Factors Affecting Seed Longevity in Storage

Factors Affecting Seed Longevity in Storage

1. Kind / variety of the Seed:

The seed storability is considerably influenced by the kind / variety of seeds. Some kinds are naturally short-lived, E. g onion , soybeans, peanuts , etc. some similar kinds, E. g Tall fescue and annual rye grass, though they look very much alike, differ considerably in storability, similarly, the genetic make-up of the lines/ varieties in the same kind also influences storability.

2. Initial Seed Quality:

The seed lots having vigorous, undeteriorated seeds store longer than deteriorated lots. Depending upon the severity of damage, or degree of deterioration, E. g extent of weathering damage, mechanical injury, flat, wrinkled or otherwise damaged seed, even seed lots of good germination, at the beginning of storage, can and do decline rapidly within a few months.

The important implication of this is that only high quality seed should be carried over. The mediocre quality seed may be retained only for the next plating seasons. The low quality seed should invariably be rejected.

3. Moisture Content:

The amount of moisture in the seeds is probably the most important factor influencing seed viability during storage, over most of the moisture range, the rate of deterioration increases as the moisture content on seed storability.

Moisture Content and Storage Life of Cereal Seeds at Temperatures not Exceeding 90 0 F for Seeds of High Germination and High Vigour at Start of Storage (Harrington and Douglas, 1970)

  
Further, if seeds are kept at higher moisture contents than mentioned in table the losses could be very rapid due to mold growth on and in the seed (12 to 14 per cent moisture content), or due to heating (18 to 20 per cent moisture content). Moreover, within the normal range, biological activity of seeds, in sects and molds further increases as the temperature increases. The higher moisture content of the seeds, the more they are adversely affected by both upper and lower ranges of temperature.

It is important to note that very low moisture content (below 4 per cent) may also damage seeds due to extreme desiccation.

Since the life of seed and its span largely revolves around its moisture content, it is necessary to dry seeds to safe moisture contents. The safe moisture content, however, depends upon storage length, type of storage structure, kind/verity of seed, type of packaging material used. For cereals in ordinary storage conditions for twelve to eighteen months, seed drying up to ten per cent moisture content appears quite satisfactory. However, for storage in sealed containers, drying up to 5 to 8 per cent moisture content, depending upon the particular kind, may be necessary.

4. Relative Humidity and Temperature During Storage:

Relative humidity and temperature by far are the most important factors determining the storage life of seeds. Seed attain a rather specific and characteristics moisture content when subjected to given levels of atmospheric humidities. This characteristics moisture content is referred to as equilibrium moisture content, for a particular kind of speed at a given relative humidity, tends to increase as temperature decreases and as deterioration progresses. Thus the maintenance of speed moisture content during storage is a function of relative humidity and to a lesser extent of temperature, at equilibrium moisture content, there is no net gain or loss in seed moisture content. Seed placed in an environment with a relative humidity higher or lower than that with which its moisture content is in equilibrium , will gain or lose moisture until an equilibrium is established with the new environment . In sealed storage, seed moisture content determines the relative humidity of the environment in the containers.

Establishment of moisture equilibrium in seeds is a time dependent process. It does not occur instantaneously. A period of time is required, the length of which varies with the seed kind, initial moisture content, the average relative humidity and the temperature. Under open storage conditions, seed moisture content, fluctuates with changes in relative humidity. However, normal diurnal fluctuation in relative humidity have little effect on moisture content. Table gives the equilibrium moisture content for important field and vegetable crops.

Absorbed Moisture Content of Cereals Seeds in Equilibrium with air ( Harrington, 1959):

Approximate Moisture Content of Vegetable Seeds in Equilibrium with Air    

Temperature also plays an important role in life of seed, although if does not appear to be a controlling one. Within the normal range of biological activity of seeds, insect and moulds increases as temperature increases. The higher the moisture content of the seeds, the more they are adversely affected by temperature. Decreasing temperature and seed moisture, therefore, is an effective means of maintaining seed quality in storage.

Low temperatures are very effective in maintaining seed quality, even through relative humidity might be quite high. Good cold storage for seed should not exceed sixty percent in relative humidity.

5.  Provenance:

 It has already been stated that a number of factors, operating before and during harvest can affect seed viability. It is suprising then that samples of seed obtained from different sources may show differences in viability behaviour. It is not always easy to know and satisfactorily assess what the causes of these differences are, or even sometimes to know how important they are, because of wide variability between samples from different sources.

Nevertheless, the seed begins its existence before it is harvest. And it is only to be expected that seeds harvested in different pre-harvest condition which will have caused different amounts of deterioration by the time seeds are harvested.

6. Effects of Fluctuating Environment Conditions on Viability:

There have been a few reports to the effect that fluctuating conditions are harmful, however, at present there is not a priori reason to suppose that change in temperature, or moisture content , would in itself be deterious save, possibly , for very rapid changes in seed moisture content.

More critical investigations are needed on the effect of fluctuating environmental conditions.

7. Special Effect of Extreme Storage Conditions on Viability:

Researches indicate that three sets of extreme storage conditions of temperature and moisture contents, say about 30 percent, in cereals provided the temperature is suitable, germination will result in loss of viability when seeds are very moist, thirdly , if seeds are subjected to extreme desiccation , the period of viability may be less than expected.

8. Oxygen Pressure:

Recent researches on the role of a gaseous environment on seed viability indicate that increases in pressure of oxygen tend to decreases the period of viability. The little work carried out on the use of antioxidants shows that heat injury to kidney bean embryos was decreased in reduced oxygen pressures, and that the application of cysteine overcame the injury to some extent. Onion and okra seeds treated with either starch phosphate or alphatocopherol suggest that starch phosphate is very effective in prolonging the viability of both spp, and alphatocopherol had some beneficial effect on onion seeds.
Effect of Storage Condition on the Activity of Organisms Associated with Seeds in Storage:

There are six main types of organism associated with seeds in storage, namely,
Bacteria- Fungi-Mites- Insects- Rodents- and Birds.

The activity of all these organism can lead to damage resulting in loss of vigour or viability or, particularly in the case of rodents, to complete loss of seed.

Bacteria and fungi. The important consideration in the control of seed micro flora, is the relative humidity of the inter-seed atmosphere. Researches indicate that all storage fungi are completely inactive below 62 percent relative humidity and that there is very little activity below about 75 percent relative humidity upwards, the amount of fungi in a seed often shows an exponential relationship with relative humidity. The storage bacteria require at least 90 percent relative humidity for growth and they , therefore , only become significant under conditions in which fungi are already very active.

With regard to effect of temperature on the growth of the micro-flora, certain organisms can grow at temperatures as high as 80 0 C. Consequently , since high temperatures rapidly decreases seed viability , the only practical method of controlling micro-flora activity by temperature alone is by deep freezing. At this time there are no satisfactory chemical methods of control of these organisms in storage.

Insect and Mites: There is no insect activity at seed moisture contents below 8 percent , but if grain is infected, increased activity may generally be expected upto about 15 % moisture content. The optimum temperatures for insect activity of more important storage insects ranges from 28 to 38 0 C. The temperatures below 17 to 22 0 C are considered unsafe for insect activity.

Although it is normally preferable to control insect and mite activity by the manipulation of the seed environment, it is possible to effect some control of these organism chemically, i.e through the use of fumigants and contact insecticides. One of the problems of chemical control is that the chemicals can have an adverse effect on seed viability, or vigour , and some of them are dangerous to handle. Nevertheless, fumigants which have been used successfully include methyl bromide, hydrogen cyanide, phosphine, ethylene dichloride and carbon tetrachloride in 1 3:1 mixture, carbon disulphide and naphthalene. Contact insecticides – used in seed storage include DDT, lindane and Malathion.

Rodents and Birds. Birds can be a constant source of seed loss, if even small openings exist at the lanes, or between the roof tiles. All openings should be sealed, or screened, if needed for ventilation. Rats and other rodents are more serious problems. Rodents may result into a complete loss of seed. Rodent , control measures include building the store so that the floor is 90 cm above ground level at the entrances ; having a 15 cm lip around the building at the 90 cm level of the floor; and providing a removable deck at the entrance for use only when seed is being loaded or unloaded.

9. Other Factors:

Factors besides those discussed above that affect storage life are the direct sunlight on the seed, number of times and kind of fumigation, effect of seed treatment, etc.

10. Storage in Transit, at the retailer’s store and user’s farm. It does little good to construct excellent warehouses if the seeds lose their viability subsequently by improper storage in transit, or at the retail store, or at the user’s farm. Adequate storage precautions at all these points, therefore, are also necessary.