Sexual plant breeding is a cornerstone of horticultural innovation in Kenya. By harnessing the natural reproductive processes of plants, horticulturists develop new varieties that meet local demands for improved yield, disease resistance, and climate adaptability. This chapter explores the principles and practicalities of sexual plant breeding, equipping professionals with the knowledge to enhance crop quality and sustainability in Kenya's diverse horticultural sectors.

2.1 Sexual Plant Breeding

Sexual plant breeding involves the deliberate crossing of plants through their reproductive organs to produce offspring with desirable traits. This method is widely applied in Kenyan horticulture to improve fruits like mangoes, vegetables such as tomatoes, and ornamental plants. Understanding sexual breeding techniques enables farmers and breeders to introduce genetic variability essential for adaptation and productivity.

2.1.1 Definition of sexual plant breeding

Sexual plant breeding is the process of creating new plant varieties by combining the genetic material from two parent plants through sexual reproduction. This involves the fusion of male (pollen) and female (ovule) gametes to produce seeds that carry genetic traits from both parents. Unlike asexual breeding, which clones a single plant, sexual breeding introduces genetic diversity, which is vital for improving horticultural crops under changing environmental conditions.

In Kenya, sexual breeding is crucial for crops such as passion fruit, where breeders cross different cultivars to enhance fruit quality and disease resistance. This process requires controlled pollination to ensure that the desired parent plants contribute genetic material.

Sexual breeding can be natural, relying on pollinators like bees, or artificial, where breeders manually transfer pollen. The artificial method allows precise control over parent selection, which is essential in research institutions that develop superior horticultural varieties.

2.1.2 Importance of sexual plant breeding

Sexual plant breeding plays a vital role in horticulture by enabling the development of improved plant varieties that meet market and environmental demands. It enhances crop productivity, quality, and resilience, which are critical for food security and economic growth in Kenya.

Genetic diversity enhancement

Sexual breeding introduces new gene combinations, increasing genetic diversity within plant populations. This diversity helps crops adapt to pests, diseases, and climatic stresses common in horticultural regions such as the highlands and the drier lowlands.

Development of disease-resistant varieties

Through selective crossing, breeders can combine resistance genes from different parent plants to produce offspring less susceptible to diseases like bacterial wilt in tomatoes or Fusarium wilt in bananas. This reduces reliance on chemical pesticides, promoting sustainable farming.

Improvement of yield and quality

Sexual breeding allows the combination of high-yielding traits with desirable fruit qualities such as size, taste, and shelf life. Example: a mango grower in a dry lowland area benefits from hybrids that produce sweeter fruits with longer shelf life, which improves marketability.

Adaptation to environmental conditions

Breeding programmes can target traits like drought tolerance or heat resistance, enabling crops to thrive in Kenya's diverse climates. This is crucial for smallholder farmers facing unpredictable rainfall patterns.

Economic benefits

Improved varieties can command higher prices in local and export markets, increasing farmers' incomes. For instance, cut flowers bred for specific colours and vase life meet the standards of export floriculture buyers.

2.1.3 Advantages and disadvantages of sexual plant breeding

Sexual plant breeding offers distinct benefits but also presents challenges that horticulture professionals must navigate to optimise outcomes.

Advantages

• Genetic variation creation: Sexual breeding generates new gene combinations, essential for developing plants with improved traits that cannot be achieved through cloning.
• Long-term crop improvement: Successive breeding cycles accumulate beneficial traits, resulting in stable, superior varieties adapted to local conditions.
• Combining multiple traits: Breeders can incorporate several desirable attributes, such as disease resistance and fruit quality, into one variety.
• Seed production: Sexual reproduction produces seeds that are easy to store and distribute, facilitating propagation and scaling of improved varieties.
• Supports hybrid development: Crosses between genetically diverse parents can produce hybrids with increased vigour and productivity, widely used in vegetable farming.

Disadvantages

• Time-consuming process: Developing a new variety through sexual breeding can take several years due to multiple generations needed for selection and stabilisation.
• Genetic unpredictability: Offspring may exhibit undesirable traits alongside desirable ones, requiring extensive selection and testing.
• Requires skilled labour: Controlled pollination and selection demand technical expertise, which may be limited in rural horticultural setups.
• Seed dormancy and viability issues: Some seeds produced sexually may have dormancy periods or reduced viability, complicating propagation.
• Risk of genetic dilution: Without proper isolation, cross-pollination with unwanted varieties can occur, compromising genetic purity.

2.1.4 Tools, equipment and materials used in sexual plant breeding

Efficient sexual plant breeding relies on specific tools and materials that enable accurate pollination, protection of reproductive organs, and seed handling. These tools ensure precision and reduce contamination risks during breeding programmes. Example: when crossing two tomato lines in a screenhouse, the breeder uses a lens to inspect the flower, forceps and scissors to emasculate it, a brush to apply pollen, and a bag and label to protect and identify the cross.

2.1.4.1 Magnifying lens

A magnifying lens enlarges small floral structures so that the breeder can see them clearly before and during a cross. A lens of 5x to 10x magnification is usually enough to examine anthers, stigmas and pollen grains without damaging the flower. Example: in a small-flowered ornamental, the lens shows whether the anthers have split and released pollen, so the breeder knows the best time to collect it.

The lens is used for these tasks:

• Identifying flower parts: It helps the breeder locate the stigma, style, anthers and ovary in small flowers.
• Checking stage of flower development: It shows whether the bud is ready for emasculation or the stigma is receptive, which is shiny or sticky in many crops.
• Assessing pollen quality: Plump, freely shed pollen suggests viability, while shrivelled or clumped pollen suggests poor quality.
• Confirming emasculation: After removing the anthers, the breeder checks that no anther fragments remain that could self-pollinate the flower.
• Detecting pests and damage: Small insects such as thrips on flower parts can be seen and removed before pollination.

The lens should be kept clean and dry, handled by its frame, and stored in a case to avoid scratches.

2.1.4.2 Forceps

Forceps are small, spring-action tweezers used to grip and lift delicate flower parts without crushing them. Stainless steel forceps with fine pointed tips are preferred because they can be sterilised and do not rust. Example: when emasculating a tomato flower bud, the breeder grips each anther with forceps and lifts it out whole, leaving the stigma undamaged.

Forceps are used for these tasks:

• Removing anthers: In emasculation, the anthers are pulled out before they release pollen, which prevents self-pollination.
• Removing petals and sepals: Gripping the petals gives clear access to the reproductive organs.
• Picking up anthers: A mature anther can be lifted and touched directly on a receptive stigma.
• Handling seeds: Forceps pick out individual seeds during counting, sorting and sowing.
• Placing labels and ties: They help to position small tags or fasten bags in tight spaces.

Forceps should be wiped with 70% alcohol between crosses so that pollen from one parent is not carried to another cross.

2.1.4.3 Scissors

Scissors are small cutting tools used to trim flower parts and open buds during emasculation and pollination. They must be sharp and have fine pointed blades so that cuts are clean and do not bruise the tissue around the stigma. Example: a breeder crossing two beans or peas can snip off the petals and sepals of an unopened bud with small scissors to expose the anthers before removing them.

Scissors are used for these tasks:

• Cutting the corolla: Removing the petals or part of the corolla gives access to the stamens and stigma.
• Trimming extra flowers: Surplus buds and flowers on a cluster are removed so that the plant's resources go to the labelled crosses.
• Cutting the male parent flower: A mature male flower or anther cluster can be cut and brought to the female parent.
• Harvesting fruit and seed heads: At maturity, the pods, capsules or fruits from each cross are cut off and collected separately.
• Preparing bags and labels: The same pair, kept clean, can trim paper bags or tags to size.

The blades should be sterilised with 70% alcohol between plants and kept sharp, and dull scissors should be sharpened or replaced.

2.1.4.4 Needles

Needles used in breeding are dissecting needles: fine, pointed stainless steel tips mounted in a handle. They are suited to very small flowers where even forceps are too large to work easily. Example: in a small-flowered vegetable or ornamental, the breeder uses a needle to slit open a bud and flick out the immature anthers.

Needles are used for these tasks:

• Opening anthers: The point splits a mature anther so that pollen is released onto a brush or onto the stigma.
• Emasculating small flowers: Immature anthers can be hooked and lifted out of buds too small for forceps.
• Separating flower parts: A needle teases apart petals, sepals and stamens for closer inspection under the lens.
• Moving pollen grains: A needle point can pick up a small amount of pollen for precise application to a stigma.
• Handling small seeds: It can separate and position very small seeds during sorting and sowing.

The needle is sterilised with 70% alcohol, and its tip is protected with a cap or cork when stored to prevent injury and bending.

2.1.4.5 Brushes

A brush is the main tool for carrying pollen from the male parent to the stigma of the female parent. Soft, fine bristles, such as those of a small camel-hair brush, pick up pollen without damaging the delicate stigma. Example: a breeder collecting pollen from an open pumpkin or squash male flower brushes it gently over the anthers, then touches the loaded brush to the stigma of the bagged female flower.

Brushes are used for these tasks:

• Collecting pollen: The bristles are stroked over ripe anthers to pick up pollen.
• Applying pollen: The loaded brush is dabbed on the receptive stigma to ensure good coverage.
• Pollinating many flowers: One brush per pollen parent allows several flowers to be pollinated quickly.
• Cleaning the work area: A dry brush can remove stray pollen from the work surface and from tools.

Brushes should be cleaned with alcohol and dried, or replaced, between different pollen parents so that pollen is not mixed. Using a separate brush for each male parent is the safest practice.

2.1.4.6 Bags

Pollination bags are made of paper, glassine or fine mesh and are tied over flowers to keep out unwanted pollen, insects and wind-blown material. They protect the emasculated female flower before pollination and the same flower after pollination until the fruit begins to set. Example: a breeder crossing two tomato lines bags the emasculated truss with a paper bag, pollinates it, and then covers it again until the fruit has set.

Bags are used for these tasks:

• Preventing contamination: The bag blocks pollen from bees, other insects and the wind.
• Protecting emasculated flowers: The bag keeps the female flower isolated between emasculation and pollination.
• Protecting pollinated flowers: It remains on after the cross so that stray pollen does not reach the stigma.
• Collecting pollen from the male parent: Male flowers can be bagged before opening so that their pollen stays pure until collected.
• Protecting developing fruit and seed: The bag holds seeds if pods burst and shields the cross from birds and pests.

The bag should be breathable so that heat and moisture do not build up and damage the flower. It is fastened loosely with a tie, a clip or soft wire, and a label with the cross details is attached.

Labels and markers

Labels and markers carry the record of each cross, which is the basis of reliable breeding. Weather-resistant tags and waterproof markers keep information legible until harvest. The label should show the female parent, the male parent, the date of pollination and the breeder's initials. Example: a tag reading "Line A × Line B, 12 June" tied below a flower identifies exactly where the seed came from when the pods are harvested.

Seed storage containers

Harvested seed from each cross is kept in airtight, moisture-proof jars or labelled envelopes inside a sealed container. Dry, cool storage maintains viability and prevents fungal growth, so seed is ready for the next breeding cycle. Each container carries the same cross details as the field label, and seed from different crosses is never mixed.

Chapter Summary

Sexual plant breeding involves the controlled crossing of plants to combine desirable traits and improve crop varieties. This method is important for enhancing yield, resistance to pests and diseases, and overall crop quality. The process has both advantages, such as genetic diversity, and disadvantages, including the time required to develop new varieties. Successful sexual breeding requires specific tools and equipment like magnifying lenses, forceps, scissors, needles, brushes, and bags to facilitate precise pollination and handling. Propagation of germ plasm can be achieved through seeds, cuttings, cultures, and tissue techniques, each contributing to the multiplication and preservation of selected plant material. Evaluating progeny for desired characteristics such as high yield, pest and disease resistance, non-shattering seeds, and improved quality is essential to identify superior plants. Proper waste management during and after breeding activities ensures environmental safety and maintains hygiene in the breeding environment. Carrying out sexual plant breeding and associated waste management practices effectively supports sustainable crop improvement.