Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Tropism is a directional growth response of a plant in which the direction of the response is determined by the direction of the external stimulus.
Phototropism is the growth response to light. Shoots show positive phototropism (grow towards light), while roots show negative phototropism.
Gravitropism (or Geotropism) is the growth response to gravity. Shoots show negative gravitropism (grow away from gravity), while roots show positive gravitropism (grow towards gravity).
Auxins, specifically Indole-3-acetic acid (), are plant hormones produced in the shoot and root tips that regulate cell elongation.
In shoots, a higher concentration of auxin stimulates cell elongation. In roots, a higher concentration of auxin inhibits cell elongation.
The distribution of auxin is affected by light and gravity. In phototropism, auxin moves to the shaded side of the shoot. In gravitropism, auxin accumulates on the lower side of both shoots and roots due to gravity sensing by organelles called statoliths.
The unequal distribution of auxin leads to differential growth rates on opposite sides of the plant organ, resulting in bending.
📐Formulae
💡Examples
Problem 1:
A coleoptile (shoot tip) is exposed to unidirectional light from the right side. Describe the movement of auxin and the resulting growth response.
Solution:
Auxin is produced at the tip and moves towards the shaded (left) side. The concentration of auxin () becomes higher on the left side than the right side.
Explanation:
Because high auxin concentration stimulates cell elongation in shoots, the cells on the shaded side elongate faster than those on the illuminated side. This differential growth causes the shoot to bend towards the light source (positive phototropism).
Problem 2:
A germinating seed is placed horizontally in the dark. Explain why the root grows downwards.
Solution:
Gravity causes auxin to accumulate on the lower side of the root tip. In roots, high concentrations of auxin inhibit cell elongation.
Explanation:
The cells on the upper side, having a lower concentration of auxin, elongate faster than the cells on the lower side. This causes the root to curve downwards, exhibiting positive gravitropism.
Problem 3:
What would happen to the phototropic response if the tip of a coleoptile is covered with an opaque cap and exposed to unidirectional light?
Solution:
The coleoptile will grow straight upwards and will not bend towards the light.
Explanation:
The receptors that detect light and produce/redistribute auxin are located in the tip. By covering the tip, the plant cannot detect the direction of light, preventing the unequal distribution of required for bending.