Plant Orientation Responses
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Transcript of Plant Orientation Responses
Introductory video
http://www.youtube.com/watch?v=zctM_TWg5Ik (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.
http://www.cartage.org.lb/en/themes/Sciences/BotanicalSciences/PlantHormones/Nutrition/3224.GIF
Coleoptile-A protective sheath enclosing the first leaf shoot (plumule)of a seedling.
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 growth of the shoot.
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/examcentre.aspx%3Fid%3D221&usg=__39UBvOkahZwkgPdJO9PGk4KOnE4=&h=220&w=440&sz=12&hl=en&start=4&tbnid=K0isqmL35T4BKM:&tbnh=64&tbnw=127&prev=/images%3Fq%3Dgeotropish%26gbv%3D2%26hl%3Den
http://www.cartage.org.lb/en/themes/sciences/botanicalsciences/PlantHormones/PlantHormones/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
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/note-book/Dictionary/aaa_Dictionary_pictures/Apical_and_axillary_bud.jpg
The process that causes the triangular shape of trees like conifers.
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%20etiolation%20corn.JPG
Plant Hormones and ApplicationsPlant 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)