Plant Tropisms and Their Influence on Growth
Explore how plants respond to environmental stimuli through tropisms such as geotropism and phototropism. Learn about the role of auxins in positive phototropism and understand how plant growth movements are influenced by factors like gravity and light.
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Presentation Transcript
ASSET TUTORING REVISION 2020 RESPONDING TO THE ENVIRONMENT: PLANT GROWTH 2 TROPISMS AND PRACTICALS
Tropisms TROPISMS are plant growth movements where the direction of the stimulus determines the direction of growth movement. A plant responds to a stimulus by growing towards the stimulus = positive tropism OR By growing away from the stimulus = negative tropism. LEARN ONLY: GEOTROPISM growth response of shoots and roots to the stimulus of gravity PHOTOTROPISM - growth response of shoots and roots to the stimulus of light
Phototropism 1 NORMAL growth in plant shoots/stems Describe how stems grow upwards (positive phototropism) AND include the stimulus in your description When light comes from all sides of the stem tip Auxins move downwards and are evenly distributed inside the tip of the stem Cells in the middle of the stem grow faster and elongate more Therefore the stem grows upwards
What is the role of the auxins in positive phototropism?
Phototropism 2 Describe how stems bend toward UNILATERAL light AND include the stimulus in your description Light comes from one side onto a plant stem tip Auxins move to the shaded side There are fewer auxins on the side with the light There is faster cell growth and more cell elongation on the shaded side Therefore the shaded side grows faster and The stem tip bends and grows towards the light This is positive phototropism Advantage of positive phototropism: Plant gets more light for photosynthesis and therefore more food
Geotropism 1 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.
Geotropism 2 Roots Describe how roots grow downwards (positive geotropism) AND include the stimulus in your description A plant is lying on its side which means the roots are lying horizontally Auxins are produced at the tip of the root More auxins diffuse/move to the lower side of the root Less auxins are at the upper side of the root Less auxins cause MORE elongation and growth on the upper side of the root More auxins (in roots) cause LESS elongation and growth Therefore the root tip grows downwards towards the stimulus of gravity Advantage: roots grow downwards to anchor the plant in the soil and plant roots grow towards water
Geotropism 3 Shoots/stems Describe how stems grow upwards (negative geotropism) AND include the stimulus in your description A plant is lying on its side which means the stem is also lying horizontally Auxins are produced at the tip of the stem More auxins move/diffuse to the lower side of the stem Fewer auxins are at the upper side of the stem More auxins on the lower side causes MORE cell growth and more elongation Less/fewer auxins cause less growth and elongation on the upper side Therefore the stem tip grows away from the stimulus of gravity Advantage: stem grows away from gravity and towards light
Clinostat for control experiments contains a cork disc which can be set to rotate in a vertical or horizontal plane by a motor to make factors uniform (evenly distributed)
Practical work on Geotropisms Independent variable: direction of force of gravity Dependent variable: direction of root growth Note the following fixed variables Identical apparatus at X and Y including clinostat and blotting paper Same number of identical bean seedlings Both X and Y are kept in the dark to prevent light affecting the experiment
plastic cover lined with moist blotting paper seedling in X X in complete darkness seedling in Y Y Does the direction of growth of the seedlings in clinostats X and Y change ? Ans: The radicles of the seedlings in clinostat X bends downwards in response to the pull of gravity BUT every side of the radicle of the seedling in clinostat Y is acted on by gravity, the radicle will grow straight horizontally.
plastic cover lined with moist blotting paper seedling in X X in complete darkness seedling in Y Y What is the purpose of setting up clinostat Y ? Ans: It serves as a control for comparison.
Practical work on Phototropism Note the following fixed variables: Same size light-proof boxes Same number of bean seedlings Same size of bean shoots/stems Same size pots Independent variables: direction of light Dependent variable: direction of growth of stem
A B C unilateral light unilateral light light-proof box clinostat in complete darkness What has happened to the shoots of plants A, B and C ? Ans: The shoots in pot A respond by growing towards the light source. The shoot in pot B do not show any curvature but grow vertically upwards. The shoots in pot C grow vertically upwards with slender and longer stems, and carry smaller leaves which are yellow in colour.
A B C unilateral light unilateral light light-proof box clinostat in complete darkness What is the effect of the clinostat on the shoot of plant B ? Ans: The revolving clinostat cancel out the effect of unilateral light on the shoot of plant B.
Go to Q & A on Plant Growth Responses Good luck with your studying for the finals
