Plant Sexual Propagation: Seeds and Germination

Lesson 4.1: Sexual Propagation of Plants

• Seeds are the fundamental component for sexual reproduction in plants
• A seed consists of a fertilized, mature ovule that includes an embryo, stored nutrients, and a protective seed coat

Importance of Sexual Reproduction

• More plant species and cultivars are reproduced through sexual propagation
• Nurseries commonly employ to produce rootstocks
• The process of seedling cultivation stands as a highly important means of creating new plant cultivars

Seed Germination

• Germination involves the renewal of active growth within an embryo
• The process of germination leads to the rupturing of the seed coat and the subsequent development of a seedling.

Germination Definitions

• Morphological: The embryo transforms into a seeding
• Physiological: Metabolism and growth, previously suppressed, restarts
• Biological: Transition of the embryo from a state similar to suspended animation
• Biochemical: Resumption of biochemical routes for typical growth
• Seed Analyst: The seed embryo develops into a normal plant under the right conditions

Stages of Germination:

• First stage: Involves awakening or activation, completed quickly, it includes absorption of water, seed coat softening, protoplasm hydration, oxygen absorption, and enzyme production

• Second stage: Features food reserves, enzyme digestion of stored carbohydrates, fats, and proteins in cotyledons and endosperms into simpler substances, and translocation

• Third stage: Characterized by cell division, seedling weight increase, and storage tissues decrease.

Types of Germination:

• Epigeal Germination: Marked by hypocotyl elongation, lifting cotyledons above ground
• Hypogeal Germination: Marked by the failure of the hypocotyl to lengthen, epicotyl emerges above the soil surface

Germination Requisites

• An embryo needs to be alive and ready for germination
• A seed should lack physical and physiological barriers
• The presence of a suitable surrounding

Internal Factors Delaying Germination

• Plant growth regulators, like chemical inhibitors
• A rudimentary embryo
• A tough seed coat
• Seed damage through organisms

Impact of Plant Growth Regulators

• Gibberellin encourages germination
• Abscisin discourages germination
• Cytokinin counteracts abscisin by promoting gibberellin and encouraging germination

External Factors Affecting Germination

• Water
• Oxygen at 20%
• CO2 at 0.03%
• Temperature
• Light
• pH of the medium
• Osmotic potential
• Other factors

Seed Attributes

• Viable seeds can germinate when environmental needs get met
• Quiescent seeds cannot germinate because conditions don't allow it
• Dormant seeds do not germinate, even if conditions are ideal

Dormany Types and Advantages

• Dormancy is categorized as primary, a combination of both exogenous and endogenous factors, or secondary
• Dormancy enables certain crop species to exist in deserts
• Dormancy reduces waste of grain seeds that are harvested

Seed Viability

• Viability is evaluated by looking at germination rate and percentage
• Depletion of reserves affects viability
• Changed food reserves affects viability
• A seed's membrane system affects viability
• A seeds natural repair affects viability

Pretreatment of Seeds

• Scarification
• Soaking in water/hot water assists the seed-softening stage of germination
• Stratification, or chilling, helps meet the climate needs of certain seeds (vernalization)
• Plant growth regulators are useful to help kickstart key processes
• Using a combination of the aforementioned treatments

Seed Quality Evaluation

• Seed quality is based on germination rate and percentage
• Germination rate is calculated using the total number of seeds germinated and values indicating the time between start and end tests
• Good quality seeds must be genetically true to species
• Good quality seeds should be pure
• Good quality seeds exhibit high viability
• Good quality seeds are free from diseases/insect damage
• Good quality seeds must be free from other seeds and materials

Seed Purity

• Purity represents the percentage of pure seed by weight in a sample
• Formula for Purity (%) = (a/a+b+c+d) x 100, where:
• a equals a kind pure seed under consideration
• b equals another crop seed
• c equals weed seed
• d equals inert material, for example empty or broken seed

Seed Viability Testing

• Excised-embryo test
• Tetrazolium testing

Seed Estimation

• The actual rate of seeding is determined by dividing the recommended seeding rate via its percentage of germination
• Weight per seeds/unit area is determined by factoring number/weight to include germination and purity

Lesson 4.2: Asexual Reproduction of Plants

• Asexual propagation effectively keeps a plant’s genetic composition intact
• Flowers and fruit are shown sooner in asexual reproduction than seedling plants
• Asexual reproduction becomes essential for growing types that barely make viable seeds
• Asexual Reproduction facilitates quicker and economical growth

Methods of Asexual Propagation

• Cuttings
• Grafting
• Layering
• Specialized vegetative structures
• Tissue culture

Cuttings

• Cuttings are made from plant vegetative parts
• Cuttings involve using stems, modified stems (rhizomes, corms and bulbs, tubers), leaves, or roots
• Many cells have the capacity to revert into their meristematic condition and form into roots

Types of Cuttings

• Stem
• Leaf bud
• Leaf
• Root cuttings

Leaf Cutting Types

• Whole leaf with petiole
• Whole leaf without petiole
• Split vein
• Leaf section

Root Development In Cuttings

• Stage 1: Cellular dedifferentiation and initiation of meristematic groups
• Stage 2: Root initial differentiating into recognizable root primordial
• Stage 3: Root growth and emergence

Influence of Plant Hormones

• Auxins are required for adventitious roots on stems
• Cytokinins help in cell growth, differentiation, and initiation
• Gibberellins promote stem elongation
• High GA concentrations restrain adventitious root formation, whereas lower concentrations promote root formation
• Ethylene is not yet well established in rooting
• Abscisic acid meddles with root creation in leaf cutting

Polarity In Cuttings

• Direction of cell division, enlargement, differentiation, and tissue orientation influences the cuttings
• Stem cuttings grow shoots on its distal end and roots at the proximal end
• Root cuttings grow shoots its proximal end and grow roots distally

Selection Factors

• Physiological condition of the stock plant is key
• Reduce nitrogen and select lateral growths shoots with decreased CHO accumulation
• Select shoots with high CHO
• Etiolate the shoot
• Girdle the plant at shoots base
• Plant age matters for optimal propagation

Wood Selection Influences

• Guide in selecting type of wood:
  • Individual plants within a species
  • Rooting capability between lateral and terminal shoots
  • Parts of the shoot
  • Flowering and vegetative wood
  • Presence of virus disease
  • Time of year in which the cutting is taken

Treatments of Cuttings

• Application of growth regulators(IBA/NAA)
• Vitamins
• Using proper rooting medium
• Mineral nutrients are needed
• Fungicides
• Wounding of leaf
• Use ant-transpirational treatments
• Consider the condition in which to grow leaf

Handling Rooted Cuttings

• Harden the leaves for a few days
• Soil must have proper moisture
• Cloudy days for digging
• Trim roots if deeply rooted

Method of Layering

• Layering involves inducing adventitious roots to form at the stem which are attached at parent plant
• Simple layering
• Tip layering
• Air layering
• Mound/stool layering
• Serpentine layering
• Trench layering

Specialized Vegetative Structures

• Runners: Modified stems growing from crown of plant
• Stolons: Modified stems that grow horizontally
• Suckers: Shoots arising from adventitious bud on root
• Crowns: Branches from a stem base

Grafting

• This is the joining of plants so they continue growing as one
• The rootstock provides the system of the root
• Grafting/budding is an operation where a piece of bark containing a bud is used as the scion
• The scion provides the upper part
• Interstock has a piece inserted in between stock and scion

Importance of Grafting

• Enables perpetuation of clones that are hard to reproduce by other means
• Permits one to obtain the benefits of certain rootstocks
• Allows flexibility in topworking
• Hastens seedling selection
• Allows unique forms of plant growth
• Aids in repairing damaged trees
• Detects virus diseases

Formation of Graft Union

• Development of callus tissue
• Interlocking of parenchyma cells
• Differentiation of callus tissues into new cambium cells
• Production of new vascular tissues

Factors affecting Graft Union

• Incompatibility
• Kind of plant
• Conditions of oxygen during grafting
• Stock activity of growth
• Grafting techniques
• Free from pests and diseases

Successful Graft Requirements

• Use sharp knife
• The scion must contain a bud
• Stock and scion must be compatible
• Ensure cambial contact
• Proper environment before/after

Lesson 4.3: Tissue Culture/Aseptic Micropropagation

• Micropropagation produces plants from cell, tissue, or organ culture
• Plantlets are cultured for study in nutrient medium under controlled conditions

Types of Aseptic Micropropagation

• Embryo culture
• Organ culture
• Tissue culture
• Protoplast culture
• Anther/haploid culture

Advantages of Aseptic Micropropagation

• Rapidly multiplies plant material
• Produces disease-free material
• Produces genetically uniform planting materials
• Allows for year-round reproduction

Sexual / Asexual propagation

• Micropropagation may be categorized into sexual or asexual

Sexual Propagation

• Sexual Micropropagation: Orchid, coconut, and embryo cultures

Asexual Propagation

• Asexual vegetative propagation uses shoot tips, nodal segments, and leaf tissues as explants