Plant responses

Responses to herbivory (being eaten by animals and insects):

• Alkaloids are chemicals with bitter tastes and poisonous characteristics that kill or deter herbivores.
• Tannins taste bitter and can bind to proteins in the guts of some herbivores to make the plant hard to digest (deterring animals from eating the plant)
• Pheromones are signalling chemicals that produce a response in other organisms (e.g. alarm pheromones are detected by nearby plants which then make chemicals like tannins; when corn plants are being eaten by caterpillars, they produce pheromones that attract parasitic wasps, which ay eggs in the caterpillars, which kills them)
• The mimosa pudica plant that folds up when it's touched. This protects it against herbivory and scare off animals)

Responses to abiotic stress (anything harmful that's natural but non-living e.g. a drought):

• E.g. carrots produce antifreeze proteins at low temperatures to stop ice crystals from growing.

Phototropism = the growth of a plant in response to light 

1. Take 3 shoots equal in height and put them in pots with the same type of soil. Cover the tip of 1 with a foil cap, 1 without foil and 1 with the base covered in foil (leaving the tip exposed)

2.Set them up in front of light and leave them there for 2 days. Make them the same distance from the light with the same light intensity. Control other variables (temperature, moisture etc.)

3.Results should show that the shoots with exposed tip grew towards the light source (+phototropism), the one with the covered tip will have growth towards the light prevented. Covering the base of the shoot still allows the tip to grow towards the light (auxin is still produced).

4.Recording the amount of growth (in mm) and its direction will give quantitative results.

Geotropism = the growth of a plant in response to gravity

1. Line 3 petri dishes with moist cotton wool (using the same volume of water + amount of cotton wool). Space out 10 cress seeds onto the surface of each dish, pushing them into the wool.

2.Tape a lid to the dishes and wrap them in foil (to prevent light reaching the seeds and affecting the results). Choose somewhere where the temperature is constant and warm to keep them.

3.Put one dish upright (90 degree angle), one on a slope (45 degrees) and one flat and horizontal. Label them all, marking which way is up.

4.Leave the seeds for 4 days and then look at the shoot and root growth. At each angle, the shoots will have grown away from gravity (- geotropism) and the roots will have grown towards gravity (= geotropism).

Leaf loss in deciduous plants:

• Deciduous plants lose their leaves in the winter to conserve water as its difficult to absorb from the soil. It's triggered by the shortening day length in the autumn and is controlled by hormones.
• Auxins inhibit leaf loss. As the leaf gets older, less is produced (leading to more leaf loss)
• Ethene stimulates leaf loss. As leaves get older, more is produced. An abscission layer develops at the bottom of the leaf stalk, eventually breaking the cell walls and making the leaf fall off.

Seed germination:

• Gibberellins stimulate seed germination by causing the breakdown of starch into glucose in the seed.
• The plant embryo uses the glucose to start respiring, releasing the energy needed for it to grow.
• Gibberellins are inhibited (seed germination is prevented) by abscisic acid (a hormone)

Stomatal closure:

• Need to be able to close stomata to reduce water loss through transpiration. It's done using guard cells. When they're turgid the pore on the leaf is open, and when they're flaccid, the pore closes.
• Abscisic acid triggers stomatal closure.
• It binds to receptors on the guard cell membranes, causing ion channels to open. These allow calcium ions to enter the cytosol, causing other ion channels to open. This raises the water potential of the cells, causing water to leave the guard cells by osmosis (making them flaccid so the stomata closes)

• Plant 30 seeds that are similar in age, height, weight etc. Count and record the number of side shoots growing from the main stems of the plants.
• For 10 of them, remove the tip and apply a paste containing auxins, apply a paste contaiing no auxins for the next 10, and leave 10 plants alone(to act as a control - so you know any observed changes are from the hormone and not another factor).
• Let them all grow for 5 days in the same conditions (light intensity, temperature etc)
• After the time has passed, count the number of side shoots growing from each plant.
• Results should show that removing the tips caused extra side shoots to grow, and that removing the tips and applying auxins prevented extra shoots from growing. This demonstrates that auxins inhibit the growth of side shoots (so they're involved in apical dominance.

• Done using genetically altererd seeds that are unable to produce gibberellins. (they're unable to germinate unless they're given gibberellins)
• Plant 40 plants that are similar in age height etc. Leave 20 of them to grow normally, and water 20 of them with a dilute solution of gibberellin (keep all other conditions the same)
• Let them grow for 27 days, measuring the length of the stems once a week .
• The results should show that stems grow more (longer stem length), when watered with a dilute solution of gibberellin. This demonstrates that gibberellin stimulates cell elongation.

• ETHENE  - stimulates enzymes that break down cell walls, break down chlorophyll and convert starch into sugars (making fruits ripe). Bananas are harvested and transported before they're ripe (less likely to be damaged) and then exposed to ethene so that they all ripe at the same time.

• AUXINS - used as selective weed killers (herbicides). They make weeds produce long stems, making them grow too fast and die.
• Used as rooting hormones (rooting powder). They made a cutting grow roots, so they can be planted and grown into a new plant.

• Auxins and gibberellins can be synergistic = work together = e.g. they work together to make plants grow tall.
• They can also be antagonistic = oppose each others actions  = e.g. gibberellins stimulate the growth of side shoots while auxins inhibit it.
• Auxins are antagonistic to ethene (auxins inhibit leaf loss and ethene stimulates it)