1. 16.2 PowerPoint 2 Plant responses 5.1.5
16.2 Plant responses to abiotic stress
Links here are Photosynthesis (chapter 17)
9.5 Plant adaptations to very dry and very watery conditions
When environmental conditions change, plants have to cope or die
Abiotic stress?
Changes in daylength, cold, heat, water availability, winds, salinity….
Plant responses can be physical: thick cuticles, hairy leaves, sunken stomata or can be physiological adaptations

2. Why is it so important for plants to be able to respond to their surroundings? 2 marks
They are rooted to the ground so cannot move their bodies (1);
Therefore very important that they are sensitive so they can grow in the right direction and make the best of the circumstances where they have germinated (1).
Leaf loss in deciduous plants as a response to abiotic stress:

3. Leaf loss in deciduous plants as a response to abiotic stress:
Plants in temperate climates experience great environmental changes in a year
Daylight hours
Temperature changes
Water availability
Seasonal changes have an impact on the amount of photosynthesis possible
Balance lost between the amount of glucose required for respiration to maintain leaves is greater than the amount of glucose produced by photosynthesis.
Winds
So most deciduous trees in a temperate climate lose all of their leaves in winter and are dormant
WHITEBOARDS: What are the survival advantages of this?

4. Leaf Abscission/fall
Abscission (from the Latin ab, meaning away, and scindere, meaning to cut) is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed
Trees in temperate countries shed their leaves in autumn.
Survival advantage
Reduces water loss through leaf surfaces
Avoids frost damage
Avoid fungal infections through damp, cold leaf surfaces
Plants have limited photosynthesis in winter

5. Stages in leaf abscission
Falling light levels results in low auxin levels
Drop in auxin causes an increase in ethene production
This causes the cells in abscission zone to be more sensitive to ethene
Gene switching occurs to stimulate new enzyme production (now required proteins)
More ethene produced, inhibits auxin production
This increases production of enzyme cellulase, which digests the cell walls in outer layer of abscission zone (separation layer)
Vascular bundles are sealed off and a fatty protective layer produced to prevent entry of pathogens
Other cells are stimulated to retain water and swell putting a strain on outer layer
Other abiotic factors play a part, winds, rain
Separates the petiole from the stem
A neat, waterproof scar is left

6. Daylength sensitivity
Plants are also sensitive to the LACK of light available: photoperidism
Many different plant responses are affected by the photoperiod:
Breaking dormancy of leaf buds so they open
Timing of flowering in a plant
Formation of tubers for overwintering
This sensitivity is due to:

7. Phytochromes: Plant photoreceptors
Like animals plants detect the quantity, direction and wavelength of light using photoreceptors and respond to the changes in light conditions
All messages in plants are chemicals
Responses are slower
Phytochromes: Plant photoreceptors
Exist in 2 forms: Pr Pfr
Each absorbs light at a different wavelength, and the ratio of both changes depending on the levels of light

8. Phytochromes
A wide range of responses are regulated by phytochromes such as
Seed germination
Stem elongation
Leaf expansion
Chlorophyll formation
Flowering
In most plants P FR is physiologically active and stimulates the activities

9. Two isomers are photoreversible:
Phytochromes:
PR : absorbs red (R; 660 nm) light
P FR : absorbs far red (FR; 730 nm) light
Two isomers are photoreversible:
Synthesised in Pr form
Redlight Pr P fr
Far red light P fr Pr
sunlight contains more RED light SO
SUNLIGHT: Pr P fr and P fr Pr
Pr P fr DOMINATES SO P fr accumulates in light
DARKNESS: P fr Pr slowly!
P fr biologically/metabolically active Pr more stable

10. Produce a flow diagram to explain the process of abscission 8-10 marks
Falling light levels = decreased concentration of auxin
So leaves produce hormone ethene
Ethene initiates gene switching in abscission zone at base of leaf stalk
Gene switching causes production of new enzymes (cellulase)
New enzymes digest and weaken cell walls in outer layer of abscission zone (separation layer)
Vascular bundles sealed off, fatty material deposited in cells on stem side of separation layer
Layer forms protective scar when leaf falls, preventing pathogen entry
Cells in separation zone respond to hormonal cues by retaining water and swelling putting more strain on outer layer
Further abiotic factors finish process (wind etc)
Strain is too much and leaf separates from plant leaving neat waterproof scar

11. Abiotic factor: decrease in temperature
Preventing freezing
Abiotic factor: decrease in temperature
If cells freeze membranes are disrupted and they will die
Many plants have developed a mechanism for survival:
Cytoplasm and sap in vacuoles contain solutes which lower the freezing point
Some plants produce sugars/polysaccharides/amino acids which act as an antifreeze to prevent the cytoplasm from freezing
Gene expression triggered due to reduction in day length and temperature fall and ABA produced, triggers gene switching so plants make chemicals (sugars,proteins) which lower freezing point of cytoplasm.

12. 1. Explain how plant hormones are involved in protecting the plant cells from damage in freezing conditions 5 marks
Chemicals such as abscisic acid (1);
trigger gene switching (1);
so plants make chemicals such as sugars or proteins (1);
which lower the freezing point of the cytoplasm (1);
or protect the cells against damage by ice crystals if they do freeze (1).

13. Stomatal control: ABA
Abiotic stress causes release of ABA (abscisic acid) which leads to stomatal opening/closure
Leaf cells produce ABA when under stress which closes stomata
Scientists also believe that water availability at roots provides a warning
As soil water levels fall, ABA produced in roots is transported to leaves
Binds to receptors on plasma membranes of guard cells
Changes ionic concentration in guard cells, reduces water potential so reducing turgor and so the guard cells close the stomata and loss of water by transpiration reduced.

14. Quick thinking: whiteboards
Why do many trees in temperate climates lose all of their leaves in winter? 6 marks
Exam question
Plant responses to environmental changes …13 marks

15. The amount of photosynthesis that can take place decreases as day length is reduced and temperatures fall (1);
so the amount of glucose produced by photosynthesis falls (1);
the amount of glucose needed for respiration to maintain leaves through the winter (1);
produce chemicals to prevent freezing damage increases (1);
it becomes more efficient to lose the leaves and become dormant until the days lengthen and temperatures increase again (1).