Well, I can't say what the narrator was basing his/her statement upon, but
it can be justified. Roots are, for the most part, programmed to grow
downward (positive geotropism), and in many if not most soil environments,
water is more abundant at lower levels. Thus, growing downward means
growing toward water. (One situation where this doesn't hold true is right
after a rain, when the upper level of soil may be wetter then a lower
level.)
Additionally, roots (like all protoplasm-based structures) need water to
grow. Thus, roots that are tentatively entering a wet zone (assuming it's
not too wet) will grow faster than roots entering a dry zone, thus setting
up a positive feed-back loop regulating root growth, which obviously has a
genetic component.
Finally, you may consider differential mortality of root cells in wet verus
dry soil regions as also being a genetic program that causes the root SYSTEM
to grow toward water.
I think this is enough to justify the statement. Of course, the statement
is a bit misleading in that it may call up the image of a little root tip
sniffing first in one direction, and then in another, and finally saying,
"Ah, the water's over there; I'd better grow in that direction." But this
sort of imagery often colors our understanding of mechanistic processes in
biology, even for scientists (for me, at least).
Martin M. Meiss
2011/5/19 Wayne Tyson <[email protected]>
> Ecolog:
>
> I just surfed across a "science" program on the "History" channel. The
> narrator said "Plant roots are genetically programmed to seek water . . ."
>
> Will someone please inform me of the basis for this statement?
>
> WT
