Thursday, May 6, 2010

Germination

Before I talk about germination I would like to start by comparing two types of seed based on their water content at maturity:

1.  Orthodox seeds are those that can withstand drying and freezing after maturity.  For example corn (Zea mays) and beans (Phaseolus vulgaris) are traditionally dried after harvesting and sometimes they may be kept in low-temperature environment for storage.  This process helps maintain the viability of the seeds for a longer period of time.  Most annual plants have orthodox seeds.  As the seed matures, its moisture content decreases while dry matter content increases.

2.  Recalcitrant seeds (as in avocado) lose their viability when their moisture content is reduced to a certain level.  Unlike the orthodox seeds, drying and freezing recalcitrant seeds damage the embryo.  Examples of plants with recalcitrant seeds are mango, cacao, and avocado.  As the fruit matures, the seed moisture content increases.  At this point the seed is capable of germination.  Recalcitrant seeds should not be dried after they are extracted from the fruit.   

Germination is the sequence of events in a viable seed starting with the imbibition of water that leads to the growth of the embryo and the development of a seedling.

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