SENSITIVITY all organisms sense and interact with their environments a stimulus is a detectable change in the internal or external environment and evokes a response Mimosa pudica
SENSITIVITYplant survival and growth are influenced by abiotic factors including: Water Light Windplants rely entirely on chemical coordination
How do Plant responses differ from those of Animals?Plant responses are: Slower
How do Plant responses differ from those of Animals? Plant responses are: slower often involve growth
Growth substances: coordinate plantsdo not necessarily move from their sites of synthesis and so by definition, should not be called hormones
Three stages of growth:cell divisioncell enlargementcelldifferentiation
Auxinsare one of the five major types of growth substancespromote stem elongation and growth
TROPISM is amovement of part of a plant in response to, and directed by, an external stimulusthe movement is always a growth movement
Tropic responses can be:Positive – Negative –if growth is towards if growth is away from the stimulusthe stimulus Growth Growth Gravity
Phototropism is a plant movement in response to unilateral light Light Light
Phototropism Stems are: positively phototropic Most roots: - do not respond to light - in exceptional cases, exhibit only a weak negative phototropic response
What must happen Shaded side ofto the two sides of coleoptilethe coleoptile for it to bend? Illuminated side of coleoptile Light GROW UNEQUALLY!!
PhototropismWhat is the adaptive value of thephototropic response of stems? To give the plant a greater exposure to available light
Auxins & Phototropism
A Coleoptile is a :hollow, cylindrical sheath that surrounds the primary leaf of a germinating monocot seed Coleoptile First leaf
Darwin and his son Francis (1880)Light Tip Base covered covered by opaque Tip by shield Tip covered removed transparent by opaque cap cap
Darwin and his son Francis (1880)• observed that a seedling could bend toward light only if the tip of the coleoptile was present• they postulated that a signal was transmitted from the tip to the elongating region
Boysen-Jensen (1913): demonstrated that the signal was a mobile chemical substance Light Tip separated by Tip separated gelatine block by mica
Boysen-Jensen (1913): Light Light Light
Excised tip placed on agar blockFrits Went Growth-promoting chemical(1926) Diffuses into agar block Agar block with chemical stimulates growth Control (no chemical in agar Offset blocks cause curvature block) has no effect Control
In 1926, Frits Went showed that the:degree of curvature of oat coleoptiles was directly proportional to the concentration of the chemical
In 1926, Frits Went called the chemical auxin chemical was identified as: IAA (Indoleacetic acid)
Where is auxin made?Continuously in the A little auxin is shoot apex and probably made in young leaves the roots Light from above Light from one side
How does auxin move?From cell to cell by: - diffusion (apparently) and is eventually inactivated and degraded by enzymesLong-distance transport via the: - vascular system (mainly phloem) from shoots to roots
Effect of unilateral illumination on distribution of auxin
Which part of the shoot detects the direction of the light? The tip of the shoot. Especially the blue wavelength is detected.
What is present at tip to detect the light?Membrane-bound proteins called PHOTOTROPINS
Phototropins:are photoreceptorsbring about signal transduction (transduction means that one form of energy is changed into another)
How do phototropins affect auxins? PhototropinsRESULT: auxin flow seem to causefrom the meristem is auxin channels todiverted to the dark close on the litside of the shoot tip side of the shoot
Mode of Action of Auxins
Acid Growth Hypothesis H 2O provides a model linking auxin to cell wall expansion HOW does the cell wall expand?
Auxin enters cell wallAuxin combines with H+ Auxin causes cell totransport H+ into cell wallCell wall becomes acidic
Why is it important to have an acidic pH in the cell wall? Auxin stimulates H+ Cytoplasm Cell wall pH 7 pH 5
Enzymes that can break the bonds between cell wall fibres are activated Expansins are proteins that have unique "loosening" effects on plant cell walls.
Proton pumps play a major role in the growth response of cells to auxin Cell wall enzymes Expansin Cross-linking CELL WALL cell wallpolysaccharides H2O Cell H+ Plasma wall H+ membraneMicrofibril H+ H+ H+ H+ H+ H+ Nucleus Cytoplasm Vacuole ATP H+ Plasma membrane Cytoplasm
Acid Growth Hypothesishas been experimentally supported in several ways: – buffers that block cell wall acidification block cell expansion – other compounds that release H+ from the cell can also cause cell expansion – the movement of H+ has been observed in response to auxin treatment
Mark the side where you expect a lot of auxin to be present.
Name the tropism & response shown: Positive geotropism by Positive phototropism the root by the shoot Negative geotropism by the shoot
Question: [SEP, 2007]1. This question concerns phototropism in plants.a) Explain the term ‘phototropism’. (1) A plant movement in response to light.b)Why are phototropic responses important for plants? (1) To grow towards light and absorb it.
c) Phototropic responses are mediated by auxins. What is an auxin? (1) A plant growth substance which promotes stem elongation and growth.d) Name ONE auxin. (1) Indoleacetic acid (IAA)
e) How do auxins cause a phototropic response in plants? (3) Auxins move away from light. A greater concentration on the shaded side of the stem causes cell elongation. Curvature of stem results due to unequal growth on both sides of the stem.
f) Why are phototropic responses not evident in plant seedlings from which the shoot tip has been removed? (1) The source of auxin has been removed.
Question: [MAY, 2009]2. Use your knowledge of biology to describe the selective advantage of the following adaptation: Phototropism in plants. (5)
Question: [MAY, 2009] Phototropism is a growth movement in response to light. Shoots are positively phototropic .e. grow towards the light. This enables plants to trap light to build food by photosynthesis. Some roots are negatively phototropic i.e. grow away from light. This results in better anchorage of the plant and more chance to absorb water and ions deep down in the soil.
3. Two experiments were undertaken to investigate aspects of phototropism in plant shoots.a. Explain, as fully as possible, the responses of shoot A and B in experiment 1. (4) A: Glass blocks auxin on the left: no growth. Opposite side grows more as auxin diffuses down stem. Stem bends to the left.
3. Two experiments were undertaken to investigate aspects of phototropism in plant shoots.a. Explain, as fully as possible, the responses of shoot A and B in experiment 1. (4) B: Glass blocks auxin on the right: no growth. Opposite side grows more as auxin diffuses down stem. Stem bends to the right.
b. Explain concisely why, in unidirectional light, shoot C exhibits a tropic response while shoot D does not. (2)Tip is uncovered Tip is covered && can detect light cannot detectand grow towards light.it.