Download Plant Orientation Responses

PLANT ORIENTATION RESPONSES
Introductory video
 http://www.youtube.com/watch?v=zctM_TW
g5Ik
(Plant Physiology: Phototropic Response).
 Biozone p166 together!
Tropisms and Growth responses
 Tropisms are plant responses to external
stimuli, where the stimulus direction
determines the direction of the growth
response.
 They can be positive or negative.
 Nastic Responses are non directional
responses to intensity of a stimulus.
 They are not described as positive or negative
 E.g. Venus fly trap, opening/ closing of stomata.
 Biozone p170
Hormones
 The ways in which a plant responds to stimuli
is coordinated by hormones.
 . Hormones also control the growth of a plant
 One plant hormone is called auxin (indole
acetic acid or IAA).
 Auxin is made by cells in the tips of the roots
and shoots.
 It speeds up growth in shoots and slows down
growth in roots.
 Unequal distribution of hormones causes
unequal growth rates in different tissues.
How Phototropism works.
ColeoptileA protective sheath
enclosing the first
leaf shoot (plumule)
of a seedling.
http://www.cartage.org.lb/en/themes/Sciences/BotanicalSciences/PlantHormones/Nutrition/3224.GIF
Phototropism mechanism.
1. Auxin(IAA) is produced in the tips of shoots.
(made in actively growing plant tissue
(meristematic) from the a.a tryptophan.
2. If light is shone from one side of the seedling, the
auxin migrates down from the tip and
concentrates on the darker side of the seedling.
3. The greater concentration of auxin on the dark
side, promotes greater cell elongation on this
side. So the cells grow faster and longer than the
cells on the bright side.
The shoot bends towards the light as a result of this
uneven distribution of auxin and hence uneven
http://plantphys.info/plant_physiology/images/darwintip.gif
Experimental findings
 The auxin is produced in the tip of
shoots.
 It is water soluble and diffuses
through agar or gelatine blocks, but
will not pass through mica.
 The auxin causes elongation of the
cells, thus promoting upwards
growth.
 Shoots bend towards light.
Geotropism- How does it work?
http://images.google.co.nz/imgres?imgurl=http://lgfl.skoool.co.uk/uploadedImages/Coord10.14_geotropism.gif&imgrefurl=http://lgfl.skoool.co.uk/examc
entre.aspx%3Fid%3D221&usg=__39UBvOkahZwkgPdJO9PGk4KOnE4=&h=220&w=440&sz=12&hl=en&start=4&tbnid=K0isqmL35T4BKM:&tbnh=64&tbn
w=127&prev=/images%3Fq%3Dgeotropish%26gbv%3D2%26hl%3Den
http://www.cartage.org.lb/en/themes/sciences/botanicalsciences/PlantHormones/Pl
antHormones/gravitrop.gif
Activity
 Write a detailed explanation of why
shoots show negative gravitropism
and roots show positive
gravitropism. You can include an
annotated diagram if you wish.
 Biozone p167-169.
Other tropisms
 Chemotropism –e.g. growth of pollen tube
towards ovary.
 Hydrotropism -strong response
 Thigmotropism-which side is growth inhibited
or slowed down in ?
 Self check p17-18 green book
Effect of auxin on shoots/ roots
 This depends on the concentration in auxin.
 Low concentrations stimulate roots, while high
concentrations inhibit (and can kill).
 Low concentrations stimulate the growth of
lateral buds, while high concentrations inhibit.
 Low concentrations do not stimulate the stems,
while high concentrations do.
Apical dominance
The process that causes the triangular shape of trees like
conifers.
 Auxin is in high concentrations at
the top of the plant. This stimulates
shoot and inhibits growth in lateral
buds.
 Lower down the auxin in less
concentrated and also deactivated
by some enzymes. Cytokinins also
stimulate lateral buds, causing
bushiness of shrubs near bottom.
 Principle can also be used in broad
leaf trees.
http://www.cactus-art.biz/notebook/Dictionary/aaa_Dictionary_pictures/Apical_and_axillary_bud.jpg
Etiolation
 Occurs when plants are grown in reduced
light or the dark.
 Leaves are small and yellow and there are
long internodes.
 Phytochlorophyll  turns to green chlorophyll.
http://biology.uwsp.edu/courses/botlab/images/Lab12Growth/12D%20etiol
ation%20corn.JPG
Plant Hormones and Applications
Plant hormones are called phytohormones.
 Gibberellins –produced in growing tips –
cause rapid elongation of stem. Can promote
germination and stem thickening.
 Cytokinins – stimulate cell division in tissue
cultures and whole plants. Proportion with
auxin can determine what grows in tissue
cultures. (Page 19 green book)
 Ethylene gas (ethene)-ripening.
http://www.crossrefrigeration.com/images/bananaripe1_14.jpg
 Abscisic acid ABA close stomata and reduce water
loss.
Inhibiting hormone against auxin,
gibberellins and cytokinins.
 Prepares deciduous plants for leaf
fall.
 Helps induce winter dormancy.
Other effects of auxin (IAA)
 Rooting powders for cuttings.
 Suppression of root elongation.
 Suppression of lateral buds (apical
dominance)
 Stimulation of mitosis in stems of woody
plants.
 Initiation of flowering in some plants.
 Prevention of premature abscission
(dropping) of leaves and fruit. (fruit growers)