38.1. Plant Responses to Stimuli (p. 676)
A. Organisms Respond to Stimuli
1. One of the defining characteristics of living organisms is their ability to respond to environmental stimuli.
2. Adaptive organisms respond to environmental stimuli because it leads to longevity and survival of the species.
3. Plants utilize a reception-transduction-response pathway when they respond to a stimulus.
4. Animals have nerves and muscles; plants respond by tropisms, nastic movements, and thigmomorphogenesis.
B. Plants Respond by Growing
1. A tropism is a growth response toward or away from a directional stimulus.
a. Directional is important in tropisms; the stimulus is coming from only one direction instead of many.
b. Growth toward a stimulus is a positive tropism and growth away from a stimulus is a negative tropism.
c. Due to differential growth, one side elongates faster, and the result is curving toward or away from a stimulus.
2. Three well-known tropisms are named for the stimulus that causes the response.
a. A phototropism is directional growth of plants in response to light; stems show positive phototropism.
b. Gravitropism is a response to earth's gravity; roots demonstrate positive gravitropism (Fig. 38.2b) and stems demonstrate negative gravitropism. (Fig. 38.2a)
c. Thigmotropism is an unequal growth due to touch (e.g., the coiling of tendrils around a pole). (Fig. 38.3)
C. Plants Respond to Light
1. Early researchers, including Charles Darwin, observed plants curve toward light.
2. Phototropism occurs because cells on the shady side of stems elongate.
3. It is believed that a yellow pigment related to riboflavin acts as a photoreceptor for light.
a. Following reception, the plant hormone auxin migrates from the bright side to the shady side of the stem.
b. It is not yet known how reception of stimulus is coupled to production of auxin (how transduction occurs).
4. Auxin is also involved in gravitropism, apical dominance, and root and seed development.
D. Plants Respond to Gravity
1. An upright plant placed on its side displays negative gravitropism when it grows upward opposite gravity.
2. Charles Darwin and his son were first to show this and that roots display positive gravitropism.
a. If the root cap is removed, roots no longer respond to gravity.
b. Later researchers showed the root cap contains statoliths, starch grains within amyloplasts.
c. Due to gravity, the amyloplasts settle to the lowest part of the cell. (Fig. 38.2c) [micro. slide 78]
3. The hormone auxin provides for both positive and negative gravitropisms.
a. The two types of tissues respond differently to auxin, which moves to lower side of both stems and root.
b. Auxin inhibits the growth of root cells; the cells of the upper surface elongate and root curves downward.
c. Auxin stimulates growth of stem cells; the cells of the lower surface elongate and the stem curves upward.
E. Plants Respond to Contact
1. Unequal growth due to contact with solid objects is thigmotropism.
2. The coiling of morning glory or pea tendrils around posts, etc. is a common example.
3. The cells in contact with an object grow less while those on the opposite side elongate.
4. This process is quite rapid; a tendril has been observed to encircle an object in ten minutes.
5. The response endures; a couple of minutes of stroking can bring about a response that lasts for several days.
6. The response can be delayed; tendrils touched in the dark will respond once they are illuminated.
a. ATP rather than light can cause the response; the need for light may be a need for ATP.
b. Hormones auxin and ethylene may be involved; they induce curvature of tendrils in the absence of touch.
7. Thigmomorphogenesis is a touch response involving the whole plant.
a. The entire plant responds to the presence of wind or rain.
b. A plant growing in a windy location has a shorter, thicker trunk.
c. Even simple rubbing of a plant can inhibit cellular elongation and produce a shorter, sturdier plant.
F. Plants Respond by Turgor Pressure Changes
1. In contrast to tropisms, nastic movements are independent of the direction of the stimulus.
2. Seismonastic movements result from touch, shaking, or thermal stimulation.
3. This response, which takes only a second or two, is due to a loss of turgor pressure within cells of some organ.
4. If you touch a Mimosa pudica leaf, the leaflets fold because the petiole droops. (Fig. 38.4)
5. A pulvinus is a thickening at the base of such leaflets where the turgor pressure can rapidly drop.
6. The mechanisms are potassium ions that move out of the cell; water follows by osmosis.
7. A single stimulus such as a hot needle can cause leaves to respond.
8. A Venus's-flytrap has three sensitive hairs at the base of the trap; if they are touched by an insect, an impulse-type stimulus triggers the trap to close; turgor pressure in the leaf cells propel the trap.
G. Some Plants Have Sleep Movements
1. Sleep movements are nastic responses that occur daily in response to daily changes in light level.
(Fig. 38.5)
2. The movement is due to changes in turgor pressure of motor cells in a pulvinus.
3. Some plant movements are periodic, corresponding to environmental changes in light, temperature, etc.
4. A circadian rhythm is a biological rhythm with a 24-hour cycle.
5. A biological clock is an internal mechanism maintaining biological rhythms in absence of environmental stimuli.
6. Biological clocks are synchronized by external stimuli to twenty-four-hour rhythms; photoperiod is more reliable an indicator of seasonal changes than temperature change.
7. Stomates and flowers usually open in morning and close at night; some plants secrete nectar at same time of day.
38.2. Plant Hormones Coordinate Responses (p. 680)
A. In order for plants to respond to stimuli, the activities of plant cells and structures have to be coordinated.
1. Almost all communication in a plant is done by hormones.
2. Hormones are chemical messengers produced in low concentration and active in another part of an organism.
3. Responses are influenced by several hormones and likely require a specific ratio of two or more hormones.
4. Hormones are synthesized in regions of transduction; they travel after reception of the appropriate stimulus.
5. Each naturally occurring hormone has a specific chemical structure.
6. Other chemicals that differ only slightly from the natural hormones also affect the growth of plants.
7. Plant growth regulators are hormone imitators and naturally occurring hormones that regulate plant growth.
B. Auxin and Its Many Effects
1. Indoleacetic acid (IAA) is the most common naturally occurring auxin.
a. It is produced in shoot apical meristem and is found in young leaves and in flowers and fruits.
b. Apically produced IAA prevents the growth of axillary buds; this provides for apical dominance.
c. When a terminal bud is removed, the nearest buds begin to grow, and the plant branches.
(Fig. 38.6)
d. The application of a weak solution of auxin will cause roots to develop from the ends of cuttings.
e. Auxin production by seeds also promotes the growth of fruit.
f. As long as auxin is concentrated in leaves and fruits rather than in the stem, leaves and fruits do not fall off.
2. Auxin-controlled cell elongation is involved in gravitropism and phototropism.
a. After gravity has been perceived, auxin moves to the lower surface of roots and stems. (Fig. 38.2)
b. Early work by the Darwins with oat seedlings revealed phototropism would not occur if the tip of a seedling is cut off or covered by a cap; they concluded the cause of curvature is moved from the coleoptile tip to rest of the shoot.
3. Frits W. Went experimented with coleoptiles in 1926. (Fig. 38.7)
a. He cut off tips and placed them on agar.
b. If an agar block was placed to one side, the coleoptile would curve away from that side regardless of light.
c. He concluded a chemical caused curved growth and named it auxin, after the Greek word for "to grow."
C. How Auxin Works
1. When a plant is exposed to unidirectional light, auxin moves laterally from bright side to shady side of a stem.
2. There it binds to receptors and activates the ATP-driven proton (H+) pump. (Fig. 38.8) [transp. 202]
3. As hydrogen ions are pumped out of the cell, the cell wall becomes acidic, breaking hydrogen bonds.
4. Cellulose fibrils are weakened and activated enzymes further degrade the cell wall.
5. The electrochemical gradient established causes the uptake of solutes and an increase in turgor pressure.
6. The turgid cell presses against the cell wall, stretching it so that elongation occurs. (Fig. 38.8)
[transp. 202]
7. Auxin-mediated elongation is observed in younger cells; perhaps older cells lack auxin receptors.
D. Gibberellins and Stem Elongation
1. Gibberellins are growth promoters that bring about elongation of resulting cells.
2. Gibberellins are a group of about 70 plant hormones that chemically differ only slightly.
3. GA3 is the most common of the natural gibberellins.
4. Gibberellins were discovered in 1926 by Ewiti Kurosawa, a Japanese scientist investigating a fungal disease of rice plants called "foolish seedling disease."
a. He found a fungus infecting plants produced an excess of a chemical named gibberellin, after the fungus name.
b. In 1956, gibberellic acid was finally isolated from a flowering plant rather than a fungus.
5. Mode of action (Fig. 38.10) [transp. 203]
a. The hormone GA3 binds to a receptor; a second messenger (Ca2+) inside cell activates a protein (calmodulin).
b. The Ca2+-calmodulin complex triggers expression of a gene that codes for production of the enzyme, amylase.
c. Amylase acts on starch to release sugars used as a source of energy by the growing embryo.
E. Cytokinins and Cell Division
1. The cytokinins are a class of plant hormones that promote cell division.
2. Cytokinins are derivatives of the purine adenine.
3. A natural cytokinin is zeatin, which is found in corn kernels.
4. Researchers discovered cytokinins in work on growing plant tissues in culture. (Fig. 38.11)
5. Oligosaccharins (chemical fragments released from the cell wall) are also effective in directing differentiation.
6. Researchers hypothesize that auxin and cytokinins are part of a reception-transduction-response pathway that leads to the activation of enzymes that release these fragments from the cell wall. (Fig. 38.10) [transp. 203]
F. Cytokinins Affect Leaves
1. Aging processes are senescence; large molecules are broken down and transported to other parts of the plant.
2. Cytokinins prevent the senescence of leaves; they also initiate the development of leaf growth.
3. When applied to lateral buds, cytokinins can initiate growth, despite apical dominance.
4. Interaction is shown by how varying ratios of auxin and cytokinins differentiate plant tissues.
(Fig. 38.11)
G. Plant Hormones That Inhibit
1. Abscisic acid (ABA) is called "stress hormone" because it initiates and maintains seed and bud dormancy and brings about the closure of stomates.
2. Dormancy occurs when a plant organ readies itself for adverse conditions by stopping growth.
a. For example, it is thought that ABA moves from leaves to vegetative buds in the fall, and thereafter these buds are converted to winter buds, which are covered by thick and hardened scales.
b. A reduction in the level of ABA and an increase in the level of gibberellins are thought to break seed and bud dormancy, resulting in seed germination and buds sending forth leaves.
3. Abscisic acid brings about the closing of stomates when a plant is under water stress. (Fig. 38.12)
a. By some unknown mechanism, ABA causes K+ ions to leave guard cells.
b. As a result, the guard cells lose water and the stomates close.
4. Although the external application of abscissic acid promotes abscision, this hormone is not believed to function in this process; the hormone ethylene is considered to have this natural function.
H. Ethylene is for Fruit Ripening
1. It was an early practice to prepare citrus fruit for market by storage in a room with a kerosene stove.
2. Later work revealed incomplete combustion of kerosene produced ethylene, which ripens fruit.
3. Ethylene is a gaseous plant hormone that ripens fruit by increasing the activity of enzymes that soften fruit.
4. Ethylene stimulates the production of cellulase, an enzyme that hydrolyses cellulose in plant cell walls.
5. A barrel of ripening apples can induce ripening of a bunch of bananas some distance away.
6. Ethylene is released from the site of a wound due to physical damage, which is why one rotten apple spoils the whole bunch.
7. The presence of ethylene in the air inhibits the growth of plants in general.
8. Ethylene is present in auto exhaust and in homes heated with natural gas.
9. Inhibition of plant growth occurs in low concentrations (one part of ethylene per 10 million parts of air).
10. Ethylene is involved in abscission, the dropping of leaves, fruits, or flowers from a plant.
a. Lower levels of auxin in these areas (compared to the stem) probably initiate abscission.
(Fig. 38.13)
b. Once abscission has begun, ethylene stimulates production of enzymes that cause leaf, fruit, or flower drop.
38.3. The Photoperiod Controls Seasonal Changes (p. 688)
A. Many physiological changes in plants (e.g. seed germination, the breaking of bud dormancy, flowering, and the onset of senescence) are related to a seasonal change in day length.
1. Photoperiodism is the physiological response to the relative lengths of daylight and darkness of this mechanism.
2. Work by the U.S. Department of Agriculture in controlled greenhouses led to understanding of this mechanism.
B. Plants can be divided into three groups, based on photoperiodism.
1. Short-day plants flower when the day length is shorter than a critical length (e.g., cockle-bur, poinsettia, and chrysanthemum); in effect, they require a period of darkness that is longer than a critical length to flower.
2. Long-day plants flower when the day length is longer than a critical length (e.g., wheat, barley, clover, and spinach); in effect, they require a period of darkness that is shorter than a critical length to flower.
3. Day-neutral plants are plants for which flowering is not dependent on day length (e.g., tomato and cucumber).
C. A long-day and a short-day plant can have the same critical length. (Fig. 38.14) [transp. 204]
1. These plants differ, in terms of their photoperiodism, in specific sequence of day lengths as a season progresses.
2. Spinach is a long-day plant that flowers in the summer when the day length increases to 14 hours.
3. Ragweed is a short-day plant that flowers in the fall when the day length shortens to 14 hours or less.
4. In 1938, K. C. Hammer and J. Bonner experimented with artificial lengths of dark and light periods.
a. The cocklebur, a short-day plant, flowers as long as the dark period lasts over 8.5 hours.
b. If a dark period is interrupted by a flash, it does not flower; however, darkness amid a day cycle has no effect.
c. Long-day plants require a dark period shorter than a critical length, regardless of the length of light period.
d. Therefore, the length of the dark period controls flowering, not the length of the light period.
D. Phytochrome and Flowering
1. U.S.D.A. scientists discovered phytochrome, a blue-green leaf pigment that alternately exists in two forms:
2. Pr (phytochrome red) absorbs red light (wavelength of 660 nm); it is converted to Pfr. (Fig. 38.15) [transp. 205]
3. Pfr is phytochrome far-red and absorbs far-red light (wavelength of 730 nm); it is converted to Pr.
4. During a 24-hour period, there is a shift in the ratio of these two pigments.
a. Direct sunlight contains more red than far-red light; therefore, Pfr is present in plant leaves during the day.
b. In shade and at sunset, there is more far-red than red light; therefore, Pfr is converted to Pr as night approaches.
c. There is also a slow metabolic replacement of Pfr by Pr during the night.
5. Phytochrome conversion may be a first step in reception-transduction-response pathway that results in flowering.
E. Phytochrome Has Other Functions
1. The Pr 
Pfr conversion cycle is now known to control other growth functions in plants.
2. In addition to being involved in the flowering process, Pfr promotes seed germination and stem branching.
3. Following germination, the presence of Pr dominates, the stem elongates and grows toward sunlight, while the leaves remain small. (Fig. 38.16)
4. Once the plant is exposed to sunlight and Pr is converted to Pfr, the plant begins to grow normally-the leaves expand and the stem branches.
5. The Pfr form of phytochrome triggers activation of one or more regulatory proteins in the cytosol.
6. These proteins migrate to nucleus and bind to "light-stimulated" genes coding for proteins found in chloroplasts.