16.2 plant responses to abiotic stress

leaf loss in deciduous plants as a response to abiotic stress

• variations in temperature and light levels can both trigger lead loss in deciduous plants

• temperature and light levels fall (winter) → rate of photosynthesis is low so not enough glucose is being produced to keep cells alive

daylight sensitivity in plants

• results from a light sensitive pigment called phytochrome with exists in two forms, Pr and Pfr

• each absorbs a different type of light and the ratio of Pr to Pfr changes depending on the levels of light

why does abcission take place

• due to shortening of light period

• less light = lower concentration of auxin

• leaves respond by producing ethene

process of abcission

1. ethene causes genes producing digestive enzymes to be activated. These weaken the cell walls in the outer absiccion zone, known as the seperation layer

2. vascular bundles that carry materials in and out of the leaf are sealed off

3. fatty material is deposited in the cells of the protective later in the absiccion zone. This forms a protective barrier so pathgogens can’t enter the stem once the leaf falls off

4. cells deep in the seperation zone retain water and swell up, putting strain on already weakened cell walls

5. environmental conditions such as strong wind and rain finally seperate the leaf from the stem

6. a neat waterproof scar is left behind

how do plants prevent freezing

• if plant cells freeze, the cell membranes disrupted and the cells will die

• many plants have therefore evolved to produce a range of solutes in their cytoplasm and sap which lower the freezing point, effectively acting as ‘anti freeze’

• many species only produce these chemicals in the winter months

• genes are suppressed and activated in response to a sustained drop in temperature

• a sustained period of warmer temperatures along with extended day length reverses there changes in spring

stomatal control

• the opening and closing of stomata in response to environmental stress is largely controlled by ABA 

• plants can cope with extreme heat by opening to stomata to increase evaporation by transpiration to cool the leaves

• plants can cope with water shortage by closing stomata to reduce water loss by transpiration

action of ABA

1. Leaf cells release ABA under stress, causing stomatal closure

2. As water levels in the soil falls, roots also produce ABA, which is transported up to the leaves where it binds to receptors in the cell membrane of the stomatal guard cells

3. ABA causes changes in the ion concentration of guard cells

4. this causes water to leave the guard cells by osmosis, reducing their turgor

5. this leads to the guard cells closing the stomata, which greatly reduces water loss by transpiration