RESPONSES TO THE ENVIRONMENT

I. Main Environmental Stimuli affecting Plant Development

     A.  Light (Intensity, Color, Duration, Periodicity)

     B.  Temperature (Absolute and Periodicity)

     C.  Gravity

     D.  Nutrients

     E.  Mechanical Forces

II. Tropisms = permanent, directed movements of plant parts brought 
               about by asymmetric growth which occur in some     
               constant relationship to an external stimulus

     A. Phototropism = external stimulus = light

          1. Positive = toward light
          2. Negative = away from light
          3. Blue light most effective (Flavoprotein?)
          4. In asymmetric light 
               -> more auxin on dark side of plant 
               -> more growth on this side
                -> bending toward light
               a. light destroys auxin?
               b. light induces auxin transport to dark side?

     B. Geotropism = external stimulus = gravity

          1. Positive = toward center of earth (roots)
          2. Negative = away from center of earth (stem)
          3. In asymmetric orientation in gravitational field
               -> more auxin on "down" side
               -> high auxin promotes cell expansion in stems
                    ->> more growth on lower side
                    ->> stem bends upward
               -> high auxin inhibits cell expansion in roots
                    ->> more growth on upper side
                    ->> root bends downward
               -> more auxin on lower

III.  Nastic movements = movements of plant parts brought about by
     asymmetric growth or asymmetric changes in turgor pressure
     which exhibit no directional relationship to an external
     stimulus

     A. Epinasty = bending down of lateral appendages (leaves,
                    flower parts)

          1. High auxin induces ethylene which inhibits cell
               expansion on lower side

     B. Hyponasty = bending upward of lateral appendages

          1. Inducible by exogenous gibberellin

     C. Nyctinasty = rhythmic change in lateral appendage position
                    in response to day/night cycle

          1. Not solely explained by presence or absence of light
          2. Circadian rhythms = periodic movements of plant parts
               that have periods of ca. 24 hrs.
          3. Mechanism and/or external stimulus remains
               unidentified at present

     D. Seismonasty (touch), Thermonasty (heat)

          1. Relatively quick responses induced by asymmetric
               changes in turgor pressure
          2. Insectivorous plants, Sensitive plants

IV. Photoperiodic Responses = growth responses induced by day/night
     variations in light

  A. Induction of reproductive transition of SAM (flowering)

     1.  Day Neutral Plants = flower when plant reaches a certain
          age.

     2.  Short Day Plants = flower when day length shorter than
          some critical length (actually night length becomes
          longer than some critical length)

     3.  Long Day Plants = flower when day length is longer than
          some critical length (night length is shorter than some
          critical length)

  B.  Seeds released from dormancy

  C.  Leaf senescence

  D.  Frost hardening

  E. Phytochrome pigment allows plants to measure length of
     photoperiod

     1. Two forms of phytochrome
          a. Pr absorbs red light (660 nm)
          b. Pfr absorbs far red light (730 nm)

     2. Pr is converted to Pfr in red light (day)

     3. Pfr is converted to Pr in far red light (experimental)
        Pfr is converted to Pr in dark (dark decay)

     4. Amount of Pfr appears to influence several unknown        
        intermediaries which elicit photoperiodic responses