BSC 314 Lecture Notes - Lecture 31: Seed Dormancy, Germplasm, Primordium
27 Jun 2018
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Dormancy
Rarely do all factors of the environment remain suitable indefinitely for plant growth. In
the temperate latitudes, for example, breaks in the growing season occur when seasons
change, bringing reduced temperatures and shorter days in the autumn and winter. In
the subtropics and tropics, where temperatures and day lengths remain equitable all
year, water availability may fluctuate between a wet and a dry season. Plants have
developed mechanisms to survive during such adverse periods.
One effective mechanism, used by annual plants, is to produce a photosynthetic and
flowering structure rapidly and then sink the resources derived from photosynthesis into
seed production and distribution. The plant body is no longer useful and is abandoned
after protected embryos are produced. The seeds withstand the changes of the next
unfavorable growth period and germinate when environmental stimuli indicate favorable
growth conditions. Perennial flowering plants also use the seed mechanism, but some
retain their photosynthetic and root structures, merely dropping the most vulnerable
parts (leaves) during the unfavorable growth period. When one or more of the plant
organs undergoes a period in which the growth processes are slowed down or
suspended, that state is termed dormancy. The growth is reactivated when
environmental stimuli are received that, in effect, inform the plant that conditions are
again suitable for growth. The signals to break dormancy are extraordinarily precise.
External stimuli combine with internal signals to ensure that renewal of growth will occur
at the most favorable time. Many plants have internal growth inhibitors that decay slowly
over time, such as ABA. Until the inhibitor has dropped to a certain low level, no growth
will take place despite external stimuli; both external and internal signals must be
correct.
Seed dormancy
Almost all seeds undergo some period of dormancy—if they did not, they would start to
grow in the fruits on the mother plant and defeat their principal purposes: dispersal and
survival of the germplasm. The period between the formation of the seed and the time
when it will germinate is called the after-ripening period, which may be a few days or
months depending on the plant.
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