Seed value is denoted by common terminology “ Seed Germination”. Meaning of seed germination varies among botanists and seed technologies. According to botanists, emergence of radicle from the seed is seed germination while seed technologists consider germination as not only emergence while seed technologists consider germination as not only emergence of radicle from the seed but also the capacity of that emergence to form normal plants. The difference between two definitions is the seed vigour. Seed vigour is the most controversial and misunderstood term in seed science.
Hiltener (1911) tried to describe the term vigour for the first time. He found that Fusarium affected seeds of cereals germinated but were not able to penetrate thick soil layer. He termed this ability as “Triebkraft” meaning driving force or shooting power. This word was later translated to French and English as “vigueur” and “ vigour” respectively. Some definitions given by different authors are as follows. Perry (1972) called vigour as a physiological property of seed. Abdullahi and vanderlip (1992) defined vigour as response of seed or seedlings under given stress condition. Jensen et.al (1992) defined vigour as a length of time the seedling is able to exert an increasing upward force. Association of Official Seed Analysis (AOSA) defined seed vigour as “ seed vigour comprises of those seed properties which determine the potential for rapid, uniform emergence, and development of normal seedlings under a wide range of field conditions.” International Seed Testing Association (ISTA): Seed vigour is “the sum of those properties which determine the potential level of activity and performance of the seed or seed lot during germination and seedling emergence.”
1) Brick gravel test: This method was originally developed by Hiltner and Ihseen (1911), so it is also known as Hiltner Brick Grit Test. This test was initially used when it was found that seeds affected by seed borne disease caused by Fusarium spp, gave short seedlings which were not able to penetrate a 3cm thick layer of brick grift without physical damage. This test has also been used to test seed vigour of the seeds injured by sprouting, seed treatment, threshing damage and frost damage. In this method, a layer of 1/5 to 1 inch of moist gravel of 2-3mm size is placed above the seed. These bricks imposed a physical stress on the seeds and seeds infected by pathogenic fungi, injured seeds and seeds weak in vigour cannot exert pressure for germination. Seedlings emerging from those bricks are considered as vigorous seeds.
2) Paper piercing test: A special type of paper is used for this vigorous test. The paper has weight of 90gm/m2, thickness of 0.4 mm, dry bursting strength 0.3 kg/cm2, wet bursting strength of 150 mm, ash content of 0.1% and filtering speed of 500ml/min. For cereal seed, seeds are placed on the moist sand of almost 0.25 inch thickness. Then these seeds are covered with the aforementioned dry paper which is then again covered with moist sand of approximately 1.25 inches. These are placed at 20 degree Celcius and the seeds which become able to emerge through paper and sand are considered to be vigourous seeds.
Seed longevity is defined as seed viability after dry storage. So, it is also known as seed storability. Higher seed longevity is important for storing and using the seeds for a long time. Following are some factors affecting seed longevity.
1) Kind/ Variety of Seed: the variety or kind of seed highly determines the storability or longevity of seeds. For example seeds of onion, peanuts etc are naturally short-lived while some seeds of similar kind such as annual ryegrass have different seed longevity even though they look similar. Genetic make-up of plants also highly determines seed longevity in the same kind.
2) Initial Seed Quality: Seed quality highly determines the seed longevity. Seeds with low quality that are damaged due to weathering, broken, cracked during handling, wrinkled, flat, mechanically injured have low storability. Even though they have good germination quality at the time of storage, it will reduce within a few weeks or months. So, during storage, high quality seed should be stored for longevity. Seed with average quality can be stored for next planting season while seed with inferior quality should not be stored.
3) Moisture Content Of Seed: Moisture content of seed highly determines the seed storability. Seeds with high moisture cannot be stored for a long time due to infestation with insect pests and diseases. Very low moisture content also deteriorates seed longevity. Moisture requirement of seeds of different crops varies. For example optimum seed moisture for wheat is 12% while that for lentil is 9%.