Seed Technology III

 

Unit – 3: Seed testing

3.1 Definition of seed vigour, viability and longevity

Seed Viability

·         Definition: Seed viability refers to a seed's capacity to germinate and produce a normal seedling under optimal conditions.  

·         It's essentially whether a seed is alive or dead. A viable seed contains a live embryo, which is the baby plant inside the seed, and sufficient food reserves to support its initial growth.  

Things makes a seed viable:

§  Live embryo: This is the tiny plant inside the seed. It's the core component for growth.

§  Sufficient food reserves: The seed needs stored food (like starch) to nourish the embryo until it can produce its own food through photosynthesis.

Seed Vigour

·         Definition: Seed vigour is a measure of a seed's overall quality and performance under challenging conditions. It's about how quickly and strongly a seed can germinate and grow into a healthy plant.  

·         Vigorous seeds are like athletes; they have the energy and potential to overcome obstacles like poor soil, diseases, or harsh weather. Factors influencing vigour include genetic makeup, growing conditions, and post-harvest handling.  

Seed Longevity

·         Definition: Seed longevity is the duration for which a seed can remain viable under storage conditions.  

·         It's how long a seed can stay "alive" while waiting to be planted.

·         Longevity is influenced by factors like seed moisture content, temperature, oxygen levels, and the presence of pests and diseases.

v  Viability is about whether a seed is alive.

v  Vigour is about how strong and healthy a seed is.

v  Longevity is about how long a seed can stay alive

3.2. Seed Sampling and Equipment

 

Seed sampling is the process of selecting a representative portion of a seed lot for testing.

This is crucial to determine the quality and purity of the entire seed lot.  

 

Types of Samples:

 

1. Primary sample: The initial portion taken from the seed lot.  
2. Composite sample: A combination of multiple primary samples.  
3. Submitted sample: The sample sent to a laboratory for testing.
4. Working sample: A portion of the submitted sample used for actual testing.  

 

Sampling Equipment:

 

The choice of equipment depends on the type of seed, container, and the size of the seed lot.

1. Triers: Used for sampling free-flowing seeds from bags or bulk. They come in various lengths and diameters.  
2. Deep bin samplers: Used for taking samples from large seed lots stored in bins.  
3. Mechanical dividers: Used to reduce the sample size while maintaining representativeness.

 

Sampling Process:

 

1. Determine sample size: This depends on the seed lot size and desired precision.
2. Select sampling points: Ensure that the samples are taken from different parts of the seed lot to represent the entire population.
3. Extract samples: Use the appropriate equipment to extract the required amount of seed from each sampling point.
4. Combine samples: Combine the primary samples to form a composite sample.
5. Reduce sample size: Use a mechanical divider to reduce the composite sample to a manageable size for testing.

 

The sample should accurately reflect the quality of the entire seed lot. The sample size should be adequate to provide reliable results. Proper handling of the sample is essential to prevent contamination and deterioration. Ensure that the sampling equipment is calibrated correctly.

 

3.3. Physical Purity Analysis

 

Physical purity analysis is a critical step in seed quality assessment. It determines the composition of a seed lot by separating it into different components and calculating their percentages by weight.  

 

Components of a Seed Lot

A seed lot typically consists of four main components:

a.       Pure seed: Seeds of the declared species or variety.  

b.       Other crop seeds: Seeds of different crop species.

c.       Weed seeds: Seeds of unwanted plant species.

d.       Inert matter: Non-seed components like chaff, broken seeds, dirt, and stones.

 

Process of Physical Purity Analysis

         i.            Preparation of the working sample: A representative portion of the seed lot is prepared for analysis.  

       ii.            Separation of components: Using tools like sieves, blowers, forceps, and microscopes, the working sample is divided into the four components mentioned above.  

      iii.            Weight determination: Each component is weighed precisely.  

      iv.            Calculation of percentages: The weight of each component is expressed as a percentage of the total weight of the working sample.  

Importance of Physical Purity Analysis

i.                     Ensures seed quality: By identifying impurities, it helps maintain seed quality standards.  

ii.                   Determines seeding rate: Knowing the pure seed content helps in accurate seed sowing.  

iii.                  Predicts crop performance: High purity levels often correlate with better crop establishment and yield.

iv.                  Facilitates weed management: Identifying weed seeds helps in implementing appropriate weed control measures.

 

Equipment Used : Sieves Blowers; Forceps; Magnifying glasses; Microscopes ; Precision balance

 

The International Seed Testing Association (ISTA) provides standardized procedures for physical purity analysis to ensure consistency and reliability of results.  

 

 

3.4 Seed moisture – importance – methods of moisture determination

 

Importance of Seed Moisture

Seed moisture content is a crucial factor influencing several aspects of seed quality and storage.

·         Viability: High moisture content accelerates seed deterioration, leading to reduced germination and vigor.  

·         Storage longevity: Lower moisture levels prolong seed life by inhibiting microbial growth and enzymatic activities.

·         Seed processing: Optimal moisture content is essential for efficient cleaning, drying, and packaging.  

·         Germination: While low moisture is generally preferred for storage, adequate moisture is necessary for germination.

 

Methods of Moisture Determination:

Several methods are employed to determine seed moisture content.  

1. Traditional Methods

a.       Oven-drying method: This is the standard method where a seed sample is dried in an oven at a specific temperature (usually 103°C) until constant weight is achieved. The weight loss represents the moisture content.  

b.       Distillation method: This method involves distilling the seed sample with a solvent like toluene. The volume of water collected is used to calculate moisture content.  

2. Rapid Methods

a.       Karl Fischer titration: This method is accurate and rapid, but requires specialized equipment and reagents.  

b.       Near-infrared spectroscopy (NIR): This non-destructive method provides rapid moisture estimates but requires calibration.  

3. Other Methods

a.       Electronic moisture meters: These devices use electrical conductivity or capacitance to estimate moisture content.  

b.       Microwave drying: This method is faster than oven drying but requires specialized equipment.

 

3.5 Seed Germination Tests: Paper, Sand, or Soil

 

The choice of medium for a seed germination test depends on several factors, including seed type, desired precision, and available resources. Each method offers unique advantages and disadvantages.

 

1.      Paper-based Germination Tests

Method: Seeds are placed on moistened filter paper in Petri dishes or germination chambers.

Advantages: Simple, inexpensive, and easy to control environmental conditions. Ideal for small seeds.

Disadvantages: May not accurately reflect field conditions, susceptible to fungal growth, and less suitable for large seeds.

2.      Sand-based Germination Tests

Method: Seeds are sown in moistened sand contained in trays or pots.

Advantages: Better mimics field conditions, provides adequate aeration, and suitable for larger seeds.

Disadvantages: More labor-intensive, requires careful control of moisture levels, and potential for contamination.

3.      Soil-based Germination Tests

Method: Seeds are sown in a specific soil type under controlled conditions.

Advantages: Most closely simulates field conditions, ideal for assessing seed performance in different soil types.

Disadvantages: Time-consuming, requires careful soil preparation, and increased risk of disease and pest problems.

Factors to Consider

·         Seed type: Larger seeds often perform better in sand or soil.

·         Test objectives: If accurate field simulation is required, soil-based tests are preferred.

·         Resources: Paper-based tests are generally more cost-effective and require less labor.

·         Contamination risk: Paper-based tests are more susceptible to fungal growth.

 

A standard germination test is a laboratory procedure designed to determine the maximum germination potential of a seed lot under ideal conditions. It's a crucial quality assessment for seeds before they are released for commercial use.

 

Procedure:

         i.            Sample preparation: A specific number of seeds (determined by seed type and testing standards) are selected from the representative sample.

       ii.            Germination setup: Seeds are placed on moistened blotter paper or sand in Petri dishes or germination trays.

      iii.            Incubation: The setup is placed in a germination chamber with controlled conditions.

      iv.            Counting and evaluation: Germinated seeds are counted at specific intervals (usually 7 and 14 days). Normal seedlings, abnormal seedlings, and dead seeds are classified.

       v.            Calculation: Germination percentage is calculated based on the number of normal seedlings.

 

Importance of Standard Germination Test:

·         Seed quality assessment: Determines the viability and potential of a seed lot.

·         Seed labeling: Provides information for seed packaging and labelling.

·         Research and development: Used to evaluate new seed varieties and treatments.

 

3.6. TZ test

 

TZ test, short for Tetrazolium test, is a rapid and reliable method to determine seed viability. It's a biochemical test that differentiates between live and dead seeds based on the activity of respiration enzymes. The TZ test is a valuable tool for seed quality assessment, providing quick and reliable information about seed viability. It is often used in conjunction with germination tests for a comprehensive evaluation of seed quality.

 

Testing Principle & Steps:

v  Seed hydration: The seeds are hydrated to activate the enzymes.

v  Tetrazolium solution: A colorless tetrazolium salt solution is applied to the seeds.

v  Enzyme activity: The active enzymes in viable seeds reduce the tetrazolium salt to a red-colored compound called formazan.

v  Color interpretation: The intensity and pattern of the red stain indicate the seed's viability.

 

Advantages of TZ test:

a.       Rapid results: The test can be completed within 24-48 hours, compared to weeks for germination tests.

b.       Early detection: It can identify non-viable seeds even in dormant seed lots.

c.       Accurate estimation: Provides a reliable estimate of seed viability.

d.       Versatile: Can be used for various seed types and sizes.

 

Limitations of TZ test:

a.       Technical expertise: Requires specific skills and equipment.

b.       Subjective interpretation: Staining patterns can sometimes be difficult to interpret.

c.       Does not predict vigor: While it indicates viability, it doesn't assess seed vigor.

 

Applications of TZ test:

         i.            Seed quality control: Assessing seed lots before planting.

       ii.            Seed storage: Monitoring seed viability during storage.

      iii.            Seed research: Studying seed physiology and germination.

 

3.7. Seed Health Testing

 

Seed health testing is a critical process to determine the presence or absence of pathogens (fungi, bacteria, viruses) and insect pests within a seed lot. It's essential for ensuring the quality and safety of the seed, protecting crops from diseases, and preventing the spread of pathogens to new areas.

 

Importance of Seed Health Testing:

·         Disease prevention: Detects and prevents the spread of seed-borne diseases.

·         Yield protection: Ensures healthy seedlings and optimal crop yield.

·         Quarantine purposes: Helps prevent the introduction of new pathogens into regions.

·         Seed certification: Verifies the quality and purity of seed lots.

 

Common Seed-Borne Pathogens:

v  Fungi: Most common, causing various diseases like damping-off, blights, smuts, and rusts.

v  Bacteria: Less common but can cause severe diseases like bacterial blights and wilts.

v  Viruses: Often transmitted through seeds, leading to crop losses.

 

Seed Health Testing Methods

Several methods are used to detect seed-borne pathogens:

a.       Blotter test: Seeds are placed on moist blotter paper and incubated under controlled conditions to observe fungal growth.

b.       Agar plate test: Seeds are surface disinfected and placed on agar plates to detect fungal and bacterial colonies.

c.       Seedling disease evaluation: Seeds are sown in sterilized media and observed for disease symptoms.

d.       Immunological methods: Use antibodies to detect specific pathogens.

e.       Molecular methods: Techniques like PCR (Polymerase Chain Reaction) are used to detect specific pathogens with high sensitivity.