While polar auxin transport establishes the orientation of growth, lateral (side-to-side) redistribution of auxin is the mechanism that explains phototropism. This redistribution is carried out by an auxin carrier protein that moves to one side of the cell and thus allows auxin to exit the cell only from that side.
When light strikes a grass coleoptile on one side, auxin at the tip moves laterally toward the shaded side. The imbalance thus established is maintained down the coleoptile, so that in the growing region below, there is more auxin on the shaded side, speeding growth on that side and causing the coleoptile to bend toward the light. This bending toward light is pho-totropism (Figure 38.10a). If you have noticed a house plant bending toward a window, you have seen phototropism.
Even in the dark, auxin moves to the lower side of a shoot that has been tipped over, causing more rapid growth in the lower side and, hence, an upward bending of the shoot. Such growth in a direction determined by gravity is called gravit-ropism (Figure 38.10b). The upward gravitropic response of shoots is defined as negative gravitropism; that of roots, which bend downward, is positive gravitropism.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.