tropisms

tropisms are tropic movements (directional responses) seen in plants, in response to variety of possible stimuli

• chemotropism: directional response to chemicals 

• phototropism: directional response to light stimuli 

• thermotropism: directional response to temperature 

• hydrotropism: directional response to water 

• thigmotropism: directional response to touch

  • shoots of climbing plants winding around other plants/solid structures ex. ivy

positive or negative

movement toward environmental stimulus is called positive tropism, movement away from stimulus is called negative tropism

shoot tips of plant that grow toward light source are positively phototropic

nastic movements

nasties are nastic movements (non-directional responses) which plants have in response to stimuli

• chemonasty

• photonasty

• thermonasty

• hydronasty

• thigmonasty 

gibberellins

• named after fungus Gibberella fujikuroi - causes rice plants, on which it is parasitic, to grow abnormally tall.

  • disease of rice called ‘foolish seedling’ investigated in 1920’s by Japanese plant pathologist Eiichi Kurosawa.

• substance causing abnormal growth was isolated from fungus + structural formulae identified

• chemical found to belong to class of 100+ naturally occurring plant hormones called gibberellins

• all acidic + usually referred to as GA (Giberellic acid)

overview: gibberellins

• gibberellins are synthesised in apical portions of roots + stems

• they play leading role in controlling stem elongation in growing stems of mature trees + shrubs, where it’s application characteristically promotes internode elongation

• elongations effect also enhanced if auxin present

• application of gibberellins to dwarf mutants is known to restore normal growth + development in many plants.

• gibberellin application also induce some plants to grow out of rosette stage of + flower early during first growth year

• also hastens seed germination, maybe bc can substitute for low light effects + temp requirements in this process

how gibberellins work

• GA: all ‘terpenes’, complex group of plant chemicals related to lipids + are also acids

• GA formed in:

  • Chloroplasts

  • Embryo of seeds

  • Buds

  • Root tips

  • Young leaves

• Found to be absent from genetically dwarf varieties of plants.

• migrate in non-polar manner that’s up/down plant in phloem + xylem

• GA is involved in the extension growth of the stem, causing elongation of internodes.

• highest concentration of GA is in fruits/seeds as they form plants + in seeds as they germinate.

• other aspects of growth + development in higher plants:

  • Dormancy

  • Germination

  • Onset of flowering

• main effects however is in stem elongation, mainly by affecting cell elongation

1. Absorption of water stimulates germination

2. Embryo synthesizes gibberellin in response to water uptake

3. Aleurone layer synthesizes amylase in response to gibberellin

4. Amylase mobilizes energy reserves: hydrolyses starch to maltose. Maltose is converted to glucose and respired to release ATP


questions

3. name plant hormone that initiates seed germination

   • gibberellin

4. which does not bring about directional growth response in plants

   • B: oxygen levels

5. when seed germinates it does not obtain energy through photosynthesis. explain where the energy for the initial growth of a seed comes from…

   • .digestion/breakdown of starch by enzymes. this produces sugar that is used in respiration

6. what is a non-directional response in a plant called?

   • nastic movement

mean height dwarf pea plant 1.18, 3.32, 4.68, 6.56, 8.54, 13.28, 15.78

mean height treated dwarf pea plant 0.82, 5.8, 14, 23.38, 26.64, 33.46, 35.2

standard deviation for day 20 on table 2 - 4.7

• days bottom 0-20

• side mean height 0-40

• 2 lines

  • control plant

  • then draw line from gibberelin plants

  • after plotted mean, plot little bar above and below to represent standard deviation