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Plant responses Plant Hormones • Like animals, plants also use hormones to control internal activities but unlike animals they have no nervous system. • The hormones they release are not from glands but from different types of cells. • Instead of blood the plant hormones travel through a series of tubes called phloem. How do plants always grow the right way up? What conditions will affect how a plant grows? What do plants respond to? Plants are very sensitive and their growth is affected by their environmental conditions. water light gravity A condition that affects plant growth is called a stimulus. What are the three types of stimuli that plants respond to? What is a tropism? Plants respond to stimuli by growing to or away from them. A growth movement in response to a stimulus is a tropism. water light gravity Movement towards a stimulus is called a positive tropism. What is a movement away from a stimulus called? A negative tropism. Different types of tropism Plant responses to light, water and gravity are given special names. water hydrotropism light phototropism gravity geotropism The question is, which parts of a plant respond to these different stimuli? Plant response to Light How do plants respond to light? A plant’s response to light is called phototropism. Is phototropism a positive or negative tropism? light Plants grow towards light, which is a positive tropism. Why do plants grow towards light? Plants need light for photosynthesis, so they respond to light by growing towards it. Phototropism and auxins Which part of a plant is involved in phototropism? A growing shoot on a plant responds to light. light Auxins are made in the tip of a growing shoot and move down the stem. These plant hormones speed up growth in a growing shoot. Phototropism and auxins – light from above light When a shoot gets light from above, the auxins produced at the tip are spread out evenly in the shoot. How will this affect the growth of the shoot? Phototropism and auxins – light from above Why does a shoot grow straight up when light is from above? There is an equal amount of auxins in all parts of the shoot, which grow at the same rate, so the shoot grows straight up. light light Phototropism and auxins – light from one side When a shoot gets light from one side, the auxins are not evenly spread out in the shoot. light How will this affect the growth of the shoot? Phototropism and auxins – light from one side Why does the shoot bend towards the light when it gets light from one side? There are more auxins on the shaded side of the shoot, so the shaded side grows faster making the bright side bend towards the light. light light Phototropism experiment Plant response to Gravity How do plants respond to gravity? A plant’s response to gravity is called a geotropism. Different parts of a plant have different responses to gravity. shoots grow up negative geotropism gravity roots grow down positive geotropism Are these responses positive or negative geotropisms? Why is important that roots grow down into the soil? Geotropism and auxins Auxins speed up growth in shoots and slow growth in roots. If a plant is laid on its side, the auxins produced collect in the lower side of the root and stem. How will the auxins affect the growth of the root and stem? Geotropism and auxins If a plant is laid on its side, why does the root grow down and the stem grow up? Auxins slow down growth on the lower side of the root, so the root curves down. Auxins speed up growth on the lower side of the stem, so the stem curves up. Plant response to water How do plants respond to water? A plant’s response to water is called hydrotropism. Roots always grow towards water, a positive tropism. dry soil wet soil Roots will grow sideways or even upwards, towards water. Do roots have a stronger response to water or gravity? Roots always have a stronger response to water than gravity to ensure that a plant gets the water it needs. Hydrotropism and auxins – water all around When roots are well watered from all sides, the auxins produced in the roots are spread out evenly. How will this affect the growth of the roots? wet soil wet soil Hydrotropism and auxins – water all around Why do roots grow out in all directions when they are well watered from all sides? There is an equal amount of auxins in all parts of the roots, which grow at the same rate, so the roots grow out in all directions. wet soil wet soil wet soil wet soil Hydrotropism and auxins – more water on one side When roots are in soil with more water on one side, the auxins are not spread out evenly in the roots. How will this affect the growth of the roots? dry soil wet soil Hydrotropism and auxins – more water one side Why do roots grow towards the wet side when they are in soil with more water on one side? There are more auxins on the wetter side of the roots, which grows slower than the dryer side, so the roots bend towards the water. dry soil wet soil dry soil wet soil Plant Hormones Hormone What it does Auxins Cytokinin •Released by the tip of shoots •Causes cell elongation (growth) •Stops side branches growing to near the tip. •Involved in phototropism. •Involved in geotropism • Involved in plant cell division Gibberellins •Growth of stems when flowers start to form. •Involved in seed germination. Aba •Produced by chloroplasts. •Inhibits the growth of the plant. •Closes stomata in times of drought. •Produced in response to auxin in ripening fruit. •Promotes ripening. •Inhibited by carbon dioxide. •Makes starch turn to glucose. Ethylene