We have discussed GA with respect to its role in seed dormancy and geminatation. GA was first discovered as the active agent produced by a rice disease that causes drastic elongation of rice. The fungus responsible for the disease is called Gibberella fujikuroi and the disease is called the "foolish seedling" disease because the plants elongate so much, they fall over (this results in crop loss).
Many drawf varieties of plants are defective in GA biosynthesis of sensitivity. For example, pea plants typically grow as a vine. Bush varieties may be defective in GA production. Spraying bush varieties with GA solution will stimulate elongation and restore vine-like growth.
GA-controlled responses include:
The effect of GA on stem elongation is perhaps the most dramatic response. The effect of GA on cell elongation is different thatn the effect of auxin. Instead of causing cell wall acidification, GA changes in cell wall plasticity in other ways. In some fruits, cell enlargement is sensitive to GA.
Effects of ethylene on plant growth were observed in the 1800's when gas street lamps were found to cause plants to produce thick stems and to lose thier leaves. Ethylene was found to be produced by plants and function in fruit ripeneing in the early 1900's.
Physiological responses to ethylene include:
Ethylene physiology has a major impact in the horticulture industry, especially in the fruit and cut flower industries where post harvest physiology determines the quality of the product.
We have examined the role of ABA in hardening, seed and bud dormancy, and regulation of stomata. Although ABA was first thought to be the hormone responsible for leaf and fruit abscision, it was later determined that it is generally not involved in abscision. However it has many other important roles in plant development.
Physiological responses to ABA include:
Plants that are defective in ABA production typically show lack of dormancy and are extremely sensitive to water deficiency. The lack of dormancy can result in vivipory (germination of immature seed while still on the fruit).