Plant Responses To Abiotic Stress

When the environmental conditions around plants change, what do they have to do?

Cope or die.

What do abiotic stresses include?

• Changes in day length.

• Cold and heat.

• Lack of/excess water.

• High winds.

• Changes in salinity.

What do plants that grow in temperate climates experience?

Great environmental changes during the year.

As light and temperature affect the rate of photosynthesis, what do seasonal changes have a big impact on?

The amount of photosynthesis posisble.

What happens in very low temperatures?

A point comes where the amount of glucose required for respiration to maintain the leaves, and to produce chemicals from chlorophyll that might protect them against freezing is greater than the amount of glucose produced by photosynthesis. In addition, a tree that is in leaf is more likely to be damaged or blown over by winter gales.

What happens to deciduous trees in temperate climates?

They lose all of their leaves in winter and remain dormant until the days lengthen and temperatures rise again in the spring.

What is photoperiodism?

Plants are sensitive to a lack of light in their environment.

What are many different plant responses affected by?

The photoperiod including the breaking of the dormancy of the leaf buds so they open up, the timing of flowering in a plant and when tubers are formed in preparation for overwintering.

What does the sensitivity of plants to day length (or dark length) result from?

A light-sensitive pigment called phytochrome. This exists in two forms - P_r and P_fr. Each absorbs a different type of light and the ratio of P_r to P_fr changes depending on the levels of light.

After summer ends, what does the lengthening of the dark period trigger?

A number of changes, including abscission of leaf fall and a period of dormancy during the winter months.

What do falling light levels result in falling concentrations in?

Auxins.

How do the leaves respond to the falling auxin concentrations?

By producing gaseous plant hormone ethene. At the base of the leaf stalk is a region called the abscission zone, made up of two layers of cells sensitive to ethene. Ethene seems to digest and weaken the cell walls in the outer layer of the abscission zone.

In abscission, what happens after ethene digests and weakens the cell walls in the outer layer of the abscission zone?

The vascular bundles which carry materials into and out of the lear are sealed off. At the same time fatty material is deposited in the cells on the stem side of the separation layer. This laver forms a protective scar when the leaf falls, preventing the entry of pathogens. Cells deep in the separation zone respond to hormonal cues by retaining water and swelling, putting more strain on the already weakened outer laver.

In abscission, what happens after strain is put on the already weakened seperation?

Further abiotic factors such as low temperatures or strong autumn winds finish the process - the strain is too much and the leaf separates from the plant.

What is left following leaf loss?

A neat, waterproof scar.

What is another major abiotic factor that affects plants?

A decrease in temperature.

What happens if cells freeze?

Their membranes are disrupted and they will die.

In response to low temperatures, what have many plants evolved?

Mechanisms that protect their cells in freezing conditions.

How to plants protect their cells in freezing conditions?

The cytoplasm of the plant cells and the sap in the vacuoles contain solutes which lower the freezing point Some plants produce sugars, polysaccharides, amino acids, and even proteins which act as antifreeze to prevent the cytoplasm from freezing, or protect the cells from damage even if they do freeze.

In the winter, what do most species only produce?

The chemicals which make them hardy and frost resistant during the winter months. It appears that different genes are suppressed and activated in response to a sustained fall in temperatures along with a reduction in day length, effectively preparing the plants to withstand frosty conditions. A sustained spell of warm weather along with extended day length reverses these changes in the spring.

What are heat and water availability?

Major abiotic stresses for plants.

What is one of the major ways in which plants can respond to heat and water availability?

By opening the stomata to cool the plant as water evaporates from the cells in the leaves in transpiration, or to close the stomata to conserve water.

What is the opening and closing of the stomata in response to abiotic stresses largely under the control of?

ABA. The leaf cells appear to release ABA under abiotic stress, causing stomatal closure.

What does ABA activate?

Changes in the ionic concentration of the guard cells, reducing the water potential and therefore turgor of the cells.

As a result of reduced turgor, what happens to guard cells?

They close the stomata and water loss by transpiration is greatly reduced.