Seedling Evaluation - Seed Testing - page 86

seed produced, particularly if applied during early stages of seed development. A more common cause of
pesticide damage occurs when seeds are stored close to pesticides in a warehouse. Seeds mailed in “empty”
pesticide bags or boxes have been observed to produce abnormal seedlings when germinated. Retesting in
sand or soil is recommended when damage due to pesticide exposure is suspected.
Deterioration and Aging. Seeds of declining vigor due to age or unfavorable storage conditions are
usually slow to germinate. Seedlings from such low vigor seeds may be weak and watery in appearance.
Essential structures may be stunted and more susceptible to infection by saprophytic fungi that further
interfere with growth.
Pathogen Injury. Although seeds infected with pathogenic organisms may initiate growth, essential
seedling structures may be damaged or destroyed by fungi and or bacteria (e.g., scutellum rot in corn caused
by Fusarium ). Since the extent of disease infection of seedlings depends on environmental conditions
during germination, test results may be erratic. When seedlings are badly infested with pathogenic organ-
isms, the analyst must be careful to distinguish between primary and secondary infection. Retests in sand
or soil are recommended for questionable seed lots.
Toxicity in Media. Sometimes toxic seed testing materials cause abnormal seedlings. If seeds are
germinated on substrata placed directly on galvanized trays or on galvanized trays coated with a thin copper
inish, the seedlings may show zinc toxicity. The most common sign of zinc toxicity is stunted, thickened,
and discolored roots. If galvanized trays must be used, they should be covered with plastic or wax paper or
seeds should be placed in a container on top of the galvanized trays. Artiicial substrata such as toweling,
blotters, and creped cellulose may also contain chemicals toxic to seedlings. Sulfuric acid not thoroughly
rinsed from paper pulp treatments (sometimes used to give paper a hard surface) and/or binders intended to
hold paper together may be sources of toxicity. Even tap water may contain toxic chemicals that cause ger-
mination failure, root inhibition, or other seedling abnormalities. On the other hand, distilled water, because
of its low pH, may also affect germination results. Germination substrata and water from new or unknown
sources should be tested for phytotoxicity prior to routine use. Plant seeds of sensitive species such as
timothy, lettuce, celery, or sorghum on the substrate to be tested as well as on a similar substrate known to
be nonphytotoxic (control). Stunted roots or hypocotyls, or roots that arch away from the substrate are signs
of phytotoxicity. A comparison of test and control samples should be made daily because the signs will be
more dificult to see once roots become entangled. Proper seedling evaluation is dificult when phytotoxic
substances are introduced into a germination test via germination substrata and/or water used during testing.
Excessive Moisture. Seedlings of certain species such as Trifolium pratense, Pinus sylvestris , and
very small-seeded genera like Begonia, Kalanchoe , and Nicotiana are sensitive to the moisture conditions
of germination substrata. If it is too wet, these species produce weak, glassy seedlings or seedlings with
brown root tips. Other species need comparatively wet conditions for normal germination and seedling
growth (e.g., Trifolium repens, Pinus palustris ); otherwise their roots curl and growth is arrested. If a
number of seedlings show such signs, the test must be repeated under more favorable moisture conditions
fACTorS AffECTInG SEEdLInG EVALuATIon
Stage of Seedling development
As a general rule, seedlings must not be removed from a germination test before all their essential structures
have had a chance to develop to allow their accurate assessment. Depending on the type of seedling tested,
the majority of seedlings should have cotyledons freed from seed coats (e.g., Lactuca ); primary leaves
expanded (e.g., Phaseolus ); or leaves emerging through the coleoptile (e.g., Triticum ). However, in many
cases of epigeal dicots (e.g., Daucus , leguminous trees), not all seedlings will have cotyledons free from the
seed coat by the end of the germination test period (inal count). Tightly adhering seed coats may indicate
that cotyledons are necrotic or decayed. At least the “neck” at the base of the cotyledons should be clearly
visible by the inal count. If there is doubt about the cotyledon condition, seed coats should be removed for