Agricultural Science (AS)·Crop science · NSSCAS 3.1c
Plant growth substances & tropisms
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A bean seedling on a windowsill bends right over, leaning towards the glass — nobody moved it, the plant moved itself. A farmer dips a bare cutting in a little powder and roots burst out where there were none. What invisible chemistry is behind both wonders? In this lesson we meet the plant's own hormones — auxins and their helpers — see how Namibian farmers put them to work rooting cuttings, ripening fruit and killing weeds, and learn why shoots grow towards the light while roots grow down into the earth.
What you'll learn in this lesson
By the end you should be able to (NSSCAS Agricultural Science (AS) 3.1c):
- Explain what plant growth substances (plant hormones) are and how they control plant growth
- Describe the roles of auxins in cell elongation, apical dominance and root initiation
- Outline briefly the roles of gibberellins, cytokinins, ethylene and abscisic acid
- Outline how plant growth substances are used in agriculture: auxin rooting powder for cuttings, ethylene for ripening fruit, synthetic auxins (2,4-D) as weedkillers, and gibberellins for larger fruit
- Define gravitropism (geotropism) as growth of plant parts towards or away from gravity
- Define phototropism as growth of plant parts towards or away from the direction of light
- Distinguish between gravitropism and phototropism in terms of stimulus, and the responses of roots and shoots
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Miss Elina and Mike talk through the whole topic — with the figure and working drawn live.