Plant Tropism: Growth Response to Stimuli

• A tropism refers to a plant's growth response to an external stimulus like light, gravity, or water
• These responses enable plants to acquire the stimulus required for a high growth rate

Auxins

• Auxins are plant hormones made in the tips of shoots and roots
• Auxins help shoots move towards light, and roots move downwards and towards water
• Growth responses are named based on the affecting factor

Types of Growth Response

• Phototropism is growth in response to light
• Geotropism is growth in response to gravity
• Hydrotropism is growth in response to water
• Growth responses can be positive, growing towards a stimulus, or negative, growing away from it

Auxin Effects

• Auxin speeds up cell growth and elongation in stems but slows it down in roots
• If auxin is absent, such as when the stem tip is cut off, the plant stops growing upwards

Phototropism

• Phototropism is more influential on shoots than roots
• Auxin moves away from light
• With light from one side, auxin concentrates on the shaded side of the stem, accelerating cell growth there
• The stem bends towards the light due to differential growth
• Positive phototropism occurs in the shoot

Phototropism and the Clinostat

• A clinostat rotates a plant, ensuring uniform auxin distribution despite a light source
• This results in the plant growing straight as all sides receive equal amounts of auxin

Geotropism

• Geotropism, the effect of gravity on growth, is more significant in roots
• Auxin diffuses downwards in the root due to gravity
• When a plant lies on its side, auxin accumulates on the lower side

Auxin

• Auxin slows down root cell growth, causing the root to curve downwards
• Auxin speeds up stem cell growth, causing the stem to curve upwards

Root Growth Under Gravity

• Auxin settles on the lower side of the root, slowing cell growth
• The top side grows faster, causing the root to bend downwards towards gravity
• Positive geotropism occurs in the root

Geotropism and the Clinostat

• Radicles of germinating seeds on a clinostat receive evenly distributed auxin while rotating
• As a result, the radicles grow straight

Positive and Negative Tropisms

• Positive tropisms involve growth towards a stimulus
• Shoots display positive phototropism, growing towards light
• Negative tropism involves growth away from a stimulus
• Roots exhibit negative phototropism, growing away from light, into the soil

Shoot and Root

• Shoot has positive phototropism and negative geotropism
• Root has negative phototropism and positive geotropism