Plant responses

• What is geotropism and it’s advantage?

  A plants response to gravity. This is important as it allows the shoots and roots to grow in the right directions.
• What is phototropism and why is it important to plants?

  A plant’s response to light. Allows the plant to grow in the direction of sunlight, maximising photosynthesis
• What is thigmotropism and what is an advantage of this?

  A plants response to touch. Can allow climbing plants to have greater access to sunlight in a dense rainforest
• What is hydrotropism and why is it important?

  A plant‘s response to moisture. Allows for the root tips to grow towards damper areas of soil, so the plant has a greater access to water
• What is chemotropism and why is it important?

  Plant‘s response to chemicals. Important as pollen tubes grow DOWN the flower’s stigma( part of a plant that accepts pollen) to the ovules because of chemotropism
• Why is the response to touch in Mimosa Pudica not a tropism?

  The leaflets (that make up the leaves in the plant) are touch sensitive and can fold inwards when touched. But the response isn’t related to the direction of the touch.
• What is nastic movement in plants and what is an example?

  Non-directional response( a part of the plant doesn’t grow towards/away from the stimulus). For example, the leaflets in Mimosa pudica curl inwards.
• Why is nastic movement an adaptation of Mimosa pudica?

  Because it can protect them from herbivorous insects or decrease water lost by transpiration when the plant is not photosynthesising
• Name three categories of chemical defences against herbivory in a plant.

  Tannins, pheromones and alkaloids
• Mode of actions of tannins.


  Has a bitter taste that can put off herbivores. Also when ingested by insects, the breakdown of it can produce toxic chemicals that are fatal to the insect
• Mode of action of alkaloids.

  Similar to tannins; bitter tasting and toxic to herbivorous insects and sometimes FUNGI.
• Mode of action of pheromones.


  May be DIRECTLY toxic to herbivorous insects but can also trigger chemical defences in some plants
• Examples of abiotic stress.

  Drought, freezing temperatures, etc. A plant can respond to drought by closing their stomata so they don’t lose further water or by dropping their leaves
• what hormones are involved in leaf loss and what are their roles?

  Ethene and auxin!An abscission layer forms at the base of a leaf stalk. Ethene stimulates break down of cell walls in this layer, leaf eventually falls off.Auxin inhibits leaf loss.As a leaf ages, the conc. of auxin in the leaf decreases, so the leaf is susceptible to falling off.
• What hormone is involved in stomatal closure and what is its role?

  ABA; it binds to ABA receptors on guard cells causing ions to leave the guard cells.Water potential of guard cells increases as a result and water leaves the guard cells. Stomata close as a result
• What role does giberellins play in seed germination?

  It is produced by the embryo of the seed when conditions are right.It stimulates the cells in the aleurone layer to synthesise amylase.The amylase hydrolyses starch to glucose. The energy from glucose can be used for growth.
• Does ABA play a role in seed germination?

  No, it has an opposite effect to giberellins. It keeps the seed dormant by inhibiting amylase production
• What do auxins cause in plants?

  Apical dominance! Auxins found at the tip of an apical shoot cause the stem to grow upwards and not sideways. This allows the plants to grow UPWARDS towards sunlight
• How can the effect of auxin in the tip of a shoot be investigated?

  By cutting off the tip of a plant shoot. Then lateral buds should start growing as a result of the abscence of auxin!

  
• What is the experimental evidence for the role of giberellins in stem elongation and seed germination?

  Giberellins can be added to dwarf plants, which have Low concentrations of giberellins, to stimulate growth.
• Describe briefly the practical used to investigate the effect of IBA on root growth.

  Make Serial dilutions of a 5% solution of IBA using alcohol(IBA isn’t soluble in water!) and take 60 plant cutting from the SAME plant.Dip 10 cuttings into the 5% solution and repeat this for the 4 diluted solutions.10 cuttings are left which are dipped into alcohol acting as a control. Then the roots from each set of 10 cuttings are removed and weighed. This is to see the effect of the concentration of IBA on root growth
• How can auxins be used commercially?

  In high concentrations, they can act as weed killers! Therefore synthetic auxins are used as it’s cheaper! Also rooting powders encourage root growth, as they contain low concentrations of auxin
• What is the commercial use of ethene?

  Ripening of fruits
• What can auxins and giberellins be used to produce?

  seedless fruits such as seedless grapes