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Auxins • Stimulate cell division and elongation in stems and roots • Regulate cell expansion during photo- and geotropisms • Found in developing leaves and seeds and shoot tips Example: indole-3-acetic acid (IAA) Cytokinins • Stimulate cell division • Prevent ageing of leaves • Root tips and leaves Example: Zeatin Tobacco plant on the left was gentically engineered to produce cytokinin in mature leaves Apical Dominance Gibberellins • Stimulate cell elongation • Stimulate seed germination • Stimulate fruit growth Example: gibberellic acid • Found in young shoots and developing seeds Grapes on the right treated with GA Ethylene • Promotes ripening of fruit • Found in all parts, especially when under stress, ageing, or ripening Abscisic acid • Induces and maintains seed dormancy • Inhibits shoot growth • Closes stomata • Found in mature leaves, fruits and plants under stress Plants not watered for 14 days - plant on the right treated with synthetic ABA Phototropism growth towards light as a result of auxins Auxin builds up on the shady side causing these cells to lengthen Gravitropism growth in response to gravity Stems grow against gravity and roots grow with gravity Auxin is involved Thigmotropsim Growth in response to touch Cells not touching the object grow longer causing the stem to bend
