Physical Purity and Seed Germination Test | Agriculture Optional Notes for UPSC PDF Download

Physical Purity

Purity Analysis

In the seed testing laboratory, purity analysis involves assessing the various components of purity within a seed sample. This typically includes identifying pure seeds, seeds from other crops, weed seeds, and inert matter.

Objective

The aim of purity analysis is to ascertain whether the submitted sample meets the specified physical quality standards concerning its various components.

Method

Using the Working Sample
Purity analysis is carried out on the working sample, which is of the prescribed weight and is obtained from the submitted sample. The analysis can be conducted on a single working sample of the specified weight or on two sub-samples, each weighing at least half of the prescribed weight, and each drawn independently.

Weighing the working sample
The number of decimal places to which the working sample and the componenets of the working sample should be weighed is given below:

Purity Separation
After weighing, the working sample is divided into its various components, which include pure seed, other seed crop, weed seed, and inert matter.

Pure Seed
Pure seed comprises the seeds of the kind or species as indicated by the sender. This category encompasses all botanical varieties within that kind or species. It even includes immature, undersized, shriveled, diseased, or germinated seeds. Additionally, broken seeds are considered pure if their size is greater than half of the original size, except in leguminous and cruciferous plants, where seeds with their seed coat entirely removed are classified as inert matter.

Other Crop Seed
This category pertains to seeds of crops that are different from the kind being examined.

Weed Seed
Weed seeds include those of species typically recognized as weeds or specified as noxious weeds under the Seed Act.

Inert Matter
Inert matter consists of seed-like structures, stem pieces, leaves, sand particles, stone particles, empty glumes, lemmas, paleas, chaff, awns, and stalks longer than florets and spikelets.

Method of Purity Separation

To separate and categorize these components, the sample is placed on the purity work board after undergoing sieving or blowing procedures. The sample is then divided into other crop seeds and inert matter. After separation, each type of weed seed and other crop seed is identified by genus and species, with their names and numbers recorded. The specific type of inert matter present is also noted during this process.

Calculation

All the four components must be weighed to the required number of decimal places. The percentages of the components are determined as follows.

If there is a gain or loss between the weight of the original samples and the sum of all the components is in excess of one percent, another analysis should be made.

Duplicate tests

If the analysis result is near the border line in relation to the seed standards, one more test is done and the average is reported. However, if a duplicate analysis is made of two half sample or whole samples, the difference between the two must not exceed the permissible tolerance. If the difference is in excess of the tolerance, analyze further (but not more than 4 pairs in all) until a pair is obtained which has its member within tolerance.

Purity analysis in groundnut
It should be carried out on pods and the size of working sample is 1000.

Determination of huskless seeds
It is required in certain crops like sunflower and paddy. 400 seeds taken from the pure seed and the number of seeds without husk are counted (partly huskless seeds are excluded) and the % is calculated as

Seed Germination Test

Germination is the process in which the essential structures necessary for the development of a normal plant emerge from the seed embryo. It indicates the seed's ability to produce a healthy plant under favorable conditions.

Principles

Germination tests are conducted using a pure seed fraction. The test typically requires a minimum of 400 seeds, distributed in four replicates of 100 seeds each, or eight replicates of 50 seeds each, or 16 replicates of 25 seeds each, depending on the seed size and container size.

The test is carried out under optimal conditions of moisture, temperature, suitable substrate, and, if necessary, light. No pretreatment is given to the seed unless recommended by the International Seed Testing Association (ISTA).

Materials Required

Substrate
The substrate serves as a moisture reservoir and provides a medium for seed germination and seedling growth. Commonly used substrates include sand, germination paper, and soil.

1. Sand

Size of Sand Particles
The sand particles should have a size that is not too large or too small. They should pass through a 0.80 mm sieve and be retained by a 0.05 mm sieve.

Toxicity
The sand should be free from toxic materials or pathogens. If any pathogens are detected, the sand should be sterilized in an autoclave.

Germination Tray
When using sand, germination trays are employed for conducting the test. These trays are typically of a standard size measuring 22.5 x 22.5 x 4 cm and can be made of zinc or stainless steel.

Method of seed placement
Seed in sand(S)
Seeds are planted in a uniform layer of moist sand and then covered to a depth of 1 to 2 cm with sand.

Top of Sand (TS)
In the top of sand method, seeds are gently pressed into the surface of the sand.

Spacing
To ensure the normal growth of seedlings and prevent entanglement and disease spread, equal spacing should be provided on all sides of the seeds. Typically, the spacing should be 1-5 times the width or diameter of the seed.

Water
The amount of water to be added to the sand depends on the seed size. For most cereals, excluding maize, the sand can be moistened to 50% of its water-holding capacity. For larger-seeded legumes and maize, the sand is moistened to 60% of its water-holding capacity.

2. Paper
The most commonly used paper substrates include filter paper, blotter paper, or kraft paper towels. These papers should have capillary movement of water in a vertical direction (rising about 30 mm per minute). They should be free from toxic substances and should not harbor fungi or bacteria. Additionally, the paper should retain sufficient moisture throughout the testing period. Its texture should be such that the roots of germinating seedlings can grow on it and not into it.

Methods
Top of Paper (TP)
In the top of paper method, seeds are placed on one or more layers of moist filter paper or blotter paper in petri dishes. These petri dishes are covered with lids and then positioned inside a germination cabinet. This method is suitable for seeds that require light during germination.

Between paper (BP)
The seeds are germinated between two layers of paper. The seeds are placed between two layers of paper and rolled in towels. The rolled towels are placed in the germinator in an upright position.

Germination apparatus

Germination cabinet / Germination room
This is called chamber where in temperature and relative humidity are controlled. We can maintain the temperature, relative humidity and light required for different crops.

Room germinator
It works with same principle as that of germinator. This is a modified chamber of larger one and the worker can enter into it and evaluate the seedlings. Provisions are made to maintain the temperature and relative humidity. This is used widely in practice.

Seed Counting Board
The seed counting board is employed to ensure precise seed counting and spacing. It comprises two plates: a stationary basal plate and a movable top plate. Both plates have an equal number of uniformly spaced holes, typically 50 or 100, when the plates are in separate positions.
After collecting the sample, the top plate is shifted in a manner that aligns the holes in a single row, allowing a set quantity of seeds to be released onto the substrate.

Vacuum seed counter

Consists of a head, pipe and wall. There are plates of 50 or 100 holes which can be fitted to the head.

When vacuum is created the plate absorbs seeds and once the vacuum is released the seeds fall on the substrate.

Impression board
Made of plastic / wood with 50 or 100 holes / pins. Here the knobs are arranged in equal length and space. By giving impression on the sand it makes uniform depth and spacing for seed.

Evaluation of germination test

The germination test is evaluated as

• Normal seedlings
• Abnormal seedlings
• Hard seeds
• Fresh and ungerminated seeds
• Dead seeds

 ISTA classified the seedlings into different categories based on the development of essential structures.

Normal seedlings
Seedlings which has the capacity for continued development into normal plant when grown in favourable conditions of soil, water, temperature and light.

Characters of normal seedlings

• A well developed root system with primary root except in certain species of graminae which normally produce seminal root or secondary root.
• A well developed shoot axis consisting of elongated hypocotyls in seedlings of epigeal germination.
• A well developed epicotyl in seedlings of hypogeal germination.
• One cotyledon in monocotyledon and two in dicotyledons.
• A well developed coleoptiles in graminae containing a green leaf.
• A well developed plumule in dicotyledons.    

Normal seedlings

• Seedlings with following slight defects are also taken as normal seedlings.
• Primary root with limited damage but well developed secondary roots in leguminaceae (Phaseolus, Pisum), graminae (Maize), cucurbitaceae (Cucumis) and malvaceae (cotton)
• Seedlings with limited damage or decay to essential structures but no damage to conducting tissue.
• Seedlings which are decayed by a pathogen with a clear evidence that the parent seed is not the source of infection.

Abnormal seedlings
Seedlings which do not show the capacity for continued development into normal plant when grown in favourable condition of soil, water, temperature and light.

Types of abnormal seedlings

• Damaged Seedlings: These are seedlings with one or more essential structures missing or severely damaged, making balanced growth unlikely. This includes seedlings with no cotyledons, as well as those with splits, cracks, lesions, or missing essential structures and without a primary root.
• Deformed Seedlings: Deformed seedlings exhibit weak or unbalanced development of essential structures. This can manifest as spirally twisted or stunted plumules, hypocotyls, or epicotyls, swollen shoots, and stunted roots.
• Twisted Coleoptiles: Twisted coleoptiles are a specific type of deformity in which the protective sheath surrounding the emerging shoot is twisted or distorted.
• Decayed Seedlings: Decayed seedlings have at least one of their essential structures showing symptoms of disease or decay due to primary infection from the seed. This prevents the normal development of the seedlings.
• Hard Seeds: Hard seeds are those that do not absorb moisture by the end of the test period and remain firm. Examples include seeds of leguminaceae and malvaceae.
• Fresh and Ungerminated Seeds: Fresh and ungerminated seeds are neither hard nor have germinated by the end of the test period. They remain firm and seemingly viable.
• Dead Seeds: Dead seeds are seeds that, by the end of the test period, have not become hard, fresh, or produced any part of a seedling. They often collapse, and a milky paste may come out when pressed at the end of the test.

Retesting 
If the results of a test are considered unsatisfactory it will not be reported and a second test will be made by the same method or by alternative method under the following circumstances.

• Replicates performance is out of tolerance
• Results being inaccurate due to wrong evaluating of seedlings or counting or errors in test conditions
• Dormancy persistence or phytotoxicity or spread of fungi or bacteria. The average of the two test shall be reported.

Use of tolerances
The result of a germination test can be relied upon only if the difference between the highest and the lowest replicates is within accepted tolerances.
To decide if two test results of the same sample are compatible again the tolerance table is used.

Reporting results
The result of the germination test is calculated as the average of 4x100 seed replicates. It is expressed as percentage by number of normal seedlings. The percentage is calculated to the nearest whole number. The percentage of abnormal seedlings, hard, fresh and dead seeds is calculated in the same way. These should be entered on the analysis of certificate under appropriate space. If the result is 'nil' for any of these categories it shall be reported as ‘0’.