Our seeds are sprouted and curving toward the light. The curvature of the coleoptile is thought to be due to a difference between the auxin levels on the two sides of the coleoptile. Went found that unilateral light caused more auxin to be produced in agar blocks under the shaded side of coleoptile tips (compared with blocks under the lighted side).
I hope you have some questions about the light causing phototropism based on your understanding of photosynthesis and seed germination in lettuce!
What photon flux of light is needed? Again, the required photon flux is very low.
How about the wavelength? This was thoroughly investigated, and phototropism is maximally stimulated by blue-light. The blue-light photoreceptor is not yet known but is suspected to be either riboflavin or a carotenoid of some sort. It expected that there will be an announcement on this very soon. Rumblings at national meetings indicate that we should know this very soon.
How does light accomplish the needed distribution of auxin across the coleoptile?
It has taken many decades of work to figure that out.
It is important that we also realize that amount of hormone is not the only determining factor in plant growth responses. In the late 1970s it was pointed out that sensitivity of the responding cells was a critical factor.
Sensitivity is different in different tissues. In the example graph above, roots are clearly more sensitive to auxin concentration than are coleoptiles. It takes more auxin to stimulate coleoptiles than it does to stimulate roots.
It is also true that sensitivity changes through development...and this is largely unexplored but important. For example the cells that respond to the light-stimulated auxin are located a certain distance down a coleoptile. Younger cells above this zone and older cells below this zone do not respond even though the auxin differential is essentially the same as for cells within the responding zone!
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