Presentation on theme: "Hormonal Control of Growth in Plants"— Presentation transcript:
1 Hormonal Control of Growth in Plants Higher BiologyHormonal Control of Growth in Plants
2 Hormonal Control of Growth By the end of this lesson you should be able to:Know the name of 1 plant growth substance.Know where IAA is produced.Know the effects of IAA or auxins:on cellson shoots and rootson bending of stemson leaf abscissionon fruit formation
3 Plant Growth Substances Also known as hormones.They are produced in one part of the plant (in tiny quantities) and carried to another part of the plant where they have their effect.
4 Indole Acetic Acid (IAA) IAA is one of a group of plant growth substances called auxins.IAA is produced in...It is produced in:Root tipShoot tipLeaf meristemsIt is transported over:short distances by diffusion.long distances via the phloem (by translocation)
5 Indole Acetic Acid (IAA) Effects of IAA at the cell level:Stimulates cell divisionPromotes cell elongation- by increasing the plasticity of cell walls, enabling them to stretch irreversibly when water enters by osmosis during vacuolation.Secondary growthin lateral meristems(cambium)Primary growthin apical meristems
6 Indole Acetic Acid (IAA) Necessary for differentiation to form specialised cells.Look at the experiment on pages of Torrance.Complete “The Effects of different concentrations of IAA on root and shoot growth” from your diagram packAdd high concentrations and low concentrations at the relevant points.
7 Indole Acetic Acid (IAA) Effects of IAA at organ level:On the shoot:Very high concentrationsHigh concentrationsLow concentrationsInhibit growthStimulate growthNo effect on growth
8 Indole Acetic Acid (IAA) Effects of IAA at organ level:On the root:High concentrationsLow concentrationsNo effect on growthStimulate growth
9 BendingWhat happens if the IAA is not evenly distributed at the root or shoot tip?Look at Figure 25.5 in your diagram pack.Shade in the areas where IAA is found on the shoot.What side is the shoot bending towards?How is this related to IAA concentration?Write a statement linking auxin concentration and curvature of the shoot.
10 Bending Why does the shoot bend? The higher concentration of auxin causes a greater cell elongation- which means the shoot bends.
11 Bending in response to light. Have you noticed that stems tend to grow towards the light?This is called phototropism.Look at Figure 25.6 in your diagram pack.Shade in the areas where IAA is most concentrated.Complete your “Phototropism” sheet from your diagram pack.
12 Bending in response to light. Phototropism is the name given to the directional growth movement by a plant shoot in response to light from one direction.phototropism videoWhy does this happen?The cells on shadedside elongatemore, so the shootbendsCausing anunevendistributionof auxinMore auxinon theshaded sideThe lightdestroys theauxin
13 Apical Dominance What is the position of the apical bud in a plant? Read pages on “Apical Dominance” in Torrance, and then answer the following questions:What is the position of the apical bud in a plant?What is the position of the lateral buds in a plant?What substance does the apical bud produce?How does this substance travel to the lateral buds?
14 Apical DominanceWhat effect does this substance have on the lateral buds?Describe what would happen to the lateral buds if the apical bud was removed.
15 Leaf AbscissionLeaf abscission is the process by which a plant sheds its leaves.It occurs in deciduous trees in autumn and in diseased leaves at any time.Prior to leaf fall, the auxin levels drop and a thin abscission layer of cells form at the base of the leaf stalk.The walls of these cells gradually become weaker, eventually snap and the leaf falls off. (e.g. when the wind blows)
16 Leaf AbscissionHigh levels of auxin travel from the leaf to prevent the abscission layer from forming, during the growing season.leaf abscission animation
17 Fruit AbscissionPut the following stages into the correct order to describe fruit abscission.Auxin levels dropThin layer ofcells form atthe base ofthe fruit stalkWalls of theabscission layerbecome weakerAuxin levels dropWalls of theabscission layerbecome weakerFruit falls offThin layer ofcells form atthe base ofthe fruit stalkFruit falls offFruit stalk snapsFruit stalk snaps
18 Fruit formationIn a flower what happens to the ovule after fertilisation?It forms the seed!In a flower what happens to the ovary wall after fertilisation?It forms the fruit!Fruit formation is caused by high levels of IAA made by the developing embryo.
19 Fruit formationUnpollinated flowers can be induced to form fruit by treating them with IAA.Since fertilisation has not happened the fruit is formed without a seed!This is called parthenocarpy.Horticulturalists use this to produce a good crop of fruit which all ripen at the same time (to make harvesting more economical).
20 Fruit formationHorticulturalists often spray fruit crops with synthetic auxin, towards the end of the growing season, to delay the formation of the abscission layer at the fruit stalk.This prevents the fruit from falling off the tree until it is fully ripe.
21 Practice QuestionsTorranceTYK page 256 Q1, 2, 4 and 5
22 Hormonal Control of Growth Can you do it?Know the name of 1 plant growth substance.Know where IAA is produced.Know the effects of IAA or auxins:on cellson shoots and rootson bending of stemson leaf abscissionon fruit formation
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