Abiotic stresses include changes in day length, cold and heat, lack of water or excess water, high winds, and changes in salinity.

Abiotic stresses can affect the growth, productivity, and survival of plants.

1. Summer - daylight hours are at a maximum and deciduous trees are in full leaf, maximising photosynthesis and growth
2. Autumn - lengthening of the dark period triggers leaf fall in deciduous trees
3. Winter - daylight hours are at a minimum and deciduous trees maintain a period of dormancy
4. Spring - days lengthen and temperatures rise, causing deciduous trees to come out of dormancy

Photoperiodism is the sensitivity of plants to the levels of light in their environment.

Phytochrome is a light-sensitive pigment that exists in two forms: P_r and P_fr, which absorb different types of light.

The ratio of P_r and P_fr changes depending on the light level, affecting many plant responses involving photoperiodism including the timing of flowering and the breaking of the dormancy of leaf buds.

Abscission is when plants lose their leaves.

Abscission is triggered by lengthening of the dark period during autumn.

1. A reduction in light levels causes auxin concentrations to reduce
2. Leaves produce the gaseous plant hormone ethene
3. Ethene helps switch on genes in cells in the abscission zone at the base of the leaf stalk
4. These cells produce enzymes that digest cell walls in the separation layer of the abscission zone
5. Vascular bundles supplying the leaf are closed off
6. Fatty material is deposited in the cells in the inner layer of the abscission zone, forming a protective scar when the leaf falls to prevent the entry of pathogens
7. Cells in the separation zone retain water and swell, leading to the leaf separating from the plant

A sustained fall in temperatures along with a reduction in day length triggers the production of sugars, polysaccharides, amino acids, and even proteins that act as antifreeze chemicals in plants.

These chemicals prevent the cytoplasm in plant cells from freezing.

1. Plants produce ABA in response to low water availability
2. ABA is transported to the leaves
3. ABA binds to receptors on the plasma membrane of guard cells
4. ABA activates changes in the ionic concentration of the guard cells, reducing the water potential and therefore the turgor of the cells
5. This closes the stomata, helping the plant to conserve water

Plants can open the stomata to cool the plant as water evaporates from the cells in the leaves in transpiration.

However, this must be done in moderation as higher temperatures will also result in more evaporation and thus water loss, so stomata close when temperatures are too high.