Agriculture Reference
In-Depth Information
The farmer is usually aware of the physical appearance of the seed and notes any external damage which
indicates roughly handled seeds that may perform poorly. Thus, it is in the interest of both the company (or
producer) and customer to know the germination capability of seeds prior to purchase.
Many factors may contribute to loss of seed germination. These may be from either physical or physi-
ological causes. Physical quality is associated with the structure or physical appearance of the seed such as
a seed coat fracture or embryo lesion. In contrast, physiological seed quality is related to changes in cel-
lular metabolism, although physical evidence of such changes may also be apparent. These may be caused
by nutrient deiciencies, environmental stress during seed development, or poor storage conditions which
inluence the physiological eficiency of the germinating seed. In some instances, the factor that determines
germination cannot be clearly distinguished as either a physical or physiological problem. For example,
some seedborne infections alter the seeds' physical appearance but their primary inluence is on the physi-
ological condition.
Physiological Seed damage
The maturation environment of the seed has a profound inluence on its subsequent germination. The time
of physiological maturity, which corresponds to the time of maximum dry weight of the developing seed, is
the point at which the seed achieves its greatest potential for rapid germination. Studies have demonstrated
that larger and heavier seeds possess greater germination capability. However, other factors may also affect
its germination. For example, the mineral nutrition of the parent plant also inluences germination capabil-
ity, as evidenced by manganese deiciencies in peas that may lead to brown necrotic areas (marsh spot) on
the inside (adaxial) surface of the cotyledons. Similarly, calcium deiciencies in peanuts result in necrosis
of the hypocotyl of germinating seeds. Boron deiciencies in vetches may result in seedlings with pale and
stunted plumules.
After physiological maturity, any environmental factor present during dry down, harvesting, storage,
and conditioning of the seed can reduce germination capacity.
Physical Seed damage
Physical (also called mechanical) seed damage can take many forms. In its severest form, it causes splitting
of cotyledons and broken seeds. This type of damage is not often encountered in the laboratory and can be
easily removed during conditioning. A more common form of physical seed damage is a seed coat fracture
which is dificult to remove by conditioning, yet is a clear indication of loss in seed quality. Intact seed
coats play an important role in mediating the entry of water in the seed during imbibition, thereby protect-
ing the sensitive embryo against imbibitional injury (McDonald, 1985). Intact seed coats are also essential
to reduce the amount of metabolite leakage from imbibing seeds. Such compounds serve as ready nutrient
sources for soil microlora that can cause rapid deterioration.
These kinds of physical damage generally mask more subtle forms that are also present. Examples
include radicle fractures or cotyledon bruises that are dificult to detect under the seed coat and result in
seedlings with short stubby roots and/or weak root systems that perform poorly in the ield. In extreme
cases, damage to the radicle can result in abnormal seedlings. Any damage to the cotyledon may retard
translocation of essential nutrients to the growing embryonic axis culminating in delayed seedling growth.
In all cases, it should be remembered that seed injury is dynamic and progressive and constantly increases
in area and severity during storage (Moore, 1972). Fast green and indoxyl acetate are examples of the tests
used to detect physical damage in seeds of many species.
