Plant Responses to StimuliPlant Responses to Stimuli

Plants versus AnimalsPlants versus Animals

► Plants as living organisms have the ability to:Plants as living organisms have the ability to:► 1)1) use energy to obtain materials from the use energy to obtain materials from the

external environment, and use energy to external environment, and use energy to rearrange those materials into new plant rearrange those materials into new plant substances.substances.

► 2)2) With very few exceptions, plants acquire With very few exceptions, plants acquire all the matter and energy they need: without all the matter and energy they need: without changing location (as must most animals, changing location (as must most animals, protists, and bacteria), without preying on protists, and bacteria), without preying on other living organisms (as animals do), and other living organisms (as animals do), and without relying on matter assembled by other without relying on matter assembled by other organisms (as fungi do).organisms (as fungi do).

How are new individuals How are new individuals formed? formed? ► Some plants, such as the Some plants, such as the

Bryophyllum Bryophyllum grow new grow new individuals from a leaf.individuals from a leaf.

► Some trees, such as Some trees, such as sumac and poplar, grow sumac and poplar, grow new individuals, called new individuals, called suckers, from the roots. suckers, from the roots.

► Spider plants and Spider plants and strawberries grow stems strawberries grow stems from which new from which new individuals can become individuals can become established at some established at some distance from the distance from the original plant.original plant.

► Many grasses grow from Many grasses grow from nodes in their roots. nodes in their roots.

Attainment of NutrientsAttainment of Nutrients► Most plants obtain their Most plants obtain their

nutrients from the air and the nutrients from the air and the soil. soil.

► Some plant species however, Some plant species however, are able to “prey”, or eat and are able to “prey”, or eat and gain nutrients from animals.gain nutrients from animals.

► The pitcher plant (the The pitcher plant (the provincial flower of provincial flower of Newfoundland and Labrador) Newfoundland and Labrador) and the Venus flytrap are and the Venus flytrap are consumers. consumers.

► They trap and digest insects in They trap and digest insects in order to obtain nutrients that order to obtain nutrients that are not available in the are not available in the nutrient-poor soil they grow in. nutrient-poor soil they grow in.

► These plants are also These plants are also producers because they producers because they photosynthesize, as well.photosynthesize, as well.

► P.PlantP.Plant► (Not (Not Newfoundland)Newfoundland)

Development in the Development in the MeristemMeristem

► What gives plants their What gives plants their amazing ability to grow amazing ability to grow throughout their lives? throughout their lives?

► Although mitosis and cell Although mitosis and cell division occurs throughout a division occurs throughout a plant as it grows, eventually plant as it grows, eventually new growth is restricted to new growth is restricted to small regions of unspecialized small regions of unspecialized tissue collectively called the tissue collectively called the meristemsmeristems

► Growth there results from the Growth there results from the accumulation of rapidly accumulation of rapidly dividing cells. dividing cells.

► When a cell in the meristem When a cell in the meristem divides, one of the two divides, one of the two resulting cells remains in the resulting cells remains in the meristem. The other cell meristem. The other cell becomes part of the plant becomes part of the plant body. body.

► Initially, all meristem cells are Initially, all meristem cells are identical in structure, and they identical in structure, and they have no specialized function.have no specialized function.

Development in the Development in the MeristemMeristem

► As they divide As they divide repeatedly, they begin repeatedly, they begin to differ in:to differ in: ShapeShape relative proportions of relative proportions of

their various organellestheir various organelles functions they can functions they can

perform. perform. ► Changes result in the Changes result in the

cells becoming cells becoming specialized for specialized for particular functions, particular functions, such as photosynthesis, such as photosynthesis, storage, and support.storage, and support.

Types of Meristem TissueTypes of Meristem Tissue► Two main types of meristem Two main types of meristem

tissue. tissue. ► 1)1) apical meristem apical meristem tissue: tissue:

located in the root and shoot located in the root and shoot tips of plants. Division of tips of plants. Division of apical meristem cells results in apical meristem cells results in growth of roots, leaves, and growth of roots, leaves, and flowers. flowers.

► Protected by a Protected by a root caproot cap in the in the root.root.

► Protected by a Protected by a terminal budterminal bud in the stem.in the stem.

► In colder climates, the In colder climates, the terminal buds stop growing in terminal buds stop growing in the winter and resume the winter and resume growing in the spring. growing in the spring.

► These buds are protected by These buds are protected by bud scales, which fall off when bud scales, which fall off when growth begins in the spring.growth begins in the spring.

Types of Meristem TissueTypes of Meristem Tissue► 2) Lateral meristem 2) Lateral meristem

results in the growth of results in the growth of tissue beneath the tissue beneath the bark of tree stemsbark of tree stems

► division of cells results division of cells results in the thickening of in the thickening of cylinders of tissue.cylinders of tissue.

► Most woody plants Most woody plants have two kinds of have two kinds of lateral meristems: a lateral meristems: a vascular cambiumvascular cambium and a and a cork cambiumcork cambium..

MeristemsMeristems

► Meristem tissue Meristem tissue enables plants to grow enables plants to grow from from cuttingscuttings. .

► Growing plants from Growing plants from cuttings is the basis of cuttings is the basis of plant cloningplant cloning: : growing genetically growing genetically identical copies of an identical copies of an organism from a organism from a single cell or part of single cell or part of an organism. an organism.

► For some species, For some species, growing plants from growing plants from cuttings can be much cuttings can be much faster than growing faster than growing them from seed.them from seed.

Internal Regulation of PlantInternal Regulation of PlantGrowth and DevelopmentGrowth and Development

► Plants can grow to their maximum Plants can grow to their maximum height when environmental conditions height when environmental conditions are optimal. are optimal.

► Optimal conditions include adequate Optimal conditions include adequate moisture, warmth, light, and moisture, warmth, light, and nutrientsnutrients. .

► FertilizersFertilizers promote plant growth and promote plant growth and development by providing additional development by providing additional nutrition. nutrition.

► PesticidesPesticides and and fungicidesfungicides promote promote plant growth by controlling numbers of plant growth by controlling numbers of insects and fungi that feed on plants. insects and fungi that feed on plants.

► Plant growth and development are Plant growth and development are also controlled by the plant’s own also controlled by the plant’s own hormoneshormones..

► HormoneHormone: a chemical compound : a chemical compound manufactured by specialized tissue in manufactured by specialized tissue in one body part of an organism but that one body part of an organism but that governs or regulates the activity of governs or regulates the activity of another body part or parts. another body part or parts.

► Even though a hormone may circulate Even though a hormone may circulate throughout an organism, it will act throughout an organism, it will act only on only on specific “target” tissues or specific “target” tissues or organsorgans..

► FertilizerFertilizer

►PesticidePesticide

Alice…R.I.PAlice…R.I.P

Hormonal Control of Plant Hormonal Control of Plant Growth: AuxinGrowth: Auxin

► In the early 1800s, experiments undertaken In the early 1800s, experiments undertaken by Charles Darwin and his son Francis by Charles Darwin and his son Francis described the effects of a mysterious described the effects of a mysterious “influence” that affected the growth of “influence” that affected the growth of grass seedlings. grass seedlings.

► The seedlings normally grew toward a light The seedlings normally grew toward a light source, however this behaviour was not source, however this behaviour was not seen if the tips of the grass seedlings were seen if the tips of the grass seedlings were covered with an opaque capsule that did covered with an opaque capsule that did not let light through. not let light through.

► The remainder of the plant, where the The remainder of the plant, where the growth actually occurs, was still exposed to growth actually occurs, was still exposed to light. light.

► If the tip of the seedling were covered with If the tip of the seedling were covered with a gelatin capsule, which allowed light to a gelatin capsule, which allowed light to pass through, then the seedling would grow pass through, then the seedling would grow towards the light as expected. towards the light as expected.

► For many years, researchers conducted For many years, researchers conducted experiments to try to explain the nature of experiments to try to explain the nature of this observation. this observation.

► In 1926, Frits Went performed a series of In 1926, Frits Went performed a series of experiments that showed there was a experiments that showed there was a chemical messenger in the grass seedlings. chemical messenger in the grass seedlings. This chemical could enhance plant growth. This chemical could enhance plant growth. He named the substance He named the substance auxinauxin, from the , from the Greek work Greek work auxeinauxein, which means “to , which means “to increase”.increase”.

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Hormonal Control of Plant Hormonal Control of Plant Growth PROMOTOR Growth PROMOTOR

HORMONESHORMONES► Other discoveries about plant hormones came as a Other discoveries about plant hormones came as a

result of people noticing unusual growth in plants. result of people noticing unusual growth in plants. ► For example, observers noticed that a rice plant For example, observers noticed that a rice plant

infected with the fungus infected with the fungus Gibberella fujikoroi Gibberella fujikoroi grew grew abnormally tall. In 1935, researchers were finally able abnormally tall. In 1935, researchers were finally able to isolate the chemical compound that caused the to isolate the chemical compound that caused the accelerated growth, and named it accelerated growth, and named it gibberellic acidgibberellic acid. . They discovered that applying artificially-manufactured They discovered that applying artificially-manufactured gibberellic acid to a plant not infected by the fungus gibberellic acid to a plant not infected by the fungus caused the plant to grow abnormally tall.caused the plant to grow abnormally tall.

► Scientists and researchers continued to search for Scientists and researchers continued to search for other plant hormones that might affect growth and other plant hormones that might affect growth and development. development.

► Two types of plant growth hormones: Two types of plant growth hormones: promoter promoter hormoneshormones, which are hormones that cause growth, , which are hormones that cause growth, and and inhibitor hormonesinhibitor hormones, which are hormones that , which are hormones that block growth.block growth.

The Classes of The Classes of HormonesHormones► AuxinsAuxins are a class of hormone are a class of hormone

that is produced in the apical that is produced in the apical meristem of shoots. meristem of shoots.

► There are both There are both naturalnatural and and syntheticsynthetic auxinsauxins that promote that promote cell elongation, the cell elongation, the development of vascular tissue, development of vascular tissue, and trigger the development of and trigger the development of above-ground stems, which help above-ground stems, which help support the plant. support the plant.

► These hormones also cause These hormones also cause leaves to drop after they are no leaves to drop after they are no longer needed. longer needed.

► The stimulation of cell The stimulation of cell elongation occurs because elongation occurs because auxin increases the plasticity of auxin increases the plasticity of the plant cell wall. the plant cell wall.

► The more plastic the cell wall is, The more plastic the cell wall is, the more it can stretch during the more it can stretch during active cell growth. active cell growth.

The Classes of The Classes of HormonesHormones► Gibberellins Gibberellins are also produced in the apical meristem and are also produced in the apical meristem and

act to increase stem length. act to increase stem length. ► Increase the uptake of starch in the embryo of germinating Increase the uptake of starch in the embryo of germinating

seeds and stimulate the development of vascular tissue. seeds and stimulate the development of vascular tissue. ► The effects of gibberellins include taller and stronger plants, The effects of gibberellins include taller and stronger plants,

plants that flower early, or genetically dwarf plants that grow plants that flower early, or genetically dwarf plants that grow to normal heights. to normal heights.

► Used in commercial crops to increase fruit size, and cluster Used in commercial crops to increase fruit size, and cluster size in grapes. size in grapes.

► They can delay the ripening of citrus fruits and speed up the They can delay the ripening of citrus fruits and speed up the flowering of strawberries.flowering of strawberries.

The Classes of HormonesThe Classes of Hormones

► Cytokinins Cytokinins promote cell division and cell promote cell division and cell differentiation. differentiation.

► Cell differentiation occurs when specialized cells Cell differentiation occurs when specialized cells are needed to perform certain functions. are needed to perform certain functions.

► They also delay the aging of leaves and fruit. They also delay the aging of leaves and fruit. ► Work by influencing the synthesis and activation Work by influencing the synthesis and activation

of proteins that are required for mitosis.of proteins that are required for mitosis.► Oligosaccharins Oligosaccharins are a recently discovered class are a recently discovered class

of growth promoters. They stimulate plants to of growth promoters. They stimulate plants to manufacture antibiotics in response to attack by manufacture antibiotics in response to attack by fungi or bacteria. fungi or bacteria.

► This allows the plant to grow to its full potential This allows the plant to grow to its full potential because the negative influences of pests are because the negative influences of pests are diminished.diminished.

The Classes of Hormones: The Classes of Hormones: InhibitorsInhibitors

► There are two classes of hormone growth There are two classes of hormone growth inhibitors. inhibitors.

► 1) 1) abscisic acid (ABA):abscisic acid (ABA): synthesized mainly in synthesized mainly in mature green leaves, fruits, and root caps. mature green leaves, fruits, and root caps.

► Inhibits the germination of seeds, inhibits the Inhibits the germination of seeds, inhibits the growth of buds in plant stems, and blocks the growth of buds in plant stems, and blocks the intake of carbon dioxide by controlling the intake of carbon dioxide by controlling the opening and closing of leaf stomata. opening and closing of leaf stomata.

► Abscisic acid also blocks the action of growth Abscisic acid also blocks the action of growth promoting hormones.promoting hormones.

The Classes of Hormones: The Classes of Hormones: InhibitorsInhibitors

► 2) Ethylene2) Ethylene: a gaseous hydrocarbon. It occurs as a : a gaseous hydrocarbon. It occurs as a natural plant hormone.natural plant hormone.

► Stimulates the aging of plant tissues, the ripening Stimulates the aging of plant tissues, the ripening and sweetening of fruit, and can also speed up the and sweetening of fruit, and can also speed up the dropping of leaves from trees. dropping of leaves from trees.

► The production of ethylene gas by plants can The production of ethylene gas by plants can stimulate other plants to ripen. stimulate other plants to ripen.

► Initially noted when bananas ripened quickly if they Initially noted when bananas ripened quickly if they were left near oranges. were left near oranges.

► The ripening action of ethylene has led to its use in The ripening action of ethylene has led to its use in agriculture. For example, tomatoes may be picked agriculture. For example, tomatoes may be picked while green and then ripened artificially by the while green and then ripened artificially by the application of ethylene.application of ethylene.

Plant TropismsPlant Tropisms► Plants exhibit the ability to orient Plants exhibit the ability to orient

themselves in response to themselves in response to external stimuli such as light. external stimuli such as light.

► A directional growth response to A directional growth response to unequal stimulation from the unequal stimulation from the external environment is called a external environment is called a tropismtropism, and it controls the , and it controls the growth pattern of the plant.growth pattern of the plant.

► Various external stimuli affect the Various external stimuli affect the production of plant hormones - production of plant hormones - results in the directional growth of results in the directional growth of a plant. a plant.

► In tropism, the plant may grow In tropism, the plant may grow either toward or away from the either toward or away from the stimulus. stimulus.

► Growth toward the stimulus is a Growth toward the stimulus is a positive tropismpositive tropism. Growth away . Growth away from the stimulus is a from the stimulus is a negative negative tropismtropism. There are . There are three major three major kinds of plant tropismskinds of plant tropisms that are that are affected by light, gravity, and affected by light, gravity, and touch.touch.

Plant TropismsPlant Tropisms► 1) Phototropism 1) Phototropism occurs when occurs when

the growth of a plant is affected the growth of a plant is affected by light. by light.

► In general, plants are positively In general, plants are positively phototropic, that is, they grow phototropic, that is, they grow toward light. toward light.

► Roots are negatively phototropic Roots are negatively phototropic and grow away from light. and grow away from light.

► The growth is caused by differing The growth is caused by differing amounts of auxin produced on the amounts of auxin produced on the light and dark sides of the stem. light and dark sides of the stem.

► Auxin accumulates on the shaded Auxin accumulates on the shaded side of the stem, which causes side of the stem, which causes the cells there to elongate. the cells there to elongate.

► This causes the stem bend toward This causes the stem bend toward the light. the light.

► Turning the plant around will Turning the plant around will cause the stem to bend in the cause the stem to bend in the other direction, however, it will other direction, however, it will not change the original curve in not change the original curve in the stem because it is the result the stem because it is the result of growth.of growth.

Plant TropismsPlant Tropisms► 2) Gravitropism 2) Gravitropism is a plant’s response to gravity.is a plant’s response to gravity.► Causes roots to grow downwards (positive Causes roots to grow downwards (positive

gravitropism) and shoots and stems to grow upward gravitropism) and shoots and stems to grow upward (negative gravitropism). (negative gravitropism).

► Benefits the plant, because shoots that grow Benefits the plant, because shoots that grow upward will receive light and roots that grow upward will receive light and roots that grow downward will receive nutrients from the soil.downward will receive nutrients from the soil.

► 3) Thigmotropism 3) Thigmotropism is the response of plants to is the response of plants to touch. This behaviour is a caused by specialized touch. This behaviour is a caused by specialized cells in the epidermis of the plant. Vining plants cells in the epidermis of the plant. Vining plants demonstrate a strong positive thigmatropismdemonstrate a strong positive thigmatropism

► The vines grow toward the object touching them The vines grow toward the object touching them causing them to coil around the object. Other plants causing them to coil around the object. Other plants demonstrate a negative thigmotropism.demonstrate a negative thigmotropism.

Nastic Responses in Nastic Responses in PlantsPlants► Another type of response, called Another type of response, called

nastic movementsnastic movements, are caused by a , are caused by a stimulus that is not directional. stimulus that is not directional.

► For example, the leaves on a mimosa For example, the leaves on a mimosa plant fold up when the plant is touchedplant fold up when the plant is touched

► This response might seem to illustrate This response might seem to illustrate a negative thigmotropism, however, it a negative thigmotropism, however, it is neither directional nor permanent. is neither directional nor permanent.

► The leaflets fold downward in the same The leaflets fold downward in the same way regardless of the direction of the way regardless of the direction of the stimulus. stimulus.

► These movements are not a result of These movements are not a result of growth, but rather a change in turgor growth, but rather a change in turgor pressure in the cells at the base of pressure in the cells at the base of each leaflet. each leaflet.

► A sudden drop in pressure causes the A sudden drop in pressure causes the cells to become limp and the leaflets cells to become limp and the leaflets fold down. fold down.

► Once the stimulus has ceased, the Once the stimulus has ceased, the turgor pressure in the cells rises once turgor pressure in the cells rises once again and the leaflets open. again and the leaflets open.

► Another example of a nastic response Another example of a nastic response is the hinged leaf of a Venus’s flytrap.is the hinged leaf of a Venus’s flytrap.

► The movement of an insect on the leaf The movement of an insect on the leaf triggers the hinged leaf to close, triggers the hinged leaf to close, trapping the insect between the trapping the insect between the leaves.leaves.

Commercial Use ofCommercial Use ofGrowth RegulatorsGrowth Regulators

► Over the past century, scientists Over the past century, scientists have learned much about plant have learned much about plant growth hormones. growth hormones.

► Horticulturists and other Horticulturists and other agricultural scientists use this agricultural scientists use this knowledge of plant growth knowledge of plant growth regulators to influence the growth regulators to influence the growth and development of crops and and development of crops and ornamental plants. ornamental plants.

► Most growth regulating hormones Most growth regulating hormones used for commercial purposes are used for commercial purposes are synthetically produced rather synthetically produced rather than extracted from plants. than extracted from plants.

► For example, there appears to be For example, there appears to be only one naturally occuring auxin, only one naturally occuring auxin, but many more synthetic auxin-but many more synthetic auxin-like growth regulators exist.like growth regulators exist.

► Although these synthetically Although these synthetically produced hormones are not produced hormones are not identical to natural auxins, their identical to natural auxins, their chemical action is similar, and the chemical action is similar, and the plants respond as they would to plants respond as they would to naturally occurring auxin.naturally occurring auxin.

Commercial Use ofCommercial Use ofGrowth RegulatorsGrowth Regulators

► A large industry is based on A large industry is based on the manufacture of artificial the manufacture of artificial plant growth regulating plant growth regulating hormones. hormones.

► Some of the growth regulating Some of the growth regulating hormones that are produced hormones that are produced can be very specialized can be very specialized

► For example, a chemical For example, a chemical treatment can be applied to treatment can be applied to ornamental trees to prevent ornamental trees to prevent them from growing too tall them from growing too tall and interfering with utility and interfering with utility lines. From the ground, the lines. From the ground, the trees look normal, but the tops trees look normal, but the tops look as if they have been look as if they have been pruned flat.pruned flat.

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