Agriculture Reference
In-Depth Information
10. Expose the embryo by gently pressing it through the previously cut opening at the distal end.
Additional cutting may be required to enlarge an incorrectly made opening in order to avoid
embryo damage during removal [
Nicotinia, Streptocarpus, Columnea, Gloxinia
and most other
Gesneriaceae; most, if not all Asteraceae].
EVALuATIon
Interpretation of the embryo staining patterns is somewhat comparable to interpretation of normal and
abnormal seedlings in a germination test. The analyst must be able to envision the type of seedling that
would develop from the embryo structures being examined. For example, unstained radicle tips are gener-
ally interpreted differently in grasses than in legumes. Most legumes have a taproot system and if the radicle
does not grow, no taproot develops. However, most grasses have seminal root buds in the embryo which
can develop and produce a normal seedling even if the radicle tip does not stain. In making interpretations,
staining patterns should be correlated with the seedling drawings and descriptions included in the AOSA
and ISTA Seedling Evaluation Handbooks or those in Chapter 6.
The embryos in a test may possess a variety of staining patterns, ranging from completely stained to
completely unstained. The uniformly stained embryos are considered viable, while the unstained embryos
are nonviable. Some, however, may be partially stained and partially unstained, indicating that they are
neither completely alive nor completely dead. These seeds must be evaluated carefully to determine their
potential to produce normal seedlings.
Some seeds are large enough to evaluate with the unaided eye, but magniication of 7 to 10X is essen-
tial for evaluating small seeds (e.g., bentgrass, bluegrass). A stereoscopic microscope may be preferred over
a hand lens since it leaves both hands free to manipulate the seeds.
Meristematic (cell division) areas of the embryo should be critically evaluated. In grasses, these are
located in the tips of the radicle and seminal roots and the base of the plumule. In legume and other dicot
seeds, early cell division occurs largely in the radicle and plumule. The degree of deterioration of these
areas is the key to how successfully the embryo develops into a normal seedling.
Color is only one of many factors that must be carefully observed when interpreting a test. Turgor of
tissues, location of fractures, bruises, insect cavities, missing embryo tissues, immature and poorly devel-
oped embryos, deformed embryos and any other physical condition that might affect germination must also
be noted. Occasionally, a cloudy, foamy or reddish precipitate may appear in the staining solution. This
usually indicates that the sample contains dead, aged, heat damaged, frozen or mechanically damaged seed.
In other cases it may mean that the sample was placed in water or the test solution without being completely
conditioned. It can also be caused by an excessive staining period, resulting in deterioration of weakened
tissues, especially in the presence of microorganism activity.
In some species, initiation of germination may occur during the moistening period. Elongation of the
root-shoot axis or emergence of the radicle is good evidence of seed viability. Analysts sometimes express
concern over this advanced stage of germination and feel the overnight moistening period is too long.
However, it is not considered improper to use these visible signs of germination to help interpret staining
patterns. It should be remembered that the goal of tetrazolium testing is to determine the number of seeds
that would develop normal seedlings in a germination test and all viability indicators available should be
used in this effort.
Proper evaluation may reveal the probable cause of viability loss as well as a quantitative estimate of
viability. Accuracy of the evaluation depends to a large extent on how well the sample is conditioned and
prepared for staining and the level of experience of the analyst. However, the actual evaluation requires
even greater attention to detail and must be carefully and precisely done. This can allow separation of viable
seed into different levels of vigor as well as viability. It is worthy to note that the subjectivity in evaluation
increases as the quality of the seed lot decreases (e.g., 30-70% viability). Currently, TZ tolerance tables are
