15 Coordination and response in plants
15.1 Coordination and response in plants
Gravitropism is a response in which parts of a plant grow towards or away from gravity
Phototropism is a response in which parts of a plant grow towards or away from light
Stimulus Name of Definition Positive response Negative response
response
Light Phototropism Growth towards or Growth towards the Growth away from
away from the light source (e.g. by the light source (e.g.
direction of the light shoots) roots)
source
Water Hydrotropism Growth towards or Growth towards Growth away from
away from water water (e.g. by water (e.g. shoots)
roots)
Gravity Gravitropism Growth towards or Growth towards the Growth away from
away from the source of gravity the source of gravity
source of gravity (e.g. by roots) (e.g. shoots)
3 Explain the role of auxin in controlling shoot growth, limited to:
(a) auxin is made in the shoot tip
(b) auxin spreads through the plant from the shoot tip
(c) auxin is unequally distributed in response to light and gravity
(d) auxin stimulates cell elongation
Auxin plays a role in a plants response to gravity, affecting plant shoots and roots in different ways
When shoots grow away from gravity it is known as negative geotropism
Gravity modifies the distribution of auxin so that it accumulates on the lower side of the shoot
As seen in the phototropic response, auxin increases the rate of growth in shoots, causing the
shoot to grow upwards
When roots grow towards gravity it is known as positive geotropism
In roots, higher concentrations of auxin results in a lower rate of cell elongation
The auxin that accumulates at the lower side of the root inhibits cell elongation
As a result, the lower side grows at a slower rate than the upper side of the root
This causes the root to bend downwards
4 Investigate gravitropism and phototropism in shoots and roots
The seedlings in A grow towards the light source
In B the effect of the light only coming from one direction has been cancelled out by
using a clinostat
o A clinostat is a device that revolves slowly and repeatedly, so the shoots are
evenly exposed to light
This means all sides of the seedlings get equal exposure to light so they do not curve
towards the light source but grow straight up
In C the seedlings grow straight up looking for light and the plant becomes tall and
slender with yellowing leaves due to the lack of light
Investigating gravitropisms
Add some damp cotton wool to two Petri dishes
Place 3 bean seedlings in the cotton wool in each petri dish
o A - radicle facing downwards
o B - horizontally
o C - radicle (root grows from here) facing upwards
Cover each dish with a lid
Attach one petri dish to a support so that it’s on its side
Attach the second petri dish to a clinostat (as shown in the diagrams above).
Place both in a light-proof box (so that the seedlings are in complete darkness), leave for
two days and then observe the growth of the seedlings
In the first petri dish, all radicles (roots) have grown downwards (positive gravitropic
response) regardless of which way they were initially facing (horizontal, up or down) and
all plumules (shoots) have grown upwards (negative gravitropic response)
In the second petri dish, all radicles and all plumules have all grown neither up nor
down but straight outwards in whichever direction they were placed as the effect of
gravity has been cancelled out by the revolving of the clinostat - they have shown no
gravitropic response at all
The experiment needs to be done in a lightproof box to cancel out the effect of light on
the growth of the seedlings
