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.
Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedling.
Auxins are of two types :
(1) Natural auxins : These are naturally occuring auxins, isolated from plants and are regarded as phytohormones e.g. IAA. NAA etc.
(2) Synthetic auxins : These are synthesized by artificially e.g. IBA 2,4-D etc.