Botanically seed is a fruit, which is a mature, ripened ovary containing one or more ovules that develop into seeds. The seed possesses an embryo, a nutritive tissue and protective cover. Embryo consists of a plumule, hypocotyls and a radicle in the form of an embryo bud. The embryo upon germination develops into seedling. The large part of the seed consists of the nutritive tissue and in most of the seeds it may be seen as the cotyledons. The storage tissue provides nutritive support to the young germinating seedling till it supports itself. The seed coat provides protection to the embryo and has a structure, which helps in dispersal and survival of seeds.
Seed is a basic tool and is the principal means to get higher yields. Seed should be distinctly superior in quality, varietal purity, and freedom from admixtures and having high germination and vigor. They should be healthy without infection and with optimum moisture content. It is always important to use fresh, quality seeds that facilitate higher germination and early establishment. Seeds, tubers, bulbs and vegetatively propagated materials are used as seed material.
Germination is the transformation of an embryo into a seedling. During the process of germination, the metabolism and growth, which were suppressed or suspended, are resumed. Seedling develops from the seed from its quiescent state. Water is the basic requirement for initiating the chemical reactions.
Optimum temperature is essential for good germination. Individual species has its own temperature requirement which occurs within a limited range. Normally the temperature range for germination is 15-40° C. Seed germination is also affected by oxygen supply since it is required for respiration. The primary role of oxygen is electron acceptor in catabolism. Many of the seeds are markedly light sensitive for germination, primarily due to the activity of the phytochrome system.
A good medium is required for better germination of seeds. The medium should have high infiltration, adequate aeration, high water holding capacity and be free from toxic substances. Seed germination and establishment are a problem in arid regions where the soil moisture availability is not adequate and there is high evaporative demand. Stress during germination and seedling establishment directly affects the crop stand and often is a major constraint, especially in small millets with limited seed reserves. Soil containing high concentration of Fe and Al has the problem of surface crusting. In such soils seed germination may not be proper, as the tender seedlings are unable to emerge through the crust. Proper seedbed preparation and compaction also enable better seed germination.
Vigor of seed is defined as the condition of the seed that permits germination to proceed rapidly and uniformly and allows production of uniform seedling stand. Seed vigor is a pre-requisite for rapid and uniform germination and fast growth of seedlings under field conditions. Vigorous seeds produce seedlings that will have good health and natural robustness. The vigor of the seeds depends on the genome, history of the individual seed and the environment in which it is sown. Seed vigor is normally determined by germination, growth and development, resistance to variations in environmental conditions and the presence or absence of abnormal seedlings.
Hybrid seeds normally possess greater vigor due to inherent genetic make up. High vigor of hybrid seeds has been associated with super efficient mitochondria and extra active enzyme
systems for assimilation, growth and development. Fully mature seeds possess complete physical and physiological development needed for maximum expression of vigor. Prevalence of high
humidity and high temperature during seed storage affect the seed vigor and cause loss of viability. Other factors such as mechanical damage, attack by pathogens and passage of time also
affect the seed vigor. Mechanically affected seeds are prone to infestation by fungi and other microorganisms. Insect incidence, particularly of seed borer (Bruchus), causes damage to the
stored seeds, which lose their vigor. Vigorous seeds are disease-free, physically sound, germinate quickly and produce rapidly developing seedlings.
Seed viability is defined as the degree to which a seed is metabolically active and capable of germinating under favorable condition. Seed viability is the highest at the time of physiological maturity. Seeds with high moisture content deteriorate quickly due to energy expenditure and accumulation of breakdown process. Aging is one of the reasons that affect the seed viability. As
the age of seed increases, the semi-permeable membrane of the cell organelles loses their selective permeability and allow the metabolites to leach out. One of the important changes that take place due to aging is the degradation of mitochondria, which permanently lose their ability of swelling and contraction. Normally the viable seeds of many of the field crops germinate within seven
days. It is important to maintain the viability of seeds during germination.
It means the duration of the viability of seeds. Normally the seeds possess maximum germination potential during the physiological maturity and the deterioration of seed quality occurs from this point of maturation onwards. The rate of deterioration increases due to mechanical injury at the time of harvesting and processing. Higher moisture content in the seed also causes deterioration of seed quality and longevity. Stored seeds exposed to microorganisms and insects cause reduction in longevity. Seeds stored in low moisture, cool temperature and low oxygen tension enable increase in longevity. However, due to aging, there is break down of compounds that are essential for germination and accumulation of toxic by-products. Lipid auto-oxidation is one of processes that destroys seed viability due to aging, particularly in oilseeds. Small millets retain their viability for a long period. The seeds of tobacco remain viable for 10-15 years, if properly stored.
Seed dormancy is any condition of prefect and viable seeds, which make them resistant to germination under environmental conditions, that are ordinarily favorable for quick germination (Evenari, 1957). A dormant seed will not germinate even under conditions, which are normally favorable for growth. Many seeds possess dormancy mechanisms, which prevent germination until the internal and external conditions for growth are suitable. Seed dormancy is an important adaptive property that promotes the survival of plants under adverse environmental conditions. Many seed species show variability in depth of dormancy. Dormancy assumes significance in some of the situations in relation to the prevention of vivipary and precocious germination. In the later situation, seeds germinate even while they are still on the mother plant. Sufficient dormancy is always required to prevent pre-harvest sprouting but not to interfere with the rapid establishment of a new crop.
Types of Dormancy
Dormancy are of two types: (i) physical dormancy and (ii) physiological dormancy
- Physical dormancy: Physical dormancy may be due to the presence of impermeable or mechanically resistant seed coat. Hard and impermeable seed coat interferes with water uptake and gaseous exchange. Impermeability is due to the presence of cuticle and the extensively developed layer of palisade cells. Seed coat also acts as physical barrier and thereby restraints the expansion of embryo. Several woody species have seed coat imposed dormancy.
- Physiological dormancy: Physiological dormancy may be due to the presence of immature embryo, need for after-ripening, specific light and temperature requirement or the presence of a substance inhibiting germination.
Germination of seeds is possible only when the embryo is fully mature. Seeds with immature embryos remain dormant until the embryo develops completely within the seed. A number of
crops produce seeds that do not germinate immediately after harvest but takes some more time under normal conditions for proper germination. This pre-requisite to germination of this type of
seed is a period of after-ripening.
Many of the seeds are markedly light sensitive for germination. This is primarily due to the activity of the phytochrome system. Studies in some of the crops showed the requirement of red light as promotive (near 660mm) and far-red radiation as inhibitory (maximum near 735 m m,) for seed germination. Temperature also plays greater role in promoting germination. A period of pre chilling is required for many seeds to germinate properly.
Abscissic acid (ABA) is an effective growth inhibitor responsible for dormancy control in seeds. It inhibits the synthesis of specific enzyme necessary for the initiation of germination.
Seeds are chemically treated with fungicides or biocontrol agents to destroy seed borne diseases. Seed treatment is also done to break the dormancy and to harden the seeds to withstand drought.
Seed treatment is practiced in cotton to remove the fuzz. Seed treatment is also practiced for inoculating the biofertilizers.
Seed treatment with fungicides: Seed treatment with fungicides is necessary to destroy seed borne infection and protect the germinating seeds from soil inhabiting microorganisms. Seed
treatment is done at least 24 hours prior to sowing. Seed treating drum is mostly used for this purpose.
Chemicals used for seed treatment: There are several types of chemicals used for seed treatment. They are disinfectant, seed disinfectants and seed protectants. Seed disinfectants kill or inactivate the pathogen present in the seed. They do not remain active for a long time. Seed disinfectants act as eradicants. They help in destroying the pathogens present deep in the tissue or embryo. Seed protectants destroy the pathogen present on the seed surface. The compound usually stays for sufficiently long time on the seed surface. Different formulations are available for seed treatment. Power formulations are predominantly used for dry seed treatment because it ensures uniform coating. The fungicides normally used for seed treatment are captan or thiram @ 4 g / kilo of seed; carbendazim is used at 2 g/ kilo of seeds
Seed treating drums: Seed treating drums are used for treating seeds with fungicides. The seed treating drum consists of a small opening in one end and two handles on either sides.
One rests over the stand and the other is used to rotate the drum kept at an angle. Seeds along with the fungicides are fed into the drum through the lid and rotated rapidly with the handle.
The tilted angle of the drum helps thorough mixing of seeds and chemicals. Seed treatment with fungicides should be done at least 24 hours prior to sowing.
Seed pelleting: This is another method of treating seeds with fungicides. If the dry seed treatment is not given, seed pelleting with fungicides is recommended. Fungicides are
dissolved in water and gum is added as adhesive (lg / 100 ml of water). Seeds are put in the solution and mixed thoroughly to ensure uniform coating of fungicidal slurry over the seeds. The seeds are shade-dried and used for sowing. Seed pelleting is particularly suitable for dryland sowing.
Seed treatment with bio-control agents: Instead of the fungicide, a biocontrol agent can be used for seed treatment. The fungal bio-control agent, Trichoderma viride is widely used for
seed treatment. It is used at 4 g / kg of seed and the treatment is done at the time of sowing. The treatment reduces the seed infestation of seed borne pathogens and increases the %of
germination and seedling vigour.
Seed treatment with biofertilizers (bacterial culture): Biofertilizers such as Rhizobium and Azospirillum are used for seed treatment. Rhizobium culture is used for the pulse seeds and Azospirillum is used for treating the seeds of cereals, millets, cotton sesame, etc. Specific bacterial culture is required for greater efficiency. Three packets (200 g x 3 = 600 g) of the
culture are recommended for seed treatment in one hectare. The bacterial culture slurry may be prepared with rice gruel or jaggery solution. Seed should be thoroughly mixed with the
bacterial culture slurry and shade-dried for 15 minutes before sowing. Fungicide treated seeds can be used for bacterial culture treatment.
Seed Treatment for Breaking Dormancy
Scarification of seeds: Some of the seeds have hard seed boat. Such seeds take more time for germination. To quicken the germination, scarification is done. Seeds are mixed with sand
and hand pounded. This operation helps to smoothen the seed coat and facilitate rapid germination. While doing this operation, care should be taken that the endosperm is not
damaged. Scarification is normally done for the seeds of wild indigo (Tephrosia purpurea), which has waxy impermeable hard seed coat.
Soaking seeds in hot water: Another way of improving the germination of the seeds with hard seed coat is by soaking them in hot water. Steeping the seeds of wild indigo in boiling
water for 2-3 minutes facilitates easy germination.
Commercial sulphuric acid @ 100 ml is used for one kilo of fuzzy seeds. Required quantity of seeds is taken in a plastic bucket and acid is added to the seeds. The seeds are stirred vigorously and continuously using a wooden stick for about 2-3 minutes till the fuzz is completely digested and the seed coat turns dark brown in color. At this stage, water is added and the seeds are washed repeatedly four to five times to remove any trace of (acid. The ill-filled and damaged seeds floating on the water are discarded. Good seeds remaining at the bottom of the bucket are collected, shade-dried and used for sowing. Acid delinting could be done well in advance before sowing.
Spacing for Seed Sowing
The spacing given to plants determines as much as any other factor how they develop. By varying spacing we have a better chance to obtain the kind of growth we want. For instance, closer spacing of lettuce gives more leaf and less heart; it also follows the golden rule of less space, smaller plants, though not necessarily lower yield.
The overall yield can still be greater, because of the larger numbers of lettuces grown in a given space. Indeed, unless you want to grow mammoth vegetables for show purposes - often good for no other - small individual vegetables may be more useful for the average family. The spacing you choose will reflect your needs and also the layout of your garden. To a considerable extent, you can pre-programme the vegetable sizes of your choice. And when you are working with beds, quite different considerations apply, since there is no need to leave access space between the rows.
Instead of the conventional rows, plants are located equidistant from each other 'on the square'. Using this system, the actual 'rows' can be closer. For example, to achieve equidistant spacing of 6 in (15 cm), the distance between the rows will ncod to be only 5 in (12.5 cm) - see drawing. This will also ensure that all the plants benefit equally from the available moisture, light and nutrients. Envisage a circle around each plant from which these essentials are drawn. Now translate this into a hexagon and see how the plants fit together most efficiently, as in a honeycomb. With the closer spacing, the vegetable foliage should just about touch, so that this pattern is also the best way to achieve weed suppression.
|Fig. 3.3: Spacing. (A) Using on the square system for maximum results, (B) Equidistant spacing ensures all the plantsj benefit equally from available moisture, light and nutrients and reduces area required.