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Plant Hormones And Their Responses to Different Stimuli
Plant Hormones: • What is hormone? • A hormone is a chemical produced in one part of an organism and transported throughout the organism to sites where it affects cells capable of responding. • Hormones act as intercellular messengers and regulate cell functioning • Hormones are effective is low concentrations. • Flowering plants have fewer hormones than animals • Hormonal effects are generally slower than nervous responses but last longer than nervous responses. • In plants: • Hormones move from cell to cell through the xylem and the phloem • Hormone movement requires the expenditure of energy. Active transport is about 10 times faster than diffusion. • Hormones in plants can move in the air.
What are tropisms? Tropisms are plant responses in which the direction of the response is related to the direction in which form which the stimulus comes. A tropism can be either positive or negative. A positive tropism grows towards the stimulus. A negative tropism the plant grows away from the stimulus.
Auxin: There is a strong tendency for a growing plant to bend and grow towards a source of light. This response is called positive phototropism (response to light). Auxin is a plant hormone that controls how plants respond to sunlight. Auxin is produced continually in the growing tip of a plant. Auxin in response to light becomes more concentrated on the dark side of the plant. This results in the cells on the dark side of the plant growing longer than the cells on the side of the plant closest to the light. This uneven growth causes the shoot bends towards the light.
Auxin is also involved in negative geotropism which is a tendency for stems to grow away from the force of gravity. If a plant is placed on its side it will still grow upwards because auxin is concentrating on the dark side (the side faced down) which causes the plant to bend upwards towards the light source. Auxin also affect lateral growth of a plant. Auxin is produced in the tip of the plant and moves down through the stem. This downward movement cause the inhibition of buds sprouting along the length known as apical dominance. This results in large trees with very few branches. Gardeners who want bushy trees remove the apical tip allowing other branches to develop. Also during a bushfire the apical tip is lost causing branches to sprout and begin the regeneration process.
Gibberellins: Like auxin, Gibberellins promote cell elongation. Unlike auxin though gibberellins promote the growth of the whole plant. They are synthesised in flowers, developing fruits, sees, actively growing buds and elongating stems. Gibberellins are responsible for promoting cell division fruit enlargement and seed germination.
Cytokinins: • In the presence of auxin, Cytokinin promotes cell division and cell differentiation in plants. • If the concentration of cytokinin is greater than auxin stems and leaves develop in plants. • If the concentration of cytokinin is less than auxin roots develop
Abscisic Acid Abscisic Acid is a growth inhibiting hormone and is produced mainly in the chloroplasts . It controls the dropping of ripe fruits and unfertilised flowers as well as the falling of leaves. In deciduous plants. This process is known as abscission hence the name of the hormone which causes it. Abscisic acid also has a role in controlling stomata movement. When plants start to lose too much water abscisic acid carries a message to guard cells to close the stomata. The concentration of abscisic acid increases under stress such as, in times of drought, high temperature and water-logging.
Ethylene: • The gas ethylene is a small molecule which is released by ripening fruits such as pears, apples, bananas and avocados. An effect of ethylene is the stimulation of fruit ripening. • The ripening of fruits is an extremely important adaptation for plants as it is used to attract animals to the plant to either eat the fruit or disperse the fruit with the objective being to release the seed s inside the fruits. • Ethylene increases the rate of cellular respiration by breaking down starches into sugars. • The production of ethylene is triggered by auxin and cytokinin.
Plant Responses to Stimuli: Stimuli to which plants respond include: For some responses (such as to light and gravity) the direction of the stimulus and the direction of the response are important.
Photoperiod and Flowering: • Photoperiod refers to the day/night cycle. • Short day plants: • Short day plants require long nights to trigger flowering. • In natural circumstances these plants would flower in late Summer, Autumn and Winter • Long day plants: • Long day plants will flower if the nights are short or if the plant is continuously illuminated. • In natural conditions these plants would flower in late Spring or early Summer
Sensing Gravity: • Shoots are negatively geotropic which • means they grow away from the pull of • gravity (up out of the soil) • In contrast to shoot, roots are positively • geotropic as they grow towards the pull • of gravity. Negative Geotropism Positive Geotropism
Sensing Touch • Thigmotropism: • Thigmotropism refers to growth responses • that occur as a result of contact with a • surface. • An example of thigmotropism is the bending • and coiling of vines around tree trunks towards • the light.
Movements and plant rhythms: • Sleep movements: • Sleep movements in plants include the closing of flower petals at night and the folding of the leaves at night time. • Stomata are usually open during the day and are closed at night. • Sleep responses are usually a response to stimuli such as light, carbon dioxide levels and humidity • These response are due to changes in turgor. • Turgor is the internal fluid pressure. If a cell is turgid it has a high water content. • Solar tracking: • Leaves can also twist their stalks in response to light. • As with sleep movements this is controlled by turgor changes. • A loss of turgor will make the stalk bend
Temperature: • The temperature affects on plants are related to the general effects of temperature on enzymes. • Seed dormancy: • Many seeds require cold temperatures for many months to break dormancy which is why many seedlings begin to grow in Spring time after three months of Winter. • Some seeds of desert adapted plants requires leaching with considerable amounts of water. The leaching removes the inhibitory substance abscisic acid that has prevented the seed from developing.
Bud dormancy: • In some plants the lowering of temperature forces the buds into a period of dormancy. This is generally in Autumn when nutrients are taken from the leaves into the roots and stems and the existing leaves turn yellow/red and fall from the tree. • The inhibitory hormone abscisic acid is thought to play a role in this process. • Vernalisation: • Vernalisation is the exposure of plants to a period of cold which stimulates flowering or seed germination. • This is a technique that can be used by growers to increase production of certain flowers and vegetable.
Glossary: Hormone Tropism Phototropism Geotropism Photoperiod Thigmotropism Dormancy Apical tip Apical dominance Abscisic acid Auxin Cytokinin Ethylene Gibberellins vernalisation
Complete the following: Key Questions: 1,2,3,4,5,6,7,8,9 Chapter Review: 1,2,3,4,7,10,12,13