It turns out that the light signal apparently causes increased synthesis on both lighted and shaded sides. However, synthesis is about three-fold greater on the shaded side. On the other hand, the synthesis was not due to NEW synthesis. 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.
Roots are clearly more sensitive to auxin concentration than are coleoptiles. It takes more auxin to stimulate coleoptiles than it does to stimulate roots. For example the cells that respond to the light-stimulated auxin are located a certain distance down a coleoptile. In the gravity perception cells, starch grains fall against the inner and outer tangential walls of the columella cells in the root cap. This perception process is shown in Figure 19.24 on Page 569 .