Coordination and Response In Plants Summarized Biology Notes

Learning objectives
At the end of this post you should be able to:

● define plant growth substances as chemicals that affect the activities of particular cells and organs
● define geotropism or gravitropism as a response in which parts of a plant grow towards or away from gravity and phototropism as a response in which parts of a plant grow towards or away from the direction from which light is coming

● define growth in terms of increase in dry mass and size, and development in terms of increase in complexity

● describe the chemical control of plant growth by auxins
● describe the environmental conditions that affect germination of seeds
● describe the effects of synthetic plant growth substances used as weedkillers
● distinguish between geotropism and phototropism
● explain geotropism and phototropism in terms of auxins regulating differential growth.

Coordination, growth and development in plants

In lower grades, you learnt that sensitivity (irritability) is one of seven characteristics
that all living organisms pertorm. This characteristic is more obvious in animals than in plants. Plants have no eyes, ears or nose like you do, but they can still detect and respond to stimuli, such as light, water, chemicals and gravity. They respond by growing parts of their bodies either towards or away from the stimuli. Chemicals called plant growth substances control plant responses. Plant growth substances can be defined as chemicals that affect the activities of particular cells and organs. Auxins are plant growth substances, which control responses in plants. They are made by the cells at the tips of the shoots and roots, and diffuse to the nearby cells where they cause differential growth. In shoots, auxins stimulate growth (increase growth rate) in roots, they inhibit growth.

Tropic responses

Tropic responses refer to the growth movements made by a plant in response to stimuli such as light, gravity, water, chemicals and touch. If a plant responds by growing towards a stimulus, then the response is described as a positive tropism. If a plant grows away from a stimulus then the response is described as a negative tropism.

Geotropism/gravitropism

The growth response in which parts of a plant grow towards or away from gravity is called geotropism.
● Radicles, and the roots they develop into, grow down towards gravity. This is described as positive geotropism.
● Shoots, which will form stems and leaves, grow upwards away from gravity. This is described as negative geotropism.

The role of auxins in geotropism

It is generally accepted that chemical substances called auxins are responsible for geotropism. Auxins are plant hormones that are made in the cells all the time and cause differential growth. In the root, auxins slow down the growth ot cells. In the shoots, auxins increase the rate of growth. If a root is laid on its side in the soil, auxin collects on the lower side of the root and slows down the growth of cells on the lower side. The growth of cells on the upper side of the root is, however, not slowed down. The cells in the upper side of the root rapidly become longer and the root curves downwards, as shown in figure below.

Investigation in geotropism

Rotating clinostat (experiment)

● This allows gravity to act equally on all sides of the root and auxins spread evenly; and auxins diffuse to the bottom part, no response to gravity. As shown below.

 Stationary clinosat (control)

● This allows gravity to act on one side onily inhibiting the growth rate there. The root responds positively to gravity. as shown below.

 Practical investigation: Geotropism in roots

Spend about 20 to 30 minutes on this activiy. You will see the effect of geotropism on the roots of germinating seeds.
● You will need:
+ a few bean seeds
+ water
+ a transparent container, such as a plastic bottle or glass jar
+ some absorbent paper, such as newspaper or kitchen paper
+ a pair of scissors (not essential).
● Method:
1. Soak five bean seeds in water overnight.
2. Cut or fold the absorbent paper so that it is the same height as the container. Roll it up and stand it in the container, as shovwn in the diagram on the right.
3. Carefully push the soaked seeds about halfway different direction.

Note it, Make sure the absorbent paper down, between the container and the paper Each seed should face a does not dry out.

4. Add a little water to the container and watch the water rise up the absorbent paper.
5. Put the container in a place for a warm few days.
6. Each day you wil need to add just enough water to the bottom or the container to keep the paper and the seeds moist.
7. Look at the seeds each day. Container soaked bean seeds between paper and Container rolled-up tube of absorbent paper

Answer the following questions in your notebook.
1. Write down in which direction the roots
are growing.
2. Does the position of the seed make any
difference to the direction in which the roots
are growing?
3. Why do you think the roots grow downwards?
4. What stimulus are the roots responding to?

# Phototropism

The growth response in which parts of a plant grow towards or away from the direction from which light is coming is called phototropism. Shoots, and the stems and leaves they develop into, grow upwards towards light. This is described as positive phototropism. Radicles, and the roots they will develop into, grow away from light. This is described as negative phototropism.

# The role of auxins in phototropism

Figure below shows an investigation to find out which part of a shoot picks up the stimulus of light shining onto it. The sensitive region is the tip of the shoot where you will find the receptors that detect light.

Note it, Some plants are grown from cuttings that are small pieces of a plant, usually Consisting of a stem and some leaves. If the cut end is dipped into hormone rooting powder, then the hormones will stimulate growth and new roots will grow from the cut stem.

1. If the tip of a coleoptile is cut off and then replaced, the coleoptile will still grow towards the light.

2. If the tip is cut off, and separated from the rest of the coleoptile by a piece of agar jelly, the coleoptile still grows towards the light.

3. But if a piece of mica separates the tip trom the rest of the coleoptile, then it does not grow towards the light. This Suggests that the response to light is caused by a substance that is made in the tip, and difuses down the coleoptile.

The results of this investigation show that the part of the shoot that responds to the stimulus is the part just belovw the tip. This is the effector. Auxin diffuses downwards from the tip of the shoot and causes the cells just behind the tip to get longer. If there is no auxin present, the cells will not elongate and grow.

# Practical investigation: To see which part of a shoot is sensitive to light

Spend about 5 to 10 minutes on this activity. A learner carried out an investigation to see which part of a shoot is sensitive to light.
1. She put several oat seeds in three pots labelled A, B and C. The seeds germinated and grew shoots called Coleoptiles. The diagram shows a coleoptile.

2. She then cut off the tips of each coleoptile in pot A and she covered the tips of each coleoptile in pot B with foil. The coleoptiles in pot C were left untreated.

3. She measured the length of the Coleoptiles and recorded the average length of the coleoptiles in each pot.

4. She put pots A, B and C into boxes that allowed light to shine in from one side only, and left them for several days. 

5. She measured the new average length of the coleoptiles in each pot and compared it with the original average length to see whether the Coleoptiles had grown or not.

6. The above table shows her results.

Answer these questions!
1. Explain why the coleoptiles in pots B and C grew, but those in pot A did not grow.
2. Why did the coleoptiles in pot C grow towards the light, but those in pot B grew straight upwards?

When a shoot receives light from all directions, it is evenly lit. The auxin that is produced in the tip diffuses evenly down to the region of elongation just behind the tip. The shoot grows straight upwards, as shown in Figure below.

When a shoot receives lignt Trom one side only, it is unevenly lit. The auxin produced the tio, theretore, diffuses away from the light to the shaded side. The cells on the side where there is a greater concentration of auxin, will elongate more than those on the side te the light. This causes the shoot to grow towards the light, as shown in Figure above. When a plant grows towards or away from a stimuli, there is a permanent increase in the size of an organism, brought about by an increase in the number of cells in the body. This is called growth. Growth is a characteristic feature of all living things. It is an irreversible increase in dry mass or biIomass of an organism. Dry mass is the mass of a living organism, once all tne water has been removed. During this process of growth, certain other changes take place. Not all the cells in a particular plant are identical. Cells with difterent structures begin to appear to carry out a variety of different functions and the plant becomes more complicated. This is the process of development. One of the simplest ways of studying the influence of environmental factors is to investigate the conditions needed for seed germination.

Practical investigation: An investigation into the conditions necessary for the germination of mustard seeds

Spend about 15 to 20 minutes on this activity.
1. A learner set up five tubes as shown below.

+ You can see each tube contains a feW mustard seeds, resting on cotton wool on a little platform. The platform is made of pertorated zinc and has tiny holes in it, like a sieve. Look carefully at the differences between the tubes.

● Tubes A, B and C contain water, but tube D contains a solution called pyrogallol in sodium hydroxide. This solution absorbs the oxygen
● from the air. Tube E has no liquid in it at all. Tubes A, B, C and D contain wet cotton wool, but in tube E the cotton wool is dry.

2. The learner put tubes A, D and E in a warm place in the light.
3. She put tube B in a refrigerator and tube C in a warm, dark cupboard.
4. She left all the tubes for several days, then looked at them to see if the seeds had germinated or not.
5. She wrote her results in a table, similar to the one shown here.

Answer Questions 2 and 3
1. Using the information in the diagram and the descriptions, complete the table to show what conditions the seeds had in each tube. The last line has been done for you.
2. What conclusions can you make about the conditions for germination needed by mustard seeds?
3. Which container acts as a control? What is the purpose of the control?  

Practical investigation: An investigation to test the hypothesis that seeds need oxygen in order to germinate

Spend about 5 to 10 minutes on this activity. The diagram below shows the apparatus set up to investigate that oxygen is needed for germination.

Answer these questions in your notebook
1. Which flask, A or B, represents the control
and what is its purpose?
2. What results would you expect:
a) it oxygen is necessary tor germination?
b) it oxygen is not necessary for germination?

Germination

Germination covers all the processes that occur from the time of planting a dry seed to the establishment of the young plant. Figure below shows stages of germination in a bean seed.

The dormant seed contains very little water, so most seeds need water to start germination.
This dryness slows down metabolic reactions, which help the seed to remain alive Jn harsh conditions. The activity of germination demands an increase in the metabolic rate, so the seed must take in water. A seed in moist soil takes in water through its micropyle. This intake of water causes the seed to swell up and the testa to split. More water is taken in through the ruptured testa. The entire embryo absorbs the water. Because the cotyledons make up most of the seed, it is their swelling that eventually breaks the testa and frees the embryo. Seeds also need warmth, up to 30 °C, and oxygen. Oxygen is obtained from the air, which also gets into the seed through the micropyle and later through the ruptured testa. Enzymes in the embryo digest the insoluble food stores in the cotyledons. Proteases break down the stored proteins to amino acids. Amylase breaks down starch to the sugar maltose. These products are soluble in water and diffuse to the growing areas, tne plumule and the radicle.

Growth and development

The numan body is in a continuous growth state from the moment the zygote forms until reaching adulthood. Growth is a permanent increase in size and dry mass, caused by an increase in cell number or cell size or both. When a zygote starts forming organs of the fetus, like the brain for example, or a seed germinates and grows into a plant, the organism is said to be developing. Development is an increase in complexity of an organism as it grows to adulthood. Refer to figures below about growth in humans and plants.

The effect of synthetic plant growth substances used as weedkillers

Weeds are plants that grow in the wrong place at the wrong time. Weedkillers containing synthetic plant hormones, such as auxins, can be sprayed onto weeds to encourage rapid, uncontrolled growth that causes the weeds to die. This type of weedkiller is described as selective because it affects only some plants. For example, when broad-leaved plants grow among narrow-leaved cereal crops, such as wheat or millet, the broad-leaved plants are considered weeds. By spraying the field with selective weedkiller that contains auxin, the broad-leaved plants grow rapidly and die, leaving the cereal crop unharmed. Selective weedkilers are also used to kill weeds growing on lawns and to clear undergrowth in tree plantations

Use of plant growth substances in food production

Plant hormones can be made artificially and used to control and requlate plant growth. Synthetic plant hormones are designed to imitate the effects of the natural plant growth substances, such as auxins. The synthetic hormones are used on a commercial basis as they are cheaper to produce and are not broken down by the plant's enzymes. Synthetic auxins can be:
● added to some plants to induce the production of seedless fruit, for example, seedless tomatoes, squashes and grapes; the added hormones can also increase the size of grapes before harvesting
● used to prevent potatoes from sprouting and growing new shoots
● used to slow down the ripening of grapefruits so that they are ripened at different times and can be sold when prices are higher
● used for rooting of cuttings and micropropagation in plants.

Farmers can control when their fruit ripens by using ethane gas, which stimulates ripening. For example, citrus fruit and tomatoes can be picked and transported to the market while they are still green and unripe. When they reach the market, they can be exposed to ethane gas to ripen before they are sold. Fruits produce ethane gas when they ripen. Bananas produce a lot of ethane gas. That is why other fruits ripen quickly when they are mixed with bananas.

Summary

●Plant growth substances are chemicals that affect activities of particular cells and organs.
● Plant growth substances called auxins
● Plants respond to stimuli by differential growth control tropisms. towards (positive) or away from (negative) environmental stimuli.
● Positive geotropism is the growth response of roots towards gravity.
● Negative geotropism is the growth response of
shoots away from gravity.
● Positive phototropism is the growth response of shoots towards light.
● Negative phototropism is the growth response of shoots away from light.
● Auxins cause the differential growth of cells in the region of elongation and this results in growth curvatures of roots and shoots. Synthetic plant substances, such as auxins, can be used as weedkillers, as well as in the production of Seedless fruits. Methane gas is used to control the ripening of fruits.

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