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VCE Biology Unit 2 Area of Study 01 Adaptations of Organisms Chapter 14 Plant tropisms and hormonal control Chapter 14.1 Environmental cues Plants must survive in the immediate environment • They depend on the immediate environment for materials and energy • They are relatively tolerant of environmental changes which they cannot escape Chapter 14.1 Environmental cues Plant growth and reproduction are synchronised with seasonal changes and climate conditions • Environment provides cues for many stages of plant growth. • Flowering, ripening of fruit, seed germination – All which must occur in the ‘most ideal’ conditions • Also respond to light and temperature Chapter 14.1 Environmental cues Communication between cells in different parts of plant are required for direction and timing of growth • Other plant responses are also co-ordinated • Plants do not have a nervous system • Internal conditions are controlled by hormones Chapter 14.1 Environmental cues Hormonal systems • A hormone is a chemical produced by cells in one part of an organism, transported throughout, accepted by specific receptor sites and affect specific cells • Hormones act as intercellular messengers and regulate cell functions Chapter 14.1 Environmental cues Hormonal systems (continued) • Target cells possess specific receptors • Involved in metabolic functions, rates of chemical reactions, transport of substances across cell membranes, secretion and cell growth • Alter specific biochemical reactions – Production of enzymes – Change in membrane transport – Switching on of off of specific genes Chapter 14.1 Environmental cues Hormonal systems (continued) • Exert their affects directly by passing through cell membrane into cell or • Indirectly by interacting with a receptor on the outside of the cell membrane • Hormone-receptor associations trigger particular biochemical events • Hormones are affective in low concentrations Chapter 14.1 Environmental cues Specificity • A particular stimulus will only affect a specific group of hormone-secreting cells. E.g. a sudden shock to humans causes the release of adrenaline • Hormones are transported throughout the organism, but only the cells with specific receptors are able to respond to the hormone. This specificity is not only restricted to specific organs, but specific cells/structures within organs. Chapter 14.1 Environmental cues Specificity (continued) • Flower plants have fewer hormones than animals and they tend to affect most cells. • Hormone producing cells in plants are not organised into glands. • The cells receiving the environmental cue (stimulus) produce the required hormone. – E.g. a growing shoot tip of a plant receiving light. Chapter 14.1 Environmental cues Specificity (continued) •Plant response is specific. –E.g. growing tip of wheat, lettuce or spinach receives increasing daylight (i.e. after winter solstice) will be stimulated to produce flowers. Chapter 14.1 Environmental cues Speed • Hormonal response generally slower than nerve • Effects last longer • Affects cells widely distributed Chapter 14.1 Environmental cues Speed (continued) • Plant hormones slower in response than animals (why?) • Transported through phloem and xylem • Transported between cells • Transported through air to other plants • Transported through soil (inhibit root growth from neighbouring plants) Chapter 14.1 Environmental cues Speed (continued) • Movement of a plant hormone requires the expenditure of energy (ATP) • Active transport is 10× faster than diffusion (passive) • Still may be as slow as 1 cm/hour (1 cmhour-1) Chapter 14.1 Environmental cues Hormones and responses of flowering plants • Orientate the growth of roots, stems and leaves • Timing of flowering, fruit ripening and seed germination Chapter 14.1 Environmental cues Hormones and responses of flowering plants • Response triggered by environmental factor • Tropism – When the response is directed towards the environmental factor, this is called “Tropism” – Positive towards – Negative away Chapter 14.1 Environmental cues Hormones and responses of flowering plants • Hormones in plants are responsible for – Phototropism – growth in response to light – Geotropism – growth in response to gravity – Apical dominance – inhibition of lateral branches – Ripening of fruit – conversion of starches to sugars – Abscission – shedding of leaves and flowers Chapter 14.1 Environmental cues Auxins • Phototropism – reaching for light • Growing plants bend and grow towards light • Stimulated by chemical – auxin (“to grow”) • Chemical diffuses downwards from growing tip (meristem) Chapter 14.1 Environmental cues Auxins (continued) • Produced continuously in meristem • Diffuses through layers of cells • Light interacts with receptors that control membrane permeability to auxin • Auxin moves laterally away from light Chapter 14.1 Environmental cues Auxins (continued) • Higher concentration of auxin on dark side of tip • Those cells grow faster (elongation not cell division) • Softens cell walls allowing elongation under turgor pressure • Cells outside growing area lack auxin receptors • Auxins used as herbicides – excess auxin causes growth to the detriment of the plant (cells rupture, etc.) Phototropism Experiment Auxin causes elongation of cells on shaded side. Chapter 14.1 Environmental cues Auxins Geototropism – responding to gravity • Auxins involved in negative geotropism (growing away from gravity) • Plant laid on side in dark will bend and grow away from gravity • Auxin concentration on lower side of lateral stems Negative Geotropism Chapter 14.1 Environmental cues Auxins Apical dominance – one main stem • Auxin produced in the apical tip of a plant • Diffuses down stem • Inhibits lateral bud development • Results in taller plants with less side branches • Gardeners cut out apical tips to promote bushiness of cultivated plant • Bushfires burn off the apical tips of eucalypts in the crown Apical dominance Chapter 14.1 Environmental cues Gibberellins • Promote cell elongation • Promote growth for whole plant • Promote cell division (mitosis and cytokinesis) – Flowering – Fruit enlargement – Seed germination Chapter 14.1 Environmental cues Gibberellins • Synthesised in flowers, developing fruits, seeds, growing buds and elongating stems • Important for triggering cereal grain germination – Produced in embryo – Moves to target cells in the endosperm – Induces the formation of enzymes that digest the endosperm to produce malt Used in the brewing of malt whisky Chapter 14.1 Environmental cues Cytokinins • In the presence of auxins, cytokinins stimulates cell division and cell differentiation • Ratio of auxins to cytokinins determines path of differentiation of new cells – Stems and leaves develop when more cytokinins than auxins – Roots develop when less cytokinins than auxins Chapter 14.1 Environmental cues Abscisic acid • Opposite action to auxins • Involved in overall regulation of plant functions • Little is known about them Chapter 14.1 Environmental cues Abscisic acid • Assist plants to tolerate or avoid adverse conditions – E.g. drought, salinity, low temperature – Promotes leaf drop, bud and seed dormancy and increased frost resistance Chapter 14.1 Environmental cues Abscisic acid (ABA) • Synthesised mainly in chloroplasts • Dropping of ripe fruit, unfertilised flowers and falling of leaves in deciduous trees known as abscission • Disintegration of special layer of cells at base of organ being dropped Chapter 14.1 Environmental cues Abscisic acid (ABA) • Development of seed dormancy and vernalisation requires ABA • Appears to act on gene expression in nucleus Chapter 14.1 Environmental cues Abscisic acid (ABA) • Short term effects – stomatal movement – Plant loses too much water, ABA stimulates guard cells to close • [ABA] increases under stressful conditions Chapter 14.1 Environmental cues Ethylene (Ethene) (C2H4) • Released by ripening fruit (apples, pears, citrus, banana and avocados) • Stimulates fruit to ripen • Ripening of fleshy fruit shows colour changes and softening of flesh Chapter 14.1 Environmental cues Ethylene (Ethene) (C2H4) • Wild plants use fruit to attract animals to eat them and disperse the seeds • Ethylene increases rate of respiration and other processes of fruit ripening (e.g. breaking down starch and oils into sugars). • Triggered by auxin and abscisic acid Summary of plant hormones Hormone Where produced Effective site Action Visible effect auxins Shoot tip (meristem) Growing region of shoot Cells elongate under Tip bends towards light, apical turgor pressure dominance, used as herbicides, stimulate cuttings to root gibberellin Fruits, seeds, growing Roots, shoots, seeds, also Growth of cells Shoot elongation, germination of buds, elongating stems found in fungi seeds, flowering fruit enlargement cytokinins Roots and developing Branch and leaf buds, moves Stimulates cell division, Growth of lateral branches, used fruits through phloem and xylem cell elongation and to promote life of vegetables in tissue differentiation storage Abscisic acid chloroplasts Gene expression in nuclei Growth inhibition Seed dormancy, vernalisation, drought-tolerance ethylene Ripening fruits, flowers, Cellular metabolism Fruit ripening and leaf Fruit ripening, leaf and fruit drop, seeds, leaves and roots drop used commercially to ripen bananas and pineapples
"VCE Biology Unit 2 Area of Study 01 .._1_"