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what is a tropism?
a plant’s growth response to directional stimuli
directional and can be +ve/-ve
give 3 examples of stimuli a plant may respond to:
light
gravity
water
what are tropisms regulated by? give an example:
specific growth factors
IAA (indoleacetic acid)
explain tropisms in plant shoots:
root tips produce IAA, which can diffuse to other cells to promote cell elongation
in directional light, IAA diffuses to the shaded side of the shoot
a higher IAA conc causes the shaded side to elongate more
this causes the plant to bend towards the light source - +ve phototropism

explain tropisms in plant roots:
in plant roots, IAA inhibits cell elongation
IAA is produced in root tips and diffuses to the lower side of the root
this means that the lower side elongates less than the upper side
this causes the root to bend towards gravity and away from light - +ve gravitropism and -ve phototropism (respectively)
