#95 Control of plant growth by auxins, weedkillers

Auxins are plant growth substances, produced by the shoot and root tips of growing plants.

- Auxins in the shoot → stimulate cell growth, by the absorption of water.

- Auxins in the root → slow down the cell growth.

Auxin in phototropism

1. If a shoot is exposed to light from one side

• More auxins are moving in the shaded side (from the tip of the shoot)
• On this side, cells are stimulated to absorb more water, plant grows more 
• Shoot bends toward the light.
• This is called positive phototropism.  

2. If a root is exposed to light in the absence of gravity

• More auxins are moving in the shaded side (from the tip of the root) →
• On this side, cells are stimulated to absorb less water, plant grows less
• Root bends away from the light.
• This is called negative phototropism.    

When exposed to light from one side

Fetures	Shoot	Root
More auxins are moving in the shaded side	(+)	(+)
Cell are stimulated to absorb (on the shaded side)	more water	less  water
Stem bends	toward  the light	away  from the light
Process	Positive phototropism	Negative  phototropism.

Auxin in geotropism

1. If a shoot is placed horizontally in the absence of light:

• Auxins accumulate on the lower side of the shoot, due to gravity.
• Cells on the lower side grow more quickly
• The shoot bends upwards.
• This is called negative geotropism.

2. If a root is placed horizontally in the absence of light:

• Auxins accumulate on the lower side of the shoot, due to gravity.
• Cells on the lower side grow more slowly  
• The shoot bends downwards.
• This is called positive geotropism.  

When is placed horizontally in the absence of light

Fetures	Shoot	Root
More auxins are moving in the lower side	(+)	(+)
Cell growth (on the lower side)	more quickly	more slowly
Bending	upwards	downwards
Process	Negative  geotropism	Positive geotropism

Darwin did the first experiments to study the effects of Auxin

Credit: plantphys.info

• Shoots and roots that have their tips removed will not respond to light or gravity because the part that produces auxins has been cut off.

• Shoots that have their tips covered with opaque material grow straight upwards when exposed to one-sided light, because the auxin distribution is not influenced by the light. 

Effects of weedkillers

Weedkillers (herbicide) are synthetic plant hormones, similar to auxins. If these chemicals are sprayed on to plants they can cause rapid, uncontrolled growth and respiration, resulting in the death of the plant.

Some plant species are more sensitive than others to synthetic plant hormones, so weedkillers can be selective.

Many weedkillers kill only broad-leaved plants (dicotyledons), leaving grasses (moncotyledons) unharmed.

Try this

In figure above, the left-hand side shows an experiment in which the coleoptiles (shoots) of similar seedlings have been treated in different ways, and the right-hand side shows the result in shoot D 24 hours later.

a) i) Name the response shown by shoot D.                            [2 marks]

    ii) Explain what has caused this response.                          [3 marks]

b) Copy and complete the right-hand side figure to show the likely results for shoots A, B and C.                                                                  [3 marks]

Answer

a) i) Positive phototropism

ii) Three points from:

- the coleoptiles have been exposed to one-sided light

- auxins have been produced by the tip

- and have passed into the block

- auxins have passed from the block to the cut coleoptile

- more auxins accumulates on the shaded side of the coleoptile

- causing more growth on the shaded side.

b)A, taller and growing vertically upwards;

   B, taller and bending towards the light;

   C, taller and growing vertically upwards.