Agriculture Reference
In-Depth Information
Pathogen Infestations
Many factors contribute to dificulties in the classiication of germinable seedlings, including lack of stan-
dardization of the germination environment. Another is the infection of seedlings by fungi or bacteria. For
seed testing purposes, infections are classiied as either primary or secondary. Primary infections originate
from the seed or seedling itself, while secondary infections originate outside the seed or seedling. Whether
the infection is primary or secondary, infected seedlings are regarded as normal if all essential structures
are otherwise normal, including seedlings that have been damaged because of proximity to a diseased or
abnormal seedling. When contamination and seed decay are suspected (usually after the irst count), counts
should be made at approximately two-day intervals until the inal count. Obviously dead and moldy seeds
should be removed to help minimize such secondary contamination and their count recorded. In addition,
when symptoms of recognizable diseases appear, their presence should be recorded.
In some instances, the use of pesticides to reduce the spread of microorganisms may facilitate a ger-
mination test. If this occurs, the chemical source should be reported and the germination results provided as
supplemental information to a nontreated germination test result. Laboratory practices that help minimize
the spread of contaminating fungi and bacteria include good general practices such as hand-washing, main-
taining clean counters, equipment and containers, as well as proper spacing of seeds, proper aeration, and
keeping the substratum on the “dry side,” while providing adequate moisture for germination. When doubt
exists about the germination of questionable seedlings on approved artiicial substrata, sand or soil tests
should be conducted to serve as the inal determination of germination.
dorMAnCy
Dormancy inluences germination in several ways. Most notably, the presence of dormancy may result in a
lower estimate of quality, suggesting that the performance potential of the seed lot is lower than it actually
is. Thus, the analyst must be able to distinguish between low germination due to dormancy and that due
to physical or physiological deterioration. Experience and intuitive reasoning are the best assets in testing
when dormancy is encountered. The analyst should suspect dormancy when there is a lack of fungal growth
or the absence of decay associated with low germination. Erratic germination of some lots, while others
grow profusely, may be another indication of dormancy. The species usually provides a valuable clue since
many characteristically exhibit more dormancy than others. Freshly harvested seed of many species may
exhibit a high degree of dormancy that will decrease with time. The AOSA and ISTA rules indicate where
special procedures may be necessary to help eliminate dormancy.
The AOSA rules deine
dormant seed
as “viable seeds, other than hard seeds, that fail to germinate
when provided the speciied germination conditions for the kind of seed in question.” The percentage of
dormant seeds may be reported in addition to the percentage germination. If dormancy is suspected, but not
determined, the statement “viability of ungerminated seeds not determined” should be written on the analy-
sis report. The importance of reporting dormancy becomes obvious in the distinction between seed
viability
and
germination
. Seed viability can be deined as “the ability of a seed to germinate under favorable condi-
tions in the absence of dormancy.” Thus, the percentage germination plus the percentage dormancy equals
the percentage viable seeds in a seed lot. So, the analyst must have techniques to break the dormancy at the
time of testing. Fortunately, there are a number of techniques that are available which are speciied in the
rules of both AOSA and ISTA.
Methods of overcoming dormancy
Any time a seed fails to germinate, the analyst must determine whether the seed is ungerminable due to
a lack of viability or whether it is dormant. If no growth occurs and if the seed does not appear diseased,
the likelihood of dormancy is increased. Seeds may not germinate because they are either dead, empty, or
