Agriculture Reference
In-Depth Information
excessively moist germination substrata can cause discoloration or decay of primary roots on species such
as Glycine max and Lupinus spp.). If seedlings are dificult to evaluate, or the analyst suspects that defects
might be exaggerated on “artiicial” germination substrata (particularly if chemical injury or the presence
of disease is involved), it is advisable to retest seeds in sand, soil, or a sand/soil mixture. There are many
possible causes of abnormal seedlings as follows:
Nutrient Deiciencies . Crops grown on soils with nutrient deiciencies may produce seeds that, upon
germination, have a high incidence of abnormal seedlings. These abnormals are usually characterized by
shrunken, hollow, brown, or pithy areas on cotyledons. They may also have decayed areas on cotyledons,
hypocotyl, epicotyl or roots, and the seedlings may be stunted and undeveloped. A calcium nitrate solution
may be useful for overcoming hypocotyl collar rot of bean seedlings. Chemical treatments have not yet
been authorized for other types of mineral deiciencies. Analysts should recognize nutrient deiciencies as
presented within respective AOSA or ISTA seedling evaluation criteria. For example, manganese deiciency
during seed development is characterized by a discolored brown indentation in the center of the inner
surfaces of cotyledons. Seedlings with “marsh spot” are considered normal by the AOSA.
Freeze Damage. Freezing temperatures can cause damage to developing seeds. The degree of damage
depends on species, stage of seed development, and severity of freezing temperatures. Germination and
growth may be initiated in freeze-damaged seeds, but the resulting seedlings are often too weak to produce
normal plants. Seedlings from freeze-damaged seeds of the Poaceae may be characterized by grainy cole-
optiles and spirally twisted leaves as well as decay of the embryonic axis where it attaches to the scutellum.
Heat Damage . Abnormal seedlings may also develop from seeds that have been over-heated. This
often occurs when seeds are harvested at high moisture content and not allowed to dry down to a level
safe for storage. Fungal activity plays a major role in increased temperature during storage of moist seeds.
Heated seeds are often observed as moldy and dead seeds during a germination test, or they produce seed-
lings that decay soon after sprouting. A seedling may have missing roots or epicotyl or, in the Poaceae,
have a missing, stunted, or empty coleoptile. Heating also may result in decay of the embryonic axis where
it attaches to the scutellum. Damage may range from minor to severe and seedlings may be dificult to
evaluate.
Mechanical Damage. Mechanical damage of seeds may occur during harvesting, threshing, loading,
hauling, unloading, and cleaning/conditioning operations. Grass seeds, especially range grasses, can expe-
rience mechanical damage during combining or in special milling processes designed to remove weed seeds
or accessory seed structures. Large-seeded legumes, such as ield and garden beans, lima beans, soybeans
and peas, are especially susceptible to threshing or combine damage. Such mechanically damaged seeds
may produce seedlings with defective primary roots, hypocotyl, epicotyl, and/or broken or detached coty-
ledons. Damage at the point of cotyledon attachment may be dificult to evaluate if seedlings are removed
too early in the germination test period. Bruised areas are usually necrotic or decayed. Other legume seeds,
(e.g., larger seeded clovers and vetches) may be damaged to a lesser extent in threshing. Damage may also
occur during scariication intended to reduce the hard seed content of many legume seeds.
Insect Damage. Seeds that have become infested with insects may produce seedlings which lack
essential structures or are weak and stunted. In some cases, an adult insect lays her egg(s) in a developing
plant ovule and damage is caused as the hatched insect larva eats away tissue from inside the seed coat.
Examples of this include weevil damage to seeds of ield peas, cowpeas, and vetch, and chalcid ly damage
to alfalfa and red clover seeds. Some insects found among stored grass seeds eat away the embryo and scu-
tellum and leave only the endosperm. These seeds will not germinate. Other insects eat only the endosperm
and leave the embryo. These seeds may germinate, but would be too weak to continue development into a
normal plant.
Chemical Injury. Pesticides (fungicides, insecticides, and herbicides) used in excessive amounts for
seed treatment may cause abnormal seedling development. Such “over-treated” seeds may produce swol-
len and/or stunted seedling roots and hypocotyls. In severe cases, essential seedling structures may be
destroyed. Insecticides and/or herbicides used during seed production may also affect the quality of the
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