Methods of Crop Propagation

Sexual Propagation

Sexual propagation is propagation by means of seed. A seed is a fertilized ovule enclosing a tiny plant and the food necessary for its germination. It is thus a living but dormant plant. A seed will germinate to produce a new plant when given all the right conditions.

Seed Germination

Germination is the process whereby the seed absorbs water, swells, and produces a radicle (embryonic root). This is soon followed by the plumule which later develops into a shoot. Before the new plant develops leaves, it uses the food stored in the cotyledon(s) to grow. Once new leaves are formed the plant uses carbon dioxide from the air and water from the soil to produce its food through a process called photosynthesis.

   Recently, we classified crops as either monocotyledons or dicotyledons depending on whether they have one or two seed leaves. In monocotyledons, the single seed leaf remains underground when the seed germinates. In dicotyledons, the seed leave appear above the ground. In some cases they may turn green and carry on photosynthesis before proper leaves are formed.

  For a seed to germinate, it must have adequate moisture, oxygen (from the air), and the right temperature. seeds of some crops require light for germination.

Seed dormancy

Seeds of certain crops fail to germinate immediately after maturity even when all the right conditions for germination exist. An example is some varieties of ground nuts. This temporary failure to germinate is called dormancy. Dormancy is caused by a number of factors:

(a) Type of seed coat - A hard or thick seed coat prevents the seed from absorbing water and possibly also oxygen. Under natural conditions the hard seed coat will soften with time.

(b) Immature embryo - In some crops, the embryo has not completed its development at the time of harvest, and so the seed remains dormant.

Dormancy caused by hard or thick seed coat can be broken by two methods:

(I) Presoaking - soaking the seeds in water to soften the seed coat so that it absorbs water. When planted in the field the seed will germinate faster.

(ii) Scarification - This is a process of breaking the seed coat so that the embryo can easily absorb water and oxygen when it is planted.

Seed longevity

Longevity is the ability of the seed to remain viable or able to germinate with time. seeds of some crops remain viable for many years while those of other crops lose their viability after a few years. Onion seeds lose viability very quickly. Cereal grains generally remain viable longer than legumes. Before planting seeds of any crop it is important to know the viability.

Seed quality

Seeds for planting must meet certain minimum qualities in order to get a good crop.

Some of these qualities are that:

I.  Seeds must be mature and well filled. Small shrivelled seeds may germinate but they produce small, weak seedlings that may not survive.

ii. The seed must have a high percentage viability.

iii. Seeds for planting must be clean and free broken seeds, dirt, weed seeds and other foreign materials.

iv. They must be free from insects and seed borne diseases.

Treatment of seeds for  planting

Although I have defined seed as a fertilized ovule, agriculturists sometimes use the word 'seed' loosely to include things like potato and yam cuttings. Seeds must be dressed with seed dressing chemicals before planting. This I'd to protect them from soil borne pests and diseases. If seeds are not treated they can be attacked by soil fungi. This will cause them to rot and fail to germinate. They may also be attacked by soil borne insects and other organisms. The two most commonly used seed dressing chemicals in Nigeria are :

(I) Fernasan D.

(I) Aldrex T.

  There are also many other chemicals tested in our research institutes. If found suitable, they will eventually be recommended to farmers for use in treating seeds before planting.

   A chemicals are toxic to a certain degree. You should therefore strictly obey the directions on the packet when using any chemicals.

Asexual propagation

Asexual propagation is propagation by means of vegetative organs. For this reason it Is sometimes called vegetative propagation. In some crops that do not produce viable seeds it is the only means of propagation. In certain other crops it is used because it is easier than growing plants from seed. Asexual or vegetative propagation can be divided into four groups , namely: Division, Cutting, layering and Grafting.

   Division - This is propagation of plants from vegetative organs that separate naturally from the parent plant. Such organs include:

(I) Rhizomes - Ginger.

(ii) Suckers -  Banana.

(iii) Crowns - Pineapple.

(iv) Runners - Strawberries.

Cutting - A cutting is a portion of a plant which when placed in a suitable medium  will produce roots and shoots and grow into a new plant. Cuttings are classified in two ways:

(a) The part of the plant from which they are made.

i. Stem cutting - for example casava , sweet potatoes and Irish potatoes.

ii. Root cutting - for example Bread fruit.

iii. Leaf cutting - For example Bryophyllum.

(b) The stage of development when the cutting is made.

i. Herbaceous cutting - these are cuttings made from the stem portion having herbaceous succulent growth. For example coleus.

ii. Softwood cutting - This is also succulent tissue but from new growth. An example is Croton.

iii. Semi hardwood cutting - this contains mature tissue which has not yet hardened. An example is lemon.

iv. Hardwood cuttings - cutting from mature , hardened part of the stem. Examples are grape and fig.

Layering - This is a method of vegetative propagation in which plant parts are made to develop roots while they still attached to thr parent plant. There are sic types of layering.

i. Simple layering - A branch of the plant is buried in the soil at one point. This is cut off from thr parent plant when it has rooted

ii. Serpentine or compound layering - in this case a long branch is selected and layered at may poi ts instead of just one.

iii. Mound or stool layering - The stem is cut off completely and the stump is covered with soil. When the buds start sprouting, more soil is added so they can form adventitious roots.

iv. Trench layering - this is similar to serpentine layering but in this case the whole branch is buried in thr ground. The axial buds sprout to produce new plants that can be separated.

v. Tip layering - Only the tip of s branch is covered with soil. This is possible where the terminal bud sprouts.

vi. Air layering - in air layering the bark is removed from a portion of the stem. That portion is covered with moist saw dust or orther soft material. This is then covered with a polythene film and the place continuously kept wet. When it had rooted, it is cutoff for planting. The stem to be layered should be mature but actively growing. The bark should be easy to remove.

   Grafting - This is a method of vegetative propagation in which two parts of different plants are  joined together so that they unite and grow as a single plant. The lower part of this union is called the root stock and the part grafted on top is called the scion. When a single but is used as a scion the  process is called budding.

Conditions for successful granting include the following:

i. Stock and Scion must be compatible. That is , they must be suitable together in order for their union to be successful

ii. Stock and scion must be in proper stage of growth.

iii. Cambium layers of stock and scion must be well aligned.

(a) Small scion with thin rind

(b) Large stock with thick rind

iv. Cut  surfaces must be protected from dying.

v. After budding or grafting adequate time should be allowed to form a union.

Sections of scion or budwood:

i. It should be from a suitable plant of variety

ii. It should be from mature wood of current seasons growth.

iii. Buds on thr scion should be in dormant condition.

iv. A round twig is generally preferred.

There are various methods of budding and grafting.

Method of budding

There are two main methods of budding.

I. Shield or T budding 

ii. Patch budding

a. Stock prepared  with T- shaped incision

b. Shield bud prepared.

c. Bud inserted in stock.

d. Technique of cutting out patch from bud stick.

e. Bud patch removed.

f. Stock showing preparation of  panel with flap and bud patch inserted.

Method of grafting

There are four common methods of grafting

i. Approach grafting - the stock and scion Are brought together as two separate growing plants. The scion is not severed from the parent plant until a successful union has been achieved. Approach graft can be either spliced  or tongued

ii. Apical grafting - this is where the top part of the root stock is cut off ad in arching. The difference is that the scion has been detached from the parent plant. There are three common types of apical grafting.

1. Whip and Tongue.

2. Saddle.

3. Wedge or cleft.

Types of apical grafting

a. Scion prepared for whip and tongue graft.

b. Stock prepared for whip and tongue graft.

c. Whip and tongue graft ready for tying.

d. Stock prepared for saddle graft.

e. Scion prepared for saddle graft.

f. Saddle graft ready for tying.

g. Scion prepared for wedge or cleft graft.

h. Wedge or cleft graft ready for tying.

Advantages and disadvantages of different propagation methods 

Sexual reproduction

i. In most field crops propagation by seed is the only economic and practical method.

ii. Horticultural crops grown from seed live longer than those propagated by asexual means.

iii. In breeding new varieties, seeds are absolutely essential.

Iv. Root stocks are mostly grown from seed without which budding and grafting cannot be done.

V. Seedlings are easier and cheaper to raise from seed.

Disadvantages

I.  Some seedlings raised from seed are not true to type. This is important in horticultural crops.

ii. Plants grown from seed are not always uniform in growth, maturity, yield and other characters.

iii. Some plants produce very few seeds. Seed germination may also be poor.

iv. Seeds may not germinate because of dormancy.

v. Plants grown from seeds take a longer time to produce fruits.

Asexual reproduction

Advantages

I. Plants are true to type and uniform in quality, growth habit and yield.

ii. All plants propagated at the same time by vegetative means start bearing at almost the same time.

iii. It is the only way of propagation for plants that cannot produce seeds.

iv. Plants mature and start bearing early.

v. A plant that is susceptible to certain soil conditions can be grafted onto a root stock that is resistant to these conditions.

Disadvantages

I. Plants do not live long and they are not vigorous.

ii. It is not possible to produce new varieties.

iii. Budding and grafting requires special skills