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daylight sensitivity

leaf fall

preventing freezing

stomata control

plant responses to abiotic stress

When the environment around plants change, they have to cope or die. Plant responses include both physical and physiological responses

DAYLIGHT SENSITIVITY

  • plants are sensitive to lack of light which is also known as photoperiodism
  • many different plant responses are affected by the photoperiod including the breaking of the dormancy of leaf buds as they open up and the timing of flowering in a plant
  • the sensitivity of plants to day length results from a light sensitive pigment called phytochrome
  • phytochrome exists in two forms; Pr and Pfr, each of these absorbs a different type of light and the ratio of Pr and Pfr changes depending on the light level

AbScission

  • Abscission is the name for leaf loss in plants.
  • Abscission is triggered by shorter days and longer nights during autumn and winter nights.

Process of abscission

  1. During winter months, less sunlight results in lower auxin concentration.
  2. The leaves respond to this by producing ethene, which initates gene switching at the abscission zone (a region at the base of a leaf stalk).
  3. Gene switching stimulates the production of new enzymes.
  4. These enzymes digest and weaken the cell walls in the outer layer of the abscission zone, known as the separation layer.
  5. The vascular bundles which carry nutrients to the leaves are sealed off.
  6. Fatty material is depostied over the separation layer, forming a protective cap, protecting the plant from pathogens.
  7. Leaves become strained making them more likely to fall off in strong winds and low temperatures.

Preventing freezing

  • Another abiotic factor which affects plants is a decrease in temperature, as this could freeze cells and cause them to die.
  • Plants have evolved mechanisms that protect their cells in freezing conditions:

  1. Cytoplasm and sap in the vacuole contain solutes which lower the freezing point.
  2. Production of sugars, polysaccharides and amino acids acts as antifreeze to prevent cytoplasm from freezing.
  3. Production of chemicals which make plants frost resistant and tougher.

Stomatal control

  • Heat and water availability are also major abiotic stresses for plants.
  • Plants can responds to heat by opening stomata to allow water to evaporate from the cells in the leaves by transpiration.
  • Plants can respond to low water availability by closing stomata to conserve water inside the plant.
  • The opening and closing of stomata is mostly controlled by the hormone ABA (abscisic acid).

  1. Water levels in the soil fall and transpiration is under threat due to limited water availability.
  2. Plant roots produce ABA which is transported to the leaves
  3. ABA binds to receptors on the plasma memebrane of the stomatal guard cells
  4. ABA activates changes in the ionic concentration of the guard cells, reducing the water potential and turgor of the cells.
  5. Guard cells become more flaccid, so close the stomata and so water loss by transpiration is reduced.

The action of ABA