plant responses to abiotic stress (a,b)

Question 1

Role of plant hormones in leaf fall/abscission

Answer 1

• autumn = nights lengthen, days shorten, temperatures fall = triggers leaf fall and a period of dormancy during winter months
• falling light levels = falling concentrations of auxin = ethene gas is produced
• abscission zone (at base of leaf stalk) is made up of 2 layers sensitive to ethene
• ethene initiates gene switching resulting in the production of enzymes which digest and weaken the cell walls in the separation layer
• vascular bundles are sealed off and fatty material is deposited into cells on the stem side of the separation layer = forms a protective scar when the leaf falls preventing entry of pathogens
• further abiotic stress strains the plant and the leaf falls leaving a neat, waterproof scar

Question 2

Role of plant hormones in stomatal control

Answer 2

• stomata open to cool plant via transpiration, close to conserve water
• leaf cells release ABA under abiotic stress
• roots provide early warning of water stress: when water levels fall, roots produce ABA which is transported to the leaves where it binds to receptors on the plasma membrane of the stomatal guard cells
• ABA activates increase in ionic concentration of guard cells, reducing water potential and thus turgor of the cells
• thus guard cells close the stomata and water loss by transpiration is greatly reduced