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F215 control, genomes and environment Module 4 – responding to the environment Learning Outcomes Explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress. Define the term tropism. Explain how plant responses to environmental changes are coordinated by hormones, with reference to responding to changes in light direction Outline the role of hormones in leaf loss in deciduous plants Plant Responses Plants have evolved a wide range of responses to a large variety of stimuli, this helps them to Survive long enough to reproduce Avoid stress Avoid being eaten Sensitivity in plants A plants responses to the external environment are mainly growth responses Tropism Growth response to a stimulus Positive Tropic Responding towards a stimulus Negative Tropic Responding away from a stimulus Examples of Tropisms Tropism Phototropism Geotropism Chemotropism Thigmotropism What is it and why is it important? Tropisms Phototropism Shoots grow towards the light - photosynthesis Geotropism Roots grow towards the pull of gravity – anchor, water, minerals Chemotropism Pollen tubes grow down the style attracted by chemicals - fertilisation Thigmotropism Response to touch e.g. Shoots of climbing plants wind round solid structures – support https://www.youtube.com/watch?v=yFSpKqiI-H8 Hormones Chemical messengers that coordinate a plants response to an environmental stimuli Act on target cells/tissues by binding to receptors in the membrane More commonly known as plant growth regulators Move around the plant by active transport, diffusion mass flow Examples Hormone Effect Auxins Promotes cell elongation, inhibit growth in side shoots, inhibit leaf fall (abscission) Cytokinins Promotes cell division Gibberellins Promotes seed germination and growth of stems Inhibit seed germination and growth, causes stomatal closure when low water availability Promotes fruit ripening Abscisic acid Ethene Tropisms Slow responses resulting in directional growth “is a directional growth response in which the direction of the response is determined by the direction of the external stimulus” Plant Growth Roots and shoots get longer Roots and shoots get wide Gives rise to side shoots Shoots get longer Auxin (IAA) Produced at the apex Travels by diffusion or active transport to zone of elongation Growth is proportional to concentration of auxin Auxin action – how cells elongate Auxin increase stretchiness of cell wall Promotes the active transport of hydrogen ions by ATPase enzyme Build up of hydrogen ions in the cell walls The Low pH activates enzymes that break cross-linkages between molecules in walls Cell takes up water by osmosis, cell swell and become longer Permanent effect Phototropism Phototropism is the response of plant organs to the direction of light. A shoot shows Positive phototropism GCSE knowledge: what causes positive phototropism? Phototropism What causes phototropism? How light causes redistribution of auxin is uncertain 2 enzymes (phototropin 1 and phototropin 2) activated by blue light. Active phototropin on the light side and the phototropin in the rest of the shoot is progressively less active. The gradient is thought to cause the redistribution of auxin. Plant movements – non directional Nastic Movements Usually brought about by changes in turgidity in cells Rapid responses examples ▪ Venus fly trap shutting ▪ Leaves closing ▪ Petals closing https://www.youtube.com/w atch?v=BLTcVNyOhUc Leaf Abscission Trees in temperate countries shed their leaves in autumn. Survival advantage Reduces water loss through leaf surfaces Avoids frost damage Avoid fungal infections through damp, cold leaf surfaces Plants have limited photosynthesis in winter How do leaves fall? Abscission (leaves being shed) Cytokinins stop leaves senescing (aging turning brown), by ensuring they have a good supply of nutrients, when levels drop senescence starts Senescence lowers auxin production Cells in the abscission zone become more sensitive to ethene Lower auxin concentration also increases ethene concentration Ethene raises cellulase production which digests cell walls in abscission zone Abscission