PRINCIPLES OF SEED PRODUCTION
Production of
genetically pure and good quality seed is a task requiring high technical
skills & comparatively heavy financial investment. During seed production
strict attention must be given to the maintenance of genetic purity and other
qualities of seeds in order to exploit the superiority of crop plant varieties.
In other words seed production must be carried out under standardized and well
organized conditions.
GENETIC
PRINCIPLES
Deterioration
of variety
Genetic
purity (trueness to type) of a variety can deteriorate due to several factors
during production cycles. The best means to ensure genetic purity would be to
overcome to the greatest possible extent the various factors responsible for
genetic deterioration. The important factors of real deterioration of variety
are as follows:-
1.
Developmental variations
2.
Mechanical mixtures
3.
Mutation
4.
Natural crossing
5.
Minor genetic variations
6.
Selective influence of
diseases
7.
The techniques of plant
breeder
Out of these mechanical
mixtures, natural outcrossing and selective influence of diseases are the most
important reasons for genetic deterioration of varieties during seed production,
followed by raising the seed crop in areas outside their adaptation which may
cause developmental variation and genetic shifts in varieties.
1. Developmental Variation: - When seed crops are
grown in different environment under different soil and fertility conditions or
different climatic conditions for several consecutive generations the
developmental variations may arise. To minimize such variations to occur in the
varieties, it is advisable to grow them in their areas of adaptation and
growing season.
2. Mechanical Mixtures:- Mechanical
mixing may often takes place at the time of sowing, if more than one variety is
sown with same drill; through volunteer plants of the same crop in the same
field or through different varieties grown in adjacent fields; or during
harvesting and threshing operations. Often the seed produced of all the
varieties are kept on same threshing floor, resulting in considerable varietal
mixtures. Secondly combining and threshing equipments is often contaminated
with seeds of other varieties in augers, elevators etc. further gunny bags,
seed bins, elevators are also contaminated with seeds of other varieties. To
avoid this kind of mechanical mixture contamination, it would be necessary to
rogue the seed fields and practice the utmost care during seed production,
harvesting, threshing and further handling.
3. Mutations: - This is not a serious
factor of varietal deterioration. In majority of cases it is difficult to
identify or detect minor mutations. The mutant can be removed from seed plots
to purify the seeds. In case of vegetatively propagated crops periodic increase
of true to type stock would eliminate the deterioration effects of mutations.
4. Natural Crossing: - In sexually propagated
crops natural crossing is another most important source of varietal
deterioration due to introgression of genes from unrelated stocks which can be
only solved by prevention. The extent of varietal contamination depends
upon the amount of natural outcrossing. The
deterioration in variety due to natural crossing occurs due to the
following 3 reasons:-
Ø Natural crossing with
undesirable type
Ø Natural crossing with
diseased plant
Ø Natural crossing with
off-type plants
In self pollinated crops, natural crossing is not a serious source
of contamination and variety deterioration unless the variety is male sterile
and is grown in close promixity to other varieties. But in cross pollinated
crops, natural crossing is the major source of genetic contamination. According to Bateman (1947) the extent of
genetic contamination in seed fields due to natural crossing depends upon
the following factors:
i)
The breeding system of species
ii)
Isolation distance
iii)
Pollinating agent
As the isolation
between varieties is increased the contamination generally decreases, although
there may be small amounts of contamination over wide distances. Isolation of
seed crops therefore is a primary factor in seed production of crop plants
cross pollinated by wind or insects. The extent of contamination depends upon
the direction of prevailing winds, number of insects present and their
activity, humidity & temperature at the time of anthesis etc.
5. Minor Genetic Variation:- Minor genetic variation
may still exist even in the varieties appearing phenotypically uniform and
homogenous at the time of their release. During later production of cycle some
of these variations may be lost because of selective elimination by
environment. Minor genetic variability may be constant features in often cross
pollinated crop species. Care during maintenance of nucleus and breeder’s seed
is necessary in such cases.
6.
Selective Influence of Disease:- The selective influence
of diseases in varietal deterioration is also of considerable importance. New
crop varieties often become susceptible to new races of diseases often caused
by obligate parasites and are out of seed programmes. Similarly vegetatively
propagated stocks deteriorate fast if infested by viral, fungal and bacterial
diseases. During seed production it is very necessary to produce disease free
seeds/stocks.
7. Techniques of plant Breeder: - In certain instances
serious instabilities may occur in varieties due to cytogenetically
irregularities not properly assessed in the varieties prior to their release.
Premature release of variety still segregating for resistance &
susceptibility to disease or other factors may also be important in the
deterioration of varieties. In addition to the factors discussed above, other
factors such as break down in male sterility, certain environmental conditions
and other heritable variation may considerably lower the genetic purity.
Maintenance of Genetic Purity during Seed Production
The various steps suggested by Horne (1953)
to maintain varietal purity are as follows:-
1. Use of approved seed
only in seed multiplication.
2. Inspection and approval
of fields prior to planting
3. Field inspection and
approval of growing crops at critical stages for verification of genetic
purity, detection of mixtures, weeds and seed borne diseases etc.
4. Providing adequate
isolation to prevent contamination by natural crossing or mechanical mixtures
5. Rouging of seed fields
prior to the stage at which they could contaminate the seed crop.
6. Periodic testing of
varieties for genetic purity.
7. Certification of seed
crops to maintain genetic purity and quality of seed.
8. Grow out test.
Seed Certification:- The
genetic purity in commercial seeds production is often maintained through a system of seed certification. The principal objective
of seed certification is to maintain & make available crop seeds, tubers or bulbs and sometimes turf grasses which
are of good seeding value and true to variety. To accomplish these
purposes, qualified and well experienced personnel of seed certification agency
carry out field inspections at appropriate stages of crop growth. They also
make inspections to verify that the seed crop/seed lot is of the requisite
genetic purity & quality, after
harvesting to verify quality & at the processing plants draw
samples for seed testing & sometimes for grow out test also
Grow-out Test:
Varieties being grown for seed production should periodically be tested for
genetic purity by grow-out tests, to make sure that they are being maintained
in their true form.
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Then how do we differentiate grow out test and seed testing?
ReplyDeleteGrow out test is to check only the seed genetic purity but seed testing include physical, genetic and germination.
ReplyDelete