Plant
species are naturally endowed with the ability to regenerate themselves through
self- or cross-pollination of their flowers to produce seeds. When
physiologically mature, seed germinate under optimum environments and generate
new individual plants to perpetuate the parent plant.
Similarly,
plant species whose seeds are not adequately viable to produce new plants
and/or sterile (e.g. plantains) and depend primarily on the induction of
vegetative sections (leaf, stem, root, flower stalks) containing viable buds
are particularly more vigorous in asexual propagation of these plant species.
Seed
propagation of crops is more ancient than asexual propagation, and evolved with
the origin of agricultural crop production in pre-historic times. Historically,
human use of seeds marks the transition from nomadic food gathering to
sedentary civilizations based on agriculture, in different parts of the world.
In
recent times, technological advances have led to the development of
micro-propagation, which involves the culturing of individual cells or groups
of cells (tissues) under highly aseptic conditions to produce whole new
disease- and insect-free plants.
In
this article, you should be able to explain:
•
The distinction between sexual and asexual plant propagation
•
The various methods of propagating different crop types, including non-food
crops
•
The advantages and limitations of the propagation techniques
•
The practical application of the techniques for self-sustenance.
Types of Propagation Methods
1. Sexual Propagation
This
is a method of plant propagation involving the fusion of distinctly different
sex cells (male, female) to produce a plant. The fusion of sex cells allows the
exchange of genetic materials leading to heterogeneity and formation of
hybrids, which vary widely in their appearance, physiological status, etc. and
conforms on the emerging plants diverse adaptation to varying environmental
growth conditions.
2. Asexual (Vegetative)
Propagation
This
method involves the induction of a vegetative section or part of a living plant
to form roots and subsequently, developing it into a whole new plant. Plant
multiplication does not involve the seed cycle (exchange of genetic materials)
and therefore, it is the best way to maintain some species as clones;
individuals identical to the parent.
Read: Systems of Crop Production: Evolution and Types
Types of Propagating Materials (Propagules)
1 Seed
‘Seed’
is the generative part of the plant used for propagation. A seed is a small
immature plant (embryo) protected by a seed coat or testa, which is formed from
the outer layers of the ovule after fertilization. The seed is the basic unit
of propagating many tropical crops, including yam and fruits (mango, pawpaw,
passion fruit).
Even
in crop species whose primary mode of propagation is by vegetative means (e.g.
mango, avocado pear), seed sowing constitutes an important method of
regenerating new plants, and obviating the limitations (e.g. poor adaptation)
of vegetative propagation.
Seeds
are sown in three different ways, namely by broadcasting, drilling/row-seeding,
and pocket drilling/ sowing in holes.
Advantages
of sexual propagation are ease of transportation of propagating materials, less
cost, skill and work to raise seedlings, ease of vegetative propagation in
mature plants, hybrid vigour and associated benefits of development of new
varieties and wider adaptation to varying environments. The disadvantages
include slow seedling growth, non-prototype offspring’s, problem of ensuring
uniform produce quality since most seeds originate from cross-pollination (by
wind, insects), weaker seedlings and longer period to plant maturity than vegetative-propagated
crop species.
2. Budding/Bud Grafting
A
process consisting of the engrafting of the bud (scion) of a plant into the
stem (stock) of another plant of the same genus.
Generally,
it is very suitable for propagating deciduous fruit (Citrus spp.) and shade
trees. Budding of improved materials on regenerated chupons is one of the new
methods of rehabilitating cacao in Nigeria. In the most common Tbudding
pattern, the desired scion from a young, actively-growing shoot of a chosen
crop variety is immediately slid into a T-shaped slit on the rootstock.
The
joined bud and rootstock are held by a winding of rubber band/special tape/wrap
which holds it until sealed, which prevents drying or contamination of grafted
materials.
Chip
budding is used for budding species whose barks do not “slip” (when cut, the
bark easily lifts in one uniform layer from the underlying wood) easily without
tearing. Bud grafting is faster, easier and less messy than other forms of grafting
discussed below.
Bud
grafting allows the production of plants identical to a parent plant. Also, it
may give increased productivity of crops through the hardness, superior rooting
capacity, drought tolerance and insect or disease resistance of the rootstock.
However,
the method is labour-intensive, requires great skill of nursery operations (and
therefore, expensive) and can only be efficient when performed at very specific
times when weather conditions and crop physiological growth status are optimum.
Also, the vascular cambium of the both the bud scion and rootstock must be
aligned to stimulate tissue growth on the basal ends before rooting.
3. Grafting
This
is similar to budding in theory, but different in the sense that grafting
involves the joining of the upper part (scion wood, 0.63-1.27 cm diameter and
only with leaf buds) of one plant to the under stock (rootstock) of another
plant of the same species (clones, varieties).
Grafting
is an old art and science of plant propagation in pears, citrus, mangoes,
grapes and other fruit tress, traceable back 4000 years to ancient China and
Mesopotamia. Some plants graft naturally, where two branches are in close
contact over several years (e.g. ivy).
Grafting
allows gardeners to produce plants identical to a parent plant, allows growers
to control size and shape of a tree or shrub (e.g. apples) and gives more
vigorous and earlier-fruiting plants.
Also,
two varieties can be grown on the same tree to facilitate pollination (e.g. in
apples). However, like budding grafting is labour-intensive, expensive,
inefficient in poor weather and plant growth conditions, and where cambiums of
both scion wood and rootstock are not precisely aligned.
Other
disadvantages are graft incompatibility, sucker production in grafted plants
and death of rootstock due to rooting of the scion arising from planting the
graft union below the ground.
There
is a need to protect the grafted area from dislodging the scion out of
alignment, especially by bracing. Also, there is a great risk of the top growth
being very brittle thus, failing to harden off before cold weather.
Read: Agricultural Development in Nigeria – All you need to know
8 Top Reasons for budding and grafting
1.
Opportunity to change varieties or cultivars for crop improvement
2.
Optimizing cross-pollination, especially in fruit trees that are not
self-pollinating
3.
Advantage of particular (desired) rootstocks, especially in respect of superior
growth habits, disease and insect resistance, and drought tolerance
4.
Benefit from interstocks, valuable in a situation of graft incompatibility
5.
To perpetuate clones by grafting onto seedling rootstocks when clones of plant
species (e.g. conifers) cannot be economically reproduced from vegetative
cuttings due to low rooting percentage of cuttings
6.
To produce certain plant forms e.g. weeping or cascading forms as in weeping
hemlock (Tsuga canadensi Carr var. pendula)
7. To repair damaged plants, arising from maintenance equipment, disease, rodents or winter storms, through in arching, approach grafting, or bridge grafting;
8. To increase growth rate of seedlings, especially in seedling progeny of
many trees requiring 8-12 years to fruit with natural development.
9.
To facilitate virus indexing, through confirmation of presence or absence of
the virus by grafting scions from the plant onto another plant that is highly
susceptible and would quickly show symptoms of infection.
4. Layering
This
involves bending a branch/part of the stem of a growing plant and anchoring
(with a rock or peg) and burying a portion of it, with a view to establishing a
new root system at the point of contact between the bent part and the earth
(i.e. on the shoots that are still attached to the parent plant).
A
light soil increases rooting success as will wounding or girdling of the buried
portion. Treatment with a rooting hormone (e.g. Rootone, HormondinR , HormonexR
) is most desirable, particularly one containing a fungicide. Plants with
flexible branches are particularly suited to this method. As soon as the new
plant is established, the connection with the parent plant is severed and the
new plant becomes independent. Layering is a good propagation choice when only
a few plants are needed.
A
heavy soil will reduce rooting success while covering the tip of the parent
plant (bent shoot) kills it. Other types of layering are serpentine/compound
layering, continuous/trench layering and mound/stool layering.
5. Cuttings
A
cutting is a vegetative plant part which is severed from the parent plant in
order to regenerate the parent plant (by regaining loss tissues), thereby
forming a new plant. Both woody and herbaceous plants are asexually propagated
by cuttings of stems, leaves and roots.
Cuttings regenerate new plants through change of mature cells into meristematic cells that are found at rapid growth sites like buds.
As in layering, the use of
rooting hormone as a dip, preferably one containing a fungicide, helps to
hasten rooting, increase number of roots, or gives uniform rooting, except on
soft fleshy stems. Rooting medium may be coarse sand, vermiculite, soil, water
or a mixture of peat and perlite.
A
major advantage of cuttings is the practical regeneration of whole plants from
actively-growing plant parts/organs.
Cutting
technique also, typically as with other asexual methods of plant propagation,
produces several whole new plants, and which are genetically identical clones
of the parent plant.
Important disadvantages are that cuttings should be made plants can be propagated from cuttings (e.g. Acacia spp.); cuttings must be shielded from direct sunlight, especially if they are under glass or plastic; stock plants (plants used for asexual propagation) should be healthy and well-branched as should the tools and conditions for preparing cuttings to ensure healthy new clones; choice of correct rooting medium to achieve optimum rooting within the shortest possible time.
There are many types of cuttings.Based on the vegetative part of the plant providing the cutting material.
There
are:
1. Stem Cuttings: This
technique is the most commonly used method of propagating many woody ornamental
plants and houseplants; sweet potato, sugar-cane and cassava are food crops
propagated from stem cuttings. Stem cuttings of many favourite shrubs are quite
easy to root whereas those of a tree species are more difficult to root.
A glasshouse is not necessary for successful propagation by stem cuttings but it is critical to maintain high humidity around the cutting. Facilities for rooting cuttings include flower pots, trays, small hoop frame and/or an intermittent mist system. Materials for making stem cuttings should be vigorous, new growth with no flower buds and free of diseases and insects.
Cuttings should be 5.08-10.16 cm long, cut from older stems and have 2-3 leaves
(2-3 nodes) attached.
Dipping
the base of the stem, including the node area, into a rooting powder stimulates
rooting. The stem should however, be dry when dipped.
Four
main types of stem cuttings are identifiable based on the growth stage of the
stock plant, which is very critical in the rooting of cuttings, namely:
i. Herbaceous cuttings-
cuttings taken from non-woody plants, such as perennials and houseplants e.g.
Chrysanthemums, rose. Cuttings are 5.08-15.24 cm stem pieces, with a terminal
bud.
ii. Softwood cuttings-
cuttings taken from soft, succulent, new growth from non-woody stock plants,
before the new growth starts to harden (mature). The cuttings are used to
propagate flowering shrubs. They must be taken after rain or water is required
to keep them cool in the morning. The larger diagonal cut gives more area to
develop roots. Cuttings should be kept in water before rooting.
iii. Semi-hardwood
cuttings- cuttings prepared from partially mature (firm) wood of
the current season’s growth, just after a flush of growth. The method is used
for propagating many broadleaf evergreen shrubs, some conifers, holly, rose and
cacao (using the tips of shoots).
iv. Hardwood cuttings-
cuttings taken from tissue that has become woody (firm) and the plant is mature
and dormant with no signs of active growth. Several cuttings can be made from
the same branch of some shrubs.
Basal
cuts should be just below a node, while the upper cut should be slightly above
a bud. Cuttings should be kept moist until rooting. The system is most often
used for deciduous shrubs and many evergreen species e.g. grape, fig and rose.
The three types of hardwood cuttings are straight, mallet and heel cuttings.
2. Leaf Cuttings: Leaf
cuttings are used almost exclusively for propagating a few indoor plants. Leaf
cuttings do not include an axillary bud, and thus, can only be used for
propagating plants that are capable of forming adventitious buds.
The
method involves the use of a healthy leaf blade or leaf without petiole in
propagating new plants, following the same procedures as for stem cuttings,
particularly treating leaf cuttings with growth hormones to stimulate rooting
and quick bud development.
There
are several types of leaf cuttings, and for all of them, the old leaf is not as
part of the new plant and is thus, usually discarded. In most cases, the old
leaf provides the energy food source for nurturing the newly-generated plant
e.g. Bryophyllum pinnatum.
i. Whole leaf with petiole-
This involves a whole leaf with about 3.81 cm of the petiole.
The
lower end of the petiole is dipped into a rooting medium after which one or
more new plants form at the base of the petiole. The old petiole may be reused
after the new plants have formed their own roots. African violets and peperomia
are propagated in this way.
ii. Whole leaf without
petiole- This method is used for propagated plants with sessile
(petiole-less) thick, fleshy leaves. The leaf is inserted vertically into the
medium after which one or more new plants will form from the auxiliary bud. The
leaf may be removed after the plant forms its own roots.
iii. Split-vein-
The veins on the lower surface of a leaf from the stock plant (e.g. Begonia and
snake plant) are slit before the leaf cutting is laid on the medium. The
rooting medium is used to hold down the margins of a curling leaf. A variation
of this method involves inserting leaf wedges cut with at least one main vein
into the medium with the main vein partially covered. In both cases, new plants
form from the base of the split vein and leaf wedge.
iv. Leaf-bud cuttings-
These are used for many trailing vines and when space or cutting material is
limited. Each node on a stem can be treated as a cutting. A leaf-bud cutting
consists of a leafblade, petiole, and a short piece of stem with an attached
axillary bud.
The
cuttings are placed in the rooting medium with the bud covered 1.27-2.54 cm and
the leaf exposed. Rubber plant, Camellia, Rhododendron and blackberry are
propagated using this method.
v. Flower stalks-
This follows the same technique as leaf cuttings. It involves plant propagation
from a flower stalk, usually with large leaf ears devoid of flower buds. This
enhances chimera production in African violet.
6. Root Cuttings
This
involves the propagation of plants from a section of a root. In some species,
the root cuttings produce new shoots which subsequently form their own root
system whereas in others, root cuttings develop root system before producing
new shoots. Plants propagated from root cuttings include blackberry and rose.
In
most cases, root cuttings of woody plants are usually taken during the dormant
(inactive growing period) season when roots have large carbohydrate levels.
Root cuttings can also be taken from actively-growing plants i.e. throughout
the growing season.
In
plants with large roots that are normally propagated outdoors in a hotbed, the
root cuttings should be 5.08-15.24 cm in length, with a straight cut at the
proximal end and slanted cut at the distal end of the root cutting. In plants
with small roots, the root cuttings are 2.54-5.08 cm in length and are laid
horizontally about 1.27 cm below the soil or sand in a flat. The flat is then
placed under shade, which is removed after new shoots appear.
7. Divisions
Divisions
are segments produced by cutting or breaking a crown or clump of suckers.
Suckers are aerial stems formed from adventitious buds. Each segment consists
of a bud and some roots, which when replanted grows into a new plant vertical
to the parent plant. In cocoyam and taro, plants are propagated from young
shoots. Pears and raspberries are propagated by suckers.
8. Bulbs and Corms
Bulbs
are specialized underground stems whose leaves are used as foodstorage organs.
The fleshy stem part is usually very short (compressed) and attached to a basal
plate while the fleshy leaves (bud scales) protect the terminal bud, which
eventually grow into a new plant under appropriate (favourable) environment.
Bulbs can be propagated by removing small bulblets (young bulblets) or offsets
(mature bulblets/large buds as in lilies) that form at the base of the parent
bulb.
The
small bulbs mature into plants that produce flower in 2-3 years. In other crops
such as grape hyacinth (Muscari spp.), scooping (complete removal of the basal
plate) and scoring (making three knife cuts cross the base of the bulb enough
to destroy the main shoot) removes apical dominance and encourage bulblet
formation, more than from offset propagation.
Also,
in daffodil (Narcissus spp.) bulblets form from bulb clipping and twin scaling,
where respectively, bulblets develop from the basal plate between scale leaves
and at the edge of the basal plate.
A
corm is the swollen base of a stem enclosed by dry scale-like leaves. It is a
solid stem structure with nodes and internodes.
Natural
increase of new corms and cormels (miniature corms) in the field enhance corm
propagation. The same procedure of propagating bulbs applies to corms, corm
bits and cormels.
Both
methods are used in gladiolus, lilies, onion, plantain and bananas.
In
bananas, corm bits are better sources of planting material because corm bits
uniquely assure clean planting material free of nematodes and weevils, easily
prevent transfer of banana weevils and nematodes from infested banana to new
plantations, provide enough planting materials from the few available scarce
suckers, are easier to treat with Furadan (nematicide/insecticide mixture) than
suckers, and are easier to transport for planting than suckers.
9. Runners/stolons
These
are the lateral stems or vines of crops such as grass species and sweet potato,
which run and grow horizontally on the soil surface to produce nodal
adventitious roots and subsequently plantlets.
The
organs are cut into smaller sections, each with one or more buds, and partially
buried in the ground to produce new plants. Examples of plants propagated
through these organs are strawberries and yarrow.
10. Tubers
The
“tuber” is specialized kind of swollen, modified and compressed stem structure
that functions as an underground food storage organ developed from either the
base of the stem (stem tuber) or the root (root tuber) of a plant.
The
tubers of root crops such as yam and Irish potato are sliced into setts from
which axillary buds (“eyes”) sprout into new plants or by planting whole
tubers. In Nigeria, sets are treated with pesticide dip containing 100 g
Mancoceb + 70 ml Basudin in 10 litre of water to ensure healthy seed yams at
harvest.
Sweet
potato is propagated from the tuberous roots (swollen secondary roots modified
for food storage) which are capable of producing shoots at the proximal end and
new roots at the distal end. The tuberous root propagation in dahlia is
achieved by crown division.
11. Suckers
A
sucker is a branch of the parent plant that will occasionally appear in a leaf
axil of the plant. Propagation can be achieved by cutting the suckers from the
parent plant and rooting in a rooting medium, e.g. African violet.
Some
trees and roots have shallow roots which produce separate plants called
suckers. Cutting through the roots around the suckers helps to separate the
suckers, more easily with smaller suckers about 60 cm.
The advantages of suckers compared to corm bits are ease of preparing suckers for planting and ease of identifying sucker qualities (bad, good) through observations of the parent plant.
Suckers are best planted at the onset of the rainy season to allow them access to sufficient and prolonged water supply.
In
banana, young plants or offsprings (side shoots/suckers) are produced by a
mature plant, namely water sucker (a weak side shoot with wide leaves, a
surface runner), sword sucker (a side shoot with narrow pale sword-shaped
leaves), maiden sucker (a large sucker with wide leaves, that has not yet
flowered) and peepers (a young shoot with scale leaves). Sword suckers are the
only suckers suitable for propagating banana.
12. Rhizomes
Rhizomes are horizontal stems running at or just below the ground surface, specially modified to food storage organs. Unlike roots, rhizomes have nodes and internodes, with the nodes containing growing points (“eyes”).
When cut into smaller
sections or segments containing one or more viable buds and scale leaves, the
buds sprout into new plants. Ginger, iris, couch grass and strawberry are
propagated from rhizomes.
13. Micropropagation or Tissue Culture
This
modern technique of plant propagation is based on the principle that each plant
cell has the potential to grow into a new plant exactly like the parent plant.
In
this method, individual or small group of plant cells (tiny pieces of bud, leaf
and stem) are manipulated in a way to enable them produce a new plant. Mass
propagation of sweet potato tubers is achieved by in vitro culture of nodal
segments in MS medium containing 9% sucrose under continuous darkness using Jar
Fermentor Technique.
Begonia
and roses are also propagated by tissue culture using the meristem-tip. The
advantages of this method are speed and efficiency of plant propagation and
production of disease-free (aseptic) plants. Disadvantages include spontaneous
natural mutations and very exacting conditions for growing tissue culture
materials, such as absolute sterile conditions, strict control of temperature,
light, humidity and atmosphere with costly electronic sensors and computer
equipment.
Conclusion on Propagation of Crops- All You Need to Know
In
this article, you have learned that:
i.
Plants can be propagated by both sexual and asexual/vegetative methods
ii.
Crop and non-crop plants are propagated differently.
This
article has discussed the distinction between sexual and asexual plant
propagation, the various methods of propagating different crop types, including
non-food crops, the advantages and limitations of the propagation techniques
and the practical application of the techniques for self-sustenance in
agriculture.
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