2b Growth and Development

Plant Growth and Development Overview

Growth Definition

• Growth is an irreversible increase in volume due to the division and enlargement of cells.

• Cells mature and differentiate into forms adapted to specific functions.

Influences on Growth and Development

• Growth and development are influenced by:

  • Genetic Factors: Impact the inherent qualities and physical performance of the plant.

  • External Environmental Factors: These include light, temperature, water, and soil conditions.

  • Chemicals: Both naturally occurring hormones within plants and synthetic plant growth regulators (PGRs).

Tropism and Plant Reactions

Types of Tropism

• Tropism: Response to unidirectional stimulus.

  • Phototropism: Response to light direction.

  • Geotropism: Response to gravity.

  • Thigmotropism: Response to touch or contact.

Nastic Movements

• General responses to external stimuli include:

  • Epinasty: Downward drooping of leaves.

  • Stomata Closure: Regulation of gas exchange.

  • Photoperiodism: Plant response to light duration.

  • Vernalization: Requirement of a period of cold for flowering.

  • Leaf Fall (Abscission): Shedding of leaves in response to changes in environmental conditions.

Plant Growth Regulators (PGRs)

Types of PGRs

• Auxins (e.g., IAA, NAA): Influence:

  • Cell elongation, shoot and bud growth, rooting.

  • Applications include promoting fruit setting and inhibiting lateral bud development.

• Gibberellins (e.g., GA3): Important for:

  • Breaking dormancy and initiating germination.

  • Stem elongation and increasing internode lengths.

• Cytokinins (e.g., BAP): Involved in:

  • Cell division and differentiation, leaf formation, and chloroplast development.

• Abscisic Acid (ABA): Responsible for:

  • Regulating dormancy and stomata closure, and assisting in leaf abscission.

• Ethylene: Triggers:

  • Flower opening, fruit ripening, and contributes to the aging process in plants.

Apical Dominance

• Concept: Inhibition of lateral buds by the presence of a shoot tip.

• Removal of the shoot tip promotes growth of lateral buds.

• Application of auxins like IAA or NAA to cut tips can maintain dormancy of lateral buds.

Agricultural Practices

• Manual removal of shoot tips can create bushier plants.

• Application of artificial auxins post-pruning mimics a dominant shoot tip to extend growth intervals before further cutting.

Auxin Effects

• Auxins enhance the vascular cambium and vascular tissue activity, promoting:

  • Development of fruit and overall plant structure.

• Uses include preventing bud sprouting on tubers during storage, as with NAA, and controlling weed growth using 2,4-D, which selectively kills dicots.

Rooting Powders

• Common Ingredients: IBA, NAA, and occasionally IAA, assist in root formation and development through rooting formulations.

Phototropism

• Definition: Growth response to directional light.

  • Positive Phototropism: Shoots grow toward light.

  • Negative Phototropism: Roots grow away from light.

Role of Auxins in Phototropism

• Auxins are distributed unevenly in response to light, causing differential growth, which leads to curvature toward light.

Other Types of Tropism

• Gravitropism: Response to gravity.

• Chemotropism: Response to chemical stimuli, notably nutrient uptake.

• Thigmotropism: Responding to touch or physical contact.

• Hydrotropism: Growth toward moisture, although it may not be a true tropism.

Nastic Movements

• General plant responses that are non-directional, primarily dependent on environmental conditions.

Photoperiodism

• Definition: Plant response to varying lengths of day and night impacting development stages like flowering and bud dormancy.

• Requires specific critical night lengths for some plants to induce flowering or dormancy.

Short Day vs. Long Day Plants

• Short Day Plants: Flower during short days with longer nights (e.g., aster, chrysanthemum).

• Long Day Plants: Require long days for flowering (e.g., lettuce, spinach).

• Neutral Day Plants: Not influenced significantly by day length (e.g., tomatoes, roses).

Manipulation of Flowering

• Breeders manage flowering through day length controls, utilizing artificial light to induce flowering outside of natural seasonal cues.

• Certain intensities (e.g., 1/1000th of full sunlight) may be sufficient.

Phytochromes

• Role in Photoperiodism: Pigments that respond to light wavelengths, crucial in signaling flowering based on red (Pr) and far-red (Pfr) light ratios.

Photoperiodic Response Mechanism

• Conversion between Pr and Pfr is essential for flowering signal initiation, regulated by light exposure and darkness stability.

Critical Night Length and Flower Induction

• Flowering signals require established critical night lengths which vary among species, with the response managed by manipulating light intensity and duration.