5.4 Plant hormones

Question 1

Phototropism and gravitropism/geotropism

Answer 1

Plants need to be able to grow in response to light (phototropism) and gravity (gravitropism or geotropism).

The shoots must grow upwards, away from gravity and towards light, so that leaves are able to absorb sunlight – shoots show a positive phototropic response and a negative gravitropic response.
Roots need to grow downwards into the soil, away from light and towards gravity, in order to anchor the plant and absorb water and minerals from the soil particles so roots show a negative phototropic response and a positive gravitropic response.

Question 2

Gibberellins

Answer 2

Gibberellins can be used to:
End seed dormancy, as a high concentration of gibberellin promotes seed germination. Gibberellin levels naturally rise after a period of dormancy (exposure to cold and dry conditions) – usually, dormancy ends with an intake of water into the seed and warmer weather.
Promote flowering regardless of the weather conditions the plant is in.
Increase fruit size – higher levels of gibberellin promote the development and growth of fruit.

Question 3

Ethene

Answer 3

Ethene is used in the food industry to control ripening of fruit during storage and transport.
It is far more effective to transport unripe fruit, as ripe fruit is softer and therefore more easily damaged and spoiled.
The production of ethene can be inhibited to delay ripening of fruits in storage; this can either be achieved directly by adding chemicals that prevent ethene from being produced, or by reacting ethene in the air around fruit with substances that can remove it.
When ripening needs to be encouraged (eg. when fruit is in the supermarket), artificially produced ethene gas can be released to speed up the process.

Question 4

Uses of plant hormones

Answer 4

Plant hormones can be extracted or artificially made and used by gardeners and farmers in horticulture and agriculture to usefully control plant growth to obtain larger yields for example.
The use of auxins, ethene and gibberellins commercially has been very beneficial is helping producing food and plants for decoration.
However the everyday use of hormones as weed killers can have a negative effect on biodiversity; as the growth of unwanted but natural plants such as weeds is inhibited.
Many different species of plants are classed as weeds commercially, but to other organisms they are a food source and potential habitat, so destroying them can have negative effects on other organisms in the ecosystem.

Question 5

Uses of auxins

Answer 5

Auxins can be used as selective weed killers; negatively affecting the growth of broad-leaved plants which are weeds in comparison to the narrow-leaved grasses and cereals grown as crops for food production (which are desired).
They are also used as rooting powders (growth supplement) and to promote growth in tissue culture.

Question 6

Distribution of auxins

Answer 6

The distribution of auxin in the shoots is affected by light and gravity, whereas the distribution in the roots is primarily affected by gravity alone.
If a shoot or root is placed on its side, auxins will accumulate along the lower side as a result of gravity; so the uppermost side has a lower auxin concentration.
In the shoots, the lower side grows faster the upper side, so the shoot grows upwards.
In the roots, the lower side grows slower than the upper side (as auxin inhibits cell elongation and growth in the roots), so the root grows downwards.
Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.

Question 7

Practical 8 (plant growth)

Answer 7

Aim: To investigate the effect of light or gravity on the growth of newly germinated seedlings.
In order to investigate the effect of gravity on germinated seedlings you will:
Set up two petri dishes with 3 mustard seeds and allow them to germinate.
Place one dish on a clinostat and the other to a support on its side.
Record the direction of growth of both the shoots and the roots for each seed.