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)