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result from movement of auxins accross shoot/root if exposed to light stronger on one side than the other
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all-round light results in straight growth, but if brighter on one side/unilateral, shoots grow towards light (positive trophic) so shoots recieve all-round light and roots grow away (negatively trophic) so if return to soil after heavy rain (where it emerges from soil)
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Auxins role in phototropisms in unilateral light
side of shoot exposed to light has less auxins than unlit side as auxins move laterally accross shoot and build up on unlit side, causing cell elongation of unlit side and so growth towards stimulus (light), auxin levels always same, just reside more on unlit sides (so disproves that light kills auxins), proved with shoots in unilateral light (one normal and other split), normal one has more auxins on unlit side, but split one has similar auxin concs both sides (shows auxin transported from lit to dark side in unilateral light)
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Practical investigations into phototropisms
- can germinate and grow seedlings in conditions of dark, all-round light and unilateral light, and observe patterns of growth
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growing in the dark
in dark, plants grow upwards rapidly to to reach light for photo (etiolation), this extreme elongation of internodes caused by gibberellins (whose levels fall in light presence), but slow growth in light valuable to form leaves, strengthen stems and grow
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