Plant Responses to Abiotic Stress

What is abiotic stress?

• Stress caused by non-living factors in the environment (e.g., drought, high salinity, freezing temperatures).

Give two common examples of abiotic stress for a plant.

• Drought (water scarcity)

• Freezing temperatures

• High soil salinity

What is a plant's main physiological response to the abiotic stress of drought?

• Stomatal closure (to reduce water loss via transpiration).

Which plant hormone is responsible for coordinating the response to drought?

• Abscisic acid (ABA)

Where does ABA bind to initiate stomatal closure?

• To specific receptors on the cell-surface membrane of the guard cells.

Briefly, how does ABA cause stomatal closure?

• ABA binds to guard cell receptors, causing ions (e.g., K⁺) to leave the cell. Water follows by osmosis, the cells become flaccid, and the pore closes.

What is a plant's main long-term (anatomical) response to seasonal drought or cold?

• Leaf loss (abscission), seen in deciduous plants.

Why do deciduous plants lose their leaves in winter? (2 reasons)

• Water conservation (Drought): The ground is frozen, so liquid water is unavailable for roots. Losing leaves stops transpiration.

• Low light & temperature: Photosynthesis would be very low, making the leaves an energy drain.

What is phytochrome and what does it do?

• A photoreceptor (light-detecting pigment) in plants.

• It has two forms: P_r (absorbs red light) and P_fr (absorbs far-red light).

• Sunlight converts P_r → P_fr 

• Darkness/shade converts P_fr →P_r 

• P_fr is the biologically active form that acts like a "switch" to control responses like germination and flowering.

• Ratio of P_r : P_fr  changes depending on level of light

What two major dangers do freezing temperatures pose to a plant?

• Cell Lysis: Ice crystals forming in the cytoplasm can pierce membranes and kill the cell.

• Drought: Water in the soil is frozen (ice) and cannot be absorbed by roots.

How do some plants prevent their cells from freezing?

• They increase the solute concentration in their cytoplasm (e.g., with sugars or amino acids). This acts as an antifreeze, lowering the freezing point of the cytoplasm.

What is the main disadvantage of stomatal closure?

• It stops gas exchange, meaning the plant cannot take in CO₂ for photosynthesis.

Which hormone controls leaf loss (abscission)?

• It is controlled by a change in hormone balance: auxin levels drop and ethene levels rise.

Explain the mechanism by which a plant closes its stomata in response to drought conditions. (5 marks)

• Drought (low water potential in soil) is detected by roots, and the hormone abscisic acid (ABA) is produced.

• ABA travels in the xylem to the leaves.

• ABA binds to specific receptors on the guard cells.

• This binding causes ion channels (e.g., $K^+$ channels) to open, and ions move OUT of the guard cells.

• This increases the water potential inside the guard cells.

• Water leaves the guard cells by osmosis, they become flaccid, and the stomata close.

  (Any 5 points)

Explain why deciduous trees losing their leaves in winter is an adaptation to abiotic stress. (4 marks)

• Winter is a period of abiotic stress, specifically low temperatures and freezing.

• When the ground freezes, soil water is ice and is unavailable to the roots (causing drought).

• Leaves are the main site of transpiration (water loss).

• By losing their leaves, the tree massively reduces water loss over winter.

• This conserves water until the ground thaws in spring, preventing death from dehydration. (Any 4 points)

Describe two ways, other than stomatal closure, that plants can respond to or are adapted for drought. (3 marks)

• Leaf Loss: Deciduous plants lose leaves to stop transpiration.

• Antifreeze: Plants increase solute concentration in cytoplasm to lower the freezing point (as freezing causes drought).

• Xerophytic Adaptations (any 1):

  • Thick waxy cuticle to reduce evaporation.

  • Sunken stomata/Hairs/Rolled leaves to trap humid air and reduce the water potential gradient.

  • Reduced leaf SA (spines) to reduce the area for transpiration.(Any 3 points from 2 different adaptations)

Explain how freezing temperatures can lead to the death of plant cells. (3 marks)

• Low temperatures cause ice crystals to form.

• If ice forms inside the cytoplasm / vacuole.

• The sharp crystals can pierce / damage the cell membrane, tonoplast, or other organelles.

• This (e.g., a burst membrane) is lethal and causes the cell to die.

Some plants are known as "antifreeze plants". Explain the physiological mechanism that allows them to survive freezing temperatures.

• They respond to cold by increasing the concentration of solutes in their cytoplasm.

• These solutes include sugars, amino acids, or specific antifreeze proteins.

• This lowers the water potential of the cytoplasm and therefore lowers its freezing point.

• This prevents ice crystals from forming inside the cell, thus preventing damage to membranes. (Any 3 points)