Plant Responses - Abiotic Stress, Herbivory

Question 1

What are chemical responses to herbivory?

Answer 1

Plants release chemicals

Tannins:

• Type of phenol that has a bitter taste - deter animals
• Toxic to insects - binds to digestive enzymes to inactivate them

Alkaloids:

• Bitter tasting, nitrogenous compounds - deter animals
• Poisons and affects metabolism of insects
• Also can spread in soil and prevent germination of other plants - protection against plant competitors

Question 2

What is a pheromone and its role in plant chemical defence?

Answer 2

• A chemical made by an organism, affecting the social behaviour of another member of the same species

Pheromones Defending Plants

• Maple tree is attacked, releases pheromone, absorbed by other leaves on plant
  
  • Stimulates callose production for protection
  • Stimulates leaves on plants of nearby trees to prepare for attack
• Pheromones communicate with neighbouring plants about water stress

Question 3

What is a physical response to herbivory?

Answer 3

• Seen in Mimosa pudica, the leaves fold in response to touch, this scares herbivores and dislodges small insects
• The leaf falls quickly, takes roughly 10 minutes to return to position
  
  • K⁺ ion movement in cells cause osmotic movement to help the leaves open up again
• An examples of a nastic response
  
  • Opposite to tropism, a nastic response has no direction
• M. pudica displays a thigmonastic response
  
  • Non-directional response to touch

Question 4

State the 2 responses to abiotic stress?

Answer 4

Abiotic Stress: insufficient light or water

• Leaf loss
• Stomatal Closure

Question 5

Describe and explain the process of leaf loss (abcission)

Answer 5

• At the end of each leaf stalk there is a region called the abcission zone
  
  • Made of protective layer, separation layer
• Cytokinins (by maintaining nutrient supply) and auxins (by making the leaf insensitive to ethene) prevent leaf senescence (ageing), in the

1. Due to lack of light and winter conditions, [cytokinin} drops, the leaf becomes senescent so auxins are not produced
2. Falling [auxin] means the leaf is now sensitive to ethene
   
   • Ethene initiates gene switching to produce the enzyme cellulase to digest and weaken the cell walls in the separation zone
   • Causing the leaf to fall off
3. Vascular bundles are sealed off and fatty materials are deposited at the protection layer to prevent pathogen entry

Question 6

What are the roles of abscisic acid?

Answer 6

ABA (abscisic acid) - plant hormone that:

• Maintains seed dormancy
• Stimulates cold protective response (e.g. antifreeze and stomatal closure)
• When soil water levels falls, ABA released by roots to leaves to bind to guard cell receptors (stops transpiration)
• Antagonist to gibberellins in seed germination (ratio of ABA to gibberellins determines the rate of seed germination)

Question 7

Describe and explain the process of stomatal closure

Answer 7

Plants close their stomata to conserve water or open to cool by transpiration

1. Once ABA binds to receptors on guard cells, it activates chemical processes
   
   • pH increase and a transfer of Ca²⁺ from vacuole to cytoplasm
2. Calcium ions stimulates loss of charged ions (K⁺, NO₃^-, Cl^-) from cell, increasing water potential, water moves out by osmosis
3. Change in turgor pressure changes cell shape closing the stomata

Question 8

What are the roles of gibberellins in seed germination?

Answer 8

1. The embryo absorbs H₂O and swells, secreting gibberellins
2. The gibberellins travel to the aleurone layer (layer proctecting the endosperm - starch store)
3. This triggers the production of amylases & proteases which hydrolyses the stored starch to form monomers: glucose, amino acids
4. The embryo uses the monomers to produce ATP to build cells to grow