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Plant Auxins Noadswood Science, 2011 Monday, May 22, 2017 Plant Auxins To understand how plant auxins function Which Way How do plants always ‘know’ which way is up when they grow?! Which Way Plants are very sensitive and their growth is affected by their environmental conditions (a stimulus) water light gravity Tropism Plants respond to stimuli by growing to or away from them – a growth movement in response to a stimulus is a tropism (towards stimulus = positive tropism, away from stimulus = negative tropism) Water Hydrotropism Light Phototropism Gravity Geotropism Tropisms Phototropism (Light) A plant’s response to light is called phototropism – plants grow towards light, which is a positive tropism (plants need light for photosynthesis, so they respond to light by growing towards it) 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 (Light) light 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 Phototropism (Light) light light 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… Phototropism Experiment Setup the phototropism experiment as shown in the diagram using seedlings and a light source on one side of the box… Geotropism (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 Roots grow down Gravity Positive geotropism Geotropism (Gravity) 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 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. Hydrotropism (Water) A plant’s response to water is called hydrotropism – roots always grow towards water, a positive tropism Roots will grow sideways or even upwards, towards water – roots always have a stronger response to water than gravity to ensure that a plant gets the water it needs Dry soil Wet soil Equal Hydration When roots 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 Unequal Hydration When roots are in soil with more water on one side, the auxins are not spread out evenly in the roots Dry soil Wet soil Dry soil 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 Wet soil Agriculture Plant hormones can be used in gardening and agriculture to control how plants grow and develop… Plant hormones are naturally-occurring chemicals but can also be produced synthetically for the following commercial uses: Growing cuttings Ripening fruit Killing weeds Cuttings A cutting is a part of plant that is removed from the main shoot A cutting does not have roots and so has no supply of water or minerals… Rooting powder contains plant growth hormones – dip a cutting into rooting powder and the plant hormones stimulate the cutting to grow new roots Rooting Powder Weed Killers Cereal crops like wheat are grasses with narrow leaves – most weeds that grow in fields and gardens have broad leaves Selective weed killers contain plant hormones at concentrations that will only affect plants with broad leaves These selective weed killers upset the normal growth of broad-leaved plants and so kill off any weeds – the growth of cereal crops and grasses is not affected Ripening Fruit Fruit is usually ripe and ready to eat when it is bought from a greengrocers or supermarket – fruit farmers are able to control when fruit ripens using plant hormones Fruit that is picked before it is ripe is usually hard – there is less chance of unripe fruit being damaged when transported, which is important to the farmer and the buyer Plant hormones can be sprayed onto unripe fruit during transportation – these hormones ripen fruit in time for its arrival in the shops (but you should wash your fruit before you eat it)!
