* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Photosynthesis
Photosynthesis wikipedia , lookup
Plant tolerance to herbivory wikipedia , lookup
Plant stress measurement wikipedia , lookup
History of herbalism wikipedia , lookup
Plant breeding wikipedia , lookup
Evolutionary history of plants wikipedia , lookup
Plant secondary metabolism wikipedia , lookup
History of botany wikipedia , lookup
Plant defense against herbivory wikipedia , lookup
Venus flytrap wikipedia , lookup
Plant use of endophytic fungi in defense wikipedia , lookup
Historia Plantarum (Theophrastus) wikipedia , lookup
Plant nutrition wikipedia , lookup
Plant evolutionary developmental biology wikipedia , lookup
Ornamental bulbous plant wikipedia , lookup
Plant ecology wikipedia , lookup
Plant morphology wikipedia , lookup
Flowering plant wikipedia , lookup
Plant reproduction wikipedia , lookup
Plant physiology wikipedia , lookup
Control of Growth and Responses in Plants Ch. 27 - Plant Responses 1 Control of Growth and Responses in Plants 2 Ch. 27 - Plant Responses Tropisms Plant growth toward or away from a unidirectional stimulus is called a tropism - Positive is towards stimulus - Negative is away from stimulus - Due to differential growth - one side of organ elongates faster than the other Three types of tropisms: - Phototropism - movement in response to light - Gravitropism - movement in response to gravity - Thigmotropism - in response to touch Phototropism 3 Control of Growth and Responses in Plants 4 Phototropism Positive phototropism: Studied by Charles & Francis Darwin Occurs because cells on the shady side of the stem elongate A pigment related to riboflavin thought to act as a photoreceptor when phototropism occurs - Plant hormone called auxin migrates from lighted side of stem to shady side of stem - Cells on the shady side elongate faster than those on the bright side, causing stem to curve toward the light Control of Growth and Responses in Plants 5 Gravitropism When a plant is placed on its side, the stem grows upward, opposite of the pull of gravity. This is an example of negative response called gravitropism Roots, in contrast, show positive gravitropism, as they grow downwards. Roots without root caps don’t respond to gravity Root cap cells contain sensors called statoliths, which are starch grains located within amyloplasts, a type of plastid. - Amyloplasts settle to lower part of cell & cause bending of root. Gravitropism 6 Gravitropism Negative gravitropism of stems Positive gravitropism of roots Sedimentation of statoliths 7 Control of Growth and Responses in Plants 8 Gravitropism Auxin is responsible for: Positive gravitropism of roots, and Negative gravitropism of shoots How does auxin do this: Amyloplasts come in contact with ER which releases stored calcium ions. This leads to activation of auxin pumps & auxin enters the cells Roots & stems respond differently to auxin: - Auxin inhibits growth of root cells, so cells on upper surface elongate so root curves downward - Auxin stimulates growth of stem cells, so cells on lower surface elongate so stem curves upward Gravitropism Negative gravitropism of stems Positive gravitropism of roots Sedimentation of statoliths 9 Control of Growth and Responses in Plants 10 Thigmotropism Unusual growth due to contact with solid objects is called thigmotropism Ex: Coiling of tendrils The plant grows straight until it touches something. Cells in contact with object grow less while those on the opposite side elongate. Response can be quite rapid; within 10 minutes Sometimes it seems to need light which might be a need for ATP for the response. Coiling Response 11 Control of Growth and Responses in Plants 12 Nastic Movements Nastic movements: Do not involve growth and Are not dependent on the stimulus direction Seismonastic movements result from: Touch, shaking, or Thermal stimulation Due to loss of turgor pressure within a few cells located in a thickening, called a pulvinus, at the base of each leaflet. Touch causes K+ to flow out of cells & then water follows. •Ex: Mimosa leaves & Venus flytrap Seismonastic Movement Mimosa pudica 13 Control of Growth and Responses in Plants 14 Nastic Movements Sleep movements: Occur daily in response to light and dark changes Ex: Prayer Plant Movement due to changes in turgor pressure of motor cells in a pulvinus located at the base of each leaf. Sleep Movement Prayer plant 15 Control of Growth and Responses in Plants 16 Circadian Rhythms Biological rhythms with a 24-hour cycle Tend to be persistent - Rhythm is maintained in the absence of environmental stimuli - Caused by a biological clock - Without environmental stimuli, circadian rhythms continue but the cycle extends to 25 or 26 hours - Believed that the clocks are synchronized by external stimuli such as length of daylight compared to length of darkness. This is called the photoperiod. Control of Growth and Responses in Plants 17 Plant Hormones Almost all communication on a plant is done by hormones Chemical signals produced in very low concentrations in one part of plants and then active in another part of the plant Hormones travel within phloem, or from cell to cell, in response to the appropriate stimulus Each hormone has a specific chemical structure Control of Growth and Responses in Plants 18 Auxins The most common naturally occurring auxin is indoleacetic acid (IAA). It is produced in shoot apical meristem and is found in young leaves and in flowers and fruits Auxins affect many aspects of plant growth & development Control of Growth and Responses in Plants 19 Effects of Auxin Apical Dominance Apically produced auxin prevents the growth of axillary buds (side buds) When a terminal bud is removed, the nearest lateral buds begin to grow, and the plant branches - Pruning the top of a plant generally achieves a fuller look by removing the apical dominance - Weak solution of auxin applied to woody cutting causes rapid growth of adventitious roots - Promotes fruit growth Apical Dominance 20 Control of Growth and Responses in Plants 21 Effects of Auxin Auxin production by seeds also promotes the growth of fruit. - As long as auxin is concentrated in leaves or fruits rather than in the stem, leaves and fruits do not fall off. - Trees can be sprayed with auxin to keep mature fruit from falling to ground - Auxin is sprayed on tomatoes to induce development of fruit without pollination creating seedless tomatoes Phototropism Experiments Control of Growth and Responses in Plants Darwin & Darwin (1880s) 1. Used coleoptiles (grass shoots) 2. Found that shoots bend if: a. Tips of shoots are present & Normal Covered with clear cap Opaque base 3. No bending if: a. Tip covered with cap b. Tip was removed 4. Concluded tip senses light 22 Phototropism Experiments 23 Control of Growth and Responses in Plants Peter Boysen-Jensen (1913) 1. Removed tips of shoots 2. Placed gelatin on stump 3. Replaced tip on top of gelatin: a. Shoots bent towards light 4. Put piece of impermeable mica between shoot and tip: a. No phototropic response 5. Concluded that some mobile chemical is responsible for the phototropic response 24 Phototropism Experiments 25 A. Paal (1918) Control of Growth and Responses in Plants 1. Removed tips of shoots 2. Put shoots in the dark 3. Replaced tips back on stumps but put them off-center on stumps 4. Tip placed on right side: a. Shoots bent towards left 5. Tip placed on left side: a. Shoots bent towards right 6. Suggested tip produces chemical that moves down shoot & causes cells below it to grow a. Light must alter its amount 26 Phototropism Experiments 27 Fritz Went (1926) Control of Growth and Responses in Plants 1. Removed tips & placed them on blocks of agar for an hour 2. Put blocks of agar only on cut ends of stumps 3. If placed in center of stump: a. Shoots grew straight upward 4. If placed off-center of stump a. Shoots grew & bent to opposite side 5. Blank agar blocks didn’t grow a. Definitive evidence of a hormone He named the hormone auxin. 28 29 Demonstrating Phototropism Went’s Experiment Phototropism 30 How Auxins Work Control of Growth and Responses in Plants 31 When a stem is exposed to unidirectional light, auxin moves to the shady side. Auxins bind to plasma membrane receptors which leads to a series of reactions & the generation of at least three specific second messengers: 1. Activates a proton, H+, pump Acidic conditions cause cell wall to loosen Cellulose fibrils are weakened 2. Activates Golgi apparatus Sends out vesicles laden with cell wall materials How Auxins Work Control of Growth and Responses in Plants 32 3. Stimulates DNA-binding protein Activates a particular gene Leads to production of growth factors Cell walls become extensible & then fill with water by osmosis. Turgor pressure increases due to the entry of water & the cell elongates. This occurs on side opposite to the light so the stem lengthens on shady side causing a bending toward the light. Auxin Mode of Action 33 34 mm Control of Growth and Responses in Plants 35 Gibberellins Growth promoting hormones Bring about internode elongation of stem cells Gibberellic acid (GA3) = most common - Stem elongation - Can cause dwarf plants to grow huge Sources of gibberellin: - Young leaves, roots, embryos, seeds & fruits Commercial uses: - Break dormancy of buds & seeds, induce flowering, increase size of flowers, produce larger seedless grapes 36 Effect of Gibberellins Treated No treatment Control of Growth and Responses in Plants 37 Gibberellins How GA3 acts as a chemical messenger: Embryo produces gibberellins Amylase, enzyme that breaks down starch, appears in cells just inside seed coat GA3 is the first messenger - Attaches to receptor in plasma membrane Second messenger, calcium ions, combines with a DNA-binding protein - Believed to activate the gene that codes for amylase. This acts on starch to release sugars used as source of energy for growing embryo Gibberellic Acid: Structure and Mode of Action 38 Control of Growth and Responses in Plants 39 Cytokinins Cytokinins A class of plant hormones that promote cell division (cytokinesis) First isolated in 1967 from corn = zeatin Produced in dividing tissues or roots & in seeds & fruits - Promotes cell division - Prevents senescence (Aging process. Leaves die and fall off) - Initiates leaf growth. Lateral buds will grow when cytokinin is applied to them. Control of Growth and Responses in Plants 40 Abscisic Acid Abscisic acid (ABA): (aka stress hormone) Initiates and maintains seed and bud dormancy Brings about closure of stomata Dormancy occurs when a plant readies itself for adverse conditions by stopping growth - ABA moves from leaves to vegetative buds in fall - Buds are converted to winter buds which get covered by thick, hardened scales - In spring, reduction in level of ABA & increases in gibberellins break seed and bud dormancy. Produced by: - Any “green tissue” with chloroplasts - Monocot endosperm, and - Roots Abscisic Acid: Control of Stoma Opening ABA binding leads to influx of Ca2+ & the opening of K+ channels. Water exits guard cells & stoma closes. 41 Control of Growth and Responses in Plants 42 Ethylene Ethylene is involved in abscission, the dropping of leaves, fruits & flowers from a plant Once abscission has begun: - Ethylene stimulates certain enzymes like cellulase - Causes leaves, fruits, or flowers to drop Also ripens fruit by increasing activity of enzymes that soften fruit Uses in agriculture: - To hasten ripening of green fruits - To create pleasing colors before sales Ethylene is a gas that can induce ripening of nearby fruits Functions of Ethylene 43 Control of Growth and Responses in Plants Photoperiodism Photoperiodism is any physiological response prompted by changes in day or night length Photoperiod 1. The relative lengths of day and night 2. This changes with the seasons 3. Flowering, germination & dormancy all occur at specific times of year 4. Thus, photoperiod is the major environmental factor that needs to be measured by plants 44 Control of Growth and Responses in Plants 45 Photoperiodism Three Types of Plants: 1. Short-Day Plants (Long-night) a. Flower when days are short (fall, winter) b. Actually controlled by night length: Night length must be longer than a critical value. c. Continuity of darkness is what matters. A flash of light will disrupt flowering d. Examples: Chrysanthemums, poinsettias, rice, ragweed Photoperiodism and Flowering 46 Control of Growth and Responses in Plants 47 Photoperiodism 2. Long-Day Plants (Short-night) a. Flower when days are long (late spring, summer) b. Night must be shorter than a critical value c. A flash of light during the night can induce flowering during the wrong season d. Examples: spinach, wheat, lettuce, iris, petunia, mustard Photoperiodism and Flowering 48 Control of Growth and Responses in Plants Photoperiodism 3. Day-Neutral Plants a. Day length doesn’t matter b. Flower year round c. Examples: Roses, carnations, dandelions, sunflowers 49 Control of Growth and Responses in Plants Photoperiodism How is photoperiod detected? 1. Involves phytochrome, a light-absorbing pigment that exists in 2 inter-changeable forms: a. Pr strongly absorbs red light (660-680 nm) b. Pfr absorbs far-red light (700–730 nm) 2. When Pr absorbs red light it is converted quickly to Pfr 3. When Pfr absorbs far-red light it is converted slowly to Pr 50 Phytochrome Conversion Cycle happens quickly (happens slowly) 51 Control of Growth and Responses in Plants Photoperiodism 4. Pfr slowly reverts back to Pr in the dark 5. At sunset, far-red light is common a. So Pfr begins to convert to Pr b. This marks end of day; start of night. c. Pr accumulates slowly all night 6. At sunrise, red-light is common a. So Pr converts to Pfr relatively quickly b. This marks end of night; start of day. 52 Phytochrome Conversion Cycle happens quickly (happens slowly) 53