Welcome to your study notes on Plant adaptations and plant hormones!
Please note: The entirety of this topic is Separate Science (Triple Biology) only. Additionally, the section on the commercial uses of plant hormones is Higher Tier only.
What you'll learn
- How plants adapt their leaf structure, stomata, and cuticle to survive in extreme environments.
- How the hormone auxin controls plant growth in response to light (phototropism) and gravity (gravitropism).
- How farmers and growers commercially use plant hormones like auxins, gibberellins, and ethene.
1. Plant Adaptations in Extreme Environments
Plants cannot walk away when their environment becomes too hot, too dry, or too salty. To survive in extreme environments, they must adapt their structure.
Plants adapted to dry (arid) conditions are called xerophytes (such as cacti or marram grass). Their main challenge is preventing excessive water loss through transpiration (the evaporation of water from plant leaves) while still allowing carbon dioxide in for photosynthesis.
Leaf Size and Shape
In hot, dry environments, broad leaves are a liability because their large surface area increases water loss.
- Many desert plants have leaves reduced to spines or needles (e.g., cacti). This dramatically reduces the surface area-to-volume ratio, minimising the area from which water can evaporate.
- Spines also protect the plant from being eaten by herbivores seeking water.
The Waxy Cuticle
The outer surface of the leaf is covered by a non-cellular, waterproof layer made of lipids called the cuticle.
- In extreme environments, plants have an exceptionally thick waxy cuticle.
- This creates a physical barrier that prevents water from evaporating directly through the epidermal cells.
Stomata and Rolled Leaves
Stomata are the tiny pores on a leaf that allow gas exchange. However, when stomata open, water vapour escapes. Xerophytes have evolved brilliant ways to keep stomata functional while saving water:
- Sunken Stomata: Stomata are located in deep pits in the leaf epidermis. This traps a pocket of moist, humid air just outside the stoma. Because the air in the pit is highly humid, the water vapour concentration gradient between the inside of the leaf and the outside air is reduced, slowing down diffusion (transpiration).
- Hairs on Leaves: Fine hairs on the leaf surface trap water vapour near the stomata, keeping the local microclimate humid and reducing water loss.
- Rolled Leaves: Some grasses roll their leaves inward so that the stomata face the inside of the roll. This creates a highly humid, sheltered inner chamber protected from wind, which dramatically lowers transpiration rates.
- Stomatal Opening Times: Some plants only open their stomata at night when temperatures are cooler and evaporation rates are lowest.
Xerophyte
A plant that is adapted to survive and grow in dry environments where liquid water is scarce.
Water vapour gradient
Remember, diffusion depends on a concentration gradient. If you trap moist air outside a stoma using hairs, pits, or rolled leaves, you lower the concentration gradient of water vapour. Consequently, water diffuses out of the leaf much more slowly.
2. Plant Hormones and Growth Control
Plants do not have a nervous system, but they still need to sense and respond to their environment. They do this using chemical messengers called plant hormones.
A growth response of a plant toward or away from an environmental stimulus is called a tropism.
- Phototropism: A growth response to light.
- Gravitropism (or Geotropism): A growth response to gravity.
If a plant grows towards a stimulus, it is a positive tropism. If it grows away from a stimulus, it is a negative tropism.
Auxins
A group of plant hormones produced in the tips of shoots and roots that control cell elongation and coordinate tropic growth responses.
How Auxins Work in Shoots
Auxins are made in the growing tip of a shoot and diffuse downwards into the surrounding tissue.
- Effect of Auxin: In shoots, auxins stimulate cell elongation (making the cells grow longer).
- Phototropic Response: When light shines on a shoot from one side, auxin concentrates on the shaded side of the shoot tip.
- As the auxin diffuses downwards, it causes the cells on the shaded side to elongate more than the cells on the illuminated side.
- Because the shaded side grows faster and longer, it forces the shoot to bend towards the light source (positive phototropism).
How Auxins Work in Roots
The mechanism in roots is different and often trips students up.
- Effect of Auxin: In roots, auxins inhibit cell elongation (preventing the cells from growing longer).
- Gravitropic Response: If a root is placed horizontally, gravity pulls auxin down so that it accumulates on the lower side of the root.
- Because auxin inhibits elongation in root cells, the cells on the lower side grow very slowly, while the cells on the upper side (with less auxin) elongate normally.
- This uneven growth causes the root to bend downwards, deeper into the soil (positive gravitropism).
Gravitropism in Shoots
If a shoot is placed horizontally, gravity also pulls auxin to its lower side. However, because auxin stimulates cell elongation in shoots, the lower side elongates faster than the upper side, causing the shoot to bend upwards (negative gravitropism).
The Shoot vs. Root Rule
Auxin behaves the same way physically (it always goes to the dark side and the bottom side), but the target cells respond oppositely:
- Shoots: Auxin promotes elongation (the side with more auxin grows faster).
- Roots: Auxin inhibits elongation (the side with more auxin grows slower).
Auxin 'destruction' by light
Students often write that "light destroys auxin." This is incorrect! Light does not destroy auxin; it simply causes auxin to diffuse sideways from the bright side to the shaded side of the shoot tip.
Worked Example: Analysing Hormone Concentration Graphs
A researcher investigated how different concentrations of auxin affected the growth of roots and shoots. The results are plotted below:
- A growth index of 000 indicates no change in growth compared to a control plant.
- Positive values show stimulation of cell elongation.
- Negative values show inhibition of cell elongation.
Using this data, explain how a concentration of 10−210^{-2}10−2 parts per million (ppm) of auxin affects the growth of roots and shoots.
Analysing hormone concentration graphs
- Locate the concentration value: Find 10−210^{-2}10−2 ppm on the x-axis of your mental or physical graph.
- Read the root response: At 10−210^{-2}10−2 ppm, the root curve is below the zero line (at a negative value). This means root cell elongation is inhibited. At this concentration, root growth is slower than the control.
- Read the shoot response: At 10−210^{-2}10−2 ppm, the shoot curve is above the zero line (at a positive value). This means shoot cell elongation is stimulated. At this concentration, shoot growth is faster than the control.
- Formulate the biological explanation: Conclude that roots are far more sensitive to auxin than shoots. A concentration of 10−210^{-2}10−2 ppm is high enough to inhibit root growth but is still supportive (stimulating) for shoot growth.
3. Commercial Uses of Plant Hormones
This entire section is Higher Tier only.
Humans have learned to harness plant hormones for agriculture and horticulture to increase crop yields, propagate plants, and control ripening.
1. Auxins
- Selective Weedkillers: Many weeds in cereal fields (like wheat) are broad-leaved plants, whereas cereals are narrow-leaved. Scientists developed selective weedkillers containing synthetic auxins. When sprayed, these auxins are absorbed mainly by the broad-leaved weeds. They cause rapid, uncontrolled growth in the weeds, which disrupts their transport systems and kills them, while leaving the grass-like crops unharmed.
- Rooting Powders: If you take a cutting of a plant leaf or stem, it won't automatically grow roots. If you dip the cut end into rooting powder containing auxins, it stimulates the rapid development of roots. This allows growers to clone prize plants quickly and cheaply.
2. Gibberellins
Gibberellins are plant hormones that control developmental processes such as seed germination and flowering.
- Germination: In nature, seeds only germinate after meeting certain conditions (like cold weather passing). Treating seeds with gibberellins can bypass this dormancy, forcing seeds to germinate at any time of year.
- Photoperiod Bypass: Gibberellins can stimulate plants to flower out of season or under light conditions that would normally prevent flowering.
- Seedless Fruit Production: Many fruits (like grapes) only grow after their flowers are pollinated and fertilized, which produces seeds. If unpollinated flowers are sprayed with gibberellins, the fruit grows anyway, resulting in larger, seedless fruit.
3. Ethene
- Fruit Ripening: Ethene is a gaseous plant hormone. It controls the ripening of fruit.
- To prevent fruit (like bananas or tomatoes) from becoming soft and damaged during long-distance shipping, they are harvested while they are still green and unripe.
- Just before they are put on supermarket shelves, they are exposed to ethene gas in controlled warehouses. This triggers rapid, uniform ripening so that they arrive in perfect condition for consumers.
Commercial applications match-up
In exam questions, make sure you do not mix up which hormone does what! A classic mistake is writing that gibberellins are used for fruit ripening—remember, ethene controls ripening, while gibberellins are used for seedless fruit and germination.
In the exam
- Name the hormone: When explaining phototropism or gravitropism, always name auxin early in your answer to secure marking points.
- Use comparative language: In tropism explanations, describe how one side elongates more than or faster than the other side. This uneven growth is what causes the bending.
- Double-check shoot vs. root: Read the question carefully to see if it asks about a shoot or a root. If you write that auxin stimulates cell elongation in a root, you will lose marks!
- Explain commercial uses fully: If asked about weedkillers, don't just say "it kills weeds." Explain how: it causes rapid, uncontrolled growth that disrupts the plant.
Check yourself
- A plant shoot is exposed to light coming from the left. In which direction will the auxin diffuse, and what effect will this have on the direction of growth?
- Why does a horizontal root bend downwards? Explain this response in terms of auxin distribution and cell elongation.
- Which plant hormone is used by supermarkets to ensure green bananas are yellow and ripe when they reach the shelves?
