Plant Hormones 1 of 34 Boardworks Ltd 2011

Plant Hormones 1 of 34 © Boardworks Ltd 2011

2 of 34 © Boardworks Ltd 2011

What do plants need to grow? This plant hasn’t been looked after very well. What do plants need to grow healthily? l sunlight l water l mineral nutrients l an air supply 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. 3 of 34 © Boardworks Ltd 2011

What controls plant growth? Plants grow in response to their environment. The shoots of these tulips grow upwards and the roots grow downwards. If a plant was unable to do this, it might not be able to get enough water or sunlight from its environment. To what environmental factors do plants respond to make sure they grow properly? 4 of 34 © Boardworks Ltd 2011

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. What are three types of stimuli that plants respond to? light water gravity Which stimulus will have the strongest effect on the plant? 5 of 34 © Boardworks Ltd 2011

Which stimulus? 6 of 34 © Boardworks Ltd 2011

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What is a tropism? 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. What is growth away from a stimulus called? A growth movement away from a stimulus is called a negative tropism. Will all the parts of a plant respond to a stimulus in the same way? 8 of 34 © Boardworks Ltd 2011

Are there different types of tropisms? Plant responses to stimuli are given specific names: light water gravity phototropism hydrotropism geotropism What do the names of each tropism mean? Which parts of a plant respond to these different stimuli? 9 of 34 © Boardworks Ltd 2011

Defining tropisms 10 of 34 © Boardworks Ltd 2011

How do tropisms affect growth? This plant does not have a very straight stem. What might have caused this? The plant was placed on a window sill and received light from one direction only. The shoots of plants grow towards sunlight. What is the name of this type of plant response? The response to sunlight is called phototropism. 11 of 34 © Boardworks Ltd 2011

Phototropism experiment 12 of 34 © Boardworks Ltd 2011

Do plants have hormones too? Like humans, plants use hormones, (chemical messengers) to control their development. Hormones produced by plants control growth, flowering and fruit ripening. The growth of shoots and roots is controlled by a group of hormones called auxins. These chemicals are produced in the tips of the shoots and roots. What environmental stimuli might cause a plant to produce auxins? 13 of 34 © Boardworks Ltd 2011

Hormones and phototropism 14 of 34 © Boardworks Ltd 2011

Discovery of auxins 15 of 34 © Boardworks Ltd 2011

Which way up? How do plants always grow the right way up? Plants respond to gravity to grow in the right direction. 16 of 34 © Boardworks Ltd 2011

Hormones and geotropism 17 of 34 © Boardworks Ltd 2011

How do roots respond to water? Roots always grow towards water, which is a positive tropism. Roots will grow sideways, or even upwards, towards water. Roots always have a stronger response to water than gravity to ensure that a plant gets the water it needs. 18 of 34 © Boardworks Ltd 2011

How do roots and shoots respond to stimuli? Plant stimuli affect certain parts of the plant in different ways. l Shoots grow towards sunlight. They are positively phototropic. l Shoots grow away from gravity. They are negatively geotropic. l Roots grow away from sunlight. They are negatively phototropic. l Roots grow towards gravity. They are positively geotropic. What experiments can be used to test these ideas? 19 of 34 © Boardworks Ltd 2011

Tropisms and auxins – true or false? 20 of 34 © Boardworks Ltd 2011

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How can plant hormones be used? Plant hormones are naturally-occurring chemicals, but they can also be produced synthetically for uses in gardening and agriculture, such as: l growing cuttings l ripening fruit l killing weeds. l controlling dormancy l making seedless fruit. Why are these purposes useful for gardeners and farmers? 22 of 34 © Boardworks Ltd 2011

How are cuttings grown? A cutting is a part of plant that is removed from the main shoot. Do cuttings have everything they need to grow? A cutting does not have roots and so has no supply of water or minerals. To stimulate root growth, cuttings are dipped into rooting powder. This contains plant growth hormones. Cuttings are genetically identical to the parent plant. This allows growers to copy successful plants. 23 of 34 © Boardworks Ltd 2011

Growing plant cuttings 24 of 34 © Boardworks Ltd 2011

How do plant hormones kill weeds? Weeds are unwelcome plants that compete with the plants being grown by gardeners and farmers. However, many chemicals that are potential weedkillers are toxic to animals and humans, as well as plants. Plant hormones can be used instead to make weedkillers that only affect certain plants. Most weeds have broad leaves. Cereals, which have narrow leaves, can be protected from weeds by using weedkillers that only effect the growth of broad-leaved plants. 25 of 34 © Boardworks Ltd 2011

How are hormones used to ripen fruit? Fruit is often picked before it is fully ripe. Why might this help keep the fruit edible? Fruit often has to travels thousands of miles from where it is picked to where it is sold in the shops. Unripe fruit is harder and less likely to bruise than ripe fruit when transported. When the fruit reaches its destination, it can then be sprayed with artificial plant hormones to encourage ripening. Why might it be a good idea to wash fruit before eating it? 26 of 34 © Boardworks Ltd 2011

Which hormone makes fruit ripen? Ethylene is a plant hormone that controls fruit ripening. It can be sprayed on unripe fruit to speed up ripening. Some fruit, such as bananas, actually produce a large amount of ethylene, which can cause other nearby fruits to ripen. Using artificial ethylene to ripen fruit is not just a modern practice. Ancient Egyptians exposed figs to natural gas, which contains ethylene, to encourage ripening. Why might it be important to keep fruit away from coal-burning engines during transportation? 27 of 34 © Boardworks Ltd 2011

Controlling dormancy Seeds 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. 28 of 34 © Boardworks Ltd 2011

Seedless fruit Plants usually reproduce using seeds found inside fruit, however people prefer to eat fruit without seeds. For some plants, farmers can produce seedless fruit by: l preventing pollination l selective breeding to create plants that have too many chromosomes, and so can’t produce seeds. Most plants only grow fruits if they have developing seeds. Plants that grow seedless fruit are often sprayed with plant hormones like auxin and giberellin to stimulate fruit growth, even though there are no seeds. 29 of 34 © Boardworks Ltd 2011