CHAPTER 39 by ewghwehws

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									CHAPTER 39

     CONTROL
      SYSTEMS
     IN PLANTS
      I. PLANT HORMONES
A.   PLANT HORMONES HELP COORDINATE
     GROWTH, DEVELOPMENT, AND RESPONSES
     TO ENVIRONMENTAL STIMULI
•    THE SITE OF ACTION, THE
     DEVELOPMENTAL STAGE OF THE
     PLANT, THE CONCENTRATION OF THE
     HORMONE, AND THE PRESENCE OF
     OTHER HORMONES ALL AFFECT
     REACTION TO A HORMONE.
•    EACH HORMONE IS PRODUCED
     PRIMARILY IN THE APICAL MERISTEM OF
     THE SHOOT.
          LIST OF PLANT HORMONES


• AUXIN: SIMULATES CELL ELONGATION IN
  DIFFERENT TARGET TISSUES.
• CYTOKININS: PRODUCED IN ACTIVELY
  GROWING TISSUES SUCH AS ROOTS, EMBRYOS,
  AND FRUITS, STIMULATE CELL DIVISION.
• GIBBERELLINS: PRODUCED IN ROOTS AND
  YOUNG LEAVES STIMULATE GROWTH IN LEAVES
  AND STEMS.
• ABSCISIC ACID: SLOWS PLANT GROWTH AND
  FAVORS THE DORMANT STATE
• ETHYLENE: HELPS CONTROL FRUIT RIPENING
  AND SENESCENCE OF PLANT CELLS AND
  ORGANS.
  II. PLANT MOVEMENTS AS MODELS FOR
        STUDYING CONTROL SYSTEMS


A. TROPISMS ORIENT THE GROWTH OF
   PLANT
• THESE GROWTH RESPONSES INCLUDE:
  1. PHOTOTROPISM: GROWTH RESPONSE
     TO LIGHT
  2. GRAVITROPISM: GROWTH RESPONSE
     TO GRAVITY
  3. THIGMOTROPISM: GROWTH
     RESPONSES TO TOUCH
      PHOTOTROPISM (A CLOSER LOOK)


•   HORMONE: AUXIN
•   STEPS:
    1. APICAL MERISTEM PRODUCES AUXIN
    2. AUXIN MOVES TO ZONE OF
       ELONGATION
    3. STIMULATION GENERATES GROWTH
GRAVITROPISM (A CLOSER LOOK)

• HORMONES: AUXIN AND
  GIBERRELLINS
• STEPS (NOT WELL DOCUMENTED):
  SCIENCE AS A PROCESS
• SEE EARLY EXPERIMENTS ON
  PHOTOTROPISM (FIG. 39.2)
III. CONTROL OF DAILY AND
    SEASONAL RESPONSES
A. BIOLOGICAL CLOCKS CONTROL
   CIRCADIAN RHYTHMS
• FREE-RUNNING CIRCADIAN
   CYCLES ARE APPROXIMATELY 24
   HOURS LONG BUT ARE ENTRAINED
   TO EXACTLY 24 HOURS BY THE
   DAY/NIGHT CYCLE.
B. PHOTOPERIODISM SYNCHRONIZES
MANY PLANT RESPONSES TO CHANGES
           OF SEASON
• PHOTOPERIODISM: RESPONSE OF
  PLANTS TO CHANGES IN
  PHOTOPERIODS
• CIRCADIAN RHYTHM: AN INTERNAL
  CLOCK THAT MEASURES THE
  LENGTH OF DAY AND NIGHT
• PHYTOCHROMES: LIGHT ABSORBING
  PIGMENTS FOUND IN PLANTS
     A CLOSER LOOK
• Pfr: RESETS THE CIRCADIAN CLOCK
• Pr: ACTIVATED IN LEAVES
• SPECIAL NOTES
  – PHYTOCHROMES ARE IN EQUILLIBRIUM
    DURING DAYLIGHT
  – Pr IS ACCUMULATED AT NIGHT
  – SUNLIGHT CONVERTS Pr BACK TO Pfr TO
    REACH EQUILLIBRIUM ONCE AGAIN
  – NIGHTLENGTH RESETS THE CIRCADIAN-
    RHYTHM CLOCK
• SOME DEVELOPMENTAL PROCESSES,
  INCLUDING FLOWERING IN MANY PLANT
  SPECIES, REQUIRE A CERTAIN
  PHOTOPERIOD.
• THE RELATIVE LENGTHS OF NIGHT AND
  DAY.
  – FOR EXAMPLE, A CRITICAL NIGHT LENGTH
    SETS A MINIMUM (IN SHORT-DAY PLANTS) OR
    MAXIMUM (IN LONG-DAY PLANTS) NUMBER OF
    HOURS OF DARKNESS REQUIRED FOR
    FLOWERING.
     IV. PHYTOCHROMES
A.    PHYTOCHROMES FUNCTION AS
     PHOTORECEPTORS IN MANY PLANT
     RESPONSES TO LIGHT AND
     PHOTOPERIOD
•    PHYTOCHROMES EXIST IN TWO
     PHOTOREVERSIBLE STATES, WITH
     CONVERSION OF Pr TO Pfr,
     TRIGGERING MANY DEVELOPMENTAL
     RESPONSES.
B PHYTOCHROMES MAY HELP ENTRAIN THE
         BIOLOGICAL CLOCK


• PHYTOCHROME CONVERSION
  MARKS SUNRISE AND SUNSET,
  PROVIDING THE CLOCK WITH
  ENVIRONMENTAL CUES.

								
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