Chapter 33: Plant Responses

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Chapter 33 Plant Responses
33-1 Plant Hormones
33-2 Plant Movements
33-3 Seasonal Responses
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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?

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33-1 Plant Hormones
I. Groups of Hormones (transported via phloem)

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

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

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(1) Growth Regulators

• Hormones stimulate or inhibit plant growth, and
  are grouped into five categories auxins,
  gibberellins, ethylene, cytokinins, and abscisic
  acid.

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(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.

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(B) Synthetic Auxins (Napthaleneacidic Acid or
NAA)

• Artificially synthesized and are used to promote
  root formation on stem and leaf cuttings.

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(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.)
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(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)

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(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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Critical Thinking
(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 friends
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?
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(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.

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Critical Thinking
(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?
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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
  walnutsharvested with mechanical tree shakers)

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Critical Thinking
(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?
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(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.
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(F) Abscisic Acid (ABA)

• Hormone that helps to bring about dormancy in a
  plants buds and maintains dormancy in its seeds.
  
• Also responsible for the closure of plants
  stomata in response to drought.

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(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 Reearch Avenue? Chemical retardants for
  the growth of grasses? Mowing once or twice a
  summer? Golf course maintenance?

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33-2 Plant Movement
I. Tropisms (positive or negative)

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

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.
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(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
  suns movement across the sky.

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(B) Thigmotropism

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

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(C) Gravitropism (auxin regulated)

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

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Critical Thinking
(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.
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(D) Chemotropism

• Plants 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)

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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).

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

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

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(B) Nyctinastic Movement (Linnaeuss 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)

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33-3 Seasonal Responses
I. Photoperiodism (i.e., more accurately,
Nyctoperiodism)

• A plants response to changes in the night
  length (i.e., flowering requires a particular
  NIGHT LENGTH to begin)

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Critical Thinking
(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?
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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)

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(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 rosesflowering 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.

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(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.

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

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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)

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(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)

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(2) Bolting

• In the 2nd years Spring, the biennial plants
  flowering stem rapidly elongates to flower.

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(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)