Growth and Development of Horticultural Crops (1+1)

Factors causing dormancy of seeds

1.Seed coats impermeable to water

• The seeds of certain plants especially those belonging to the families leguminosae, solanceae, malvaceae, etc. have very hard seed coats which are impermeable to water.
  
• The seeds remain dormant until the impermeable layer decay by the action of soil micro-organisms.
2.Seeds coats impermeable to oxygen

• In many plants such as cocklebur and many grasses, the seed dormancy is due to the impermeability of the seed coat to oxygen.
  
• However, during the period of dormancy the seed coat gradually becomes more permeable to oxygen so that they may germinate.
3.Immaturity of the Embryo

• In certain orchids, the seed dormancy is due to the immaturity of the embryos which fail to develop fully by the time the seeds are shed.
  
• In such cases, the seeds germinate only after a period or rest during which the development of embryo inside the seed is completed.
4.Germination Inhibitors

• In certain seeds, the dormancy of the seeds is due to the presence of certain germination inhibitors like coumarin, ferulic acid, abscisic acid, etc. These may be present in endosperm, embryo, testa or juice or pulp of fruit.
5.Chilling or low temperature requirement

• In certain plants such as apple, rose, peach etc, the seeds remain dormant after harvest in the autumn, because they have a low temperature or chilling requirement for germination.
  
• In nature this requirement is fulfilled by the winter temperatures.
  
• In such case, the seeds remain dormant throughout the winter season and germinate only in the following spring.
6.Light sensitive seeds

• In many species, the germination of the seeds is affected by light resulting in seed dormancy.
  
• Such light sensitive seeds are called photoblastic.
  
• Seeds of lettuce, tomato and tobacco – are positively photoblastic and germinate only after they have been exposed to light.
  
• On the other hand, the seeds of certain plants are negatively photoblastic and their germination is inhibited by light.