Question

Topic: Seed Germination

-Definition of process:

-Hormone(s) involved:

-Cells or tissues that produce the hormone:

-Briefly describe the nature of the response (use diagrams if you wish):

-Evolutionary explanation for the response (i.e why wold plants capable of responding in this way leave more offspring than ones that cannot):

Answer 1

The seed represents, the embryonic form of plant in the resting phase. By influence of various factors, the seed resumes its growth into a complete plant. This process s called seed germination.

Seed germination may comprise of phases:

1. Imbibition: involves rehydration of seeds by absorption of water. The seed coat ruptures, radicle emerge to form root.

2. Seed respiration: resumption of metabolic activities. Initial anaerobic respiration followed by anaerobic respiration.

3. Seed germination: effect of light: light dependent or sensitivity observed in photoblastic seeds. Phytochrome and some hormonal regulation.

4. Mobilization of seed reserves: stored reserves from endosperms are mobilized. Hormonal control is required.

5. Development of embryonic axis to seedling: Activation of growing regions, cell growth, division and differentiation.

Several hormones are involved in the process of seed germination:

	Hormones involved in seed Germination	Producing cells or tissues	Response
1.	Auxin: Endogenous forms are- a. Major Form-Indole Acetic Acid (IAA), b. CIAA, c. IBA, d. Phenyl acetic acid.	Apical meristem tissues, embryos	1. Embryo development Formation of: -shoot apex -primary leaves -cotyledons -stems -roots 2. Phototropism 3. Gravitropism 4. Apical dominance 5. Fruit development 6. Abscission prevention 7. Root initiation and development 8. Shade avoidance effect
2.	Gibberellins: Gibberellic acids (GA) Bioactive forms are-GA1, GA3, GA4, GA7	Apical meristems of root, stems, young leaves, Outer tissue layer of seeds	1. Controlling phytochrome activity 2. Breaking seed dormancy 3. Synthesis of hydrolytic enzymes that promotes seed germination. 4. mobilization of enzymes and nutrient release 5. Stem elongation, mutant dwarf variety to tall. 5. Bolting 6. Fruit development.
3.	Cytokinin: Adenine, Phenyl urea	Seeds, Root tissues, transported to other parts by xylem.	1. Cytokinesis or cell division. 2. Seed germination- produced in high level during seed growth, later declines at maturity. 2. Cell growth and differentiation of primary tissues 3. Regulates meristem activity. 4. Apical dominance, Growth of lateral shoots 5. Nodule formation. 6. Alleviation of stresses, like salinity, drought, heavy metals, oxidative stress
4.	Abscisic acid: Abscisin II, Dormin	Seeds, immature fruits, flowers, roots, stems, leaves, desiccated tissues	1. Growth inhibitor, inhibits germination and Vivi parity 2. Seed dormancy 3. Prevents premature germination of seeds. 4. Bud dormancy 5. Antitranspirant
5.	Ethylene	Seeds, all parts of higher plants	1. Seed germination, in counter effect of Abscisic acid. 2. Inhibit dormancy 3. fruit ripening 4. induce root hair growth 5. Sex expression 6. thigmomorphgenesis

Changes in hormone physiology, affect embryonic, as well as maternal tissues of plants. Hormones are responsible for recalcitrance and vivipary in plants families. The variation of response towards hormones, determine the physiological effect, inturn leading to genetic variation.

The response of plants towards hormones, contribute to the environmental factors and genetic variance associated with the environment. The mechanism or phototropism, geotropism, and other responses in term of environmental changes, allows the plants to adjust and evolve according to the factors changing in the environment.