In this article we will discuss about:- 1. Physiology of Seed 2. Seed Viability 3. Relationship between Viability and Vigour 4. Seed Dormancy 5. Seed Priming 6. Seed Coating (Pelleting).
Physiology of Seed:
Metabolic activity requires available water. As orthodox seeds dry during maturity and later on during processing, the available free water is lost. The little water left in the seeds (e.g. 4-6% depending on desiccation rate) is ‘bound’ to macromolecules, i.e. it is immobile and does not enter into chemical reaction.
In desiccation sensitive (recalcitrant) seeds, moisture content is always high and the seeds are concurrently metabolically active. The seeds continue to accumulate dry weight upto the time of dispersal. Hence, recalcitrant seeds are metabolically active, but the rate of metabolism can usually be reduced by storing at reduced temperature and moisture content.
As long as free water is available, metabolism is strongly related to temperature. If moisture content and temperature are high, metabolism will increase significantly. Low temperature will drastically decrease metabolism but metabolic process will still continue as long as free water is available. Even when moisture content has declined below the level where metabolic activities have ceased, both temperature and moisture content continue to influence seed longevity in storage through the ageing process.
Seed is the unique organ of a plant. The potential plant of next generation is stored in a juvenile (embryonic) state along with a reserve of food and energy molecules in inactive state but prepared for function only under favourable environmental conditions. Until favourable condition is available the mature seed remains physiologically inactive except maintaining the lowest rate of respiration.
Such a quiescent state can be sustained for a longer period through the mechanism of dormancy. On the other hand, with the availability of favourable conditions of moisture, temperature and light physiologically inactive seed bursts into the regime of full activation of its reserve molecules conducive to the growth and development of embryo. In fact, therefore, the complete domain of development and maturation of seed towards germination depends on specific physiological event.
The development of seeds is associated with the sink function in the ovule after fertilization. Translocation of simple carbohydrate, fatty acids and proteins and their condensation into various complex compounds in the storage tissues have provided the stored substances as energy substrates for future development of the viable seeds. The stored carbohydrates are usually starch, amylase, amylopectin and hemicelluloses.
Stored lipids are triglycerides of a number of fatty acids and stored proteins are complex polypeptides characteristic of different plant groups. In addition, there are auxins and various vitamins and various mineral salts. Physiologically starch and oil are interchangeable as energy substrate. Fat has richer source of carbon than starch and therefore, smaller quantity of fat would produce comparable energy produced by larger quantity of starch.
The balance of auxins which promote germination and inhibitors which prevent germination play an important role in the maintenance of seed viability and dormancy. The common promoters of seed germination are potassium nitrate, gibberellins and kinetin, besides thiourea and ethylene also play important role in promotion of seed germination. Whereas, the natural inhibitors found in some seeds are coumarin, para-ascorbic acid, ammonia, ferulic acid and absiscic acid.
Physiological Changes during Ageing:
No matter how optimal storage conditions are, seeds will sooner or later die. Ageing denotes the progression of deteriorating event that take place within the seed and which ultimately lead to the death of the seed. The term ‘progression’ suggests that ageing takes place over a prolonged period, during which cytological and biochemical deterioration accumulate.
Ageing does not include momentary loss of viability due to an instant damage, e.g. by temperature or mechanical impact. Causes of physiological ageing may be grouped into extrinsic factors, which are external factors influencing viability, and intrinsic factors where the ageing is a result of events within the seed only. A summary of the two types of factors appears in Fig. 6.1.
Ageing- effect is associated with loss in viability and rate of respiration and increase in membrane permeability. Ageing is generally caused by production and accumulation of various toxic products. The maintenance of seed longevity, however, depends primarily on prevention of destruction of some of the components which are essential in triggering the biochemical steps needed to initiate germination.
In fact, the enzymes necessary for the production of GA, cytokinin and ethylene which are necessary for triggering germination are present in cell pool or are being produced by the genes which are activated with moisture regain in dry seeds. Destruction of these enzymes by protein denaturation or the inability to produce the enzymes lenovo due to respiration of transcriptional mechanism would lead to the loss of production of hormones and thereby loss in germinability.
Depending on the intensity of ageing effect from mild to less severe, different crop plants show different expression in ultimate result. Mild ageing- effect may produce abnormal seedlings in lettuce and onion.
Genetic changes involves expression of deleterious or undesirable characters through gene recombination or mutation, alteration caused by pathogenic infection, ageing in storage accelerated by impairment of cell structures, mitochondrial function, biochemical changes altering metabolic pathway. The influence of moisture, temperature, oxygen etc. control deterioration of seeds.
Seed deterioration starts immediately after maturity, but it only influences viability when progressed to an advanced stage because seeds are capable of repairing damage that has not reached a critical point. Ageing events can be slowed down by appropriate storage. Temperature and moisture content are the two major factors determining the rate of ageing. Oxygen pressure and light may have some influence on ageing in some instances.
Their relation to ageing is as follows:
i. Temperature:
Biological processes are generally slowed down at low temperature; the lower the temperature, the slower the process. That also includes processes leading to deterioration. Further, low temperature (i.e. less than 8° C) inactivates most seed insects and storage fungi.
ii. Moisture Content:
Most biochemical and cytological deterioration is likely to take place at high moisture content.
iii. Oxygen Pressure:
Seeds stored at high moisture content do not tolerate low oxygen pressure because oxygen is necessary for respiration.
iv. Light:
Ionizing radiation has been mentioned as a factor influencing seed ageing in nature. For dry orthodox seeds there is, however, little evidence that light conditions play any role in seed longevity except for species with photo-dormancy i.e. where light is necessary for breaking dormancy and for germination.
Seed Viability:
Seed viability can be defined as the ability of embryo to live, grow and develop into a seeding under favourable environmental conditions. In favourable environmental conditions the respiration rate in seed and particularly in the viable embryo increases many fold, thereby increasing the rate of metabolism and growth.
Seed viability is therefore a physiological parameter depends on the factors which have controlled the maturity of seeds, storage of metabolites as well as the specific conditions of germination required for the particular kind of seeds. Seed vigour is described by normal seedling morphology and the rate at which seeds germinate and grow in the early stages.
Strong seed vigor has many advantages especially for the organic growers, as vigorous seeds are less likely to be overtaken by diseases, weeds, and insects than weak ones. During the time that seed is held in storage, there is a gradual decline in germination and vigour. When seeds are stored at ambient temperature, they are tested every six months to a year, depending on the conditions of storage.
Seed that is stored at 40°F (14°C), or at subfreezing temperatures is tested on a longer cycle. Every time a germination test is done, the germination per cent and test date is recorded on the stock container. What that label doesn’t show is the degree of seedling vigor. There should be a germination test number on the label so that the test log can be consulted if there are concerns about vigour.
Loss of seedling vigour is often apparent in the germination test. When the seeds take longer to germinate than usual, the seedlings are smaller, and sometimes malformed.
Information about the vigour of a variety can be partially inferred from the germination data if you are familiar with the general relationship between viability and vigour. The relationship between vigour and viability is similar except that vigour declines before viability.
There are three distinct phases to this relationship:
(1) The first when germination is approximately 80% or above, when seed is both vigorous and viable.
(2) The second stage when deterioration progresses rapidly.
(3) The third stage when deterioration slows at approximately 20% or below, and all seeds slowly die.
It should be noted that the curve shown in this graph is a generalized curve, and that there are significant differences among varieties and types of seeds in regard to viability and vigour. Large differences in vigour and viability are more likely to show up under substandard storage conditions, especially when the relative humidity permits the growth of storage molds.
Seed dormancy in general sense is the inactive period of seed embryo which has attained maturity upto when it begins to germinate. In this sense, all seeds which can be retained in long-term storage are dormant. Seed dormancy in specific sense is the condition in a viable seed by which it prevents germination even under optimal conditions provided for germination.
Seed dormancy ensures not only the optimal situation in a season for germination and growth of embryo but also safeguards storability of seed during unfavourable conditions.
Relationship between Viability and Vigour:
As ageing progresses, the seed losses its ability to germinate altogether, i.e. it loses its viability. Loss of vigour hence fore runs losses of viability. Vigour is a continuous character observed on the individual performance. If the two features are to be compared directly a germination test may be carried out under stressed conditions. The theoretical relation between vigour and viability is expressed in Fig. 9.1.
Reduction of seed vigour as preceding reduction of viability has two practical implications:
1. Seeds with a relatively high, yet reduced, viability under test condition (e.g. 80%) may show poor germination performance when grown under field conditions.
2. Seed lots in which the viability under test conditions is <50% may show deterioration beyond recovery for most of its seed. Therefore, seed with very low viability should generally be discarded and not used for planting programmes.
While the lifetime of an individual seed in a seed lot cannot be predicted, a seed lot usually displays a characteristic pattern as shown in Fig. 9.2.
Seed Dormancy:
Every type of seed goes through a period of dormancy, which is a mechanism for delaying germination until the seed is dispersed and exposed to favorable growing conditions. It is not evolutionarily advantageous for all seeds from the mother plant to germinate at the same time. Environmental conditions, such as drought, disease, and predation may wipe out a generation of seed.
Dormancy provides a mechanism for seeds to be able to adapt to their environment by germinating at different times and under different conditions. It is not a mere resting state of a seed, but rather an active physiological state. Dormancy helps seeds to germinate at the proper time of year, temperature, pH, lighting conditions, nutrient levels, or other environmental conditions that affect survival.
It allows weed seeds to remain un-germinated in the soil for many years, until the conditions are conducive to growth. Most vegetable crops have a long history of domestication, a selection process which has eliminated or reduced dormancy in many of our major food crops.
Nevertheless, some of our common crops still exhibit some dormancy, and this poses a problem for seeds men. For example, some varieties of brassicas, tomatoes, peppers, especially varieties with wild ancestry, may not germinate well, or at all, shortly after harvest. A period of dry storage may be required before the seeds germinate.
The process of artificially breaking dormancy is called conditioning. For 95% of seeds, the basis of dormancy is biochemical or physiological in nature, whereas for the remainder of dormant seeds, the cause for dormancy is an impermeable seed coat.
The mechanism for breaking dormancy varies from species to species (and even variety to variety) and most often involves drying (curing), exposure to light, leaching of chemical inhibitors, exposure to high or low temperature, or alternating temperatures. The germination rate of many vegetable seeds may be increased if exposed to alternations in daily temperature.
In some species, dormancy is broken by exposure to nitrate ions in the soil, and in others, exposure of seeds to gibberellins (plant hormone) produced by soil fungi. Seeds that are produced by fleshy fruits have dormancy broken by fermentation and leaching of the chemical inhibitors from the gelatinous coat surrounding the seed. Pre-harvest growing conditions can also affect dormancy.
For example, lettuce grown under different conditions of water and nutrients exhibit differences in the amount of dormancy. Squash and pumpkins have the highest germination percentage when seed is harvested from fruit that is twenty days past peak ripeness. In many instances, seed dormancy is released by dry storage.
The seeds of crop species that exhibit dormancy after harvest can be released from dormancy within two to six months after harvest. This is important for some pepper varieties. If the seeds are frozen, the seeds remain dormant until they are subjected to above freezing temperatures for several months.
It is important for seed growers and seed suppliers to understand the dormancy characteristics of crops, especially in regard to germination testing. A seed grower testing the germination of a lot of freshly harvested seed may be surprised to discover that the tested seed doesn’t germinate. Rather than assuming that seed is dead and needs to be discarded, it is important to test the possibility that the seed is dormant, and that the seed may need to be retested after a period of conditioning.
Seed Priming:
Seed forms a critical input for improving productivity in vegetable crops and with increasing cost of hybrid seed, we have come to a stage where one plant is expected from each seed. Modern crop production systems also require high degree of precision in crop establishment especially under greenhouse conditions.
The need for uniform plant stand and high plant population densities for machine harvest has led to growing interest in direct field sowings. The use of module system for raising plant nursery in vegetable seed industry also necessitated the use of high quality seed.
Seed priming is a pre-sowing treatment in which seeds are soaked in an osmotic solution that allows them to imbibe water and go through the first stage of germination, but does not permit radical protrusion through the seed coat. The seed then can be dried to their original moisture contents and stored or planted. Various pre-sowing treatments have been used to reduce the time between seed sowing and seedling emergence and to improve synchronization of field emergence in annual as well as perennial crops.
In the last two decades, seed priming has become a common seed treatment to increase the rate and uniformity of emergence in many vegetable species. In some report, terms such as halo-priming (soaking in salt solutions) or osmopriming (soaking in other osmotic solutions) were proposed as alternatives to priming.
If we go through the history of priming, soaking seeds in water before planting has been an old practice. One early report in 1918 recommended the placement of seeds of radish, bean, corn, cucumber and squash in Luke warm water overnight to increase germination velocity.
Seed Coating (Pelleting):
Seed coating (including pelleting) was being practiced as much as 2000 years ago. The ancient Chinese coated rice in mud balls to anchor the seed in a flooded paddy field. The coating eliminated the problem of seed drifting when seeds were sown on the surface of the flooded paddy field. The principles and concept have changed little with time. Improvements have been gained through better technology given by a better understanding of seed biology.
Seed coating (pelleting) have evolved from those which protect the seed from fungal and insect attack to a diverse range of coating, the objectives of which includes the protection of rhizobia, supply of micro and macronutrients, protection from birds and rodents, supply of growth regulators, attraction of moisture, supply of oxygen, germination stimulation, germination delay, increase in seed weight or size and the supply of selective herbicides or antidotes.
Inspite of a considerable amount of research, reliable and effective seed coatings are not intensively studied. There is still no universally accepted practice of inoculation, particularly for coating applied well in advance of planting.
Selection of Materials Used for Coating (Pelleting):
The materials used for coating should be perfect as the physical integrity of coating is decided by the type of adhesive and this was highly influential during handling, transport and planting operations of the pellets.
The adhesive recommended by various authors are gum, gum Arabic, methyl cellulose, gelatin, casein, casemate salts, plastic rexins, polyvinyl acetate, methyl ethyl cellulose, polyurethane polyvinyl alcohol, polyvinyl acetate, poly electrolyte or dextrin and poly ethylene oxide. Some of cheap and low cost binding materials are the rice gruel, maida gruel, sago gruel and starch gruel.
The concentration recommended for methylcellulose is 3 % W/V, gum Arabic is 45% and nitric coat is 4.3% W/V solution in water. The low cost adhesive such as rice gruel, Maida gruel etc. are used at the concentration of 5% or 10% depending upon filler material used for pelleting.
Selection of adhesive is also based on selective purposes. Plastics resins, poly ethylene oxides is to prevent erosion of surface sown seeds, polyurethane to bind lime in a way that resists coat abrasion, blends of polyvinyl alcohol and polyvinyl acetate to bind vermiculite and poly electrolytes or dextran to aggregate soil around the seeds, thereby improving the aeration of sown seeds.
The materials used as filler for pelleting must be beneficial and harmless to both seed and rhizosphere. The most common materials used as protectants for Rhizobia include lime, gypsum, dolomite and rock phosphate. Other materials include clay minerals such as montmorillonite and vermiculite, besides blood, peat, poultry manure; moss and mucilage are also used.
The size of particle selected for filler material varies with the type of filler material. The particle size is also important for resistant coating on the seed material. The activated clay, lime and dolomite should pass through 300 mesh sieve. The dried blood, wood charcoal, milk powder and yeast extract should pass through 150 mesh sieve size.
Dried blood, milk powder and yeast extract are used in combination with dolomite in different proportions:
i. Dried blood-15:85(W/W) Blood: Dolomite
ii. Milk powder- 3:85 (W/W) Milk powder: Dolomite
iii. Yeast extract- 1:99 (W/W) Yeast extract: Dolomite.
In addition to the above explained materials, bio fertilizers are also used as filler materials.
For low cost investment and environmental benefits, botanic leaf powder are used as filler materials. The common botanies used for pelleting are Arappu (Albiza mar a), Pungan (Pongamia pinnata), Notchi (Vitex negundo), Prosopis (Prosopis juliflora), Neem (Azadirachta inidca), Moringa (Moringa pterygosperma) and Tamarind (Tamarindus indica). These botanies are recommended @200-300 g/ kg of seed. The fineness of the powder should be in such way that it passes through muslin cloth. These leaf powders contain an auxin like substances which regulates the growth of seedling in initial establishment.
Characteristic of Ideal Filler Materials:
i. It should be non- toxic
ii. Friendly to both seed, adhesive and environment
iii. Easily soluble in water
iv. Easily available for commercial production
v. Low cost.
Based on either bio-active chemicals or filler material added for pelleting, seed pelleting is classified into different types:
1. Inoculant Pelleting:
Different bio-fertilizers viz. Rhizobia, Phosphobacteria, Azospirillum and Azotobacter are used as filler materials which are fixed to the seed with the help of an adhesive. This type of pelleting will help in improving the activity of micro-organisms and help in nitrogen fixation. VAM (Vesicular Arbuscular Mycorrhizae) can also be used for pelleting the seeds.
2. Protective Coating:
The disease controlling, bio-control agents like Rhizobacteria or Bacillus spp. or Streptomyces spp. can also be used for pelleting the seed. Antibiotics are also used for control of diseases through pelleting. Seed pelleting is also done with fungicides and pesticides and they are added to the adhesive and coated on the seeds.
3. Herbicides Coating:
Filler antidote or absorbent coating can be used before sowing. Herbicide antidote like 1, 8 napthalic anhydride (NA) is found the best as seed pelleting. The absorbent, activated carbon, can also be used as filler material which is found good in protecting the seed from 2, 4-D and alachor herbicide damage.
4. Nutrient Coating:
The nutrient coating with micro and macro nutrients enhances the germination and seedling growth. The micronutrients are required in less quantity than soil or foliar application. The micronutrients used are ZnSO4, CuSO4, KCL, borax etc.
5. Hydrophilic Coating:
Starch graft polymers and magnesium carbonate are capable of improving movement of air and water. The increase in germination is due to increase in the rate of imbibition’s where the fine particles in the coating act as moisture attracting materials.
6. Oxygen Supplier Coating:
Seed are coated with peroxides of zinc or calcium which aids in increased oxygen supply to the germinating seeds. The seeds which has seed coats impermeable to oxygen will be highly benefited for enhancing germination.
The three basic steps involved in pelleting are stated as stamping, coating and rolling. The materials need for pelleting are seed, adhesive and filler materials. The seed are uniformly coated with adhesive in correct quantity initially. Then the filler materials are sprinkled on the coated seeds and are rolled on the filler material for effective and uniform coating.
Role of Seed Pelleting in Vegetables:
Seed germination in vegetables especially in gourds are affected by various soil types and conditions such as soil moisture content, soil temperature, salinity and alkalinity of the soil. Soil is quite heterogeneous. Germination and seedling establishment may be altered by micro heterogeneous soil. Among the vegetables, cucurbitaceous seeds are more sensitive to soil moisture levels hence field emergence in these crops are always problematic even with seeds of high germinability. Either a little high or low soil moisture results in poor emergence, necessitating resowing.
Seed pelleting with Arappu powder @ 500g per kg of seed registered a spectacular improvement in seed quality attributes in terms of germination, speed of germination, root and shoot length, vigour index in the gourds. Seed pelleted with N, P, K, Fe, Cu, Mn gave higher field germination over unpelleted seeds in tomato. According to Eric and Roos (1979) pelleted seeds of carrot and onion can safely be stored for upto three years under low temperature (10° C) and low RH (50%) condition.
Advantages of Pelleting:
i. Pelleting regulates the sizes of seeds for precision planting.
ii. Reduces the amount of seeds required.
iii. Attraction of moisture
iv. Supply of plant growth regulators, micronutrients.
v. Influences micro-environment.
vi. Saving of chemicals/ fertilizers.
vii. Uniform seed establishment.
viii. Increase yield.
ix. Remedy for sowing in problematic soils.
x. Uniform sized seeds which is easy to handle.
Disadvantages of Pelleting:
i. High cost.
ii. Delayed field emergence at low moisture level.
iii. More moisture is required.