BIO 104 4/18/96

What do plants perceive?
1) Gravity
     a. gravitropism
     b. geotropism
2) Light (phototropism)
     a. direction
     b. intensity
     c. photoperiod
3) Wind/Touch (thigmotropism)

Meristem -  cell division; Growth = cell expansion
Water enters the vacuole which creates pressure on the cell walls,
loosening of the cell wall enables the cell to expand.

Charles Darwin and Francis Darwin (1880's) Did the first experiments
with plant hormones.
4 experiments with Oat Coleoptiles (see page 610 Darwin's Key Experiment)

Experiment 1
     They cut off the tip of the coleoptile and
there was no bending of the plant towards the light.
(This indicated that the tip may regulate the
movement of the plant towards the light).




Experiment 2
     They put an opaque cap over the tip of the
coleoptile and the was no bending of the plant
towards the light.




Experiment 3
     They put a translucent cap over the tip of
the coleoptile and bending occurred towards the 
light.




Experiment 4
     They put a cover around the base of the 
coleoptile and bending towards the light occurred.

Conclusion:  There is something produced in the 
tip that produces bending in the coleoptile. SEE DARWIN'S DISCOVERY OF AUXIN
Page 610

It was later discovered (1946) that auxin (IAA) is what causes this bending.

In 1926 Fritz Went conducted a series of experiments -
He cut off the tip of a coleoptile and placed it on agar.  He then put the
agar on the plant whose tip had been cut off.  The plant grew straight and
did not bend.

He then cut off the tip of a coleoptile then placed the agar containing tip
material onto the plant, but off-center and in the DARK!  The plant bent to
the right, but the agar was placed on the left half of the coleoptile.  The
agar "substance" caused cell expansion of the left side of the coleoptile,
but not the right side, thus the plant bent to the right. SEE WENT'S
DEMONSTRATION OF AUXIN  on Page 611

Apical Dominance
     terminal buds
     lateral buds
     -a high concentration of auxin inhibits cell expansion
     -cut off terminal bud (apical meristem) and the lateral buds would begin
      to grow and produce their own side shoots

Plant Hormones:
1) Auxin (indole acetic acid)
     -isolated in 1946
     -allows for expansion, loosens cell walls
     -auxin is a regulator of growth
     -auxin moves away from light (This is why the Darwin's plants bent).
     The auxin moved away from the light thus expanding those cells making the
     plant bend towards the light.
     -responsible for fruit development
          -fertilized ovule give off auxin which stimulates growth of fruit.
          THUS: seedless fruit

2) Gibberellins
     -responsible for the germination of seeds
     -sprouting of buds
     -stem elongation
     -fruit development
     
     Barley Seed germination Example -
     1) embryo gives off gibberellic acid which effects the cells just under
        the seed coat and turns on the synthesis of the enzyme amylase.
     2) amylase breaks down starch and forms sugars which are taken up by the
        embryo for growth and energy. Starch breaks down to the disaccharide 
        maltose -used in the making of beer
     -auxin softens cell walls
     -gibberellic acid turns on protein synthesis - formation of amylase
     (enzyme)

3) Ethylene
     -simple hydrocarbon
     -ethylene gas works to promote fruit ripening
     
4) Abscisic Acid
     -causes the leaves to fall of trees
     -released during water stress - controls the pumping of potassium ions
      out of guard cells (closing stoma)

Flowering Plants
Three types of plants
     1) long-day plants
     2) short-day plants
     3) day-neutral plants
Long day plants flower in early summer, and they need short nights.
Short day plants flower in fall, and they need long nights.
Scientists working for the U.S. Agricultural Department came upon these
discoveries while trying to grow tobacco plants.  

If you flash a light in the middle of a long night period short day plants will
not bloom because they see it as two short nights, while long day plants will bloom.
This shows that plants measure the length of the night not the length of the
day.

Phytochrome:  this is what perceives light in the plant
     Red light absorbing form (Pr) absorbs light at 660 nm and converts it
     to Far red form (Pfr). Pfr absorbs light at 730 nm and converts it back
     to (Pr).  During the day the Pr is converted to Pfr. During the night the
     far red form is converted back to the red form.
So, if there is a flash of light during the night then the conversion of far
red to red light is stopped and reversed.  The conversion process is reset, so
the flowers are not able to bloom
     The ratio of red light to far red light appears to be the trigger for the
      flower to bloom.
     - See Figure 32.7

     This is a metabolic process.