Chapter 24 Plant Response to Stimuli 1

Objectives – What you will need to know from this section Describe the organs used by plants to respond to stimuli Explain the term: growth regulation Define the following terms: tropisms, phototropism, geotropism, thigmotropism, hydrotropism, chemotropism Name examples of phototropism & geotropism Define the term: growth regulator Outline the transport of regulators via the vascular system Responses in the Flowering Plant 2

7. 7. Explain the term: Combined Effect Explain the term: Growth Promoter Explain the term: Growth Inhibitor Name 4 methods of anatomical/chemical adaptations that protect plants State 2 examples of the use of plant regulators. Mandatory Activity: Effects of I.A.A. Growth Regulators on Plants 3

The ability to detect change and to respond is called sensitivity. Response is a form of defence that allows organisms to survive. Plant adapt to new situations by modifying their growth, by means of chemicals called growth regulators [hormones]. Responses in the Flowering Plant 4

A hormone is a chemical produced in one part of an organism, transported to other tissues where it has its effect. External factors that regulate the growth of plants are: 1. 1.light intensity, 2. 2.day length, 3. 3.gravity, 4. 4.temperature 5. 5.and others. 5

Plant growth regulators [hormones] are chemicals that interact with one another to control a particular development or response. Plants only grow at their tips, in small regions of active cell division called meristems— the tips of shoots, roots and side buds. Plant growth regulators [hormones] 6

Plant Growth Regulators 1.Chemicals that control the growth of plants 2.They are produced in the meristems 3.They are transported in the xylem and phloem 4.Even a small amount of growth regulator can have great effect on growth 7

LEARNING CHECK 1. 1.The ability to detect change and to respond is called? 2. 2.Plants adapt to new situations by? 3. 3.What is a growth regulator? 4. 4.List the properties of plant growth regulators What is a meristem? 8

Tropisms A tropism is a plant’s response to a stimulus e.g. sunlight, gravity. Phototropism is a growth response of a stem towards light, so that it can receive the maximum amount of light for photosynthesis 9

Geotropism is the growth of a plant in response to gravity—it allows the root to get the water and nutrients it needs from the soil, and raises the leaves up into the light for photosynthesis. 10

11 Thigmotropism is the growth of a plant in response to contact, e.g. ivy wraps around objects which help support it.

Hydrotropism is the response of roots to water – they will grow towards it. 12

Chemotropism is the response to chemicals, e.g. pollen tubes growing down the carpel in response to chemicals released by the ovule. 13

LEARNING CHECK 1. 1.What is meant by the term “tropism” What is a stimulus Name 5 different tropisms and the stimulus involved in each case List the advantage to the plant of each tropism. 14

New roots developing Growth shut down for winter Plant growth regulators interact with one another to control a particular development or response. Some regulators promote growth, i.e. speed up growth e.g. auxins, gibberellins, cytokinins. Some regulators inhibit growth, i.e. slow down growth e.g. abscisic acid and ethene. Plant growth regulators 15

Auxins APICAL DOMINANCE PHOTOTROPISM Growth promoters i.e. promote cell enlargement and growth, are involved in phototropism and apical dominance 16

Apical dominance Auxins are responsible for apical dominance, where the main bud inhibits the growth of buds lower down stem. This photograph shows side buds sprouting when the main stem is cut off [pruned]. 17

When light shines on plants it causes auxin to move down the shaded part of the stem, causing the shoot to bend and grow towards the sunlight - phototropism Quicker growth here due to more hormones Phototropism 18

Gibberellins 1. 1.cause stem lengthening, 2. 2.mobilise the stored food in germinating seeds, 3. 3.break dormancy in buds and seeds in Spring. 19

Cytokinins stimulate cell division (mitosis) and trigger leaf growth in spring. Cytokinins 20

Abscisic acid triggers bud and seed dormancy in autumn and inhibits cell growth. Abscisic Acid 21

Ethene promotes ripening of fruit and the fall of leaves, flowers and fruits. Ethene 22

Uses of plant regulators 1. 1.Artificial auxins are used in rooting powders to stimulate root formation on stem cuttings A synthetic auxin [2,4-D] is used as a weedkiller Fruit is transported green and unripe, and can then be quickly ripened by spraying it with ethene. 23

LEARNING CHECK 1. 1.What is a plant growth regulator? 2. 2.Explain the terms “promote” and “inhibit” Name 5 growth regulators and a function of each Give 3 examples of how we make use of plants growth regulators [natural or artificial] 24

Plants can adapt themselves for protection in two ways: 1.Structural or anatomical adaptations 2.Chemical adaptations Adaptations for Protection in Plants 25

Some adaptations that plants use to protect themselves include: 1. Anatomical: Spines, thorns or stinging hairs to deter animals from eating them, e.g. cacti, nettles. 26 Plant Defences

Structural Adaptations Bark/epidermis prevents entry of microbes and reduces loss of water Thick cuticle on stem or leaves in plants that live in dry places Thorns prevent plants from being eaten by herbivores Stinging cell in epidermis prevent plants been eaten Guard cells change shape when they lose water which causes stomata to close and this reduces water loss 27

2. Chemical: Toxins that cause illness or death, e.g. Oak produces tannins in their leaves to protect them against caterpillars. 28 The leaves and acorns of the oak tree are poisonous to cattle, horses, sheep, and goats in large amounts due to the toxin tannic acid, and cause kidney damage and gastroenteritis.

Chemical Adaptations 1.Production of Tannins to make the plant indigestible 2.Production of toxic chemicals to prevent growth of insect larvae 3.Production of nicotine in legumes that damage nerves and muscles 4.Production of heat shock proteins to protect enzymes when temperatures are high. 29

LEARNING CHECK 1. 1.Why is defence important to plants? 2. 2.List four methods used by plants to defend themselves Explain the term “structural adaptation” What is a toxin? 5. 5.What advantage is it to a plant that it grows throughout its life [from meristems]? 30

Auxins affect virtually every aspect of plant development, and how they respond to environmental stimuli. Auxins influence includes 1. 1.phototropism, 2. 2.geotropism, 3. 3.cell enlargement and growth, 4. 4.apical dominance, 5. 5.root growth, 6. 6.fruit development, 7. 7.vascular development, 8. 8.and senescence [aging]. AUXINS -- HIGHER LEVEL 31

AUXINS 1. 1.Growth – the zone of elongation grows, extending the shoot & root Apical dominance – auxin produced in the shoot tip inhibits the growth of buds lower down stem Tropisms - involved in phototropism —stem bends towards light due to cell enlargement 32

IAA (Indoleacetic acid) is an auxin that is made in the meristems of shoots, buds and roots, and in the tips of coleoptiles [of grasses]. 33

The coleoptile is the protective sheath around the leaves and shoot of grass seedlings, such as corn or oats. 34

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Auxin Animations of auxin 36

LEARNING CHECK 1. 1.What is an auxin? 2. 2.List 4 effects of auxins in a plant List the properties of auxins What does elongation mean? 5. 5.What is IAA? 6. 6.What is a meristem? 7. 7.What is a coleoptile? 37

If a plant receives light from one side only: the light causes auxin to travel down by active transport in the phloem tissue from the stem / meristem down the ‘dark’ side of the stem (or coleoptile). Quicker growth here due to more auxin 38 Phototropism & Auxins

The extra auxin loosens the cellulose fibres in plant cell walls & this allows the cells to elongate (get longer) faster than cells on the ‘bright’ side. causing the stem to bend towards the light 39 Phototropism & Auxins

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