PLANT HORMONES

What do plants need to grow?This plant hasn’t been looked after very well. What do plants need to grow healthily? sunlight water mineral nutrients

If a plant cannot get these from its environment, can it move to somewhere else?

Plants are rooted to one place. However, they can control their direction of growth.

an air supply

What controls plant growth?Plants grow in response to their environment.

The shoots of these tulips grow upwards and the roots grow downwards.When seeds are spread they may fall any way up in the soil. But when they start to grow (germinate) the roots grow downwards into the soil to anchor and take up water.At the same time the shoots need to grow upwards towards the light so they can photosynthesis.

What do plants respond to?Plants are very sensitive and their growth is affected by their environmental conditions.

Any condition that affects plant growth is called a stimulus.

light water gravity

Which stimulus will have the strongest effect on the plant?

What are three types of stimuli that plants respond to?

A tropism is a growth movement in response to a stimulus.Plants grow towards or away from stimuli.

A growth movement towards a stimulus is called a positive tropism.

A growth movement away from a stimulus is called a negative tropism.

What is a tropism?

Will all the parts of a plant respond to a stimulus in the same way?

What is growth away from a stimulus called?

Plant responses to stimuli are given specific names:

What do the names of each tropism mean?

water gravitylight

hydrotropism Geotropism (Gravitropism)

phototropism

Which parts of a plant respond to these different stimuli?

Are there different types of tropisms?

How do roots and shoots respond to stimuli?

Shoots grow towards sunlight. They are positively phototropic.

Plant stimuli affect certain parts of the plant in different ways.

Shoots grow away from gravity. They are negatively geotropic.

Roots grow away from sunlight. They are negatively phototropic.

Roots grow towards gravity. They are positively geotropic.

What experiments can be used to test these ideas?

Gravitropism and Auxins

Auxins speed up growth in shoots and slow growth in roots.

If a plant is laid on its side, the auxins produced collect in the lower side of the root and stem.

Auxins slow down growth on the lower side of the root, so the root curves down.

Auxins speed up growth on the lower side of the shoot, so the shoot curves up.

growing cuttings

killing weeds.

controlling dormancy

Why are these purposes useful for gardeners and farmers?

Plant hormones are naturally-occurring chemicals, but they can also be produced synthetically for uses in gardening and agriculture, such as:

How can plant hormones be used?

making seedless fruit.

ripening fruit

Plants responses happen as a result of a plant hormone which coordinates and controls growth. These responses are easy to see in young seedlings, but they also happen in adult plants. The respond of plant roots to light, gravity and moisture are controlled by a group of hormones called Auxin. These chemicals are produced in the tips of the shoots and roots. The response happens because of an uneven distribution of this hormone, causing an unequal growth rate.

In the root, more auxin gathers on the lower side

The roots grows more on the side with least auxin,

therefore bends down towards the force of garvity

Controlling dormancySeeds can lie dormant for many years. Dormant seeds don’t germinate, even in warm moist conditions.

Dormancy is important as it ensures seeds don’t all germinate at the same time. This means that if there is a short period of poor growing conditions, such as a drought, some of the dormant seeds will survive.

Dormancy is controlled by plant hormones. Farmers can add artificial hormones to seeds to prevent them from germinating.

The record for the longest dormancy is held by a 1,300-year-old lotus seed which finally sprouted in 1995.

The responses of plant roots and shoots to light, gravity and moisture are controlled by a hormones called auxin.The response happens because of an even distribution of auxin in the growing shoots or roots of plants

A) When light shines directly above tip, growth is equal towards light source

What Causes Tropisms?