Grafting and Micropropagation

GRAFTING

Detached Scion Grafting – Apical Graftage

Tongue Grafting:

• • Purpose: Ideal for grafting relatively small materials, around 6-13 mm in diameter.
  • Advantages: Provides considerable vascular cambium connection, heals quickly, and forms a strong union.
  • Procedure:
    • Choose rootstock and scion of equal diameter for the best union.
    • The scion should contain 2 or more buds.
    • Typically done during the dormant season.
    • Rootstock Preparation: After heading back, a smooth 2.5-6 cm long cut is made. A reverse cut is made about one-third down from the top to form a tongue.
    • Scion Preparation: A matching tongue cut is made at the base of the scion.
    • Union Formation: Insert the scion tongue into the rootstock tongue, interlocking them.
    • Additional Tips: Avoid scions larger than the rootstock to prevent large callus formation. This method is commonly used for temperate fruits like apple, pear, etc.

Whip Grafting (Splice Grafting):

• • Purpose: Similar to tongue grafting but without the tongue cut.
  • Procedure:
    • Make a slanting cut on both the scion and rootstock of equal length and angle.
    • Join the stock and scion tightly, ensuring the cambium layers match.
    • Wrap or tie securely to ensure a good vascular connection.
    • Commonly used in greenhouse vegetable production with disease-resistant rootstocks.

Cleft Grafting:

• • Purpose: Most effective in early spring, just before active growth begins.
  • Rootstock and Scion Preparation:
    • Rootstock: Make a vertical split of 5-8 cm in length.
    • Scion: Prepare dormant one-year-old wood, about 8-10 cm long, 10-13 mm thick, with 2-3 buds. Shape the base of the scion into a wedge about 5 cm long.
    • Insert the scion into the split rootstock, ensuring a smooth fit.
    • Wrap the graft tightly with grafting tape and thoroughly wax the union.
    • Uses: Rejuvenating old orchards and propagating species like walnut, hazelnut, pecan nut, and grape.

Wedge Grafting:

• • Purpose: Similar to cleft grafting, but with a V-shaped cut on the stock.
  • Procedure:
    • Make a V-wedge (5 cm long) into the side of the stub on the rootstock, with two cuts meeting at the bottom.
    • Shape the base of the scion to match the V-wedge.
    • Insert one or more scions based on the rootstock’s diameter (1-2 for 5 cm, 3 for 10 cm).
    • Wax all exposed cut surfaces.
    • Uses: Typically used for top-working older orchards.

Detached Scion Grafting – Side Graftage

Side-Stub Grafting:

• • Purpose: Useful for trees too large for whip/tongue grafting but too small for cleft or wedge grafting.
  • Best Rootstock Size: About 2.5 cm in diameter.
  • Procedure:
    • Make a 20-30° angled cut into the rootstock about 2.5 cm deep.
    • When the branch is pulled back, the cut will open slightly but close when released.
    • Scion Preparation: The scion should be 7.5 cm long with 2-3 buds.
    • At the base of the scion, create a wedge about 2.5 cm long.
    • Insert the scion into the rootstock and secure it with wrapping material.

Micropropagation in Fruit Crops

Micropropagation is the process of producing plants from small plant parts, tissues, or cells under aseptic (in vitro) conditions. This method relies on the principle of totipotency, the ability of a single plant cell or tissue to regenerate into a whole plant. Strawberry was the first fruit crop to be commercially propagated using micropropagation techniques.

Stages of Micropropagation:

1. Shoot Initiation: The initial stage where small explants (tissues or cells) are cultured to induce the formation of shoots.
2. Shoot Multiplication: The process in which multiple shoots are produced from the initial explant.
3. Shoot Elongation: The shoots produced in the previous stage grow in length.
4. Rooting: Formation of roots from the elongated shoots.
5. Acclimatization: The final stage where the tissue-cultured plants are gradually adapted to external environmental conditions before planting in soil.

Advantages of Micropropagation:

1. Large-Scale Multiplication: Enables rapid production of virus-free planting materials in a short time and small space.
2. Year-Round Production: Micropropagation can be done throughout the year, independent of seasonal variations.
3. Useful for Difficult-to-Propagate Plants: Especially beneficial for plants like papaya, where vegetative propagation is not easy.
4. Secondary Metabolite Production: Allows for the production of secondary metabolites like alkaloids, essential oils, and other valuable compounds.
5. Long-Term Storage: Plants can be stored for longer periods in small spaces, aiding in preservation.
6. Seedless Varieties: Techniques like embryo rescue can help in producing seedless varieties, such as in grapes.
7. Shortening Breeding Cycles: Speeds up the breeding process, allowing for faster development of new cultivars.
8. Homozygous Plant Production: Helps in generating genetically uniform (homozygous) plants, important for breeding programs.

Methods of Micropropagation:

Meristem Tip Culture:

• Involves culturing the meristem (growing tip) of a plant, which consists of 1 or 2 pairs of leaf primordia.
• This method is effective in eliminating viruses from infected plants.

Callus Culture:

• Callus is an unorganized mass of parenchymatous cells that forms when a plant explant is cultured.
• Organogenesis in callus culture occurs in two steps:
  • First, the formation of meristematic cells.
  • Second, the active growth of stem, bud, and root from the callus.

Anther Culture:

• Used to produce homozygous
• The technique was first reported by Guha & Maheshwari (1959) in Datura.
• It involves culturing pollen grains during the mitotic stage when they pass from uninucleate to binucleate conditions.

Cell Suspension Culture:

• Involves homogenizing callus tissue into a liquid medium, followed by shaking to suspend the cells in the medium.
• This allows for the growth of suspended cells in the culture.

Ovule Culture: Involves culturing unfertilized or just fertilized ovules, which helps obtain plants from self-incompatible species.

Embryo Culture: The embryo is excised at an immature stage (before degeneration) to induce plant development.

Applications of Micropropagation:

• Banana: The technique is commonly used for shoot tip culture from rhizoes of sword suckers.
• Grape: Involves shoot tip culture with two-nodal micro-cuttings, along with mbryo rescue techniques for seedless variety production.
• Papaya: Shoot tip culture is used to propagate papaya.