Lecture 11

Plant Adaptations to Climate

Plants adapt to their environments in various ways to survive extreme conditions. Key adaptations include:

• Succulent Leaves: Helps retain water.

• High Concentration of Solutes: Assists in water uptake.

• Spines: Provide shade and help dissipate heat.

• Reduced Leaf Surface: Little to no leaves minimize water loss.

• Annual Habits: Adapt to low nutrient soils. Examples include carnivorous plants or those associated with nitrogen-fixing bacteria like rhizobium.

Defense Against Herbivores and Pathogens

Plants have evolved numerous defense mechanisms to protect against herbivory and pathogens:

• Constitutive Defenses: Always present and include:

  • Outer Surfaces: Cuticle, thick cell walls, epidermis, and bark create barriers.

  • Trichomes (Hairs): Can be sharp or sticky, deterring pests.

  • Laticifers: Latex channels produce a sticky substance that blocks the mouths of feeding animals.

  • Thorns/Spines: Modified leaves or stems provide physical defenses.

• Ant-Plant Symbiosis: Mutualism where ants inhabit the plant and defend it in exchange for food sources (Beltian bodies).

• Secondary Metabolites: Chemicals that can repel invaders, often derived for purposes beyond primary needs.

Induced Responses and Hormonal Control

Plants can sense changes in their environment and respond accordingly, often through hormonal signals:

• Auxin: This hormone regulates growth direction and arrangement, influencing:

  • Phototropism: Growth towards light.

  • Gravitropism: Growth direction in response to gravity (negative gravitropism goes up, positive down).

  • Apical Dominance: Encourages growth at the stem’s apex, inhibiting side shoots.

  • Root Formation: Auxin aids in developing larger root systems.

• Ethylene Gas: Influences leaf abscission (falling) and fruit ripening via positive feedback.

• Apical Hook: Controls surface cell elongation, which guides growth directions.