Seeds and Seedlings - Seed Testing - page 26

GErMInATIon - TrAnSITIon froM SEEd To SEEdLInG
Seed germination is deined as the resumption of growth of the seed embryo from a state of quiescence to
the development of a seedling. This is a complex process of growth and development and requires a favor-
able environment of moisture, temperature and oxygen availability. Additionally, some seeds require light
for germination while others do not. In any case, seed testing laboratories must be equipped with germina-
tors or controlled environment rooms which provide optimum germination conditions (see Chapter 5).
Water Availability
Water is perhaps the key factor enabling a seed to begin to germinate. Moisture imbibition results in hydra-
tion of the seed tissues and establishes a medium for enzyme activation, breakdown of food storage reserves
and the translocation of nutrients to meristematic regions. This permits seedling growth and development.
favorable Temperature
Other than imbibition, none of the events in germination could occur without a favorable temperature. The
process of germination is dependent on a complex sequence of chemical changes, all of which require a
favorable temperature. Thus, processes of enzyme activation, breakdown of food reserves, their transloca-
tion to the growing points and their utilization in seedling growth and development are dependent on a
favorable temperature.
oxygen (Air) Availability
The breakdown of food reserves in the seed is essentially an oxidative process. That is, oxygen is required.
Fortunately, most species have evolved to germinate best in an oxygen environment equivalent to that of
ambient air. In a practical sense, seeds can generally germinate satisfactorily if a consistent source of fresh
air is available. Most seeds will not germinate if the oxygen supply is curtailed.
Light and Seed Germination
The role of light in seed germination can be quite complex. Some species germinate readily in the complete
absence of light (e.g., corn), while others will not germinate without speciic light regimes (e.g., lettuce).
Fortunately, years of experience and research in germination testing have resulted in a wealth of informa-
tion in the light requirements of species commonly encountered in the laboratory. This information has been
incorporated into the rules of both the AOSA and ISTA.
Other than the inluence of light in promoting germination, the reduction or complete absence of light
can result in weak, spindly, etiolated seedlings characterized by yellowish growth devoid of the normal
green color. While this does not in itself cause abnormality, it can make evaluation of germination more
dificult.
SEEdLInGS
Upon water uptake (imbibition), seeds may germinate if they are physiologically mature and if favorable
environmental conditions are present.
Germination is of two types; epigeal or hypogeal . Epigeal germination describes the condition in
which the cotyledons are raised above the soil as in garden beans and conifer seedlings. Hypogeal germina-
tion describes the condition where the cotyledons (or endosperm) remain below the soil surface. Seedlings
with hypogeal germination include peas and cereals.