Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedling.
Seed germination : It is the process by which the seed resumes its metabolic activities and grows under favourable conditions.
The following steps are involved in seed germination :
1. Imbibition of water : The seeds imbibe or uptake water and swell. Imbibition takes place against great compressive forces. Due to the imbibition pressure seed coats ruptures.
3. Resumption of metabolic activities : As the protoplast is rehydrated, the metabolic activties resume. The respiration is in the beginning anerobic which later becomes aerobic When the anerobic respiration reaches peak then mitoehondria differentrates in the embryo cell.
4. Mobilisation of reserve food : Activated embryo has long lived RNAs. They induce the production of hormones which synthesise hydrolytic enzymes like amylases, proteases, lipases. The enzymes move into the storage cells and digest the nutrients which are translocated to the tips of embryo axis.
6. Growth of the embryo : On receiving the food the embryo cells metabolise actively. DNA replicates and transcribe RNAs. The cells divide. The radical end enlarges, ruptures the seed coat and passes downward to form primary root which later on forms the root system.The plmule elongates to form the stem, new leaves are formed and cotyledons fall.
Growth is most important phenomenon because of following reasons :
1. Due to growth, size of the organism, increases.
2. The growth results in increase in weight, mass and size of the organism.
3. Growth allows the cells to be differentiated. e.g. In roots, the cells first elongate in the zone of elongation and then in the zone of differentiation they are differentiated into tissues and organs.
4. It is due to growth that radical forms root and plumule forms the stem during germination of the seeds.
5. It is due to growth that buds sprout to form the branches.
8. It is due to the growth that floral buds sprout to form flowers.
9. It is due to the growth of pollen grains and functional megaspore cells that male and female gametophytes are formed respectively.
10. It is due to the growth that pollen tubes grow and siphonogamy occurs.
11. The fruits are formed due to the growth of the ovary.
1. They bring growth.
2. They stimulate cell division.
3. They bring opening of stomata.
4. They check abscission of leaves except ethylene.
5. They break seed dormancy.
6. They induce formation of fruits and flowers in all types of plants.
7. Gibberellins overcome vernalization.
Inhibitor hormones :
1. They retard growth.
2. They inhibit cell division.
3. They bring closing of stomata.
4. They bring abscission of leaves.
5. They induce seed dormancy.
6. They generally do not induce fruit and flower formation.
7. They have no impact.
Vernalization ; It is the treatment of low temperature for reducing vegetative phase and hastening the reproductive phase. It was first discovered by Lysenko.
Vernalisation refers specially to the promotion of flowering by a period of low temperature. For example - Subjecting the growing of a biennial plant to a cold treatment stimulates a subsequent photoperiodic flowering response.
Significance of vernalization :
1. The reproductive phase can be quickened.
2. The yield is increased.
3. The plants become more resistant towards diseases.
4. It prevents precocious reproductive development late in the growing season, and enables the plant to have sufficient time to reach maturity.
Types of plants depending upon photoperiodism :
1. Short day plants : These plants produce flowers in winter. They are also called long night plants e.g. Chrysanthemum, tabacco etc.
2. Long day plants : These plants produce flowers in summer, when the days are long and duration of light is above critical point e.g. Melon, Momordica, Water-melon, Lady's finger plant etc.
3. Short long day plants : These plants need short photoperiod for initiation and long photoperiod for blossoming of flowers e.g. Rye, Campanula etc.
4. Long short day plants : These plants need long photoperiod for initiation and short photoperiod for opening of floral buds. They give flowers between summer and winter e.g. Cestrum, Bryophyllum etc.
5. Intermediate plants : These plants need definite photoperiod for flowering e.g. wild kidney bean.
6. Day neutral plants : These plants produce flowers in both winter and summer seasons. They do not need any specific photoperiod e.g. Sunflower Tomato etc.