seeds must have the ABA washed out of the seed coat, as do plants of marshy areas; temperate area plants have ABA degraded by light_ stimulatedenzymes. In other cases breaking dormancy is relative to the ratio of ABA (which keeps seeds in dormancy) and gibberellins (which promote germination).
Low levels of ABA in maturing seeds proniote premature germination. Similar to seed dormancy, ABA promotes winter bud scale formation on woody plants in preparation for winter dormancy. ABA derivatives, called dormins, are used in commercial nurseries to keep materials to be shipped in dormant conditions. The dormancy can be reversed with gibberellins.
ABA is also referred to as the stress activity hormone. For example, ABA promotes stomata closure during leaf water deficit conditions by activating K+ ion transport out guard cells. This involves signal transduction pathways with Calcium secondary messengers. ABA in this case originates in roots, and detects low water level in root tissues. ABA moves upward into leaves and activates stomata] closure.
Etnymn Ethylene is the sole plant growth regulator known that is a gas. Ethylene is produced in many plant organs, though its effects are most studied in fruits. Ethylene is the signal molecule for signal transduction pathways that activate a number of tissue growth and development activities throughout the plant, including fruit maturation, teat abscission and senescence.
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