Model Answer
0 min readIntroduction
Seed quality is paramount for successful crop establishment and yield. While a seed may appear physically sound, its ability to germinate and produce a healthy seedling depends on several factors. Two critical components of seed quality are viability and vigour. Recent advancements in seed technology, including hybrid seed production and genetic engineering, have highlighted the importance of accurately assessing these qualities to ensure optimal crop performance and minimize yield losses. This answer will differentiate between seed vigour and viability, and outline the assessment methods used in a seed testing program.
Defining Seed Viability and Vigour
Seed Viability refers to the ability of a seed to germinate. It is an all-or-nothing characteristic; a viable seed either germinates or it doesn't. It primarily depends on the integrity of the embryo and the surrounding endosperm. A seed is considered viable if it has the potential to germinate under suitable conditions. Viability is largely determined by the structural integrity of the seed components, particularly the embryo.
Seed Vigour, on the other hand, is a more complex characteristic representing the overall seed quality, encompassing its potential for rapid, uniform, and friendly emergence. It reflects the seed’s ability to germinate and produce normal seedlings under less-than-ideal conditions. Vigour is influenced by factors like seed age, storage conditions, and the physiological status of the embryo and endosperm. A vigorous seed will germinate quickly and uniformly, even under stress conditions (e.g., low moisture, high temperature).
Key Differences Between Viability and Vigour
| Feature | Viability | Vigour |
|---|---|---|
| Definition | Ability to germinate | Seedling density & uniformity under stress |
| Nature | Binary (yes/no) | Graded (high, medium, low) |
| Physiological Basis | Structural integrity of embryo | Metabolic activity, reserve food mobilization |
| Assessment | Germination tests | Germination tests under stress, seedling growth parameters |
| Influence of Environment | Less affected by environmental stress | Highly affected by environmental stress |
Assessment Methods in a Seed Testing Programme
Seed testing programs typically follow guidelines established by organizations like the International Seed Testing Association (ISTA). The assessment involves both viability and vigour tests:
Viability Assessment
- Germination Test (ISTA Procedures): This is the primary method. Seeds are placed on a suitable substrate (e.g., filter paper, sand) under controlled temperature and moisture conditions. Germination is monitored over a specific period (usually 7-14 days). The percentage of seeds that germinate is recorded as the germination percentage, indicating viability.
- Tetrazolium Test (TZ): This is a rapid viability test, especially useful for small seeds or seeds that are difficult to germinate. Seeds are immersed in a tetrazolium solution, which stains the living tissues (embryo) red. The extent of staining indicates viability.
Vigour Assessment
- Rapid Emergence Test (RET): Seeds are placed between filter paper and a porous membrane, and emergence is measured over a short period (e.g., 72 hours). Faster emergence indicates higher vigour.
- Seedling Growth Potential (SGP): Seeds are germinated under controlled conditions, and seedling growth parameters (root length, shoot length, dry weight) are measured. Larger, healthier seedlings indicate higher vigour.
- Cold Germination Test: Seeds are germinated at low temperatures (e.g., 5°C) to assess their ability to germinate under cold stress.
- Electrical Conductivity Test: This test measures the leakage of ions from seeds. Higher leakage indicates lower vigour, as it suggests damage to cell membranes.
The ISTA Seed Testing Regulations (currently in its 2020 edition) provide detailed protocols for these tests.
Example: Assessing Rice Seed Quality
In rice seed production, vigour is particularly important due to the relatively long germination time and susceptibility to environmental stress. The RET and SGP tests are routinely used to differentiate between lots of rice seeds, ensuring farmers receive high-quality seeds that establish quickly and uniformly.
Case Study: Hybrid Maize Seed Quality Control
Hybrid maize seed production involves complex genetic crosses. Seed companies utilize vigour tests like RET and SGP to ensure the hybrid seeds possess adequate vigour to overcome the challenges of field conditions and provide consistent germination and seedling establishment. Failure to do so can lead to significant yield losses for farmers.
Conclusion
In essence, viability defines whether a seed *can* germinate, while vigour determines *how well* it germinates under varying conditions. Both are crucial for ensuring successful crop establishment. Seed testing programs utilize a combination of tests, with ISTA guidelines providing a standardized framework for assessment. Increasing awareness and stringent quality control measures are vital to ensure farmers receive seeds with both high viability and vigour, contributing to food security and sustainable agriculture.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.