hydrotropism

David Hershey dh321 at MailAndNews.com
Wed Jan 31 21:48:11 EST 2001


>===== Original Message From Jon Greenberg <jongreen at bluemarble.net> =====
>Thank you for your respnse, David. I'm a bit confused by all this. The 
question
>about this lab activity was actualy raised by someone who had seen your 
paper.
>In light of all this, I am still left with some basic questions:
>
>1) How should we interpret experiments in which roots seem to grow toward 
water
>in soil, or at least to grow more in wetter soil? Is this hydrotropism or 
isn't
>it?

Greater root growth in wetter soil zones compared to drier soil zones is not 
evidence of root hydrotropism, although it has often been misinterpreted as 
such. It merely demonstrates that root growth requires water. By analogy, 
greater plant growth under sunny compared to shady conditions would be not be 
considered an example of phototropism. Salisbury and Ross's 1985 "Plant 
Physiology" text (p. 115) has drawings showing a similar greater root growth 
in soil zones with higher mineral nutrient levels than in soil zones with 
lower mineral nutrient levels. That is not considered a nutrient tropism, just 
greater growth of roots in soil zones rich in nutrients.

To demonstrate positive hydrotropism of roots in soils, one needs to observe a 
bending response where roots grow from wetter into drier soil and then bend 
back into the wetter soil zone. The classic study on root hydrotropism in soil 
was by Loomis and Ewan (1936). In their introduction, they noted that "A 
positive response of roots to moisture stimuli is very commonly assumed.... 
The hydrotropic responses of roots in air are of considerable academic 
interest, but they appear to have little relationship to the development of 
plants under normal conditions."

They concluded that "The primary roots of many plant species have shown no 
tendency to bend when growing from the moist soil layer with available 
moisture into the dry soil with insufficient moisture to maintain growth. The 
typical response in this group of plants has been a growth along the lines 
determined by the positions of the seed and the normal response to gravity, 
until the root has grown into the dry soil. Under these conditions branching 
was stimulated in the portions of the root remaining in the moist soil. These 
branches grew normally outward and down without showing hydrotropic response, 
and continued in the moist soil or passed from it into the dry soil, depending 
upon their initial direction of growth. Those which grew into the dry soil, 
with favorable food conditions, again formed branches which repeated the 
response of the earlier roots."

In some experiments, Loomis and Ewan did find some weak hydrotropic responses 
but only with a few species and never at the virtually 100% level you find 
with gravitropism and phototropism.  Their overall conclusion was "Apparently 
these results demonstrate the possibility of a response in soil similar to 
that which has been obtained in the air. On the other hand, they seem to show 
equally clearly that hydrotropism is not a universal and probably under field 
conditions not a common plant response."


>
>2) If roots simply grow more in wetter soil because they can better maintian
>turgor there that they need for growth, does it follow that hydrotropism is 
not
>a true tropism any more than a growth repopnse to, say, N or other nutrients, 
or
>is it the case on the other hand that this is merely the mechanistic 
explanation
>of the response, but knowing how it works makes it no less a tropism?
>


You could make a case that hydrotropism is a true tropism but it apparently 
differs from phototropism and gravitropism in that it occurs only in certain 
species and under more exacting conditions. Also, unlike phototropism and 
gravitropism, hydrotropism has not been shown to play a major role under 
natural conditions, i.e. roots growing in soil. Another problem with 
hydrotopism as a true tropism is that water is not as clearly a directional 
factor, a required condition for a tropism, as are gravity, light and solid 
objects (for thigmotropism). Water flows readily in soils and soil moisture 
varies over time so a stable moisture gradient is not that common in soils. In 
comprison, the direction of gravity and solid objects are constant, and light 
direction is also much more consistent. Even Loomis and Ewan's experiments 
were somewhat artificial because they had an unnaturally sharp transition from 
wet to dry soil zones. In one experiment, seeds in a small zone of moist soil 
were surrounded by dry soil below and on 3 of 4 sides.

Many of the hydrotropism experiments I have seen in the teaching literature do 
not actually look at root bending so they are not proper tropism experiments. 
Those that do look at root bending use artificial situations such as roots 
growing out the bottom of a flowerpot. There is no shortage of well-designed, 
inexpensive, easy, and relatively sure-to-work teaching experiments with 
plants, such as those on mineral nutrition, soil pH, soil salinity, 
hydroponics, plant hormones, photosynthesis, photoperiodism, germination, 
water relations, gravitropism, phototropism, carnivorous plants, etc. Given 
the technical difficulty of hydrotropism experiments, I don't see an advantage 
of a hydrotropism experiment at the high school level to demonstrate a 
phenomenon that appears to be of minor importance.

For some reason, hydrotropism has generated a lot of bad scientific reasoning 
in the biology teaching literature. A good example is in the otherwise good 
book by Suzuki (1985) who makes the illogical claim that "In a way that we 
don't fully understand, roots can sense where water is and usually head right 
for it. Roots sometimes cause trouble for us by wrapping themselves around 
sewer pipes - they're just trying to get at the water inside."  Roots cannot 
sense water inside nonleaking pipes. If roots grow around sewer pipes, it is 
usually because the pipes are leaking and providing both water and mineral 
nutrients required for root growth, not because roots have some sort of ESP to 
detect water inside a nonleaking pipe.

References

Loomis, W.E. and Ewan, L.M. 1936. Hydrotropic responses of roots in soils. 
Botanical Gazette 97:728-743.

Suzuki, D. 1985. Looking at Plants. New York: Warner Books.



>(Please respond to my other e-mail address (and the list, if you like)--
>
>jon.greenberg at phschool.com
>
>Many thanks,
>
>Jon Greenberg
>
>David Hershey wrote:
>
>> The recent research on hydrotropism that I have seen has been done with
>> roots
>> grown in humid air rather than in soil, thus "in humid air" must become a
>> part
>> of the modern definition of hydrotropism. Even at close to the soil
>> permanent
>> wilting point of -1.5 MegaPascals water potential, the soil relative
>> humidity
>> is about 99%. Based on the humid air hydrotropism research, it seems that
>> there would not be a large enough relative humidity gradient in soil to
>> induce
>> a hydrotropic response. Therefore, hydrotropism seems more of a laboratory
>> phenomenon rather than one important under natural conditions. It may have
>> application in microgravity conditions of outer space.
>>
>> I wrote an article examining some of the classic precollege teaching
>> experiments on hydrotropism:
>>
>> Hershey, D.R. 1992. Is hydrotropism all wet? Science Activities 
29(2):20-24.
>>
>> >===== Original Message From jongreen at bluemarble.net (Jon Greenberg) =====
>> >Hello again, plant folks.
>> >
>> >I am editing a high school biology lab manual that is under revision,
>> >and came across an activity to demonstrate hydrotropism. I recall
>> >learning in grad school about 20 years ago that roots grow toward water.
>> >However, I understand that this has been questioned and some do not
>> >agree that there is such a thing as hydrotropism.
>> >
>> >Can anyone enlighten me on this point/
>> >

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