Many viable seeds germinate immediately after harvest if provided with suitable conditions of germination i.e., water, oxygen and suitable temperature (some seeds e.g., Lettuce need light also). However perfectly viable seeds of many plants do not germinate immediately after harvest even when provided with suitable conditions of germination i.e., their germination is blocked. This block to the process leading to germination is called dormancy. The seed dormancy may be due to many causes some of which are as follows :
Causes of seed dormancy
1. Impermeability of seed coats to oxygen e.g., Xanthium
2. Impermeability of seed coats to water e.g., Many plants of Leguminosae,
3. Hard seed coat, which does not allow proper growth of developing embryo e.g., Mustard
4. Immature embryo, some seeds contain an imperfectly developed embryo.
5. Embryo requiring After-ripening in dry storage: These embryos although developed fully, do not germinate unless kept in storage in a dry place for sometime after harvest.
6. Germination Inhibitors : Some plants produce such chemical compounds that inhibit the germination of their own seeds e.g., Tomato - The pulp of the fruit produces an inhibitor.