Edexcel Biology Gcse - Plant Hormones Part 2

They are mostly made in the tips of the growing stems and roots, which are known as apical meristems, and can diffuse to other parts of the stems or roots.

Unequal distribution of auxin can cause unequal growth rates in plants and shoots. Auxins change the rates of elongation in plant cells and control growth of stems and roots.

cells in stems grow more

- cells in roots grow less

In a stem, the shaded side contains more auxin and grows longer, which causes the stem to grow towards the light. The plant does NOT bend towards the light. It grows because the auxin causes the cells to elongate on the shaded side, so this side grows more. This unequal growth of the two sides, results in the growth of the stem towards the light.

If lit from above, the plant will grow upwards.

Phototropism is a response to the stimulus of light, whereas gravitropism (also called geotropism) is a response to the stimulus of gravity.

when the stem grows against the force of gravity, this is known as a negative gravitropism

when a root grows in the direction of the force of gravity, this is known as a positive gravitropism

In a root placed horizontally, the bottom side contains more auxin and grows less - causing the root to grow in the direction of the force of gravity.

The opposite happens in a stem. When a stem is placed horizontally, the bottom side contains more auxin and grows more - causing the stem to grow upwards against the force of gravity.

Germination is the beginning of plant growth, and the plant will continue to grow if the correct conditions are available.

There are many types of plant hormones. They are used in agriculture and horticulture to have a specific effect.

Auxins were the first class of plant hormones to be discovered. Their main function is to help plants grow and auxin stimulates plant cells to elongate. The apical meristem of a plant is one of the main places where auxin is produced. The apical meristem is also the location that all other parts of a plant grow from - the stem, leaves, and flowers.

as weed killers

| - as rooting powders

Selective weed killers kill some plants, but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass. The selective weed killer contains a growth hormone that causes the weeds to grow too quickly and die. Because the weeds have broader leaves, the weed killer is absorbed in larger quantities by the weeds than it is by the grass. Selective weed killers can reduce biodiversity within treated areas due to specific plants being killed.

Rooting powder contains plant hormones to promote growth.

A small potted plant bring dipped in rooting powder

Plant cuttings can be dipped in hormone rooting powder before planting.

Synthetic plant hormones are used to control plant growth. For example, rooting powder contains growth hormones that make stem cuttings develop roots quickly.

Gibberellins are a group of plant hormones responsible for growth and development. They are important for initiating seed germination. Low concentrations can be used to increase the speed of germination, and they stimulate cell elongation so plants grow taller.

They are naturally produced by barley and other seeds.

end seed dormancy

promote flowering

increase fruit size

Seed dormancy must be broken for seeds to germinate, and this can be done by using gibberellins.

Gibberellins can promote flowering, which can result in more financially profitable flowers to sell due to the increased speed of flower growth. More attractive flowers and larger specimens are also produced.

Flowering also has an impact on the rate of fruit growth. Gibberellins are used in the production of seedless fruit such as seedless grapes, which are normally sprayed with gibberellin to increase the size of each grape.

Some hormones slow the ripening of fruits and others speed it up. These hormones and their inhibitors are useful for delaying ripening during transport or when fruit is displayed in shops. Ethene is a plant hormone which causes fruit to ripen.

Ethene is used routinely within the food industry to provide controlled ripening during storage and transport or when fruit is displayed in shops, in sealed packages. Bananas are picked when they are green and unripened. Ethene is a hydrocarbon gas and it speeds up ripening in bananas and other fruit. The effect of ethene released from bananas is clearly visible if you keep them in a bowl with other fruit, as it causes other fruits to ripen very quickly.

Auxin

Ethene

Plant hormones can speed up or slow down ripening

A response to light

Auxins are made in the growing tip of a stem or root. Only cells in the tips of roots or shoots can make auxins.

The shaded side of a stem contains more auxin. This makes the cells elongate, causing the stem to grow towards the light.

When a root grows in the direction of the force of gravity, its top side grows more than its bottom side. This is because the top side contains less auxin, making it grow longer.