Gravitropism in Plants

[[DDS] Samer]

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I was currently reading the following in Cliffs and I don't really understand the two points mentioned about the horizontal stem and root. For the point about the stem, what is meant by "the lower side" and "upper side" if the stem is horizontal?

And for the root point, what does the concentration of auxins being higher in the roots than in the stems have to do with anything?

Any help is much appreciated.

Gravitropism (or geotropism), the response to gravity by stems and roots, is not well understood. In general, both auxin and gibberellins are involved, but their action depends on their relative concentrations 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), moves up the roots, and, as in stems, concentrates on the lower side of the root. However, in roots, auxin inhibits growth. This is because concentrations of auxin are higher in roots than in stems.

 

[sshar43]

Hello! To start off, auxins promote growth in the shoots/stems, while inhibiting growth in the roots.

When talking about stem elongation, if you look at a stem that is horizontal (as in this picture: http://assoc.garden.org/courseweb/course1/week2/images/c1w2-t.gif ), the auxins will accumulate towards the bottom side of the stem to increase the growth/elongation of cells there and thereby make the stem grow upwards (as seen by the upwards curve of the stem).

Since auxins inhibit growth in the roots, more auxins will accumulate towards the bottom of the root (as in this picture: http://home.earthlink.net/~dayvdanls/AuxinInhib.gif), causing inhibition of growth at the bottom and causing elongation of the cells at the top. This causes the root to grow downwards.

Hope this helps!