CliffsNotes AP Biology by Phillip E. Pack

Author:Phillip E. Pack
Language: eng
Format: epub
Publisher: Houghton Mifflin Harcourt
Published: 2013-03-19T16:00:00+00:00

2. Gravitropism, the response to gravity by stems and roots, is not well understood. Amyloplasts, specialized, starch-containing, cell organelles, sink to the bottom of cells in response to gravity, and, thus, may be associated with the detection of a gravitational field. In general, auxin is involved, but its action depends on its relative concentration and the target organ (root or stem). The role of auxin appears to agree with the following:

If a stem is horizontal, auxin produced at the apical meristem moves down the stem and concentrates on its lower side. Since auxin stimulates cell elongation, growth of the lower side is greater than that of the upper side, and the stem bends upward as it grows.

If a root is horizontal, auxin is produced at the apical meristem (root tip). The obvious hypothesis is that the auxin moves away from the root tips, and, as in stems, concentrates on the lower side of the root. But in order for roots to bend down, auxin must inhibit growth. Perhaps the concentration or sensitivity to auxin is different in roots than it is in shoots. Additional research is necessary to provide a more complete explanation of gravitropism in roots.

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