Biology - Plant hormones

Define tropism

A type of plant growth in a particular direction in response to a stimulus

What is a positive tropism

Growth towards a stimulus

What is a negative tropism

Growth away from a stimulus

What is the stimulus for phototropism

light

What are the potential benefits of phototropism

Maximising light absorption for photosynthesis → faster growth

What is the stimulus for gravitotropism

gravity

What are the potential benefits of gravitotropism

Roots grow down → more water + better anchorage

How does auxin control growth in a shoot

Auxin is produced in the tip of the shoot. It travels down the stem through the phloem, where it causes cells to divide by mitosis and makes cell walls more elastic. More auxin is transferred to the shady side of the stem, so there is a higher auxin concentration there than on the sunnier side of the stem. As a result the cells in the shade have more elastic cell walls, which allows them to absorb more water be osmosis, causing them to grow more This causes the shady side to grow faster, which makes the stem bend towards the light.

How does auxin control growth in a root

Auxin is made in the root tip. Auxin builds up on the lower side. This makes the cells on the lower side less elastic, therefore less able to absorb water by osmosis, and less able to grow. This means that the cells on the upper side are able to elongate and grow. This causes the root to grow downwards

Plant responses to light: independent variable

Light level

Plant responses to light: dependant variable

Plant growth

Plant responses to light: control variable

Water amount, the number of plants in each dish, minerals, type of cotton wool, size of dish, carbon dioxide levels

Plant responses to light: method

Three petri dishes are prepared with seeds placed on damp cotton wool and the dishes covered to prevent the cotton wool drying out. The seeds are left to germinate - this will take a day or two depending on the temperature. One dish is placed on a window sill, one in a dark cupboard and one in subdued lighting e.g. under a bench. The heights of the seeds in each dish are measured each day for about seven days.

In what 3 ways can auxin be used

Weedkillers, rooting powder, tissue culture

Why does auxin work as a weedkiller? Why does it not kill surrounding plants?

In high concentrations, the auxin causes uncontrolled growth, which causes the plants to die. Weeds typically have broad leaves, which means they take in a lot of the auxin, while crops or other plants may have narrower leaves so stay largely unaffected

What can cause issues with using auxin based weedkillers

Plants may develop resistance, being able to produce enzymes to break down the weedkiller before it takes effect

What properties of herbicides are undesirable? Why?

Non specific → may kill desired plants Long lasting → may contaminate soil, water, desired plants Accumulate in body tissues → can mutate DNA, cause long term diseases

How can auxin be used as rooting powder

It is a growth hormone → causes plants to develop roots

How can auxin be used in tissue culture

Growth hormone → changing concentration encourages the growth of shoots and roots

What is tissue culture

The growth of plant cells or tissues in a sterile, artificial environment

In what way can ethene be used for plants? Why does this work

Controlling fruit ripening. Fruit is picked before it is ripe, so it will last longer, and is less likely to bruise during transport. Ethene is then added to the fruit to make it ripen quickly

In what 3 ways are gibberellins used in plants

Ending seed dormancy, promoting flowering, increasing fruit size

Why is it helpful for gibberellins to end seed dormancy

Why is it helpful for gibberellins to promote flowering

Buds and flowers can be dormant, gibberellin can make plants grow bushier and make them flower, often to make them look more attractive

Why is it helpful for gibberellins to increase fruit size

So farmers can make a larger profit generally. Also used a lot to produce seedless fruit. In seedless fruit, the seeds (in which gibberellins are normally produced) fail to develop, and the fruit would grow in tight clusters and be very small. Gibberellins are then applied so the fruit grows to a normal size