Sexual & Asexual Reproduction

Introduction  Plant reproduction is necessary for the survival and perpetuation of plant species.  Plants have the capability to propagate or multiply themselves by means of sexual and/or asexual reproduction.

Types of Reproduction  Sexual reproduction involves using seeds to produce new plants.  Asexual or Vegetative reproduction involves using plant parts other than seeds to produce new plants.

 Agronomic and horticultural crops reproduced primarily by seeds include:  Field crops (corn, wheat, rice, sorghum, cotton  Flowering greenhouse and bedding plants  Vegetable transplants

 Horticultural crops and plants produced more quickly and efficiently by vegetative means include:  Landscape and ornamental plants (flowering perennials, groundcovers, shrubs)  Turf grasses  Fruit Crops

Sexual Reproduction  Seeds are the means by which plants sexually reproduce.  Although seeds contain the genetic characteristics of the parent plant, characteristics of plants grown from seeds may vary from the parent plant.

 Pollination and fertilization are part of the sexual process that occurs within the flowers or reproductive structures of a plant.  These processes result in the formation and development of seeds.

Pollination  Pollination occurs when mature pollen grains (male sex cells from the stamen) come into contact with the moist, sticky surface of a flower stigma (female flower part). stamen stigma

 Pollen grains transfer to the stigmas of flowers by means of: gravity, insects, wind, and animals.

 Self-pollination occurs when pollen comes into contact with a stigma within the same flower or other flowers on the same plant.  Cross-pollination occurs when pollen from the flower of one plant transfers to stigmas of flowers on another plant.

 After a pollen grain anchors on the moist, sticky surface of a flower stigma, it germinates and produces a pollen tube.  The pollen tube grows down through the style and into the ovary where it eventually reaches an ovule

Fertilization  In angiosperms (flowering plants), two haploid nuclei or sperm cells are released from the pollen tube.

 Double Fertilization is known as, one sperm cell unites with the egg cell in the ovule to form a zygote (diploid), while the second sperm cell unites with two separate polar nuclei to form a primary endosperm nucleus (triploid).

 The zygote, formed through the union of the sperm cell and egg cell, proceeds through cell division and develops into a seed embryo.  The primary endosperm nucleus develops to form a food source to be used by the embryonic plant during seed germination.

 For each seed a plant produces, a separate grain of pollen must reach and unite with an ovule.

Asexual Reproduction  Asexual or vegetative reproduction involves the production of new plants by means of vegetative parts of an existing plant.

 The vegetative parts of many plants have the ability to produce new roots and/or shoots to form a new plant.

 Asexual reproduction is often advantageous over sexual reproduction because: new plants reach maturity in less time; disease-free plants can be produced in controlled environmental conditions;

reproduction is possible for plants that do not develop reproductive parts or viable seeds; plant selection for desired characteristics is more reliable; and new plants are genetically identical to the parent plant.

 Vegetative plant parts used in asexual reproduction include:  Leaves  Stems  Buds  Roots

 Methods of vegetative plant reproduction include:  Cuttings  Layering  Separation  Division  Grafting  Budding  Tissue Culture

Cuttings  Propagation by cuttings is the most widely used method of vegetative or asexual reproduction.

Cuttings Cont’d  A cutting is any part severed from the parent plant, including:  stem cuttings,  root cuttings, and  leaf cuttings.

Layering  Layering involves forcing a vegetative plant part to form roots while still attached to the parent plant.

Types of Layering  Two types of layering are:  Air Layering, which is the process of forcing roots to form on a stem, outside the soil  Ground layering, which is the process of extending a plant part into the ground, covering it with soil, and allowing it to root.

Types of Layering Cont’d

Separation  Separation involves removing new plants formed on specialized stems and separating them from the parent plant.

Division  Division is the technique of cutting specialized plant structures into sections and forcing each section to grow into a new plant.

Grafting  Grafting consists of uniting a hardwood scion from one plant with the rootstock of another similar hardwood plant to form a vascular connection between the two plant parts.

Types of Grafting Techniques  The following techniques could be used in the grafting method.

Budding  Budding, which is similar to grafting, consists of removing buds from one plant and placing them on stems of other closely related plants to form a new plant.

Budding Techniques

Tissue Culture  Tissue culture, or micro-propagation, involves placing a very small piece of plant tissue on a sterilized culture medium.  Under sterile conditions, the plant tissue multiplies and grows into new plants.

Advantages of Tissue Cultures  Advantages of tissue culture as a means of asexual reproduction are that:  It allows large numbers of offspring to be produced quickly  it allows growers to produce disease-free plants;  it is a cost-efficient method of reproducing plants; and  it allows plants to be produced that have the same characteristics as the parent plant.

Disadvantages of Tissue Cultures  Disadvantages of tissue culture include:  The costs of necessary tools and equipment  The preciseness of establishing and maintaining sterile conditions needed for plant development  The requirement of additional time and labor as compared to other methods of asexual propagation.

Summary  Growers use several methods to multiply or increase the numbers of plant species.  Propagation methods can be sexual (reproducing from seeds) or asexual (vegetative production).