Plants respond to stimuli by growing toward or away from the source of the stimulus. Plant response to stimuli is called tropism. When the plant grows towards the stimulus, the response is called positive tropism but when it grows away from the stimulus, it is called negative tropism.
Phototropism
A plant seedling, like that of a maize plant, shows positive phototropism by having its coleoptile grow towards the direction of light.
A plant seedling, like that of a maize plant, shows positive phototropism by having its coleoptile grow towards the direction of light.
We will explore this phenomenon by examining the following elegant experiments that demonstrated that auxin was responsible for the bending of the plant towards light.
Examining the diagrams above, A shows a normal seedling with its coleoptile beginning to bend towards the source of the light to the left. Seedling B, which has had the tip of its coleoptile cut, remains unaffected by the light. When the tip of the coleoptile of seedling C is covered by an opaque cup which does not permit light to pass through, it continues to grow upwards. However, if a transparent cup is used instead of an opaque one, seedling D, the coleoptile grows towards the light. If an opaque tube is placed below the tip of the coleoptile as in seedling E, the coleoptile grows towards the light. From these experiments, it is obvious that the tip of the coleoptile is responding light.
Now if the tip of the coleoptile is cut off, a layer of gelatin jelly placed on the top of the stump, and then the cut off tip is replaced, the coleoptile grows and bends towards the light as if it had not been cut, seedling F. This suggests that the bending of the coleoptile towards the light is due to a chemical that can diffuse through the jelly. If the tip of the coleoptile is cut off, a layer of mica placed on the top of the stump and the severed tip replaced, the coleoptile does not bend towards the light. This is because the mica does not allow the chemical substance to pass through.
To demonstrate that a chemical substance causes the bending of the coleoptile towards the light, seedlings are grown in the dark and then the tips of the coleoptiles are cut off.
e next step is to place one cut off tip to the left of the centre of the cut end of the coleoptile. Observation shows that the coleoptile grows towards the right, showing that the cells of the seedling to the left are diving faster than those to the right, making the left side of the plant longer relative to the right. The result is that the plant bends to the right. Similarly, if the cut off tip is placed onto the right half of the cut end, the coleoptile will grow towards the left.
The Role of Auxin
The chemical substances involved are hormones that stimulate the growth of plants, and are called auxins. The most well studied auxin is indole acetic acid. The growing plants bend towards the light because the light disperses the auxin, making it to be more concentrated on the side of the plant not facing the light. This results in a differential growth of the seedling so that one side of the seedling is elongating faster than the other.
Phototropism. A, auxin is not dispersed; B, there is more auxin on the side not facing the light
Gravitropism
Response of plants to gravity is known as gravitropism. It is also known as geotropism. This denotes the ability of plants to grow downwards to the ground. The shoot of a growing plant lying on its side will always grow upwards while the roots bend downwards. The shoot is said to show negative gravitropism while the roots are said to show positive gravitropism. The shoot grows upwards because the auxin becomes concentrated in the lower side of the shoot. This stimulates the elongation of the cells in that region, causing the shoot to bend upwards.
It is not clear why the roots bend downwards. It has been suggested that amyloplasts (grains of starch also known as plastids) present in the apical meristems, move to the lower side of the cells when the plant is left to lie on its sides. This induces the release of calcium ions, which cause the redistribution of the auxin.
Gibberellins
Gibberellins are plant hormones that are produced by the apical meristems of buds, roots and leaves. They are also present in seeds. Their role is to promote fast growth and the elongation of stems. Dwarf plants treated with giberellins grow to normal height. They are also important in ending seed dormancy by stimulating enzyme actions at germination. They also act together with the auxins in inducing fruit formation.
Cytokinins
These compounds are synthesised by the roots and transported to other parts of the plant. They stimulate cell division and growth in young plants.
Abscisic Acid (ABA)
Abscisic acid is an inhibitor of growth produced in the roots. It is known to slow cell division. It is called the hormone of stress because it causes the stomata to close when plants lack water and encourages dormancy in plants during the cold winter months of the temperature regions.
Ethylene
Ethylene is a gas that is used to ripen citrus fruits. It is a product of incomplete combustion of kerosene and gasoline. It acts by increasing the activity of enzymes concerned with fruit ripening. Ethylene is also involved abscission. This term refers to the falling off of leaves, flowers and fruits from the plant.
5.Define photoperiodism and discuss its relationship to flowering.
Thigmotropism
Climbing plants such as yams, tomatoes and pumpkins have tendrils that coil around supports. What triggers the coiling response is the contact made with the support. This directional growth due to touch is known as thigmotropism.
Leaf Movements
When touched the leaves of Mimosa plant collapse and its leaflets fold together. This is due to a loss of turgor by cells within specialised motor organs called pulvini, located at the joints of the leaf. These cells become flaccid on touch, probably because they lose potassium, which draws away from the cells by osmosis. Electrical action potentials, similar to animal action potentials may also be involved. The cells usually take about ten minutes to regain their turgidity.
Some plants like the pea and other legumes show what appears like sleep movements. These plants lower their leaves in the evening and raise them to horizontal position in the morning. Such sleep movements are probably due to changes in turgor pressure in cells in the pulvini motor organs.
PHOTOPERIODISM
Seed germination, flowering, and the onset and breaking of bud dormancy are examples of stages in plant development that usually occur at specific times of the year. The environmental stimulus that plants use to detect the time of the year when to flower is called photoperiod, which is the relative length of the daylight compared to the length of the darkness. The effect of daylight on plants is more obvious in the temperate zones where seasonal changes are quite distinct than in the tropics where days and darkness are almost of equal duration. The length of the daylight regulates flowering in some plants and since the duration of daylight in the temperate zones differs according to the season, plant flowering will have to conform to the different seasons.
The earliest indication of how plants detect the progression of seasons came from the studies of two American agriculturists W.W. Garner and H. A. Allard who were trying to propagate an exceptionally tall (3 m) tobacco plant. This tobacco variety grew well but did not flower at the same time in summer when normal tobacco plants flowered. It was not until November-December that it grew buds and flowered. When seeds from the tall plant were sown, the tobacco plants again took a long time to flower, flowering around November-December.
Another plant the researchers studied was the soybean seed, which they planted at intervals of two weeks, one after the other, between May through July. All the soybeans flowered early September even when the periods of planting differed by as much as 60 days. It seemed as if the plants were waiting for a signal from the environment to begin flowering.
After studying the effects of different factors such as temperature, moisture and mineral nutrition on flowering, Garner and Allard concluded that their effect was minimal but noted that the length of daylight influenced flowering in plants.By studying the effect of daylight on various plants in a greenhouse where they could control the length of the light, it was concluded that plants could be divided into three groups:
Short-day plants - plant that require a photoperiod of short-days and long nights to flower. Examples are cocklebur and chrysanthemums and soybean, tobacco, dahlia.
Long-day plants - plants that will flower when the days are long and the nights short. Examples are wheat, barley and spinach, lettuce.
Day - neutral plants- plants in which flowering is not dependent on a photoperiod. Examples are dandelion, sunflower, tomato, corn, and rice.
Flowering Hormone
The substance that triggers flowering in plants has been identified to be a hormone called florigen. Unfortunately, it has not been possible to isolate this hormone and determine its structure. Florigen is found in the leaves of plants from where it flows to the buds to initiate flowering.
This was confirmed through a number of experiments. For example, it was found that if all the leaves of a plant are removed, the plant fails to respond to the photoperiod. However, if the leaves are removed leaving only a single leaf, the plant will respond to photoperiod and flower, indicating that the substance that triggers photoperiodism is present in the leaves.
Phytochrome
How does a plant measure the length light or darkness in order to be able to flower? Studies show that a green pigment called phytochrome is the photoreceptor that plants use to detect light. Phytochrome is a protein found in the leaves of most plants, including all higher plants.
This pigment is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering. It also regulates other plant responses including seed germination, elongation of seedlings, the size and size of a number of leaves and the synthesis of chlorophyll.
Study Questions
PHOTOPERIODISM
Seed germination, flowering, and the onset and breaking of bud dormancy are examples of stages in plant development that usually occur at specific times of the year. The environmental stimulus that plants use to detect the time of the year when to flower is called photoperiod, which is the relative length of the daylight compared to the length of the darkness. The effect of daylight on plants is more obvious in the temperate zones where seasonal changes are quite distinct than in the tropics where days and darkness are almost of equal duration. The length of the daylight regulates flowering in some plants and since the duration of daylight in the temperate zones differs according to the season, plant flowering will have to conform to the different seasons.
The earliest indication of how plants detect the progression of seasons came from the studies of two American agriculturists W.W. Garner and H. A. Allard who were trying to propagate an exceptionally tall (3 m) tobacco plant. This tobacco variety grew well but did not flower at the same time in summer when normal tobacco plants flowered. It was not until November-December that it grew buds and flowered. When seeds from the tall plant were sown, the tobacco plants again took a long time to flower, flowering around November-December.
Another plant the researchers studied was the soybean seed, which they planted at intervals of two weeks, one after the other, between May through July. All the soybeans flowered early September even when the periods of planting differed by as much as 60 days. It seemed as if the plants were waiting for a signal from the environment to begin flowering.
After studying the effects of different factors such as temperature, moisture and mineral nutrition on flowering, Garner and Allard concluded that their effect was minimal but noted that the length of daylight influenced flowering in plants.By studying the effect of daylight on various plants in a greenhouse where they could control the length of the light, it was concluded that plants could be divided into three groups:
Short-day plants - plant that require a photoperiod of short-days and long nights to flower. Examples are cocklebur and chrysanthemums and soybean, tobacco, dahlia.
Long-day plants - plants that will flower when the days are long and the nights short. Examples are wheat, barley and spinach, lettuce.
Day - neutral plants- plants in which flowering is not dependent on a photoperiod. Examples are dandelion, sunflower, tomato, corn, and rice.
Flowering Hormone
The substance that triggers flowering in plants has been identified to be a hormone called florigen. Unfortunately, it has not been possible to isolate this hormone and determine its structure. Florigen is found in the leaves of plants from where it flows to the buds to initiate flowering.
This was confirmed through a number of experiments. For example, it was found that if all the leaves of a plant are removed, the plant fails to respond to the photoperiod. However, if the leaves are removed leaving only a single leaf, the plant will respond to photoperiod and flower, indicating that the substance that triggers photoperiodism is present in the leaves.
Phytochrome
How does a plant measure the length light or darkness in order to be able to flower? Studies show that a green pigment called phytochrome is the photoreceptor that plants use to detect light. Phytochrome is a protein found in the leaves of most plants, including all higher plants.
This pigment is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering. It also regulates other plant responses including seed germination, elongation of seedlings, the size and size of a number of leaves and the synthesis of chlorophyll.
Study Questions
1.Name three types of hormones that promote growth processes, and give specific functions of each.
2.Describe the experiments that show that auxin is responsible for tropism in plants.
3.Define the following terms: gravitropism, thigmotropism and turgor.
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