The quality of a seedlot determines how well seeds store or perform in the field. To assess the quality of a seedlot, draw a sample from a properly stirred or mixed seedlot and conduct any of the following tests:
Seed purity is a measure of the cleanness and authenticity ('trueness-to-type") of the seedlot. It may be known by inspecting the composition of a particular sample.
1. Scoop out a handful of seeds from a well-mixed seedlot.
2. Separate the seeds of interest (pure seed fraction) from other components like seeds of other varieties and/or species (including weeds), immature, broken, undersized, shriveled, diseased (with molds/fungus or fungal stains) and infested seeds (with holes, insects' eggs and larvae, or are partly eaten by insect), chaff, stone, soil, etc. If a large portion of the sample consists of impurities, clean the seedlot first before storing, shipping or planting.
1. Most seed species sink in water and flotation serve to separate seeds of poor quality. Soak seeds in tap water until all seeds are thoroughly wet. This may take a few minutes to a day.
2. Take out floating seeds and retain sinkers. Poor quality (low viability) seeds often float while those of better quality often sink. However, some species are natural floaters (e.g., coconut, nipa and teak); hence, quality must be assessed through other means.
Seed germination is the most reliable method of determining seed viability. However, this takes some time before results can be obtained. Aside from the problem of slow germination, some alive seeds are dormant and need special treatments to promote germination. Germination results of samples without any pre-germination treatment will serve as guide in determining the amount of seeds to plant or the need for any pre-germination treatment of the seedlot.
1. Sow 50-200 small seeds in moist, clean and absorbent paper or cloth. Roll the paper or cloth loosely (ragdoll method) or keep the medium flat but covered (usually with another layer of the same material). Larger seeds like Gmelina are better sown in a seedbox or seedbed containing soil, sand or sawdust for more even seed wetting.
2. Keep the media moist by constant watering, but take care not to displace the seeds or to flood the medium. If wooden soil/sandboxes are used, the media may be kept moist by letting the container sit in a bigger container with water. Paper or cloth media may also be maintained moist by keeping in a partially opened plastic bag. Keep in the shade (no direct sun and away from mice, ants and rain.
3. Count normal seedlings after 1 to 3 weeks depending on species. Normal seedlings are those with well-formed roots and shoots. Percentage germination (G) is calculated as:
G = (No. of normal seedlings) / (Total no. of seeds tested) x 100
Seeds with a germination percentage of 50 percent or less should either be planted immediately or replaced in storage. The lower the germination of seed samples, the poorer is the storability or field performance of the seedlot.
RAPID VIABILITY TEST
The Tetrazolium Chloride (TTZ) test is a quick method of testing seed viability (1-2 days) This is usually resorted to when seed germination takes more than a month or when quick assessment about the seedlot needs to be made The TTZ may be purchased from chemical stores in 5 or 10 9 bottles costing about P1,400/10g bottle.
1. Preparation of solution
a. Dissolve 5 9 of TTZ in a liter of water (approximately 1 motor oil can). This volume is enough to use for 15-30 tests.
b. Keep the solution in a clean bottle wrapped in black sheet (use carbon paper or black plastic) and store in a refrigerator for longer effectivity. If a refrigerator is not available, prepare only the needed amount, maintaining the same proportions.
2. Sample enough seeds (100-200) from a well-mixed seedlot and soak overnight (8-12 hours). For species or seedlots with hard seeds, dip seeds in boiling (1-3 seconds) or hot (5-15 minutes) water, or nick off seedcoats (at the tip away from the radicle/root end or initial) before soaking in water.
3. Remove softened seedcoat. Carefully separate the cotyledons (expanded leaf-like structure) and retain the part with radicle for testing. For species with one cotyledon or with difficult to separate cotyledons, cut the seed lengthwise revealing all the essential parts.
4. Add enough TTZ solution to thoroughly immerse the seeds. Cover the set-up with a black sheet and let stand for 3-6 hours under ordinary room condition. Other species may require longer soaking time (820 hours) for a more thorough reaction.
5. Drain the solution, rinse seeds with water and spread in a wide container with water (enough to cover the seeds) to allow individual seed inspection.
6. Seeds with completely colored bright red tissues are alive. Some seeds with unstained parts are also viable if more than one half of the cotyledon from the radicle end is bright red, if unstained parts do not include those that would develop into roots (radicle) and shoots, or if only 1/3 or less of the radicle from the tip is unstained. Percentage viability (V) is taken as percentage of seeds judged germinable through TTZ and would reflect that of the original seedlot. It is calculated as:
V = (No. of viable seeds) / (Total no. of seeds tested) x 100
Interpretation of TTZ results is subjective and needs considerable experience. Viability estimated through TTZ is often slightly higher than actual germination.
SEED MOISTURE TEST
The amount of moisture in the seed determines how fast the seed deteriorates and how long it can be stored Moisture determination is necessary, especially in seedlots whose drying and/or storage history is unknown, to know if further drying is needed before packaging, storage or shipping. The general rule is that seeds will have approximately 12 percent moisture if dried for 2-3 days in the sun. Oven-drying with controlled temperature is the most common technique to determine moisture but is not practical at the farm level. Practical approximations of seed moisture include biting (not recommended especially if seeds are treated), pinching or cracking of seeds, depending on the species.