Chp 27 - Plant Growth & Response CO 27 Chp 27 - Plant Growth & Response

Plant Responses Organisms Respond to Stimuli One defining characteristic of life is an ability to respond to stimuli. Adaptive organisms respond to environmental stimuli because it leads to longevity and survival Animals have nerves and muscles; plants respond by growth patterns.

perception of the stimulus How A Plant Responds to Stimuli Pg 479 reaction by the plant to a stimulus perception of the stimulus the stimulus is changed into a form that is meaningful to the plant

Tropisms A tropism is plant growth toward or away from a directional stimulus. Direction is important; the stimulus comes from only one direction instead of many. Growth toward a stimulus is a positive tropism Growth away from a stimulus is a negative tropism By differential growth, one side elongates faster; result is curving toward or away from a stimulus.

Tropisms Three well-known tropisms are named for the stimulus that causes the response. Phototropism is growth of plants in response to light; stems show positive phototropism. Gravitropism is response to earth’s gravity; positive gravitropism – roots negative gravitropism – stems Thigmotropism is unequal growth due to touch (e.g., coiling of tendrils around a pole).

Tropisms Plants in Motion Website

Phototropism Phototropism occurs because cells on shady side of stems elongate. It is believed that a yellow pigment related to riboflavin acts as a photoreceptor for light. Following reception, plant hormone auxin migrates from bright side to shady side of a stem. How reception of stimulus causes the production of auxin is not yet known. Auxin is also involved in gravitropism, apical dominance, and root & seed development

Gravitropism (Geotropism) Negative gravitropism - plant grows upward, opposite gravity Charles Darwin and his son were first to show that roots display positive gravitropism. If root cap is removed, roots no longer respond to gravity. Later researchers showed root cap cells contain statoliths, starch grains within amyloplasts. Due to gravity, amyloplasts settle to lowest part of cell.

Gravitropism (Geotropism) The hormone auxin underlies both positive and negative gravitropisms. The two tissues respond differently to auxin, which moves to lower side of both stems and root. Auxin inhibits growth of root cells; cells of upper surface elongate and root curves downward. Auxin stimulates growth of stem cells; cells of lower surface elongate and stem curves upward.

Unequal growth due to contact with solid objects is thigmotropism Ex. Coiling of morning glory or pea tendrils around posts, etc., is a common example.

Cells in contact with an object grow less while those on opposite side elongate. *This process is quite rapid; tendrils have been observed to encircle an object in ten minutes. *Hormones auxin and ethylene are involved; they induce curvature of tendrils in absence of touch.

Thigmotropism Thigmomorphogenesis is a touch response involving the whole plant. Entire plant responds to presence of wind or rain. A plant growing in a windy location has a shorter, thicker trunk. Even rubbing of a plant inhibits cellular elongation and produces a shorter, sturdier plant.

Nastic Movements In contrast to tropisms, nastic movements are independent of the direction of stimulus. Seismonastic movements result from touch, shaking, or thermal stimulation. This response takes only a second or two, is due to a loss of turgor pressure within cells. Ex. Venus’s-flytrap - 3 sensitive hairs triggers an impulse-type stimulus. Turgor pressure in leaf cells propel the trap. Mechanisms are potassium ions that move out of the cell; water follows by osmosis

When a Mimosa pudica leaf is touched, leaflets fold because petiole droops. A pulvinus is a thickening at base where turgor pressure can rapidly drop.

Sleep Movements Sleep movements are nastic responses to daily changes in light level. Movement is due to changes in turgor pressure of motor cells in a pulvinus Circadian rhythm is a biological rhythm with a 24-hour cycle. Biological clock is an internal mechanism maintaining biological rhythms in absence of stimuli. Synchronized by external stimuli to 24-hour rhythms. Photoperiod is more reliable an indicator of seasonal changes than temperature change

Prayer Plant – Before dark and after dark when its leaves fold up

Plant Hormones For plants to respond to stimuli, activities of plant cells and structures have to be coordinated. Most plant communication is done by hormones Hormones are low concentration chemical messengers active in another part of an organism. A response is influenced by several hormones; requires a specific ratio of two or more hormones. Hormones are synthesized in one part of a plant; they travel in phloem after plant receives appropriate stimulus.

Plant Hormones Growth Promoter *(Florigen – May induce flowering) Auxins – Promotes cell growth Giberellins - Promotes cell growth Cytokinetins – Stimulate Cell Division Growth Inhibitors Abscisic Acid – Growth Inhibitor Ethylene – Promotes ripening of fruit *(Florigen – May induce flowering)

Auxins Indoleacetic acid (IAA) is most common naturally-occurring auxin. Produced in shoot & root apical meristem & found in young leaves, flowers, and fruits. Promotes Cellular Elongation Promotes softening of cell wall Involved in Phototropism, Gravitropism, Apical dominance - prevents growth of axillary buds Roots to develop from ends of cuttings. promotes growth of fruit.

Auxin in the apical bud inhibits lateral bud development & plant exhibits apical dominance When the apical bud is removed, lateral branches develop.

Stem cutting placed in a solution of IAA Stem cutting placed in pure water

Auxins Auxin-controlled cell elongation is involved in gravitropism & phototropism. When gravity is perceived, auxin moves to lower surface of roots and stems. Darwin discovered with oat seedlings, phototropism would not occur if tip of a seedling is cut off or covered by a cap; they concluded cause of curvature moved from coleoptile tip to rest of shoot.

What is the significance of this experiment? Frits W. Went experimented with coleoptiles in 1926. He cut off tips and placed them on agar. Auxin diffuses into agar block. Auxin’s Animation Agar block was placed to one side; coleoptile would curve away from that side regardless of light. He deduced a chemical caused curved growth and named it auxin after Greek word for “to grow” What is the significance of this experiment?

Auxin binds to receptors and activates the ATP-driven proton (H+) pump Cellulose fibrils are weakened & activated enzymes further degrade the cell wall As hydrogen ions are pumped out of the cell, cell wall becomes acidic, breaking hydrogen bonds

Turgid cell presses against cell wall, stretching it so that elongation occurs Electrochemical gradient established causes of uptake of solutes; water follows by osmosis

Figure 27.8

Gibberellins Gibberellins are a group of 70 plant hormones that chemically differ. GA3 (gibberellic acid) is the most common of natural gibberellins. Gibberellins: Produced in young roots, stems & leaves growth promoters that elongate cells. Produces bolting (rice) Stimulates production of starch digestion enzymes Release buds from apical dominance Promotes fruit development & seed germination

Gibberellins are often used to promote stem elongation Plant on the right treated with Gibberellins

Gibberellins Mode of action Hormone GA3 binds to a receptor; a second messenger (Ca2+) inside cell combines with the protein calmodulin. Ca2+-calmodulin complex activates a gene coding for the enzyme amylase. Amylase acts on starch to release sugars used as a source of energy by the growing embryo.

Reception Transduction Response

Ca2+-calmodulin complex activates a gene coding for the enzyme amylase. Amylase acts on starch to release sugars used as a source of energy by the growing embryo. Hormone GA3 binds to a receptor; this opens up Ca2+ channels. Ca2+ acts as a second messenger inside cell & combines with the protein calmodulin.

Calmodulin may act as some sort of transcription factor … causing the production of Amylase that breaks down starch causing a breakage of seed dormancy.

Cytokinins Cytokinins are a class of plant hormones that promote cell division (cytokinesis) Cytokinins are derivatives of purine base adenine zeatin - a natural cytokinin kinetin - a synthetic cytokinin. Produced in roots (& elsewhere) Promotes growth in size of leaves Stimulates cell division Works with auxin to control cell differentiation - Releases buds from apical dominance

Senescence Aging processes are senescence; large molecules break down and transported elsewhere. Cytokinins prevent senescence of leaves; they also initiate development of leaf growth. Cytokinins initiate growth of lateral buds despite apical dominance.

Inhibitory Hormones Abscisic acid (ABA) - Growth inhibitor “Stress hormone”; it maintains seed and bud dormancy (plant organ readies itself for adverse conditions by stopping growth) Promotes closure of stomates. Promotes seed and bud dormancy Promotes resistence to water stress Formerly thought to promote abscission

The hormone abscisic acid (ABA) is believed to bring about the closing of the stomates when water stress occurs.

Inhibitory Hormones Ethylene - A gaseous plant hormone Promotes the ripening of fruit by activity of enzymes that soften fruit. Involved in leaf abscission (the dropping of leaves, fruits, or flowers) Lower levels of auxin in these areas (compared to stem) probably initiate abscission. Once abscission begins, ethylene stimulates production of enzymes such as cellulase that cause leaf, fruit, or flower drop Working with auxin, inhibits elongatin of roots, stems and leaves

“A Rotten Apple Spoils the Whole Bunch” In the presence of an ethylene producing ripe apple abscission of the holly leafs occur. “A Rotten Apple Spoils the Whole Bunch” No abscission when a holly twig is placed under glass jar for a week

Photoperiodism Many physiological changes in plants are related to a seasonal change in day length. seed germination the breaking of bud dormancy flowering onset of senescence

Phytochrome and Plant Flowering U.S.D.A. scientists discovered phytochrome, a blue-green leaf pigment that exists in two forms. Pr=(phytochrome red) absorbs red light (wavelength of 660 m); it is converted to Pfr. Pfr=(phytochrome far-red) absorbs far-red light (wavelength of 730 m); it is converted to Pr.

Phytochrome and Plant Flowering Pfr appears to reset the circadian-rhythm clock Pr is the form of phytochrome synthesized in plant cells Pr & Pfr are in equilibrium during daylight Pr accumulates at night At daybreak, light rapidly converts Pr to Pfr Night length is responsible for resseting the circadian-rhythm clock

The inactive form Pr is prevalent during the night The inactive form Pr is prevalent during the night. At sunset or in the shade, when there is more fare-red light Pfr is converted to Pr. Pfr, the active form of of phytochrome, is prevalent during the day because at that time there is more red light than far-red light. Also, during the night, metabolic processes cause Pfr to be converted to Pr.

Other Functions of Phytochrome The Pr ® Pfr conversion cycle controls other growth functions in plants. Pfr promotes seed germination and stem branching. Following germination, presence of Pr dominates; stem elongates and grows toward sunlight while leaves remain small

Grown in shade (Far-red light) Grown in sunlight (red light) Grown in shade (Far-red light)

Photoperiodism Animation #1 Photoperiodism is a physiological response to relative lengths of daylight and darkness (called a photoperiod) Plants can be divided into 3 groups, based on photoperiodism. Short-day Plants Long-day Plants Day-neutral Plants Animation #1

Photoperiodism & Flowering

Photoperiodism Short-day Plants These flower when day length was shorter than a critical length. In effect, they require a period of darkness that is longer than a critical length to flower. Ex. cocklebur, poinsettia, and chrysanthemum.

Photoperiodism Long-day Plants These flower when the day length is longer than a critical length. In effect, they require a period of darkness that is shorter than a critical length to flower. Ex. wheat, barley, clover, and spinach.

Photoperiodism Day-neutral Plants These are plants for which flowering is not dependent on day length. Ex. include tomato and cucumber.

Photoperiodism Animation

Photoperiodism In 1938, K. C. Hammer and J. Bonner experimented with artificial lengths of dark and light periods. Cocklebur, a short-day plant, flowers as long as the dark period lasts over 8.5 hours. If dark period is interrupted by a flash, it does not flower; darkness amidst day cycle has no effect. Long-day plants require a dark period shorter than a critical length regardless of length of light period. Therefore, length of the dark period controls flowering, not length of the light period.