Seed and Sowing- Seed Testing
The various classes of improved seeds are recognized to facilitate the maintenance of genetic purity of the variety and to ensure a continuous supply of good quality seed at a reasonable cost. It also helps in the multiplication of the seed rapidly while maintain its purity.
Seed Testing: Seed tests consist of a series of tests designed to determine the quality of seed. Seed tests are done in seed testing laboratories. Almost every state has a seed testing laboratory which performs the following function:
1. Conducting research on seed testing methods,
2. Training of personnel in seed testing,
3. Determining the standards for seed purity and seed quality for various crops,
4. Seed testing for certification and for implementation of seed laws of the country.
Following tests are conducted to determine the quality of seeds:
1. Purity test,
2. Germination or seed viability test and
3. Moisture content test.1.
1. Purity test: Purity denotes the percentage of seeds (by weight) belonging to the variety under certification.
Purity (%) = Weight of pure seed (g) x 100
Total weight of working samples (g)
2. Seed viability or Germination test: It is determined as per cent of seeds that produce or are likely to produce seedlings under a suitable environment.
The two tests most commonly used for the determination of seed viability are germination test and tetra zolium method.
3. Germination test determines the percentage of seeds that produce healthy root and shoot. Temperature requirement varies from 18 to 22°C. The duration of germination test varies from 7 to 28 days depending upon the crop species.
Germination % = Total no. of seeds germinated x 100
Total no. of seeds kept
For convenience, 100 seeds are planned in each sample. From each seed lot 4 or more samples are plated for a reliable germination estimate. If there is difference of 10% or more in the germination of different samples from the same lot, it is desirable to repeat the germination test.
Tetra Zolium Method: It determines the percentage of viable seeds which may be expected to germinate.
The chemical 2, 3, 5 – tetrazolium chloride in short, is colourless but it develops intense red colour when it is reduced by living cells.
Seeds are soaked in tap water overnight and are split longitudinally with the help of a scalpel so that a portion of the embryo is attached with such half of the seed. One half of each seed is placed in a Petridis covered with 1% aqueous solution of tetrazolium chloride for 4 hours. The seeds are then washed in tap water & the no. of seeds in which the embryo is stained red is determined.
Viable seed % = No. of half seeds stained red * 100
Total no. of half seeds.
The tetrazolium method is faster than the germination method and it does not require a controlled environment which is necessary for the germination test. It is relatively cheaper than earlier. Bu it cannot be applied to all the species, particularly to those species that have very small seeds & embryos, because splitting & examination of such seeds is tedius.
Real value of seed: It is the percentage of a seed sample that would produce seedlings of the variety under certification. This is also known as utility percentage of the seed & is a function of the Purity (P) and germination (G) percentage of the seed sample.
Real value of seed (%) = P x G / 100
1. Moisture content: It is determined as % water content of the seeds. Optimum moisture content reduces the deterioration during storage, prevents attack by moulds & insects and
Moisture content (%) = W1 – W2 x 100
Where, W1 – Wt. of seed sample before drying
W2 – Wt. of seed sample after drying
Facilitates processing. It is determined by drying the seed in oven at 130°C temperature for 90 minutes. The loss in weight represents the weight of water lost due to drying.