Seed and Seedling Vigor Testing - Seed Testing - page 163

Brick grit test. The brick grit test is also known as the Hiltner test. It was originally developed by
Hiltner and Ihssen (1911) for detecting seedborne Fusarium infection in cereals. Results of further stud-
ies indicated that the test also detected seed weaknesses other than those caused by fungi. For example,
it revealed cereal injury caused by frost, preharvest sprouting (Schoorel, 1960), and hot-water treatment
(Tempe, 1963) which makes it useful as a vigor test (Schoorel, 1960; Tempe, 1963).
In the Hiltner test, seeds are planted on damp brick grit or in a container of sand and covered with 3
cm of damp brick grit, then germinated in darkness at room temperature for a speciic time. Seeds weakened
by pathogenic fungi, mechanical injury, or storage deterioration are unable to penetrate the brick grit layers.
The percentage of normal seedlings from this test is considered to be an indication of the vigor level.
The Hiltner test has not been popular in the United States. Comparative trials between germination in
sand and ground brick (similar to brick grit) have shown that it fails to provide any more information about
vigor than does the standard germination test (Fritz, 1965). The test also has several disadvantages includ-
ing high cost, large space requirement, and variability in test results, as well as dificulties in obtaining,
washing, and drying of brick grit (Perry, 1973).
Osmotic stress. When seeds are sown in the ield, they are often subjected to drought stress which
results in poor emergence. Such drought conditions can be simulated in a laboratory test by use of soil, soil
solution, and other solution systems (Parmar and Moore, 1968; McWilliams and Phillips, 1971; Sharma,
1973). Since standardization of soil conditions is dificult to achieve, a solution system is preferred. Seeds
are germinated in solutions such as sodium chloride, glycerol, sucrose, polyethylene glycol (PEG), and
mannitol (Parmar and Moore, 1968; Sharma, 1973) with speciic osmotic potentials. There is evidence,
however, that some low molecular weight osmotic substances (sucrose, sodium chloride, glycerol, and
mannitol) enter germinating seeds and cause toxicity. High molecular weight PEG (4000 or more) is a
satisfactory compound for simulating true drought (Manohar, 1966; Parmar and Moore, 1968) without
causing toxic side effects. The osmotic potentials of PEG 6000 solutions at various concentrations and tem-
peratures have been determined (Michel and Kaufmann, 1973). The rate of germination under such condi-
tions is markedly reduced, and emergence of the plumule is generally more affected than that of the radicle
(El-Sharkawi and Springuel, 1977). Since vigorous seeds can tolerate greater osmotic stress, this method
has been suggested as a vigor test (Hades, 1977).
The advantage of the osmotic test is that no special equipment or training is required. However, small
corn seeds reportedly germinate better than large seeds under such conditions because of their lower water
requirement (Muchena and Grogan, 1977). A signiicant interaction of osmotic stress and temperature of
germination has been reported (El-Sharkawi and Springuel, 1977).
Respiration. Seed germination and seedling growth require the use of metabolic energy acquired
from respiration. Thus, a decrease in the rate of respiration of germinating seeds has been shown to precede
a decline in the rate of seedling growth (Woodstock, 1968). Respiration rate, measured during the irst 18
hours of germination, can be used to detect injury from gamma radiation in corn, sorghum, wheat, and
radish (Woodstock and Combs, 1965; Woodstock, 1968) and chilling injury in lima bean (Woodstock and
Pollock, 1965) and cacao (Woodstock et al., 1967). Positive correlations have been reported between rate
of oxygen uptake during imbibition and seedling growth (Woodstock and Grabe, 1967). However, this rela-
tionship has not been conirmed by other studies (Abdul-Baki, 1969; Anderson, 1970; Byrd and Delouche,
1973; Bonner, 1974).
Respiration tests are rapid and quantitative, but require a respirometer and trained personnel.
Furthermore, mechanical injury (which lowers seed vigor) may increase respiration rates (Woodstock,
1969), thus producing confusing results.
Glutamic acid decarboxylase activity (GADA). Research has shown that vigor in cereal seeds is
related to the level of glutamic acid decarboxylase activity (Grabe, 1965; Woodstock and Grabe, 1967).
This is a proteolytic enzyme that is involved in the breakdown of proteins into amino acids. Conversely,
James (1968) and Burris (1969) showed that no such relationship existed in soybean. Although GADA