Seed Vigor and Vigor Tests
Seeds, as reproductive units, are expected to produce plants in the field. However, farmers and seed producers have long recognized that the labeled percent germination often overestimates the actual field emergence of seed lots. This observation is attributed to the objective of a standard germination test which states that germination is the emergence and development from the seed embryo of those essential structures which, for the kind of seed in question, are indicative of the ability to produce a normal plant under favorable conditions (AOSA, 1999). As a result, the standard germination test fails to provide accurate information concerning a seed lot's field performance potential for at least four reasons. These include the following:
1. The definition of seed germination emphasizes that the seed analyst must focus on essential structures which lead to the production of a normal plant. But, this emphasis on seedling morphology may have little relationship with rapidity of growth; a prime criterion of the potential for successful stand establishment.
2. The standard germination test is designed to provide a first and final count. The first count has a purpose of basically removing most of the strong seedlings that have already germinated. The final count is designed to provide a sufficiently long period that even weak seeds are coaxed or provided every opportunity to be considered germinable. The germination percentage, therefore, is the sum of strong and weak seedlings. The difficulty with such a process is that weak seedlings seldom perform adequately when provided environmental stresses associated with field emergence.
3. By definition, germination is scaleless. A seed is considered either germinable or it is not. There are no distinctions provided for strong or weak seedlings. Those considered germinable may vary from weak to semilame to robust in field performance. This inability to document the quality of the seed fails to take into account the progressive nature of seed deterioration, which has a major impact on stand establishment.
4. Methodology for the conduct of a germination test is standardized so that test results are reproducible within and among seed-testing laboratories. This process means that favorable conditions are utilized as described in the definition to ensure greater uniformity in test results. Tests must be conducted on artificial, standardized, essentially sterile media in humidified, temperature controlled chambers; conditions that are so synthetic they seldom relate to field conditions. In essence, because the standard germination test is conducted under favorable conditions, it basically establishes the maximum plant-producing ability of the seed lot. When field conditions are optimum, the standard germination test may correctly predict field performance of the seed lot. For the most part, however, standard germination values overestimate actual field emergence. We know, for example, that when the standard germination test result is 80%, actual emergence under field conditions seldom reaches 80%. In most instances, the field emergence is considerably less.
These deficiencies have led to a continuously disquieting murmur for years that not all facets of seed quality were being properly identified by the standard germination test. As a result of these deficiencies, another aspect of the physiological quality of seeds has been developed called seed vigor. Seed vigor is defined as "those seed properties which determine the potential for rapid, uniform emergence and development of normal seedlings under a wide range of field conditions."
FACTORS INFLUENCING SEED VIGOR
The development of a seed encompasses a series of important ontogenetic stages from fertilization, to accumulation of nutrients, to seed dry down, to dormancy. Each of these stages represents a change in morphological and physiological development that can alter seed performance potential. The point at which the seed achieves its maximum dry weight is called physiological maturity. At this point, it has its greatest potential for maximum germination and vigor (Delouche 1974). However, since seeds generally achieve physiological maturity at high moisture levels unsafe for storage, seed is typically not harvested until it attains harvest maturity, which is low enough for safe storage, but high enough to minimize mechanical injury. Between physiological maturity and harvest maturity, the seed is essentially stored on the plant where it may be exposed to severe environmental conditions that adversely affect seed quality.
SEED VIGOR TESTS
The challenge of vigor testing has been to identify one or more quantifiable parameters that are common to seed deterioration. Although not all changes that occur during seed deterioration are understood, we can speculate on the probable sequence of events. A hypothetical model has been developed by Delouche and Baskin (1973) that outlines some of the major parameters used in measuring seed vigor.
Because a vigor test is a more sensitive index of seed quality than the standard germination test, any of the events that precede loss of germination could serve as a basis for vigor tests. The earlier the parameter can be measured during the loss of germination, the more sensitive the index of seed vigor. Thus, since the onset of membrane degradation precedes loss of germination, the most sensitive vigor test should be one that monitors membrane integrity.
Considerable experimental evidence supports this contention. Membranes are essential for many metabolic events occurring in the seed, including respiration (cristae in mitochondria), which provides the seed with the energy required for subsequent growth. The endoplasmic reticulum is also a membraneous organelle on which many enzymes are formed as ribonucleic acid is translated. Thus, any impairment of membrane function can decrease the amount of ATP formed as an energy source, as well as retard the synthesis of specific enzymes essential for growth. Subsequent to the loss in respiration and biosynthetic capacity, the rate of germination declines, culminating in a loss of seed lot uniformity. Other associated events that occur during deterioration are loss in storability and ability to resist disease infection. Deteriorated seeds that are subjected to biological and environmental stresses also exhibit reduced field emergence which may be related to final yields of certain crops. Eventually, these subtle manifestations of loss in seed quality are expressed by an increasing incidence of abnormal seedings - a component of the germination test. The final parameter of seed deterioration (and the one most often employed) is seed germination, underscoring the need for seed vigor tests to supplement routine standard laboratory germination tests.
As a result of the biochemical and physiological changes known to occur during seed deterioration, most vigor tests have focused on measuring one or more of these parameters.Characteristics of a Seed Vigor Test
A vigor test should possess certain essential characteristics that can make it useful to the seed producer and consumer. These characteristics have been described by McDonald (1980) as follows:
Inexpensive. Due to limited budgets for seed testing, it is important that a vigor test be reasonably priced and require a minimum investment in labor, equipment, and supplies.
Rapid. Every seed laboratory has periods of peak activity, thus it is important that the vigor test be conducted rapidly to minimize analyst time and germinator space. Furthermore, seed producers desire a quick turnaround time for samples submitted for vigor tests since such quick information on seed quality can provide them with a competitive marketing advantage.
Uncomplicated. Where possible, vigor test procedures should be simple so that they can be performed in seed laboratories without requiring additional staff with special backgrounds and training.
Objective. For a vigor test to be easily standardized, a quantitative or numerical index of quality that avoids subjective interpretations by analysts should be utilized.
Reproducible. The success of any test depends on its reproducibility. If these results cannot be repeated because of intricate procedures or subjectivity of interpretation, then comparison of results among laboratories becomes meaningless.
Correlated with Field Performance. Most definitions of seed vigor emphasize the relationship between seed vigor and field performance, and many studies have demonstrated this association exists. Consequently, the ultimate value of any vigor test may be its ability to predict field performance.
Types of Seed Vigor Tests
The standard germination test is conducted under optimum conditions for seed germination. Consequently, when field conditions at planting are near optimum, the results usually correlate well with field emergence. However, under suboptimal field conditions, standard germination results usually overestimate field emergence. Therefore, additional tests are needed to better predict seedling emergence under a wide range of field conditions. Many vigor tests have been suggested; however, only a few have attained acceptance by seed analysts and seed testing organizations. This discussion will focus on the two most popular vigor tests: cold test and accelerated aging test.
Literature Cited
AOSA. 1999. Rules for Testing Seeds. Association of Official Seed Analysts, Lincoln, NE.
Byrum, J. R. and L. O. Copeland. 1995. Variability in vigour testing of maize (Zea mays L.) seed. Seed Sci. & Technol. 23:543-549.
Delouche, J. C. 1974. Maintaining soybean seed quality. In: Soybean: Production, Marketing, and Use. NFDC, TVA, Muscle Shoals, Alabama. Bull. Y-69:46-62.
Deluche, J. C. and C. C. Baskin. 1973. Accelerated aging techniques for predicting the relative storability of seed lots. Seed Sci. & Technol. 1:427-452.
ISTA. 1987. Cold Test. Pp. 28-37. Handbook of Vigour Test Methods. International Seed Testing Association, Zurich, Switzerland.
ISTA. 1999. International Rules for Seed Testing. Seed Sci. & Technol. (Supplement) 27:1-333.
McDonald, M. B. 1980. Assessment of seed quality. HortScience 15:784-788.