In this article we will discuss about:- 1. Concepts of Seed Deterioration 2. Important Theories of Seed Deterioration 3. Respiration and Heating 4. Responsible Factors 5. Prevention.
Concepts of Seed Deterioration:
Deterioration is a natural phenomenon in living material. The rates of deterioration in seeds vary in different species and depend on several factors, including storage conditions. It causes reduced seed quality, viability, vigour and finally seed death. Seed deterioration is an irreversible process; it cannot be prevented, but it can be slowed under specific conditions.
Seed deterioration is greater under improper storage conditions, such as higher moisture and temperatures. Evidence of seed deterioration includes discoloration of seeds, poor germination, poor seedling growth, production of more abnormal seedlings, and poor yield.
Seed deterioration can be characteriszed by the following three concepts:
1. Seed Deterioration is an Inexorable Process:
All living things must eventually deteriorate and die. Although death still remains an inevitable result of life, it is possible to retard the rate of deterioration through optimum storage practices.
2. Seed Deterioration is an Irreversible Process:
Once seed deterioration has occurred, this catabolic process cannot be reserved. Simply stated, low-quality seed cannot be made into high-quality seeds. Some mechanisms for preconditioning or treating seeds with fungicides improve field emergence. However, these treatments only allow the optimum expression of seed potential; they do not alter the basic physiological quality of the seed.
3. Seed Deterioration Varies among Seed Population:
It is now well established that certain varieties exhibit less deterioration than others. Even within a variety, the storage potential of individual lots varies, and even within a seed lot, individual seeds have differing storage potential.
Important Theories of Seed Deterioration:
i. Changes in Protein Structure:
Ewart (1908) theorized that seed longevity depend not on available food reserves. According to this, protein molecules should disintegrate excessively when seeds are dried to very low moisture levels. Crocker (1938) suggested that protein coagulation caused loss of viability. Later in 1948 he reported that this protein coagulation theory was too general because of the many kinds of protein in embryos and because his studies did not show which protein coagulates with aging.
ii. Development of Fat Acidity:
Increased fat acidity is a major cause of seed deterioration.
iii. Enzymatic Activity:
Attempts have been made to use enzymatic activity as a measure of seed viability. However, only a few of the many enzymes in seeds have been investigated. The general decrease in enzyme activity in the seed lowers its respiratory potential, which in turn lowers both the energy (ATP) and food supply to germinating seed.
iv. Membrane Damage:
The immediate damage rendering aged seeds incapable of germination is extra nuclear. Free radical damage to membranes and enzyme system could affect essential metabolic process when the seed become imbibed for germination.
v. Respiration and Heating:
The theories for seed deterioration are related to respiration.
Respiration and Heating:
Respiration has been defined as an oxidative- reduction process occurring in all living cells, whereby chemical action occurs producing compounds and releasing energy that are partly used in various life process.
The following phases of respiration are important:
(1) Depletion of food reserves
(2) Formation of intermediate or end products that may affect seed storage and
(3) Release of energy, much of which is in the form of heat.
1. Depletion of Food Reserves:
Since respiration is an oxidation process, there must be a substrate (the seed) with which the oxygen can combine. As respiration proceeds, more and more of the seed food reserves are used up. Over the life of stored seed, depletion of food reserves is inconsequential.
2. Release and Accumulation of Gasses:
It may affect viability of seeds in storage. The accumulation of respiratory end products such as carbon dioxide is an advantage of storing seeds in sealed containers where carbon dioxide replaces oxygen in the air thereby slowing down respiration and increasing longevity.
C6H12O6 + 6CO2 = 6 CO2 + 6H2O + 673 kg calories.
This equation shows that for oxidation of 1 mole of a hexose, 6 mole of oxygen are required. Six moles each of carbon dioxide and water result from this oxidation and 673 kg calories of energy are released. Moreover, the amount of heat released per unit of substrate varies with kind of material involved. The important point here is that heat is evolved from respiration. Under favorable storage conditions the heat of respiration is little. However, at higher moisture levels the heat of respiration can cause much damage to the seed.
3. Release of Energy:
The energy is mostly released in the form of heat. When seeds are stored under favorable conditions, respiration is of little consequence. When moisture is high, respiration increases, which in turn increases the production of heat thereby decreasing longevity.
Responsible Factors for Seed Deterioration:
The apparent respiration of seed lots and the associated heat may arise not only from the seed but also from the fungi and bacteria on the seeds, especially at seed moisture contents in equilibrium with approximately 75-per cent relative humidity and higher. The loss of viability which accelerates ageing is influenced by different factors like moisture, disease infection, fungi and others.
It is identified that seed deterioration occurs from single or combination of several factors, some important factors are as follows:
1. Internal Factors:
The physical condition and physiological state of seeds greatly influence their life span. Seed that have been broken and cracked, deteriorate more rapidly than undamaged seeds. Several kinds of environmental stresses during seed development and prior to physiological maturity can impair the longevity of seeds for example, deficiency of mineral (N, K, Ca), water and temperature extremes.
2. Relative Humidity and Temperature:
The two important factors that influence the life span of seeds are seed moisture and temperature. The effects of relative humidity and temperature of the storage environment are highly interdependent. Most crop seeds lose their viability rapidly at relative humidity’s approaching 80% and temperature 25 to 30° C or lower.
According to Harrington (1973), the sum of the percentage of relative humidity plus the temperature in degree Farenheit should not exceed 100 for safe storage. It has been suggested that the relative humidity should not be higher than 60% for seeds at 21°C and not higher than 70% for seeds at 4 to 10° C. However in general at 5°C and 45 to 50% relative humidity, many crop seeds can be safely stored.
3. Fungi, Bacteria and Pests on Seed Storage:
The process of seed harvest and cleaning removes most debris and insects, but certain fungi, bacteria, and insects make their way into stored seed. Fortunately, the same conditions that are favorable to seed preservation inhibit fungi and bacteria and kill insects.
Bacteria:
Bacteria do not have a significant role in seed deterioration because free water is required for bacterial growth, and if the moisture content of the seed is high enough to support bacteria, the seed is more likely to succumb to deterioration due to other causes such as fungi, respiration, heating or premature sprouting.
Fungi:
Most seed storage fungi are inhibited when the relative humidity is kept below 65%. At this relative humidity the moisture content of starchy seeds is about 13%, and oily seeds about 7%.
The major effects of fungi are:
i. Decrease viability of seed.
ii. Produce toxins that affect seed viability and germination.
iii. Increase heat production – important in large seed lots.
iv. Cause discoloration.
Insects:
In hot, humid climates mites, weevils, flour beetles, and borers can be a serious problem in stored seed, but if the seed is dried to 8% moisture content and the temperature reduced to 64 to 68°F i.e. 18 to 20°C, insects should not be a problem. At a moisture content of 15% and a temperature of 86 to 95°F (30 to 35°C), they can become very destructive. Mites will not survive when the relative humidity is below 60%.
4. Variety and Harvest Conditions:
Different varieties of a particular crop specie may have different longevity when stored under the same conditions. This may be explained by differences in enzyme activity, or differences in the chemical constituents of unique varieties. Harvest conditions may also affect longevity of seeds in storage.
For example, under poor conditions of harvest, seed may have more fungi on the seed coat. Differences in harvest conditions are insignificant if storage conditions are good, but under adverse conditions, harvest differences can be very significant.
5. Seed Moisture Levels:
Seeds stored at moisture contents above 14% begin to exhibit increased respiration, heating and fungal invasion that destroy seed viability more rapidly. On the other hand, below 5% seed moisture a breakdown of membrane structure hastens seed deterioration.
Thus, storage of most vegetable seeds between 5 and 6% seed moisture appears to be ideal for maximum longevity. Seed can be stored for long-term in sealed storage provided that the seed moisture content is less than 8%, which means that the relative humidity must be kept below 35%.
Prevention of Deterioration:
The rate of ageing and deterioration can be greatly reduced by controlling the external storage conditions like temperature, humidity as well as diseases and insect infestation. Certain treatments like soaking followed by drying, dry dressing with calcium peroxide, GA etc., of partly deteriorated seeds have effectively retrieved them to a better viability status facilitating crop growth.
Seeds stored under special conditions of dehumidified environment in low temperature (-10° to -30° C) or kept in cryopreservation in liquid nitrogen (-190° C) retain their viability by checking the factors influencing deterioration.