ch16 - plant responses to abiotic stress

what is an abiotic stress

factors in the environment that can negatively affect an organisms growth, productivity and survival

name abiotic stresses which affect plants

changes in daylight hours due to seasonal changes

excessive heat/ cold

lack/ excess of water

what is photoperiodism

a plants’ sensitivity to the levels of light in the environment

what type of plant is highly sensitive and is affected by levels of light

deciduous trees

what occurs to deciduous trees as a result of photoperiodism

during summer - daylight hours are at maximum = full of leaves to maximise photosynthesis

during autumn - daylight hours start to decrease = causes leaf fall

during winter - daylight hours are at minimum = enters period of dormancy

during spring - daylight hours start to increase = exits period of dormancy

what are phytochromes

light sensitive pigments which detect the duration/ length of light and darkness

where are phytochromes found

in the cytoplasm of plant cells

name the 2 forms of phytochromes

Pr

Pfr

what occurs when there is a change in daylight hours

decrease in daylight hours = leaf fall - abscission

outline the process of leaf abscission

1. phytochromes detect decrease in daylight hours/ light levels leading to decrease auxin levels & increase ethene levels

2. ethene activates genes in cells of the abscission zone to produce digestive enzymes e.g. cellulase

3. cellulase digests the cellulose cell wall in the separation layer

4. vascular bundles are sealed off

5. wind and other environmental factors cause leaf to fall

6. fat is deposited which forms a scar at the protective layer to prevent entry of pathogens

2 why is abscission so useful during winter

many leaves fall > reduced SA as there is less total leaves on plant > reduced respiration and metabolism > less energy required to maintain them plant tissues

fallen leaves act as leaf litter which reduces heat loss by insulating the root

what can excessive cold cause in a plant

cellular freezing

what can a plant do to prevent cellular freezing and describe the mode of action

synthesize sugars, amino acids, and proteins

> sugars and amino acids = increase solute concentration in the cytoplasm, lowers temperature of freezing - prevents cytoplasm from freezing

> proteins = act as antifreeze by binding to small ice crystals which may have formed to prevent them growing larger reducing damage to cell

explain how sugars and amino acids lower the temperature of freezing

To become ice, water molecules lose kinetic energy to slow down so hydrogen bonds can form 

Solutes like sugars and amino acids disrupt hydrogen bonds forming, which means water molecules must lose even more kinetic energy - for this to happen temperatures of freezing will be lower

what do plants do when there is lack or excess of water

cause stomatal opening or closing

outline what occurs in plants when there is a lack of water

1. root hair cells detect there is very small amount of water present in the soil and produce ABA

2. ABA is transported from the roots (up the stem) to the leaves

3. ABA binds to the complementary receptors on the surface of guard cells causing conformational shape change = enabling K+ ion channels to open and diffuse out

4. As K+ ions leave cell = water potential increases so water leaves guard cells via osmosis

5. guard cell becomes flaccid = stomata close = minimises water loss as there is less transpiration (water is retained)

outline what 2 things can occur in plants when there is high temperatures

1. stomata open

2. more water evaporates from leaves via transpiration removing heat and cooling down the plant

VERY HIGH TEMPS

1. stomata may also close since higher temperatures will mean much more evaporation so much more water loss