Plant Tropism and Hormones

Learning Objectives

  • Investigate how plants know where to grow up and down.
  • Phototropism.

Learning Outcomes

  • Must: Explain how geotropism and phototropism act on a growing plant.
  • Should: Investigate how auxins help a plant to grow in the right direction.
  • Could: Create an investigation to prove that light and gravity are required for plants to grow in the correct direction.

Approaches to Learning

  • Research skills

Real World Application

  • Plant growth, food industry.
  • Biology Unit.
  • Hormones

Key Terms

  • Hormones
  • Phototropism

Challenge 1: Plant Growth Direction

  • Investigate how stems and roots grow in the correct direction.
  • How do plants always grow the right way up?
  • What environmental conditions will affect how a plant grows?
  • Why do some parts of the plant want to grow towards light and other parts want to grow towards the soil?
  • Investigate the plant hormone responsible for making the stem and root ‘bend’ in the correct direction.
  • Find out what the terms geotropism and phototropism mean.

Challenge 2: Auxins

  • Investigate how auxins (hormones) make a plant stem grow towards the light and how they make a root grow towards the ground.
  • Create a comic strip to show how the auxins make the stem grow towards the lights.
  • Create a comic strip to show how the auxins make a root grow towards the ground.

Challenge 3: Space X Investigation

  • Investigate how gravity and light affects the growth of plants in space.

Sensitivity

  • Responding to the environment
  • Plants respond to stimuli to:
    • Avoid stress
    • Avoid being eaten
    • Enhance survival
    • Improve chances of having offspring
  • The way a plant grows is controlled by the hormones
  • These are sensitive to the environment & can make adjustments to help plant survive

Plant Adaptations

  • Different parts of a plant are adapted to assist its growth and survival.
    • Flowers: Adapted to enable reproduction.
    • Stems: Support the plant and transport nutrients.
    • Leaves: Adapted for photosynthesis, which makes materials for growth.
    • Roots: Anchor the plant to the ground. They also take up water and minerals from the soil.

Germinating Seedling

  • If a seed lands where conditions are good, it will start to grow.
  • The embryo plant ‘germinates’ and begins to grow
  • The embryo grows a new root and then a new shoot.

Tropism

  • Plants respond to stimuli by growing to or away from them.
  • Movement towards a stimulus is called a positive tropism.
  • Movement away from a stimulus is called a negative tropism.
  • A growth movement in response to a stimulus is a tropism.
  • Examples of stimuli include: light, water, gravity.

Types of Tropism

  • Hydrotropism: Response to water.
  • Geotropism: Response to gravity.
  • Phototropism: Response to light.

Examples

  • Positive Phototropism & Negative Geotropism
  • Positive Geotropism & Negative Phototropism

Tropism Summary

  • Roots Growing Down into Dark Soil: Negative Phototropism.
  • Roots Growing Up towards the Light: Positive Phototropism.
  • Roots Growing Down towards Water: Positive Hydrotropism.
  • Shoots Growing Upwards: Negative Geotropism.
  • Roots Growing Downwards: Positive Geotropism.

Plant Requirements

  • Plants need:
    • Sunlight and an air supply
    • Water
    • Mineral nutrients
  • Plants are rooted to one place, but they can control their direction of growth.

Plant Growth Control

  • Plants grow in response to their environment.
  • Examples: Shoots of tulips grow upwards and roots grow downwards.
  • Plants respond to environmental factors to ensure proper growth.

Hormones and Tropisms

  • Tropisms are growth responses.
  • Growth is controlled by hormones.
  • Hormones are chemicals that are transported to target cells where they have their effect.
  • Auxin is the plant hormone that is responsible for these growth responses.

Plant Hormones

  • The growth of shoots and roots is controlled by a group of hormones called auxins.
  • These chemicals are produced in the tips of the shoots and roots.
  • Like humans, plants use hormones (chemical messengers) to control their development.
  • Hormones produced by plants control growth, flowering and fruit ripening.
  • Environmental stimuli can cause a plant to produce auxins.

Auxin Mechanism

  • Auxin is made at the tips of stems and roots.
  • It's moved in solution to older parts of the stem and root where it changes the elasticity of the cells.
  • If the cells become more elastic they absorb more water and grow longer.
  • This causes bending in the stem or root.
  • Light and gravity can interfere with the transport of auxin causing it to be unevenly distributed.

Auxin Effects

  • Auxin has different effects in different types of cells.
  • It SPEEDS UP cell division (growth) in SHOOTS.
  • It REDUCES cell division in ROOTS.

Phototropism

  • A plant’s response to light is called phototropism.
  • Phototropism is a positive tropism.
  • Plants grow towards light because they need light for photosynthesis.

Auxins and Phototropism

  • Auxins are made in the tip of a growing shoot and move down the stem.
  • These plant hormones speed up growth in a growing shoot.

Phototropism - Light From Above

  • When a shoot gets light from above, the auxins produced at the tip are spread out evenly in the shoot.
  • The shoot grows straight up because there is an equal amount of auxins in all parts of the shoot, which grow at the same rate.

Phototropism - Light From One Side

  • When a shoot gets light from one side, the auxins are not evenly spread out in the shoot.
  • There are more auxins on the shaded side of the shoot, so the shaded side grows faster making the bright side bend towards the light.

Tropisms - Light from an Angle

  • Sunlight breaks down Auxin
  • The side of the tip that has the most direct sunlight will have the least amount of Auxin
  • Area of the plant that is more shaded will have more Auxin
  • More growth on shaded side
  • Lop sided growth & plant bends towards light

Phototropism

  • Auxin accumulates on the shaded side causing elongation of these cells leaning the plant shoot towards the light

Phototropism Task

  • Complete phototropism worksheet
  • Explain how and why plants grow towards the light (6 marks)
  • Collect “B&Q needs you” sheet and apply your phototropism knowledge to a real life situation

How and Why Plants Grow Towards the Light

  • How:
    • Auxin from tip of shoot
    • moves to the shaded side of the shoot
    • Auxin causes cells on shaded side to elongate and plant grows towards the light
  • Why:
    • Plants need to absorb light for photosynthesis
    • To make glucose
    • Glucose required for cell processes/starch/respiration – for energy

Geotropism

  • A plant’s response to gravity is called a geotropism.
  • Shoots grow up (negative geotropism).
  • Roots grow down (positive geotropism).
  • Auxins speed up growth in shoots and slow growth in roots.
  • If a plant is laid on its side, the auxins produced collect in the lower side of the root and stem.
  • Auxins slow down growth on the lower side of the root, so the root curves down.
  • Auxins speed up growth on the lower side of the stem, so the stem curves up.

Gravitropism

  • In the root, more auxin gathers on the lower side.
  • In the shoot, more auxin gathers on the lower side.
  • The root grows more on the side with least auxin, making it bend and grow down towards the force of gravity.
  • The shoot grows more on the side with most auxin, making it bend and grow up away from the force of gravity.
  • Uneven distribution of auxin causes unequal growth rates.
  • roots grow down and the shoots to grow up

Advantages

  • Positive phototropism (light, stem tip, towards the light): To get maximum light for photosynthesis.
  • Negative phototropism (light, root tip, away from the light): Less chance of drying out.
  • Positive gravitropism (gravity, root tip, down): More chance of finding moisture.
  • Negative gravitropism (gravity, stem tip, up): More chance of finding light.

Hydrotropism

  • Roots always grow towards water, a positive tropism.
  • Roots will grow sideways or even upwards, towards water.
  • Roots always have a stronger response to water than gravity to ensure that a plant gets the water it needs.

Hydrotropism - Water All Around

  • When roots are well watered from all sides, the auxins produced in the roots are spread out evenly.
  • Roots grow out in all directions because there is an equal amount of auxins in all parts of the roots, which grow at the same rate.

Hydrotropism - More Water One Side

  • When roots are in soil with more water on one side, the auxins are not spread out evenly in the roots.
  • There are more auxins on the wetter side of the roots, which grows slower than the dryer side, so the roots bend towards the water.

Phototropism Explanation

  • Auxin is made in the TIP and moves down to the target cells
  • Auxin ACCULMULATES on the SHADED side of the stem.
  • This gives an UNEQUAL DISTRIBUTION of Auxin
  • Auxin SPEEDS UP GROWTH RATE in shoot cells
  • The Shaded side has more auxin so has a higher growth rate.
  • The stem GROWS towards the light

Gravitropism Explanation

  • Auxin is made in the TIP and moves down to the target cells due to gravity
  • Auxin ACCULMULATES on the LOWER side of the stem.
  • This gives an UNEQUAL DISTRIBUTION of Auxin
  • Auxin REDUCES the GROWTH RATE in root cells.
  • The lower side has more auxin so has a lower growth rate.
  • The root bends down as the growth is faster on the upper side.

Plant Responses to Stimuli

  • Shoots grow towards sunlight (positively phototropic).
  • Shoots grow away from gravity (negatively geotropic).
  • Roots grow away from sunlight (negatively phototropic).
  • Roots grow towards gravity (positively geotropic).

Other Plant Hormones

  • Gibberellins: Important in initiating seed germination.
  • Ethene: Controls cell division and ripening of fruits.

Seed Experiment

  • 3 groups of seeds are grown in a cardboard box.
    • A - the tips are removed: no auxin is made, so the stems do not grow.
    • B - the tips are covered: auxin moves to all parts of the stem causing all parts to grow.
    • C - the tips are lit from one side only: auxin accumulates on the shaded side causing it to grow more than the illuminated side.

A/02 Question

  • Level 3 (5-6 marks)
    • Good explanation of the effect of hormone on growth of shoots A, B and C.
    • Quality of written communication does not impede communication of the science at this level.
  • Level 2 (3-4 marks)
    • Good account of the mechanism involved.
    • Quality of written communication partly impedes communication of the science at this level.
  • Level 1 (1-2 marks)
    • Good description of the appearance of the shoots.
    • Quality of written communication impedes communication of the science at this level.
  • Level 0 (0 marks)
    • Insufficient or irrelevant science. Answer not worthy of credit.

Poster Task

  • Create a poster to explain positive phototropism

Plenary

  • A student grew a plant in an upright pot. She then put the pot in a horizontal position and left the plant in the dark for two days.
  • Explain fully why the plant responded in this way. (4 Marks)
    • gravity accept gravitropism / geotropism 1
    • caused redistribution of auxin / hormone to lower side of stem 1
    • these hormones stimulate growth of cells on the lower side of the stem only 1
    • so the stem grows upwards 1 [4]

Using Plant Hormones

  • Roots grow downwards
    Plants grow towards a source of light
    Auxins can diffuse from cell to cell in solution
    Shoots grow upwards
    Auxiins cause cell growth
    Hormone rooting powder causes roots to grow
    Cuttings can produce clones of the parent plant
    Hormone weed killers are not usually harmful to animals
    Flowering can be induced by the use of plant hormones
    Plant hormones can be used to ripen fruit