Plant Growth and Development

Growth and Development

• Growth is a continuous, irreversible change in size, volume, and weight of a plant.
• Development is a phase to phase irreversible, qualitative change in plant body.
• Plant growth and development are influenced by internal factors including nutrients and chemical messengers such as hormones and vitamins.

Hormones and Vitamins

• Hormones are synthesized in one part of the plant and have an action in a different part.
• Vitamins are used in the same part without transport.
• Vitamins function as co-factors for enzymes.

Plant Growth Regulators

• The term "phytohormone" was suggested by Thimann (1948) for plant hormones.
• Plant hormones are also known as growth hormones, growth regulators, growth promoting substances, or growth factors.
• Plant growth regulators include all plant hormones but also non-nutrient chemicals that influence growth.

Classification of Plant Growth Regulators

• Plant hormones are classified by origin and function.
• Natural Hormones are naturally produced by plant tissues (e.g., Indole Acetic Acid - IAA).
• Synthetic Hormones are artificially produced and have similar physiological activity to natural hormones (e.g., 2,4-D, NAA).
• Postulated Hormones are spontaneously produced in the plant but their structure and function are not yet fully understood (e.g., Florigen, Vernalin).
• Growth Promoting Hormones (e.g., Auxins, gibberellins, cytokinins) increase growth.
• Growth Inhibiting Hormones (e.g., ABA, ethylene) inhibit growth.

### Auxins

• Derived from "Auxein," meaning "to grow/increase".
• Promote growth along the vertical axis when applied in low concentration.
• Discovered by Arpad Paàl in 1919 who observed bending of oat coleoptiles after asymmetrically placing cut tips.
• F.W. Went isolated the growth substance from oat coleoptile tips (1926) and Kogl and Haagen-Smit (1931) named it "auxin."
• Auxins promote cell elongation and division, apical dominance, and root initiation.
• Natural auxins are produced continuously by plants (e.g., IAA) and synthetic auxins are manufactured (e.g., 2,4-D, NAA).

### Cytokinins

• Promote cell division and promote lateral bud growth (counteract apical dominance).
• Synthesized mainly in roots and transported upwards to the shoots.
• Used in tissue culture to induce shoot development.
• May slow or prevent leaf senescence.

Ethylene

• A gaseous plant hormone.
• Produced in actively growing meristems, ripening fruits, senescing flowers, germinating seeds, and injured tissues.
• Diffuses readily throughout the plant.
• Promotes leaf senescence and abscission.
• Can increase the number of female flowers in some plants.
• Degreens citrus fruits - breaking down chlorophyll to reveal underlying color.

Abscisic Acid (ABA)

• A growth inhibitor.
• Synthesized in leaves and moves readily through the plant.
• Counteracts growth-promoting effects of auxins and gibberellins.
• Involved in leaf and fruit abscission, seed dormancy, and bud dormancy.
• Induces stomatal closure in response to stress (e.g., water, heat, or salinity).

### Photoperiodism

• Discovery: W.W. Garner and H.A. Allard discovered photoperiodism studying flowering in a tobacco plant in 1920.
• The biological measurement of relative lengths of day and night.
• The control of flowering is based on changes in day length.
• Mechanism: Phytochrome pigment in plant leaves is responsible for sensing light and initiating flowering.
• Phytochrome exists in two forms: Pr (absorbs red light) and Pfr (absorbs far red light).
• Pr is converted to Pfr upon exposure to red light, and Pfr converts back to Pr in darkness.
• Photoperiodism is used commercially to manipulate plant growth. For example, increasing day length can be used to prevent winter dormancy and autumnal fall.

### Classifications of Photoperiodic Responses

• Short-Day Plants (SDP) flower when day length is shorter than a critical day length (e.g., soybean, chrysanthemum).
• Long-Day Plants (LDP) flower when day length is longer than a critical day length (e.g., oat, radish, spinach).
• Long-Short Day Plants (LSDP) flower after a sequence of long days followed by short days (e.g., Jasmine, Bryophyllum).
• Short-Long Day Plants (SLDP) flower after a sequence of short days followed by long days (e.g., winter rye).
• Day-Neutral Plants (DNP) are insensitive to day length and flower based on internal factors (e.g., Balsam, Maize).

### Critical Day Length

• The photoperiod required to induce flowering
• Varies from species to species.
• Must not be exceeded in short-day plants and must be exceeded in long-day plants.
• A single inductive cycle is necessary for flowering.
• Night break phenomenon: A brief flash of light during the night can disrupt the short-day response in plants.
• Light quality: Far Red light promotes short-day responses, while red light promotes long-day responses.

### Vernalization

• Promotion of flowering with cold treatment.
• Dry seeds don't respond to cold treatment.
• Shortens the vegetative period, resulting in early flowering.
• Plants without cold treatment may have delayed flowering or remain vegetative.
• The cold treatment is usually applied to seeds or growing plants for a period of time.