PRELIMINARY
About one million
hectares (2.47 million acres) in the Tropics are planted in tree seedlings each
year, but only a small portion
of these seedlings are indigenous species. A common barrier to the use of
indigenous species is the unavailability of high-quality seeds arising from the
lack of seed technology information. Seed problems of tropical forest trees
that need more study include phenology of flowering and fruiting, collection,
cleaning, storage, and pretreatment for germination. A number of capable
research centers around the world are working to find the answers.
Fruit is a matured or ripened ovary which contains one or
more ovules that develop into seed. Botanically seed is defined as matured
(after fertilization) and ripened ovule which contains an embryo with food
reserve and protective coat. As per seed technology or agriculture seed is a plant
part which is used for raising or propagation or multiplication of commercial
crops e.g. true seed, tubers, suckers, bulbs, cuttings, setts and grafts.
Progress
of agriculture depends on production and distribution of good quality seeds, of
best yielding varieties with favorable characteristics. At the same time
quality of agriculture depends on good quality seed. Seed technology takes care
of this.
In simple words, seed technology is the science dealing with
the methods of improving physical and genetical characteristics of seed. The
various aspects coming under seed technology are seed production, seed processing, seed
certification, seed testing, seed storage, seed biology, seed entomology, seed
pathology and seed marketing.
Prerequisite for any seed production program is to maintain
genetic purity and other characteristics of seed. Therefore seed production
should be conducted with some underlying principles. Seed technology categories such principles considered during production
and seed multiplication under
two main headings.
Seed production is organized process. Different stages during
the process ensure that seed multiplied at each stage meet all seed certification
standards for that hybrid or crop variety. Various stages of seed
multiplication are as follows.
Seeds
are the delivery systems for agricultural biotechnology. High quality seed
leads to excellent seedling performance in the field. It is the ultimative
basis of successful companies that breed crop plants for seed production. Seed quality is a complex trait that is
determined by interactions between multiple genetic factors and environmental
conditions. Modern approches to improve seed quality therefore combine
classical genetics, plant molecular biology and a variety of seed technologies.
These "seed biotechnologies" enhance physiological quality, vigor and
synchronity to establish a crop in the field under diverse environmental
conditions.
The
methods to improve seed quality by mechanical techniques include polishing off
or rubbing off seed coat (testa) or fruit coat (pericarp) projections or hairs
(abgeriebenes Saatgut), sorting into
defined seed size classes or sorting by seed density. Examples: seeds of horticultural
species. Sugar beet fruits, where polishing removes projections of the
pericarp/perianth, which is followed by sorting into defined seed size classes.
SUBJECT MATTER
Seed technologies (seed
enhancements, seed treatments) include priming, pelleting,
coating, artificial seeds, and other novel seed treatment methods of applied
seed biology. Our basic and applied seed research projects focus on embryo
growth and on the different seed covering layers (e.g. testa, endosperm,
pericarp), which are determinants of seed quality and exhibit the biodiversity
of seed structures. Seed germination is controlled by environmental factors
(light, temperature, water) and on plant hormones as endogenous regulators
(gibberellins, abscisic acid, ethylene, auxin, cytokinins, brassinosteroids).
The utilization of plant hormones and inhibitors of their biosynthesis and
action in seed treatment technologies affects seed germination and seedling
emergence. The genes, enzymes, signaling components and down-stream targets of
plant hormones provide molecular marker for seed quality and seedling
performance.
Important
methods to enhance seed and seedling performance are through addition of
chemicals to protect the seed from pathogens and/or to improve germination. Different
techniques of seed
coating (Saatgutbeschichtung) and seed pelleting (Pillierung) are used for this. Film-coating methods allow the chemicals to
be applied in a synthetic polymer that is sprayed onto the seeds and provide a
solid, thin coat covering them. The advantage of the polymers is that they
adhere tightly to the seed and prevent loss of active materials like
fungicides, nutrients, colorants or plant hormones. Some novel applications of
film coating are used to modify imbibition and germination. They can confer
temperature-sensitive water permeability to seeds or affect gaseous exchange. They control the timing of seed germination
and seedling emergence. Certain temperature-dependent water-resistant polymers
can delay imbibition until the climatic conditions become suitable for
continued seedling growth.
Seed pelleting adds thicker artificial
coverings to seeds, which can be used to cover irregular seed shapes and add
chemicals to the pellet matrix, e.g. of sugar beet or vegetable seeds. The
pellet matrix consists of filling materials and glue. Loam, starch, tyllose
(cellulose derivative) or polyacrylate/polyacrylamide polymers are commercially
used. A film coat can be added onto the pelleting layer as shown in the figure
above. Seed pelleting is also
used to increase the size of very small horticultural seeds. This provides
improved planting features, e.g. singulate planting, the use of planting
machines, or precise placement and visibility in/on the soil. The images below
this text are examples for pelleting of very small horticultural seeds.
Seed
priming is the most important physiological seed enhancement method. Seed
priming is an hydration treatment that allows controlled imbibition and
induction of the pregerminative metabolism ("activation"), but
radicle emergence is prevented. The hydration treatment stopped before
dessication tolerance lost. An important problem stop the priming process in
the right moment, this time depends on the species and the seed batch.
Molecular marker can be used to control the priming process. Priming solutions
can be supplemented with plant hormones or beneficial microorganisms. The seeds
can be dried back for storage, distribution and planting. Germination speed and
synchronity of primed seeds are enhanced (see figures below) and can be
interpreted in the way that priming increases seed vigor (short or no
"activation" time). A wider temperature range for germination,
release of dormancy and faster emergence of uniform seedlings is achieved. This
leads to better crop stands and higher yields. A practical drawback of primed
seeds is often a decrease in storability and the need for cool storage
temperatures.
Several types of seed
priming are commonly used:
ü
Osmopriming (osmoconditioning) is the
standard priming technique. Seeds are incubated in well aerated solutions with
a low water potential, and afterwards washes and dried. The low water potential
of the solutions can be achieved by adding osmotica like mannitol,
polyethyleneglycol (PEG) or salts like KCl.
ü
Hydropriming (drum priming) is achieved
by continuous or successive addition of a limited amount of water to the seeds.
A drum is used for this purpose and the water can also be applied by humid air.
'On-farm steeping' is the cheep and useful technique that is practized by
incubating seeds (cereals, legumes) for a limited time in warm water.
ü
Matrixpriming (matriconditioning) is the
incubation of seeds in a solid, insoluble matrix (vermiculite, diatomaceous
earth, cross-linked highly water-absorbent polymers) with a limited amount of
water. This method confers a slow imbibition.
ü
Pregerminated seeds is only possible with a few
species. In contrast to normal priming, seeds are allowed to perform radicle
protrusion. This is followed by sorting for specific stages, a treatment that
reinduces dessication tolerance, and drying. The use of pregerminated seeds
causes rapid and uniform seedling development.
One major seed
problem in the Tropics is the lack of definitive information on the phenology
of flowering and maturation of fruits and seeds. Unlike most temperate zone species,
many tropical trees flower over a period of many months, so that multiple
stages of seed maturity may be present at any one time on the same tree. This
condition complicates seed collection, for there is no single definable period
of seed maturation within a species or even among trees of the same species.
Other collection
problems are presented by the spatial distribution or size of trees in natural
stands. In moist tropical forests, fruit-bearing limbs of desirable trees may
be as much as 35 meters above the forest floor. Unless seeds can be collected from
the ground after natural seedfall, climbing is the only practical option.
Predators
present another major problem in the Tropics. Animals are natural seed
dispersal mechanisms in tropical ecosystems, but they complicate things for
human seed collectors. And when seeds are dispersed on the ground, numerous
birds, rodents, and insects are there to eat them. Timely collections are needed
to avoid these losses, but incomplete knowledge about fruiting phenology and
wide spatial distribution of trees combine to make this difficult.
In contrast to
the common image of rapid germination in tropical forests, there are many
species that exhibit seed dormancy. While many species germinate promptly when dispersed,
others exhibit long delays in germination. Seed dormancy is most common among
leguminous species and species of dry tropical forests. Seed coat dormancy, the
most common cause, is easily overcome with scarification, but other, more
complex dormancies may be encountered.
Dormancy of crop
and horticultural seeds
is an unwanted trait for horticulture. However, a certain degree of dormancy is
required to prevent viviparous germination on the plant, e.g. preharvest
sprouting of cereal crops. Dormancy is the failure
of fully developed, mature, viable seed
to germinate even under favorable physical conditions (like moisture and
temperature). The seed with dormancy is termed as dormant seed.
The most
challenging problem for seed science in the Tropics is storage of recalcitrant seeds. The
intolerance of tropical recalcitrant seeds to both low moisture content and low
temperature prevents the use of these conditions for storage. Temperate
recalcitrant seeds fare better in storage because low temperatures can be used,
but both groups have short storage lives. Solution of the storage problem for
one of the recalcitrant seed groups should benefit the other group as well.
CONCLUSION
·
Supply high quality seeds, means seeds
of high yielding varieties, varieties with resistance to diseases and pests.
·
To increase agricultural production by
supply of quality seed.
·
Timely supply of seeds, i.e. well before
the sowing season.
·
Supply of seeds at reasonable prices.
Some principles are considered during production and seed
multiplication. Those are given on the page Principles of
seed production.
There are plenty
of challenges in seed technology of tropical species, and all of them can be
done without massive expenditures for laboratories and equipment. Most of the research
suggested in this article must be done on-site in the Tropics, not in the
comfort of well-equipped temperate-zone laboratories. There are many competent
seed researchers and institutions in tropical regions around the world. This is
not a complete list, by any means, and new programs seem to be continually
coming on line. While seed research is not the primary focus of some of these
institutions, they all have the capability to solve their respective problems.
Research papers
on tropical tree seeds appear now in scientific journals at an ever increasing rate,
which is evidence of two trends. One is that many more researchers from the
temperate zone are working on tropical seed problems these days, and the second
is that the capabilities of research staffs and institutions in tropical countries
are increasing. In the long run, it will be the scientists from the tropical
countries who contribute the most to meeting the challenges of tree seed
technology in tropical forestry.
BIBLIOGRAPHY
Bonner,
F.T. 1992. Seed Technology: A Challenge
for Tropical Forestry. Tree
Planters' Notes 43(4):142-145.
Project leader, USDA Forest Service Southern Forest Experiment Station, Starkville, Mississippi.
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