Plant Growth: Water and Auxin Effects

Water Absorption and Cell Elongation

• Absorption of water leads to cell swelling and elongation due to weakened cellulose bonds.

Effects of Auxins

• Two primary effects are phototropism and apical dominance.

Phototropism

• Controlled by the tip of the coleoptile, discovered by Charles Darwin and his son.
• Curving of the stem occurs distant from the tip, indicating a transmitted signal.
• Peter Boysen-Jensen demonstrated this signal is a mobile chemical using gelatine, allowing chemical diffusion without direct contact.
• Unilateral light exposure increases auxin diffusion to the shaded side, resulting in enhanced elongation and directional growth towards light.

Apical Dominance

• Auxins, produced at the stem's meristem, inhibit growth of lateral buds near the growing tip.
• As auxin concentration decreases down the stem, lateral buds are released from inhibition and can develop into side shoots.
• Removal of the apical bud halts IAA production, allowing all lateral buds to grow, resulting in bushier plants.

Auxin Concentration Effects

• Auxins affect different plant parts variably; a growth-promoting concentration in one area may inhibit another.
• Roots are more sensitive to auxins than shoots.
• A concentration of 10^-1 ppm promotes shoot growth while inhibiting root growth.
• A concentration of 100 ppm inhibits both shoot and root growth.

Uses of Growth Regulators

• Auxins: Used in rooting powders to stimulate adventitious root development for faster establishment of cuttings.
• Cytokinins: Promote cell differentiation in micropropagation.
• Ethylene: Utilized for fruit ripening.

Tropisms

• Tropisms are growth responses to external stimuli that optimize plant positioning.
• Stems exhibit positive phototropism (growth towards light) and negative geotropism (growth against gravity).
• Roots demonstrate positive geotropism (growth downwards) and negative phototropism (growth away from light).

Types of Tropisms

• Phototropism: Response to light, e.g., stems growing towards light.
• Chemotropism: Response to chemicals, e.g., roots growing towards nutrients.
• Thigmotropism: Response to touch, e.g., ivy wrapping around supports.
• Geotropism: Response to gravity, e.g., roots growing deeper into the soil.
• Hydrotropism: Response to water availability.

Plant Adaptations for Protection

• Chilies produce oils causing a burning sensation to deter herbivores.
• Nettles have stinging cells containing formic acid to discourage contact.
• Hawthorn bush sports sharp thorns to prevent herbivory.
• Gorse has stiff, pointed leaves to deter browsers.

Plant Responses to Stimuli

• Plant responses involve interactions among various growth regulators and must be coordinated effectively.
• Growth factors can be classified as external (temperature, day length, light intensity, water availability) or internal (growth regulators, injury response).

External Growth Factors

• Temperature: Unusual warmth can stimulate growth; excessive heat induces the production of protective heat shock proteins.
• Day Length: Longer days prepare plants for growth and reproduction, while shorter days signal leaf shedding in deciduous trees.
• Light Intensity: Increased light enhances photosynthesis and growth rates.
• Water: Insufficient water triggers stomatal closure to conserve moisture.

Internal Growth Factors

• Growth regulators include auxins, gibberellins, cytokinins, and ethylene.
• Responses to injury or disease can trigger the production of phytoalexins, chemicals used in plant defense.

Growth Regulatory Categories

• Promoters (e.g., auxins) enhance growth.
• Inhibitors (e.g., abscisic acid) impede growth.

Auxin Overview

• Indole Acetic Acid (IAA) is the most common auxin.
• IAA produced in the apical meristem influences various processes like stem elongation, root and shoot development, and fruit formation through cell elongation and water absorption enhancement.

Explore the intricate processes of water absorption and the role of auxins in plant growth. This quiz covers vital concepts such as phototropism and apical dominance, illustrating how these mechanisms influence plant development. Test your understanding of how auxins facilitate directional growth and control lateral bud development.