Differentiate between seed vigour and viability. How are the two assessed in a seed testing programme?

Seed Viability vs. Seed Vigour: A Differentiation

Seed Viability refers to the ability of a seed to germinate. It's a binary characteristic - a seed is either alive (viable) or dead (non-viable). It's primarily a measure of the potential for germination, irrespective of seedling health post-emergence. It's often expressed as a percentage.

Seed Vigour, on the other hand, represents the overall quality of seed, encompassing its potential for rapid, healthy emergence and early seedling growth under favorable conditions. Vigour reflects the seed's ability to withstand environmental stresses like drought, temperature fluctuations, and nutrient deficiencies. It's a more holistic measure than viability and is often assessed through accelerated aging or stress tests.

Assessment of Seed Viability

The standard test for seed viability is the Germination Test. It involves placing seeds on a moistened substrate (e.g., filter paper, sand) in a controlled environment (temperature, light, humidity) and counting the number of seeds that germinate over a specified period (usually 7-14 days).

• Principle: The test simulates natural germination conditions and provides a direct measure of the percentage of seeds that can germinate.
• Procedure: Seeds are placed in Petri dishes or trays, moistened, and incubated under defined conditions.
• Standards: The minimum germination percentage required for different crops is defined by the Seed Act, 1966, and subsequent amendments. For example, cereals generally require a minimum of 80% germination.

Assessment of Seed Vigour

Seed vigour is not directly measured; instead, it's inferred through various Vigour Tests that simulate stress conditions. These tests assess the speed of emergence and seedling growth.

• Rapid Emergence Test (RET): Measures the time taken for seeds to emerge from the substrate. Faster emergence indicates higher vigour.
• Accelerated Aging Test (AAT): Seeds are exposed to high temperatures and humidity for a defined period (e.g., 42°C for 72 hours) to simulate premature aging. The germination percentage after aging is then assessed. A higher germination percentage after aging indicates better vigour.
• Electrical Conductivity Test (ECT): Measures the leakage of ions from seeds. Higher ion leakage indicates membrane damage and lower vigour.
• Cool Germination Test (CGT): Assesses germination at low temperatures to evaluate seedling tolerance to cold stress.

Comparison Table

Parameter	Viability	Vigour
Definition	Ability of a seed to germinate	Seed's ability to produce rapid, healthy seedlings under stress
Measurement	Germination percentage	RET, AAT, ECT, CGT
Direct/Indirect	Direct measure	Indirect measure (inferred from stress tests)
Information Provided	Potential for germination	Seedling health and stress tolerance

Significance in Seed Testing Programs

Seed testing programs, mandated under the Seed Act, 1966, are crucial for ensuring the quality of seeds available to farmers. These programs include viability and vigour testing to provide farmers with reliable information about seed performance. The National Accreditation Board for Testing and Calibration Laboratories (NABL) accredits seed testing laboratories to ensure quality and standardization.

In conclusion, while seed viability confirms the potential for germination, seed vigour provides a more comprehensive assessment of seed quality, reflecting seedling health and stress tolerance. Both are vital components of a robust seed testing program, ensuring farmers receive seeds that contribute to higher yields and sustainable agricultural practices. Further research into non-destructive vigour assessment methods remains crucial, especially in the face of increasingly unpredictable climatic conditions.