B3.2 The Endocrine System - Plant Hormones

Question 1

What is a tropism?

Answer 1

A growth response made by a plant in response to a particular stimuli in their environment.

Question 2

What is the difference between a positive and negative tropism?

Answer 2

• Positive = growing towards stimulus
• Negative = growing away from stimulus

Question 3

Define ‘phototropism’.

Answer 3

• Growth in response to light
• Stems = positively phototropic
• Roots = negatively phototropic

Question 4

Define ‘gravitotropism’.

Answer 4

• Growth in response to gravity
• Stems = negatively gravitrophic
• Roots = positively gravitrophic

Question 5

Another term used for gravitropism.

Answer 5

Geotropism

Question 6

A plant hormone that enables a plant to grow towards, or away from a stimulus.

Answer 6

Auxin.

Question 7

Where is auxin made?

Answer 7

Roots and shoots.

Question 8

How does auxin cause a response to a stimulus?

Answer 8

The stimulus causes an uneven distribution of the hormone auxin, which causes an unequal growth rate, causing the root or shoot to bend.

Question 9

How does the response to auxin differ between roots and shoots?

Answer 9

• Auxin causes increased growth in shoots
• Auxin causes decreased growth in roots

Question 10

How do plants respond to light?

Answer 10

• When light hits one side of a shoot the auxin on that side both moves away from the light, and is broken down (denatured) by the light
• This lead to a build up on the shaded side
• The cells on the shaded side elongate
• The shoot bends towards the light

Question 11

How do plants respond to gravity?

Answer 11

• Gravity causes auxins to gather on the lower side of the root and shoot
• In roots auxins inhibit growth so the side with the least auxins elongates and grows down towards the force of gravity.
• In shoots auxins stimulates growth so the side with the most auxins elongates and grows down away from the force of gravity.

Question 12

State three plant hormones commonly used in agriculture and horticulture?

Answer 12

1. Auxins
2. Ethene
3. Gibberellins

Question 13

Why is ethene unique to the other two hormones?

Answer 13

It is a gas.

Question 14

This hormone is used to help to regulate fruit development, without it fruit is often small.

Answer 14

Auxins.

Question 15

What is ethene?

Answer 15

• Ethene is a hormone that stimulates the conversion of starch into sugar which causes fruit to ripen – this is why unripe fruit is not as sweet as ripe fruit.
• This hormone is the only plant hormone that is a gas

Question 16

How is ethene used commercially?

Answer 16

• To ripen fruit
• Stimulate the conversion of starch into sugar which causes fruit to ripen – this is why unripe fruit is not as sweet as ripe fruit.

Question 17

This hormone ends the dormancy period for seeds and buds, causing shoots and flowers to open.

Answer 17

Gibberellins

Question 18

List the common agricultural and horticultural uses of plant hormones.

Answer 18

• Growing cuttings
• Tissue culture
• Ripening fruit
• Killing weeds
• Kromoting flowering
• Producing seedless fruit

Question 19

How can plant hormones be used to kill weeds?

Answer 19

• Most weeds have broad leaves
• Auxin in herbicides causes the broad-leafed weeds to grow too quickly.
• They are unable to sustain the high growth rate, causing them to die

Question 20

Explain how a cutting can be grown.

Answer 20

• A cutting is a part of plant removed from the main shoot, so it does not have roots, so it is unable to absorb water or minerals.
• Rooting powder containing auxin can be used to stimulate root growth in cuttings.

Question 21

What is a tissue culture?

Answer 21

A method that uses auxin to grow whole plants from a plant tissue sample.

Question 22

How have gibberellins made farming more productive?

Answer 22

• Gibberellins can be used to end seed dormancy by promoting germination
• Enables farmers to control when crops begin to grow - can trigger winter germination in a greenhouse
• Enables farmers that plants all grow at the same time - all ready to harvest at same time
• Can use to extend growing season/have two growing seasons

Question 23

How can seedless fruit be produced?

Answer 23

Gibberellins can be sprayed on flowers to stimulate fruit growth, even though there are no seeds.

Question 24

How can ripening in fruit be delayed?

Answer 24

• By spraying plants with auxins.
• Allows harvesting at the same time
• Prevents fruit from dropping from trees to early

Question 25

Why do horticulturists and gardeners take cuttings and develop them with auxins?

Answer 25

The cuttings produce identical plants, or clones, so a good plant can be quickly and cheeply reproduced in this way.