Chapter 33: Plant Responses

33-1 Plant Hormones

33-2 Plant Movements

33-3 Seasonal Responses

Revisiting Evolution

• Plants have evolved a number of adaptations to their environment that help with their reproduction and survival.

Assessing Prior Knowledge

• Identify stem and leaf structures that may reflect adaptations to allowing movement.

• Differentiate between the locations of primary and secondary growth in a plant. Why might each be beneficial in its own way?

I. Groups of Hormones (transported via phloem)

• Hormones are chemical messengers that affect a plant’s ability to respond to its environment (e.g., sunlight, gravity, water, nutrients, and temperature).

33-1 Plant Hormones

• The effect of a hormone on a “target cell” is influenced by the hormone’s concentration and its interactions with other hormones.

(1) Growth Regulators• Hormones stimulate or inhibit plant growth, and are grouped into five categories: auxins, gibberellins, ethylene, cytokinins, and abscisic acid.

(A) Auxins (e.g., Indoleacetic Acid or IAA, regulate plant growth)• Hormones involved in plant-cell elongation, apical dominance, and abscission;

NOTE: Developing seeds in fruit naturally make IAA which stimulates the development of a fleshy fruit (removing seeds stops development).• Commercial Usage: Kill weeds, stimulate root growth, prevent sprouting of potato tubers, increase the size of fruit, and prevent fruit from falling prematurely.

(B) Synthetic Auxins (Napthaleneacidic Acid or NAA)• Artificially synthesized and are used to promote root formation on stem and leaf cuttings.

(1) Apical Dominance• The inhibition of lateral buds by the presence of a shoot tip (i.e., if NAA is applied to the cut tip of the stem, the lateral buds remain dormant)NOTE: A.D. keeps lateral buds in reserve. By keeping the lateral buds dormant, the stem can use MORE energy to grow taller, enhancing photosynthesis capabilities. (i.e., If the terminal bud is injured, the dormant lateral buds start growing.)

(2) Agent Orange (and 2,4-D) • Auxin used to defoliate jungles in the Vietnam War (NOTE: A non-auxin contaminant in Agent Orange has been cited as causing a number of health problems in exposed people)

(C) Gibberellins

• A class of hormones that stimulate ELONGATION and GROWTH (e.g., plants treated will usually cause the plant to grow to a larger than normal height).

• Commercial Usage: Increase the size of seedless grapes and stimulate seed germination.

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(1) Suppose a friend who lives in North Dakota gives you some seeds from a plant that you admired when you saw it in your friend’s backyard. You plant the seeds at your home in Georgia, but they fail to germinate. What do you suggest may be preventing the germination of the seeds?

Critical Thinking

(D) Ethylene (unlike other hormones, a GAS at room temperature) • Hormone responsible for the RIPENING of fruits. (i.e., one bad apple spoils the bunch)

• Commercial Usage: Ripen bananas and tomatoes, color ripe citrus fruits, promote the dropping of mechanically harvested fruits, and promote the flowering of pineapples.

(2) Suppose you placed a green banana in each of several plastic bags, and either placed a ripe pear or a rotting pear with each banana, sealing every bag. What would you hypothesize about the ripening rate of both classes of green bananas?

Critical Thinking

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(1) Ethephon (synthetic solution of ethylene)• Breaks down to release ethylene gas, used to ripen citrus fruits, bananas, melons, and tomatoes.

(2) Abscission (promoted by ethylene)• The detachment of leaves, flowers, or fruits from a plant (e.g., cherries and walnuts—harvested with mechanical tree shakers)

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(3) The seasonal loss of leaves by trees and shrubs serves the adaptive advantage of conserving nutrients. What other adaptive advantages might loss of leaves provide?

Critical Thinking

(E) Cytokinins

• Class of hormones that promote cell division in plants; produced in the developing shoots, roots, fruits, and seeds of a plant.NOTE: In tissue cultures, a HIGH auxin-to-cytokinin ratio promotes ROOT formation while a LOW ratio promotes SHOOT formation. In a whole plant, a high auxin-to-cytokinin ratio INHIBITS lateral bud growth, while a low ratio PROMOTES lateral bud growth.

(F) Abscisic Acid (ABA)

• Hormone that helps to bring about dormancy in a plant’s buds and maintains dormancy in its seeds.

• Also responsible for the closure of plant’s stomata in response to drought.

(G) Other Growth Regulators

• Synthetic and geared towards ornamental plants, not agriculture.(1) Growth Retardants

• Chemicals employed by utility companies which prevent plant growth (in trees), in order to prevent them from interfering with power lines.

• Potted flowers treated with GRs are kept more compact, resulting in stronger stems (easier to ship and resistant to wind damage).

NOTE: Re$earch Avenue? Chemical retardants for the growth of grasses? Mowing once or twice a summer? Golf course maintenance?

I. Tropisms (positive or negative)

• A plant movement that is determined by the direction of a specific environmental stimulus.

33-2 Plant Movement

EX: Auxin moves into the cells of the SHOOT TIP that are NOT directed toward the light, causing the cells to elongate, which causes the shoot to LEAN toward the light.

(A) Phototropisms (i.e. Heliotropism, initiated by auxins)• Plant movement in response to light coming from one direction.(1) Solar Tracking (maximizing photosynthesis)

• Motion of leaves or flowers as they follow the sun’s movement across the sky.

(B) Thigmotropism

• Plant’s growth response to touching a solid object. (e.g., tendrils and stems of vines coil when they touch an object; climbing movement)

(C) Gravitropism (auxin regulated)

• Plant’s growth response to gravity. (e.g., roots are POSITIVELY gravitropic while stems are NEGATIVELY gravitropic)

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(4) If a whole potato tuber is planted, only one or two buds at one end will sprout. However, if the potato is cut into pieces that each have a bud, all the buds will sprout. Explain why this may be.

Critical Thinking

(D) Chemotropism

• Plant’s growth in response to a chemical (e.g., the growth of a pollen tube in response to chemicals produced by the tube cell and ovule)

II. Nastic Movements (rapid changes in cell water pressure)• Plant movements that occur in response to environmental stimuli BUT that are INDEPENDENT of the direction of stimuli (unlike tropisms).

EX: Nastic movements allow carnivorous plants (i.e, Venus Fly Trap) to trap insects; Also allow “Sensitive Plants” to discourage insect predators.

• K+ ions move out of certain cells, causing osmotic movement of water to follow, forcing neighboring cells to shrink (e.g., leaf movements)

(A) Thigmonastic Movements

• Movement that occurs in response to TOUCHING or shaking a plant. (e.g., Venus Fly Trap and Sensitive Plant folding its leaves)

(B) Nyctinastic Movement (Linnaeus’s flower clocks)• Plant movements in response to the daily cycle of light and dark. (e.g., prayer plant, bean plant, honeylocust trees, and silk trees)

I. Photoperiodism (i.e., more accurately, Nyctoperiodism)• A plant’s response to changes in the night length (i.e., flowering requires a particular NIGHT LENGTH to begin)

33-3 Seasonal Responses

(5) The growth of most deciduous trees in the northern United States and Canada, where winters are severe, is regulated strictly by photoperiodism. (i.e., Temperature plays NO part in the regulation of their yearly growing cycle) Why might this phenomenon be crucial to the long-term survival of deciduous species?

Critical Thinking

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(A) Critical Night Length (night NOT day, regulates angiosperm flowering)• The uninterrupted period of darkness that causes a plant to flower (dependent on species)

(1) Short-Day (SD) Plants and Long-Day (LD) Plants• Angiosperms who flowers when the days are short (LONG NIGHTS) or vice-versa. (e.g., Chrysanthemums are SD)

(B) Responding to Day Length and Night Length• Angiosperms can be divided into three groups, depending on their response to the photoperiod (acts as a season indicator).(1) Day-Neutral Plants (DNPs)

• Are NOT affected by day length; include tomatoes, dandelions, and roses—flowering seasons are spring TO fall.• SDPs flower in the spring OR fall when the day length is SHORT; include ragweeds, poinsettias, and soybeans.

• LDPs flower in the SUMMER when the day length is LONG; include wheat, petunias, and radishes.

(C) Adjusting the Flowering Cycles of Plants• Flower cultivators who want to obtain winter flowering of LDPs simply expose them to a LOW level of incandescent LIGHT in the middle of the night.

NOTE: Summering flowering of SDPs is obtained by covering the plants in the late afternoon with an OPAQUE cloth so that the SDPs receive enough darkness.

(6) Potted poinsettias purchased for Christmas will often survive in people’s homes for many years but will rarely bloom again. Explain why this may be so.

Critical Thinking

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(D) Regulation by Phytochrome (two pigments, light and dark)• Plants monitor changes in day length with a bluish, light-sensitive pigment that is involved in bud dormancy and seed germination.

II. Vernalization (critical to biennial plants)• Low-temperature stimulation of flowering (e.g., farmers often use vernalization to grow and harvest their crops before a summer drought sets in)

(1) Biennial Plant (many species require vernalization)• Lives for two years, usually producing flowers and seeds during the SECOND year (e.g., carrots, beets, celery, and foxglove survive their first winter as short plants)

(2) Bolting• In the 2nd year’s Spring, the biennial plant’s flowering stem rapidly elongates to flower.

(A) Fall Colors (caused by a photoperiodic AND temperature response)• Chlorophyll production CEASES and DEGRADES, revealing carotenoids (orange carotenes & yellow xanthophylls) as well as anthocyanins (reds and purples produced in cool, sunny weather)