16.2 Plant responses to abiotic stress

Question 1

define abscission

Answer 1

shedding of an old/mature organ from a plant

Question 2

describe deciduous plants

Answer 2

• lose leaves when very hot or very dry
• reduces water loss (even in winter when water may be frozen)
• no photosynthesis (less light, lower temp)

Question 3

how do hormones control leaf loss

Answer 3

• abscission layer at the bottom of a leaf or stalk develops (layer of parenchyma cels with thin walls)
• ethene breaks down cells walls in abscission layer and leaves fall
• auxins produced by young leaves inhibit leaf loss and make the leaf stalk insensitive to ethene
• as leaf gets older, conc of auxin decreases and allows leaf loss in response to ethene

Question 4

describe abscisic acid (ABA)

Answer 4

• plant hormone that controls stomatal closure
• produced in roots in response to decrease soil water potential
• translocated to leaves

Question 5

how do hormones control stomatal closure

Answer 5

Guard cells lose water and become turgid by:

1. ABA binds to receptors on the cell surface of guard cells and causes cascade of reactions
2. Ca2+ channels open, influx of Ca2+ into guard cells which raises pH of cytoplasm
3. causes K+, NO3- and Cl- ions to leave the cell
4. water potential of cell increases
5. water removed from guard cell by osmosis

Question 6

define germination

Answer 6

mobilisation of food reserve and promotion of growth

Question 7

what stimulates production of gibberellins

Answer 7

biotic factors such as:

• water
• temperature
• light

Question 8

define dormancy

Answer 8

stage in which seeds do not germinate

Question 9

how do hormones control seed germination

Answer 9

1. gibberellins will allow seeds to come out of dormancy
2. ABA helps maintain seed dormancy by inhibiting the production of amylase
3. auxins are produced early in the germination to stimulate cell growth (however high conc will inhibit germination)