Plant Reproduction: Asexual vs Sexual Propagation

• Characteristics of a living organism are growth, movement, respiration, sensitivity, excretion, nutrition, and reproduction.

Modes of Reproduction

• Plants can multiply using seeds, known as sexual propagation.
• Plants can also reproduce by vegetative plant parts through asexual propagation.

Definition & Differences

• Plant propagation is the process of increasing the number of plants of a particular species or cultivar.
• Plant breeding is the art and science of improving the genetic pattern of plants in relation to their economic use.

The Asexual Propagation

• Asexual propagation involves cloning a new individual from a part of a single parent plant.
• Asexual propagation use planting materials which are vegetative parts of any plant.

The Advantages of Asexual Propagation

• Asexual propagation ensures plants are true to type and uniform in growth, yield, and fruit quality.
• This can help to avoid diseases and damaged portions can be repaired using asexual propagation approaches.
• It also allows for rapid multiplication in a very short period of time.

The Disadvantages of Asexual Propagation

• Asexual propagation requires technical knowledge and skills that can be difficult and expensive.
• Clones produced through asexual propagation are generally not so vigorous and long-lived as seedling plants.
• Asexual propagation does not allow for hybridization, which is not ideal for developing new varieties.

Forms of Asexual Propagation

• These include Apomixis, Seperation/Division, Cutting, Layering, Grafting, Budding, and Micropropagation

Apomictic Embryos

• Apomictic embryos are the production of viable fruits without fertilization, originating from Greek words "Apo + mixis," meaning away from mixing.
• Obligate apomictic examples are L. domesticum and G. mangostana
• Facultative apomictic examples include M. indica and Citrus spp.

Parthenocarpy

• Parthenocarpy refers to the development of fruit without fertilization.
• This type of reproduction produces a sterile fruit that lacks seeds.
• Parthenocarpy occurs in plants like pineapple, banana, cucumber, grape, orange, grapefruit, persimmon, and breadfruit.

Types of Seedlessness

• Seedlessness occurs through parthenocarpy (without fertilization)
• In some cases, seedlessness is created in species such as Watermelon.

Separation and Division

• Separation and Division involves separation of naturally detachable organs from the mother plant.
• Division is done by cutting vegetative structures into sections.

Modified Organs for Division

• A bulb is an underground stem with fleshy, scale-like leaves surrounding a center flower bud.
• Examples are onion, garlic, lilies and tulip
• Corm is a vertical, fleshy, underground stem that acts as a food-storage structure.
• Examples include: Gabi, Banana, and Gladiolus
• Rhizome is a creeping underground stem that produces roots and leaves at the nodes.
• Examples: Ginger, Banana, Bamboo, Sugarcane & Purple Nut Sedge
• Stolon or runner is a creeping above ground stem that grows horizontally and produces roots and shoots at the nodes.
• Strawberry & Spider plants are examples of stolons
• Tuber is a thickened part of an underground stem of a plant, with buds from which new plant shoots (stems and young leaves of a new plant) grow.
• Sucker is an adventitious shoot that arises from the underground stems below the ground.
• Examples are Banana and Pineapple.
• Offset or offshoot is a shortened, thickened stem of rosette-like appearance
• Examples: Pistia spp. & Cactus spp.
• A Pseudobulb is an enlarged stem in many orchids, especially tropical and epiphytic ones
• Bulbil - aerial plantlet formed on the axil of the leaves or flower stalk .
• Examples: Agave & Lilies

Propagation through Cuttings

• A portion of a plant is cut from the parent plant and produced under favorable conditions.
• This is to form roots and shoots, thus producing a new independent plant.

Root Formation in Cuttings

• A necrotic plate forms to seal the wound and plug the xylem to protect the surface from desiccation.
• Living cells on this plate divide to form callus.
• Cells around the vascular cambium and phloem begin to initiate adventitious roots.

Type of Plant Parts for Cutting

• Stem cutting must include a node, a point along a plant stem where leaves or other stems are attached.
• Hormones help speed up rooting.
• Dipping the cutting in fungicides help prevent rotting.
• The best time to cut is early morning because plants have more moisture.

Propagation through Stem Cuttings

• Agronomical Crops: Sugarcane (seed cane)
• Rare Plants: Philodendron Thai Cons
• Herbaceous Plants: Basil & Mints
• Ornamental Plants: Songs of India

Propagation through Leaf Cuttings

• Leaf cuttings only include a leaf blade or the blade and a portion of the petiole.
• Leaf cuttings first must form roots and later shoots.
• Depending upon the species, it can take several months to produce a plant suitable for transplanting from a leaf cutting.

Propagation through Root & Leaf-Bud Cuttings

• Root cutting example: Breadfruit and Apple
• Leaf-bud cutting example: Black Pepper and Vanilla

Rooting Hormones

• Indoleacetic acid (IAA) is a natural plant hormone (auxin) that causes root formation.
• Chemical hormones, indolebutyric acid (IBA) or naphaleneacetic acid (NAA)
• Rooting powder & Liquid Form examples of rooting hormones

Qualities of an Ideal Rooting Media

• The substance must have appropriate physical and chemical properties.
• They must retain water and air to allow sufficient drainage.
• The substance must be free from pests, weed seeds, and diseases.
• The acidity and alkalinity of the medium should be in optimum for different species.

Propagation through Layering

• "Air-Layering" refers to a propagation method by which adventitious roots are included to form on a stem while it is still attached to the parent plant.

Propagation through Grafting

• Connecting plant parts together to form one plant.
• The vascular cambium of the scion or must be aligned with the vascular cambium of rootstock.

Type of Grafting

• Inarching, or approach grafting is where a scion and stock of independently rooted plants are grafted

• The scion is later severed from its original stock

• Top working grafting scions high up in the canopy of an established tree, to change over to a new (fruit) variety

• Splice graft a plant graft made by cutting both stock and scion across obliquely

• Cleft grafting - the union of a rootstock limb that is much larger in size than the scion piece

• Side graft - the scion is inserted into the side of the stock and the aerial head of the stock is permitted to grow until union is established between stock and scion

Propagation through Budding

• Joining two plant parts such that the size of the scion is reduced to only one bud and a small section of bark, with or without wood

• A "T" shaped cut is made in the bark, near the base of the rootstock in T-budding and shield budding.

• The bark is peeled from the wood, creating a "pouch"

• Patch budding is a rectangular patch of bark containing a single bud

• The patch is taken from the scion and placed into a similar patch

• Chip budding is a similarly shaped chip is cut out of the rootstock

• The scion bud is placed in the cut, in such a way that the cambium layers match

Propagation through Micropropagation

• A method of using small pieces of plant tissue from the mother plant.
• Growing these under laboratory conditions to produce new plants

The Sexual Propagation

• Involves the formation of seeds through the fusion of male and female reproductive cells, typically from flowers

The Advantages of Sexual Propagation

• Genetic Variation: Sexual propagation introduces diversity, potentially leading to offspring with beneficial traits.
• Hybrid Vigor: Seeds can produce offspring with superior performance compared to their parents.
• Stronger and Longer: They are hardy with deep root system, and has longer lifespan
• Rootstocks: Typically raised from seeds for citrus or mango.
• Seed Propagation: Used when vegetative propagation fails or is expensive (e.g., papaya, coconut, Areca nut).
• Large-scale Seedling Production: Easiest method when a large number of seedlings are needed (e.g., dry land fruit, forest species).

The Disadvantages of Sexual Propagation

• Time and Resources: Slower and more resource-intensive.
• Genetic Variation: Can produce offspring with undesirable traits.
• Unpredictability: Traits of offspring can be unpredictable.
• Hybrid Traits: Seeds can't reliably maintain hybrid characteristics, except in cases of apomixis.
• Viability Loss: Seeds lose viability rapidly, requiring prompt planting for successful germination.

FLOWER REPRODUCTION

• Perfect Flower: A flower that has both male (stamen) and female (pistil) reproductive parts.
• Imperfect Flower: A flower that lacks either stamens (male parts) or pistils (female parts).
• Complete Flower: A flower that has all four main parts: sepals, petals, stamens, and pistils.
• Incomplete Flower: A flower that lacks one or more of the main parts (sepals, petals, stamens, or pistils).

Parts of a Fruit

• Pollination and Fertilization: After pollination, the ovary develops into a fruit as the fertilized ovules become seeds.

• Fruit Growth: The ovary walls thicken and expand, forming the flesh of the fruit.

• Fertilized ovule turns into seeds

• Maturation and Ripening: The fruit reaches its full size, develops final characteristics, and ripens, changing color, texture, and taste.

Parts of a Seed with Functions

• Embryo: The young, developing plant within the seed, consisting of the embryonic root (radicle), embryonic shoot (plumule), and one or two cotyledons (seed leaves).
• Endosperm: The nutrient-rich tissue that surrounds the embryo, providing nourishment for the germinating seedling until it can photosynthesize.
• Seed Coat: The protective outer covering of the seed, formed from the integuments of the ovule, which helps protect the seed from physical damage and desiccation.

Different Types of Seeds

• Orthodox Seeds: Naturally dry to low moisture content (≤ 20%), can be dried further to <5% without damage, and tolerate storage at low temperatures (e.g., rice, corn, beans).
• Recalcitrant Seeds: Do not naturally dry out, are damaged by low moisture levels (<12-30%), and are sensitive to freezing (<0°C) or chilling temperatures (e.g., tropical fruit crops like jackfruit, cacao).
• Intermediate Seeds: Can withstand desiccation to about 10-12% moisture content and are stored under hermetic conditions.
• They lose viability more rapidly at low temperatures (<10°C) than at warm temperatures (12-21°C) (e.g., coffee, oil palm).

Seed Germination Process

• Step 1: Imbibition: water fills the seed.
• Step 2: The water activates enzymes that begin the plant's growth.
• Step 3: The seed grows a root to access water underground.
• Step 4: The seed grows shoots that grow towards the sun.
• Step 5: The shoots grow leaves and begin photomorphogenesis.