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Transcript
Plant Responses to Stimuli Plants versus Animals ► Plants as living organisms have the ability to: ► 1) use energy to obtain materials from the external environment, and use energy to rearrange those materials into new plant substances. ► 2) With very few exceptions, plants acquire all the matter and energy they need: without changing location (as must most animals, protists, and bacteria), without preying on other living organisms (as animals do), and without relying on matter assembled by other organisms (as fungi do). How are new individuals formed? ► ► ► ► Some plants, such as the Bryophyllum grow new individuals from a leaf. Some trees, such as sumac and poplar, grow new individuals, called suckers, from the roots. Spider plants and strawberries grow stems from which new individuals can become established at some distance from the original plant. Many grasses grow from nodes in their roots. Attainment of Nutrients ► ► ► ► ► Most plants obtain their nutrients from the air and the soil. Some plant species however, are able to “prey”, or eat and gain nutrients from animals. The pitcher plant (the provincial flower of Newfoundland and Labrador) and the Venus flytrap are consumers. They trap and digest insects in order to obtain nutrients that are not available in the nutrient-poor soil they grow in. These plants are also producers because they photosynthesize, as well. P.Plant ► (Not Newfoundland) ► Development in the Meristem ► ► ► ► ► What gives plants their amazing ability to grow throughout their lives? Although mitosis and cell division occurs throughout a plant as it grows, eventually new growth is restricted to small regions of unspecialized tissue collectively called the meristems Growth there results from the accumulation of rapidly dividing cells. When a cell in the meristem divides, one of the two resulting cells remains in the meristem. The other cell becomes part of the plant body. Initially, all meristem cells are identical in structure, and they have no specialized function. Development in the Meristem ► As they divide repeatedly, they begin to differ in: Shape relative proportions of their various organelles functions they can perform. ► Changes result in the cells becoming specialized for particular functions, such as photosynthesis, storage, and support. Types of Meristem Tissue ► ► ► ► ► ► Two main types of meristem tissue. 1) apical meristem tissue: located in the root and shoot tips of plants. Division of apical meristem cells results in growth of roots, leaves, and flowers. Protected by a root cap in the root. Protected by a terminal bud in the stem. In colder climates, the terminal buds stop growing in the winter and resume growing in the spring. These buds are protected by bud scales, which fall off when growth begins in the spring. Types of Meristem Tissue ► 2) Lateral meristem results in the growth of tissue beneath the bark of tree stems ► division of cells results in the thickening of cylinders of tissue. ► Most woody plants have two kinds of lateral meristems: a vascular cambium and a cork cambium. Meristems ► ► ► Meristem tissue enables plants to grow from cuttings. Growing plants from cuttings is the basis of plant cloning: growing genetically identical copies of an organism from a single cell or part of an organism. For some species, growing plants from cuttings can be much faster than growing them from seed. Internal Regulation of Plant Growth and Development ► ► ► ► ► ► ► Plants can grow to their maximum height when environmental conditions are optimal. ► Fertilizer Optimal conditions include adequate moisture, warmth, light, and nutrients. Fertilizers promote plant growth and development by providing additional nutrition. Pesticides and fungicides promote plant growth by controlling numbers of insects and fungi that feed on plants. Plant growth and development are also controlled by the plant’s own hormones. Hormone: a chemical compound manufactured by specialized tissue in one body part of an organism but that ►Pesticide governs or regulates the activity of another body part or parts. Even though a hormone may circulate throughout an organism, it will act only on specific “target” tissues or organs. Alice…R.I.P Hormonal Control of Plant Growth: Auxin ► ► ► ► ► ► In the early 1800s, experiments undertaken by Charles Darwin and his son Francis described the effects of a mysterious “influence” that affected the growth of grass seedlings. The seedlings normally grew toward a light source, however this behaviour was not seen if the tips of the grass seedlings were covered with an opaque capsule that did not let light through. The remainder of the plant, where the growth actually occurs, was still exposed to light. If the tip of the seedling were covered with a gelatin capsule, which allowed light to pass through, then the seedling would grow towards the light as expected. For many years, researchers conducted experiments to try to explain the nature of this http://www.tutorvista.com/content/biolo observation. gy/biology-iv/plant-growthIn 1926, Frits Went performed a series of movements/growth-regulators.php experiments that showed there was a chemical messenger in the grass seedlings. This chemical could enhance plant growth. He named the substance auxin, from the Greek work auxein, which means “to increase”. Hormonal Control of Plant Growth PROMOTOR HORMONES ► ► ► ► Other discoveries about plant hormones came as a result of people noticing unusual growth in plants. For example, observers noticed that a rice plant infected with the fungus Gibberella fujikoroi grew abnormally tall. In 1935, researchers were finally able to isolate the chemical compound that caused the accelerated growth, and named it gibberellic acid. They discovered that applying artificially-manufactured gibberellic acid to a plant not infected by the fungus caused the plant to grow abnormally tall. Scientists and researchers continued to search for other plant hormones that might affect growth and development. Two types of plant growth hormones: promoter hormones, which are hormones that cause growth, and inhibitor hormones, which are hormones that block growth. The Classes of Hormones ► ► ► ► ► Auxins are a class of hormone that is produced in the apical meristem of shoots. There are both natural and synthetic auxins that promote cell elongation, the development of vascular tissue, and trigger the development of above-ground stems, which help support the plant. These hormones also cause leaves to drop after they are no longer needed. The stimulation of cell elongation occurs because auxin increases the plasticity of the plant cell wall. The more plastic the cell wall is, the more it can stretch during active cell growth. The Classes of Hormones ► ► ► ► ► Gibberellins are also produced in the apical meristem and act to increase stem length. Increase the uptake of starch in the embryo of germinating seeds and stimulate the development of vascular tissue. The effects of gibberellins include taller and stronger plants, plants that flower early, or genetically dwarf plants that grow to normal heights. Used in commercial crops to increase fruit size, and cluster size in grapes. They can delay the ripening of citrus fruits and speed up the flowering of strawberries. The Classes of Hormones Cytokinins promote cell division and cell differentiation. ► Cell differentiation occurs when specialized cells are needed to perform certain functions. ► They also delay the aging of leaves and fruit. ► Work by influencing the synthesis and activation of proteins that are required for mitosis. ► Oligosaccharins are a recently discovered class of growth promoters. They stimulate plants to manufacture antibiotics in response to attack by fungi or bacteria. ► This allows the plant to grow to its full potential because the negative influences of pests are diminished. ► The Classes of Hormones: Inhibitors ► There are two classes of hormone growth inhibitors. ► 1) abscisic acid (ABA): synthesized mainly in mature green leaves, fruits, and root caps. ► Inhibits the germination of seeds, inhibits the growth of buds in plant stems, and blocks the intake of carbon dioxide by controlling the opening and closing of leaf stomata. ► Abscisic acid also blocks the action of growth promoting hormones. The Classes of Hormones: Inhibitors ► ► ► ► ► 2) Ethylene: a gaseous hydrocarbon. It occurs as a natural plant hormone. Stimulates the aging of plant tissues, the ripening and sweetening of fruit, and can also speed up the dropping of leaves from trees. The production of ethylene gas by plants can stimulate other plants to ripen. Initially noted when bananas ripened quickly if they were left near oranges. The ripening action of ethylene has led to its use in agriculture. For example, tomatoes may be picked while green and then ripened artificially by the application of ethylene. Plant Tropisms ► ► ► ► ► Plants exhibit the ability to orient themselves in response to external stimuli such as light. A directional growth response to unequal stimulation from the external environment is called a tropism, and it controls the growth pattern of the plant. Various external stimuli affect the production of plant hormones - results in the directional growth of a plant. In tropism, the plant may grow either toward or away from the stimulus. Growth toward the stimulus is a positive tropism. Growth away from the stimulus is a negative tropism. There are three major kinds of plant tropisms that are affected by light, gravity, and touch. Plant Tropisms 1) Phototropism occurs when the growth of a plant is affected by light. ► In general, plants are positively phototropic, that is, they grow toward light. ► Roots are negatively phototropic and grow away from light. ► The growth is caused by differing amounts of auxin produced on the light and dark sides of the stem. ► Auxin accumulates on the shaded side of the stem, which causes the cells there to elongate. ► This causes the stem bend toward the light. ► Turning the plant around will cause the stem to bend in the other direction, however, it will not change the original curve in the stem because it is the result of growth. ► Plant Tropisms ► ► ► ► ► 2) Gravitropism is a plant’s response to gravity. Causes roots to grow downwards (positive gravitropism) and shoots and stems to grow upward (negative gravitropism). Benefits the plant, because shoots that grow upward will receive light and roots that grow downward will receive nutrients from the soil. 3) Thigmotropism is the response of plants to touch. This behaviour is a caused by specialized cells in the epidermis of the plant. Vining plants demonstrate a strong positive thigmatropism The vines grow toward the object touching them causing them to coil around the object. Other plants demonstrate a negative thigmotropism. Nastic Responses in Plants ► ► ► ► ► ► ► ► ► Another type of response, called nastic movements, are caused by a stimulus that is not directional. For example, the leaves on a mimosa plant fold up when the plant is touched This response might seem to illustrate a negative thigmotropism, however, it is neither directional nor permanent. The leaflets fold downward in the same way regardless of the direction of the stimulus. These movements are not a result of growth, but rather a change in turgor pressure in the cells at the base of each leaflet. A sudden drop in pressure causes the cells to become limp and the leaflets fold down. Once the stimulus has ceased, the turgor pressure in the cells rises once again and the leaflets open. Another example of a nastic response is the hinged leaf of a Venus’s flytrap. The movement of an insect on the leaf triggers the hinged leaf to close, trapping the insect between the leaves. Commercial Use of Growth Regulators ► ► ► ► ► Over the past century, scientists have learned much about plant growth hormones. Horticulturists and other agricultural scientists use this knowledge of plant growth regulators to influence the growth and development of crops and ornamental plants. Most growth regulating hormones used for commercial purposes are synthetically produced rather than extracted from plants. For example, there appears to be only one naturally occuring auxin, but many more synthetic auxin-like growth regulators exist. Although these synthetically produced hormones are not identical to natural auxins, their chemical action is similar, and the plants respond as they would to naturally occurring auxin. Commercial Use of Growth Regulators ► ► ► A large industry is based on the manufacture of artificial plant growth regulating hormones. Some of the growth regulating hormones that are produced can be very specialized For example, a chemical treatment can be applied to ornamental trees to prevent them from growing too tall and interfering with utility lines. From the ground, the trees look normal, but the tops look as if they have been pruned flat. Things to do when you have no life