Plant Hormones Grd 12 CAPS Notes 2014 PDF

1 PLANT HORMONES/PLANT GROWTH SUBSTANCES DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  2 Plant Responses to the environment: Plants respo...

1 PLANT HORMONES/PLANT GROWTH SUBSTANCES DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  2 Plant Responses to the environment: Plants respond to certain stimuli (changes) in the environment, e.g.  Light;  Water;  Gravity and  Touch. Since plants cannot move towards or away from these responses by locomotion, they use various growth patterns to bring about their response. It is important to understand the differences between growth and development. Growth is the increase in the:  Number and Increase  Size of cells in size Increase And is brought about by: in number  Cells dividing by mitosis and  Cell elongation. Elongation Development is the change of simple meristematic cells into specialised cells Mitosis such as:  Xylem,  Phloem,  Schlerenchyma,  Chlorenchyma etc. Development And is brought about by:  Differentiation/specialisation of cell structure. H2O & Dissol- mineral ved salts food Xylem Phloem DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  3 Plant Hormones/Growth substances: A hormone is:  a chemical substance; that is  produced in one part of the body;  in minute quantities/amounts; and  brings about a response in another part of the body. Plant hormones are different because they bring about a reaction in the organ in which they are produced. They are therefore referred to as plant growth substances. Animal hormones are transported, by the blood, from where they are produced (endocrine gland) to another part of the body (the target organ). (see later) Botanists generally recognise five groups of plant hormones. Fill in the groups below: PLANT HORMONES We will only study Auxins, Gibberellins and Abscisic Acid (ABA). Functions of plant hormones: Table summarising the functions of Auxins, Gibberellins and Abscisic Acid. Hormone Where found Functions Auxins In actively growing regions of  Promote cell elongation in the region of plants (meristems), e.g. new elongation behind the shoot and root tip. leaves, tips of roots and  Promote apical dominance by inhibiting lateral shoots. bud development.  Control tropic movements (direction of plant growth – tropisms)  Promote fruit development by inhibiting abscission (falling) of fruit.  Inhibit leaf abscission (falling). Gibberellins In young leaves and  Promote growth through cell elongation developing seeds and fruits. especially in stems.  Promote germination of seeds.  Promotes cell division in meristems.  Stimulates flower and fruit development in some plants. DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  4 Abscisic Acid In roots, mature leaves and  Inhibits germination of seeds, root and shoot fruits and terminal buds. growth.  Promotes abscission of ripe fruit from plants.  Promotes leaf abscission.  Promotes the winter dormancy of plants.  Promotes dormancy in seeds. Role of Auxins in Tropic responses: A tropic movement or tropism is a growth response/movement of part of a plant to an external one sided stimulus. The direction of the stimulus determines the direction of growth. Tropisms are brought about by the uneven distribution of auxins in that part of the plant. This uneven distribution results in one side/part of the plant growing/elongating faster than the other side/part. If the part of the plant grows towards the stimulus it is said to be a positive response. If the part of the plant grows away from the stimulus it is said to be a negative response. We will investigate the role of auxins in two tropic responses, the response of plants to:  light (phototropism) and  gravity (geotropism). Auxins and phototropism: Under normal conditions (A): Auxins are produced at the tip of a stem from which they are evenly distributed down the stem to the region of elongation. The stem therefore elongates equally on both sides and continues to grow straight up. Under conditions of unilateral (one sided) illumination (B): Auxins are produced at the tip of a stem from which they are unevenly distributed down the stem to the region of elongation. The auxins are redistributed to the dark side of the stem. The dark side therefore has a higher concentration of auxins and elongates more rapidly. The stem therefore grows towards the light and is said to be positively phototropic. DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  5 Auxins and geotropism: Under normal conditions (A): Auxins are produced at the tip of a root from which they are evenly distributed up the root to the region of elongation. The root therefore elongates equally on both sides and continues to grow straight down. Under conditions of unilateral (one sided) gravity (B): (i.e. the root is lying on its side) Auxins are produced at the tip of a root from which they are unevenly distributed along the root to the region of elongation. The auxins are pulled down by gravity to the bottom side of the root. The bottom side therefore has a higher concentration of auxins and elongates more slowly. The root therefore grows towards the gravity and is said to be positively geotropic. NOTE: A high concentration of auxins:  stimulates elongation in stems but  inhibits elongation in roots. Stems are positively phototropic but negatively geotropic and roots are negatively phototropic but positively geotropic. Discuss the relevance of this to plants. Auxins stimulate elongation/ growth in the shoot so the shoot curves up away from Auxins slow elongation/ If a plant is laid on its side, gravity – negatively geotropic growth in the root so the root auxins accumulate on the lower curves down towards gravity half of the stem and roots – positively geotropic DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  6 Experiments to investigate Tropic responses: Experiment 1: Interesting fact: If plants do not receive sufficient light, Aim: To demonstrate phototropism in stems of plants. e.g. if they are growing in the shade of Apparatus: other plants, auxins stimulate elongation of their stem internodes. hole in box This is called etiolation and results in unilateral tall, slender, spindly stems. plant light box clinostat Apparatus used to demonstrate phototropism Method: 1. Set up TWO sets of apparatus as shown in the diagram above. 2. For the experiment use a stationary clinostat. 3. For the control use a revolving clinostat. 4. Leave both sets of apparatus in sunlight for a few days. Results: In the experiment the plant grew __________________________ (because it gets unilateral light) and is therefore _______________ _________________. In the control the plant continued to grow ______________ ______________ (because it gets light from all sides due to the _______________ clinostat). hole in hole in box box unilateral light unilateral plant growing light plant growing upwards towards light box box stationary clinostat revolving clinostat Experiment Control Diagrams showing results of the experiment to demonstrate phototropism. DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  7 Conclusion: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ Questions: 1. Give a possible hypothesis for this experiment. (2) 2. Name the dependant variable in this experiment. (1) 3. Name the independent variables in this experiment. (1) 4. Name TWO other variables NOT mentioned in this experiment that would need to be kept constant. (2) 5. Explain why a control is required in this experiment. (4) 6. Explain how you could do this experiment at home without any clinostats. (4) 7. Explain, in terms of auxins, why the plant stems grow towards light. (4) 8. Explain the significance to plants that shoots are positively phototropic. (2) 9. Are roots positively or negatively phototropic? Give evidence from nature that supports your answer. (2) Experiment 2: Aim: To demonstrate geotropism in plants. Apparatus: damp cotton wool cork disc clinostat germinating seeds Apparatus used to demonstrate geotropism. DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  8 Method: 1. Set up TWO sets of apparatus as shown in the diagram above. 2. For the experiment use a stationary clinostat. 3. For the control use a revolving clinostat. 4. Leave both sets of apparatus in the dark for a few days. Results: In the experiment the roots (radicles) grew downwards ____________ ___________ and are therefore ______________ ______________. The shoots (plumules) grow ________ _______ ___________ and are therefore ________________ ______________. In the control the roots and shoots continued to grow ____________________________________ (because it is affected by gravity equally on all sides due to the _______________ clinostat). shoots all grow shoots all grow upwards away shoots and upwards away from gravity roots continue from gravity growing in the roots all grow same direction downwards as placed towards gravity Diagram showing results of the experiment to demonstrate geotropism Conclusion: ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ Questions: 1. Give a possible hypothesis for this experiment. (2) 2. Name the dependant variable in this experiment. (1) 3. Name the independent variables in this experiment. (1) 4. Name TWO other variables NOT mentioned in this experiment that would need to be kept constant. (2) DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  9 5. Explain why a control is required in this experiment. (4) 6. Explain how you could do this experiment at home without any clinostats. (5) 7. Explain, in terms of auxins, why the plant roots grow towards gravity. (4) 8. Explain the significance to plants that roots are positively geotropic. (3) 9. Are Shoots positively or negatively geotropic? Give evidence from nature that supports your answer. (2) glass jar moist cotton wool rolled tube of germinating seed absorbent paper DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  10 Use of Plant Hormones in Agriculture: Chemicals that closely resemble and imitate natural plant hormones are synthesised by man for use in agriculture. The advantages of using these synthetic growth substances are:  They are cheaper to produce than to extract from plants, and  Are not broken down by the plant’s enzymes in the way that the natural growth substances are. Auxins can be used:  As herbicides: A synthetic auxin (2,4D) is used to kill broad-leaved weeds growing in lawns. It does so by stimulating uncontrolled growth, which causes the plant to die because it cannot sustain this growth rate.  To control apical dominance: The auxins, produced in the apical bud of the plant, inhibit the growth of lateral (side) buds. This results in rapid upwards growth and limited lateral growth. Removal of the apical bud and its auxins therefore encourages rapid growth of the lateral branches. Farmers use this knowledge and therefore prune (remove the apical buds from) their plants so that they form thick but short hedges with many side branches. E.g. A farmer will prune his fruit trees so that they develop more lateral branches and therefore produce more fruit which is also easy to reach. apical bud removed apical bud present lateral buds inhibited lateral buds grow Diagram illustrating apical dominance. Both auxins and gibberellins can be used to:  Stimulate fruit development. If farmers spray their grapes with gibberellins or auxins the fruit produced are larger. Gibberellins and auxins are used to stimulate seedless fruit to develop. Gibberellins also stimulate shoot elongation. Sugar cane, sprayed with gibberellins, therefore grows longer stems which contain more sugar. DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  11 Plant defence mechanisms: Plants are producers and are therefore likely to be eaten by animals. They are also prone to infections by pathogens. Plants have therefore developed various defence mechanisms to protect themselves. These include:  Thorns and  Chemicals. Thorns as defence mechanisms:  Plants reduce attacks by herbivores by having many thorns on their leaves and stems which make them difficult to eat.  Thorns slow down the herbivore’s rate of nutrition and some thorns can destroy the herbivores molars.  Some plants, e.g. some acacias, have a mutualistic relationship with ants. The ants live in the swollen bases of the thorns and get food and shelter from the tree. The in turn attack herbivores in an attempt to protect their nests.  Some thorns have poisonous chemical or pathogenic bacteria that can kill herbivores. Chemicals as defence mechanisms:  Firstly the waxy cuticle prevents entry of pathogens.  Some plants produce poisonous chemical secretions in their stems, leaves or fruit that can kill or frighten off insects and other herbivores. e.g.  Wild tobacco plants produce high levels of nicotine that is deadly to the hornworm that eats them, or  The poinsettia secretes a milky sap that destroys the red blood cells of herbivores.  Some plants secrete lignins and silica that are indigestible and so hard that they will break the mandibles of insects.  Secreting sticky secretions which make it difficult for insects and animals to eat them.  Some plants secrete substances, e.g. camphor, citronella and menthol, which attract the herbivores natural predators as soon as they start eating them. e.g. wasps that lay their eggs on/in caterpillars that are eating the plant. The wasp larvae hatch and eat the caterpillars. DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  12 Consolidation: Total: 100 1. Give the correct term for each of the following: 1.1. An increase in the number and size of cells. 1.2. The plant tissue that transports water. 1.3. The development of meristematic cells to permanent tissues. 1.4. A chemical substance, produced in minute quantities that brings about a response in some part of the body. 1.5. The plant growth substance that: 1.5.1. Promotes leaf and fruit fall. 1.5.2. Stimulates elongation of stem internodes. 1.5.3. Promotes germination of seeds. 1.5.4. Promotes apical dominance. 1.6. A growth response to an external stimulus. 1.7. The growth response to the stimulus of gravity. 1.8. Increased elongation of stem internodes. 1.9. The apparatus especially designed to test tropic responses. 1.10. A growth response towards a stimulus. 2. The following diagrams show the tips of two plant stems at the beginning and at the end of an experiment. 2.1. Explain what happened to plant A and why this happened. (6) 2.2. Explain why plant B continues to grow straight up. (4) 2.3. Give an aim for this experiment. (2) 2.4. Which plant, A or B, represents the control? Explain your choice. (4) 2.5. What stimulus was experienced by the experimental plant? (2) 2.6. What is the name given to the response of the plants in A? (2) DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  13 3. A young germinating bean seed was placed on its side as shown below. 3.1. Draw a labelled diagram to show what the seed will look like in a week’s time. (5) 3.2. What stimulus affected the growth of the plant? (1) 3.3. Explain why the shoot and the root grew as shown in your diagram. (10) 4. Four potted plants, which were the same length at the start of an investigation, were grown under different light conditions. After four weeks the growth patterns shown in the diagrams were observed. A B C D Describe the light conditions that would have caused the growth response in each of the plants. Give visible reasons for your answers. 5. The diagrams below show six different experiments using coleoptiles (young shoots). (Agar absorbs auxins whereas mica is used to block the movement of auxins.) Explain the results in each experiment. DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  14 6. Study the graph below and answer the questions that follow. Key: Shoot Lateral bud root 10-5 10-4 10-3 10-2 10-1 6.1. Between which concentrations of auxins are roots stimulated to elongate? (2) 6.2. Are shoots stimulated by high or low auxin concentrations? (1) 6.3. What concentration of auxin will stimulate maximum elongation in lateral buds? (2) 6.4. At what concentration of auxin is elongation neither inhibited nor stimulated in roots? (2) 6.5. Give the maximum percentage elongation in lateral buds. (2) DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  15 7. Study the following case study and answer the questions that follow. Case study: Paperbark acacias respond to browsing by giraffe In Africa, acacia trees form an important part of the diet of many large herbivores, including giraffe. Acacias are more nutritious than many other trees and they are a popular source for giraffe, kudu and nyala. For this study acacias, (Acacia sieberiana), were chosen from three study sites. One of these sites is heavily browsed by giraffe, the second site undergoes medium browsing and the third site is lightly browsed by giraffe. The researchers predicted that trees in the high level browsing site would have the longest, most dense spines and the smallest, the most dense leaves. The researchers also predicted that these trees would have the highest concentration of tannins and hydrogen cyanide. a Table showing the characteristics of acacias different intensity giraffe browsing sites Variable Low Medium High intensity intensity intensity Ave leaf length 11,0 8,8 6,3 (cm) b c Ave leaf 5,7 3,8 3,5 width (cm) Ave leaf density 30,6 27,8 29,1 (50cm-1) Ave spine density 46,1 30,4 47,4 (50cm-1) Ave spine length 0,8 0,9 1,9 (mm) Tannin 17,7 11,3 15,7 a) The paperbark acacia is the conc (QE) favourite food of the giraffe. Hydrogen b) A branch of a paperbark acacia that cyanide 119,2 324,2 564,5 has not been browsed. conc c) A branch of a tree that has been (ppm) browsed. DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS  16 7.1. Name three mechanisms employed by acacia trees to protect themselves from browsers. (3) 7.2. The researcher collected samples from branches that were more than 2,5m above the ground. Why did he do this? (3) 7.3. In the table, the intensity of browsing is the independent variable. What effect does the level of browsing apparently have on the following dependent variables? a) Leaf length: b) Leaf density: c) Spine length: d) Leaf width: (8) 7.4. Give the scientific name for the acacia trees used in this study. (2) 7.5. Look back at the researchers predictions. a) Which predictions are correct? b) Which seem to be wrong? (6) 7.6. Some acacias use a symbiotic relationship as a means of protection. Explain. (3) Total: 100 DPGH Grade 12: Life Processes in Plants & Animals: Responding to the environment - Plants NOTES CAPS