2.84: Describe the Geotropic and Phototropic Responses of Roots and Stems

GEOTROPIC RESPONSE: Growth in response to direction of gravity

PHOTOTROPIC RESPONSE: Growth in response to direction of light

AUXINS: Plant hormone produced in the tip of stem and roots which controls the direction of growth

• Auxin changes the elasticity of cells to alter the growth of cells, therefore changing the direction of growth (increase in elasticity will allow cell to absorb more Water, causing size of cell to increase and bend towards specific direction)
• Factors including light and gravity can alter the distribution of Auxin within stem and root, causing different responses

GEOTROPIC RESPONSE OF PLANTS:

STEMS	ROOTS
Diagram Showing Positive Geotropism in Stems	Diagram Showing Negative Geotropism in Roots
During growth, Auxin produced at the tip will accumulate on the underside of the Stem due to gravity As Auxins promote the growth of Stem cells, cells on the underside of the Stem will continue to grow larger and elongate (increase in elasticity allows cell to absorb more Water to grow larger in size), whilst cells on the upperside will grow at a slower rate As a result, Stem will grow upwards against the direction of gravity - Negative Geotropism	During growth, Auxin produced at the tip will accumulate on the underside of the Root due to gravity As Auxins inhibit the growth of Root cells, cells on the underside of the Root will not grow, whilst cells on the upperside will continue to grow larger and elongate (increase in elasticity allows cell to absorb more Water to grow larger in size) As a result, Root will grow downwards towards the direction of gravity - Positive Geotropism

PHOTOTROPIC RESPONSE OF PLANTS:

STEMS	ROOTS
Diagram showing Positive Phototropism in Stems	Diagram showing Negative Phototropism in Roots
During growth, Auxin produced at tip will be redistributed by light receptors to the shaded side of the Stem As Auxins promote growth of Stem cells, cells on the shaded side will grow larger and elongate (increase in elasticity allows cell to absorb more Water to grow larger in size), whilst cells on the unshaded side exposed to light will grow at a slower rate As a result, Stem will grow towards the direction of light - Positive Phototropism, allowing Plant to maximise the absorption of light for Photosynthesis	During growth, Auxin produced at tip will be redistributed by light receptors to the shaded side of the Root As Auxins inhibit growth of Root cells, cells on the shaded side will not grow, whilst cells on the unshaded sight exposed to light will grow larger and elongate (increase in elasticity allows cell to absorb more Water to grow larger in size) As a result, Root will grow against the direction of light - Negative Phototropism