Agriculture Reference
In-Depth Information
dormant. Dormancy can be physical due to the impermeability of the seed coat to water or physiological
due to the presence of inhibiting mechanism(s). In many instances such as legumes, seed coat imperme-
ability restricts water absorption.
Hard seeds
are deined by the AOSA rules as “seeds that remain hard at
the end of the prescribed test period because they have not absorbed water due to an impermeable seed
coat.” Speciic germination testing requirements for species that typically have hard seeds are included in
Table 6A of the AOSA rules. The percentage hard seeds
should be reported in addition to
the percentage
germination. In species such as freshly harvested red clover, lespedeza, and ield peas, the trait is short-lived
and may be lost in the irst few weeks or months of laboratory storage. Several methods of reducing hard
seededness are commonly used in seed testing.
In some instances, seeds may not be hard but still fail to germinate due to a physiological mechanism(s).
These seeds are particularly common among grasses and are often called “irm” seeds. When the analyst is
unable to differentiate between a irm and dead seed, the seed should be tested for an additional ive days
beyond the inal count. This additional time usually permits differentiation of dead seeds which will typi-
cally decay while irm seeds will either germinate or remain irm with no decay. Distinctions among hard,
irm, and dead seeds can also be made in other ways. Individual irm and dead seeds may be distinguished
using forceps and slight pressure. Dead seeds have often already initiated the decay process and are easily
destroyed by the pressure, whereas irm seeds retain their integrity. The tetrazolium test (Chapter 7) is often
used to determine whether a seed is dormant or dead.
Once dormancy has been recognized in a seed lot, its level must be determined. However, seeds have
evolved many rather complex ways to maintain dormancy. Thus, the analyst must employ methods to break
various dormancy mechanisms. In some cases, a single treatment may be suficient; in others, a combina-
tion of techniques may be necessary. The AOSA and ISTA rules specify appropriate dormancy breaking
techniques in the germination table.
Low-Temperature Treatment (Prechill, or Stratification)
Low temperatures may break the dormancy of a number of species, especially those from temperate regions
which have evolved mechanisms to avoid germination during cold, winter conditions. Thus, germination
is delayed until the spring when environmental conditions are more favorable for seedling survival. In the
laboratory, this low temperature treatment is called “prechilling,” and is deined by the AOSA rules as “a
cold, moist treatment applied to seeds to overcome dormancy prior to the germination test.” It is also known
as “stratiication,” a term borrowed from the nursery industry in which dormant propagating materials have
traditionally been held during winter months between alternating layers of sand and sawdust. The procedure
in germination testing is usually accomplished by treating imbibed seeds at 5 or 10°C for a speciied period
(from a few days to months). Following the prechill, seeds are then germinated using standard germination
testing procedures.
Prechill requirements for tree and shrub seeds are generally different from most smaller seeded species
because of their larger size. These seeds can be placed directly on the substratum in an enclosed dish or into
a loosely woven bag or screen which is inserted into a medium of peat, sand, or vermiculite. The substra-
tum may be soaked for 24 hours in tap water at room temperature (18-22°C) after which the excess water
is drained and the seeds are placed in a suitable plastic vial or polyethylene bag. Following the appropriate
seed imbibition procedure, the seeds are placed at 2-5°C for the length of time speciied in the germination
table. Suficient aeration and moisture must be provided throughout the prechill period to prevent drying.
However, even after prechill, there are instances when the germination of the seeds remains suspiciously
low. In such cases, a cutting test is recommended to determine viability. This test is performed by cutting
through the seed and observing the internal structures. Seeds containing fully developed, irm tissue with
the proper coloring are considered viable. Those possessing shriveled, decayed, and discolored tissue or
lacking embryos are considered nonviable. When a high percentage of viable seeds are detected following
prechill, a retest is advisable.
