Vegetative Propagation and Micropropagation

• Bryophyllum reproduces vegetatively through its Leaves.
• Special Features: The margins of mature Bryophyllum leaves are deeply notched or serrated. Within these marginal notches lie dormant adventitious foliar buds.
• When a mature leaf detaches from the plant and falls onto moist soil, these marginal buds absorb water, break dormancy, and sprout tiny green shoots with adventitious roots.
• Each bud eventually separates to establish an independent, mature clone plant.

• Grafting: An artificial vegetative propagation technique in which the shoot system of one plant is surgically united with the root system of another plant so that they fuse and grow as a single composite organism.
• Stock: The lower, rooted plant portion that provides the root system, anchoring the plant and absorbing water and minerals (chosen for hardy, disease-resistant roots).
• Scion: The detached shoot or stem cutting possessing desirable buds that is grafted onto the stock (chosen for superior fruit, flower, or yield quality).
• Essential Precaution: The Vascular Cambium layers of both the stock and scion must be aligned in direct, intimate physical contact, tightly bound with twine, and sealed with grafting wax to prevent desiccation and fungal infection.

• (i) Potato: Stem Tuber (using axillary buds called "eyes").
• (ii) Ginger: Rhizome (underground horizontal stem).
• (iii) Onion: Bulb (condensed stem surrounded by fleshy scale leaves).
• (iv) Sweet Potato: Tuberous Root (adventitious root).

• Definition: An advanced modern laboratory technique of propagating large numbers of genetically identical plants in vitro under aseptic, sterile conditions from tiny plant tissue fragments.
• Four Sequential Steps:
1. Explant Isolation: A tiny tissue fragment (e.g., shoot apex) is excised and surface-sterilized.
2. Inoculation on Culture Medium: Explant is placed in sterile nutrient agar containing mineral salts, vitamins, glucose, and hormones (auxins and cytokinins).
3. Callus Formation: Cells divide rapidly to form an unorganized, undifferentiated cell mass called a Callus.
4. Organogenesis & Hardening: Hormonal ratios induce root and shoot development, forming Plantlets, which are hardened in greenhouses before field planting.

• Advantages:
1. Seedless Propagation: Allows commercial propagation of seedless varieties that cannot reproduce sexually (e.g., Cavendish bananas, seedless grapes, sugarcane).
2. Genetic Uniformity (Clonal Purity): Offspring are genetically identical clones of the parent plant, preserving desirable agricultural traits reliably.
• Disadvantages:
1. Zero Genetic Variation: Lack of genetic recombination leaves crops highly vulnerable to catastrophic epidemics (e.g., Panama disease in bananas).
2. Overcrowding & Rapid Degeneration: Offspring grow tightly clustered around the parent plant, causing severe competition for nutrients and light.

• In a plant infected with systemic viruses, the Apical Meristem (the actively dividing tip of shoots and roots) is almost always completely free of viral pathogens.
• This is because cells at the meristem divide and expand much faster than viruses can replicate and travel through the vascular bundles (which have not yet differentiated at the apex).
• Culturing this virus-free meristem explant produces completely healthy, disease-free clones.

• Auxins (e.g., IAA, NAA): Stimulate cell enlargement and induce root differentiation (rhizogenesis).
• Cytokinins (e.g., Kinetin, BAP): Stimulate rapid cell division and promote shoot differentiation.
• By precisely modulating the Auxin-to-Cytokinin ratio, scientists can command a neutral callus to form either roots, shoots, or both simultaneously.

• Method: A healthy, pencil-thick aerial woody branch is selected. A ring of bark (about $2-3\text{ cm}$ wide) is completely stripped off (girdled) to expose the cambium. Rooting hormone paste is applied, and the stripped section is wrapped with moist sphagnum moss and enclosed tightly in a polythene sheet tied at both ends.
• After several weeks, adventitious roots emerge through the moss. The rooted branch is then severed below the bandage and potted as an independent tree.
• Examples: Practiced on fruit trees like Litchi, Pomegranate, Guava, and Lemon.