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Assessment of Seed Quality




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    Prior to storage or sowing, the physiological quality of the seed should be assessed; this is especially important if seed is to be stored for a long period of time before sowing.

    The following quality tests should be carried out by trained personnel:

    Sampling

    If a large volume of seed is to be tested, it must initially be properly mixed to have a homogenous seedlot. After this mixture, appropriate samples of seed are drawn for quality assessment. As a minimum, two samples should be used in each quality test, so that the results can be analyzed, compared, and evaluated.

    Purity test

    As a measure of the cleanness of seed, pure seed is separated from impure seed, and then separately weighed. Seed is considered pure if it appears normal in terms of size, shape, and general outward appearance. Conversely, seed that is too small, has been partly eaten by insects, or exhibits fungal stains is regarded as impure. A sample for a purity test may consist of 100 to 1,000 seeds.

    A purity percentage is calculated as:

    Although a purity test is simple, it may not be as accurate in assessing quality as other tests. One problem encountered in applying a purity test is the subjectivity injected in separating pure seed from impure seed.

    Number of seeds per unit weight

    It is important to know the number of seeds per unit weight in a seed collection to be stored, because this number and the germinative energy percentage, to be discussed below, can be used to calculate the approximate weight of seed required to produce a desired number of seedlings.

    To determine the number of seeds per unit weight, two or more random samples of, for example, 100 grams are taken from the seedlot. The number and weight of pure, full seeds are then obtained. As in the purity test, impure seed is rejected and all other retained.

    The number of pure seeds per kilogram can easily be calculated as:

    Number of pure seeds per kilogram =

    The approximate number of Prosopis seeds per kilogram range from 10,500 to nearly 30,000, depending on the species.

    Viability test

    The viability of a seedlot, as well as the maximum possible germination to be expected, are indicated by the results of a viability test.

    The general procedure of conducting a viability test includes the following steps: remove a sample of 100 (or a multiple of 100) pure, full seeds from the seedlot; open each seed by cutting it into halves with a knife, or crushing the seed coat with a small hammer; and, inspect and record the number of seeds with a healthy, well developed, and full endosperm and embryo. For this latter inspection, a hand lens is quite helpful.

    A full seed percentage is calculated as follows:

    Full seed percentage =

    Germination test

    Often, full seeds that appear sound will not germinate because they were not fertilized or are too old. Therefore, the most reliable way to assess the quality of a seed collection is to actually germinate a sample of seeds from the collection.

    To determine the germination percentage, random samples of seed are tested by subjecting them to favorable germination conditions. A germination test can be made in containers, i.e., pots or tins used in nurseries, or in a simple, covered Petri dish. A variety of germination media are acceptable for a germination test, although the one used must furnish proper aeration and sufficient, but not excessive, moisture for each seed. Furthermore, it is important that the medium be sterile to prevent damage by fungi.

    A temperature of 30°C for 16 hours (day) and 20°C for 8 hours (night) during a germination test is prescribed by the International Rules for Seed Testing (ISTA 1976). Also, these rules specify exposure of the seeds to light during a test.

    Samples of seed need to be large enough to ensure that at least a few of the seeds will germinate. For tree species in the genus Prosopis, between 200 and 500 seeds per sample are suggested.

    As to be discussed subsequently, pre-treatment is frequently required to overcome dormancy in Prosopis seed. Once dormancy has been broken, the period of active germination is usually less than 30 days.

    Until the end of the active germination period, the number of seeds that have normal germination should be recorded at regular intervals. Once a seed has germinated and been counted, it should be removed to avoid a possible chance of being counted again.

    The results of a germination test can be expressed in different ways, including germination percentage, germinative energy, and germination capacity.

    Germination percentage, or the actual percentage of the total number of seeds in the sample that germinated during a test, is useful in comparing the quality of seed collections for testing programs and in research.

    A more valuable expression is the germinative energy. Germinative energy refers to the percentage of seed in the sample that has germinated in a test up to the time when the number of seeds germinating per day reaches its peak. The number of days required to reach this peak is the energy period. In general, seedlings that originate from seed that germinates within the energy period make the best quality planting stock.

    The germination capacity is the total number of seeds in the sample that have germinated in a test, plus the number of seeds remaining ungerminated but still sound at the end of the test, expressed as a percentage. From a practical viewpoint, this statistic is similar to the full seed percentage value obtained in a viability test.

    The results of a germination test are often used to calculate the quantity of seed that must be sown to obtain a given number of seedlings. However, it must be remembered that the actual number of surviving seedlings is likely to be much smaller than indicated by the germination test, because of losses due to unfavorable weather, rodents and birds, insects, and diseases.

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