Seedling Evaluation - Seed Testing - page 83

In some instances, pesticides can be used to reduce the spread of microorganisms to aid in seedling
interpretation in a germination test. If this is done, the pesticide should be reported and the germination test
results provided as supplemental information to a nontreated germination test result. Laboratory practices
that minimize the spread of contaminating fungi and bacteria include proper protocol for spacing seeds,
aeration, storage of germination substrate, cleaning of equipment, dishes, trays, use of distilled or “pure”
water, periodically changing ilters on building ventilation systems, and keeping the substratum on the “dry
side,” yet providing adequate moisture for germination. When doubt exists as to the germination status of
questionable seedlings on approved artiicial substrata, seed or soil tests should be conducted as the inal
determination of germination status.
Seedlings with clear evidence of secondary infection are classiied as normal if all their essential
structures are otherwise normal. Secondary infection is deined as infection which originates from adjacent
diseased seeds, seedlings, or adhering structures (such as the cluster of Beta ).
Abnormal Seedlings
The AOSA deines an abnormal seedling as: “A seedling that does not have all the essential structures
or is damaged, deformed, or decayed to such an extent that normal development is prevented.” The
ISTA deines an abnormal seedling as one that does not have the capacity to develop into a normal plant
when grown in soil under favorable conditions because one or more of the essential structures are irrepara-
bly defective. Three major classes of abnormal seedlings resulting from distinctive causes are distinguished
by ISTA: damaged seedlings; deformed seedlings; and decayed seedlings.
Damaged seedlings have one or more of the essential structures missing or so badly damaged that bal-
anced development does not occur. Embryo damage usually results from external causes, such as mechani-
cal handling, heat, drought, or insect damage. The resulting abnormalities are, for example, cotyledons or
shoot cracked or completely separated from other parts of the seedling; cracks and splits in the hypocotyl;
coleoptile with damaged or broken tip; and/or split, stunted or missing primary roots.
Deformed or unbalanced seedlings are seedlings with weak or unbalanced development. These abnor-
malities might have been caused by various physiological and/or biochemical reactions that culminated in
such abnormal seedling development. This is due to earlier external inluences such as unfavorable growing
conditions of the parent plant, poor seed maturation conditions, premature harvesting, pesticide effects,
improper cleaning/conditioning procedures, and/or poor storage conditions. In some instances, deformed
or unbalanced seedlings may result from genetic defects or natural aging processes. Characteristic seedling
abnormalities might include: retarded or spindly primary roots; short and thick, looping, twisted or spiralled
hypocotyl, epicotyl or mesocotyl; curled, discolored or necrotic cotyledons; short and deformed, split, loop-
ing, twisted or spiralled coleoptile; inverted direction of growth (shoot bending downward, roots with nega-
tive geotropism); chlorophyll deiciency (yellow or white seedlings); and/or spindly or glassy seedlings.
Decayed seedlings are those with any essential structures so diseased or decayed as a result of a disease
infection (primary infection) that normal development is prevented. This damage may result from attack by
fungi or bacteria, often as a consequence of external damage or internal weakness to the seed and does not
include damage as a result of secondary infection. Fig. 6.1 illustrates some examples of abnormal seedlings.
Refer to Figs. 6.2 to 6.22 for detailed descriptions of evaluation criteria for representative families.
It may not be possible to assign individual abnormal seedlings to any of the categories without a
thorough knowledge of the seed lot's history. Certain abnormalities can indicate inappropriate preharvest
conditions or seed handling. For example, broken and/or cracked cotyledons often are a result of rapid
moisture loss prior to harvest or careless harvesting and/or conditioning; a thickened and shortened cole-
optile or roots may result from over-treatment with pesticides. Information such as this may be of value
to seed users. However, for a majority of laboratory seedling evaluations, this is not essential. Usually,
analysts only have to determine whether or not a seedling must be classiied as abnormal, except where a
particular type of abnormality indicates incorrect germination conditions and thus necessitates a retest (e.g.,