Re: What is Hydrotropism when you have a been seedling?

This is a question that I have spent a lot of time considering because I wrote 
an article about it for a science teaching journal. Hydrotropism does not occur 
in soil grown roots as shown in a classic study in 1936. Hydrotropism would 
require that a root in dry soil bend and grow away from dry soil toward moist 
soil. However, that generally did not occur in the 1936 study of 26 species. 
What does happen is that roots that happen to grow into a zone of moist soil 
grow more and branch more than roots that happen to grow into a zone of dry 
soil. This occurs because roots require water to grow. Many people have 
misinterpreted that pattern to be hydrotropism, even though they did not observe 
roots that grew into dry soil actually bending and growing away from the dry 
soil toward moist soil. The hydrotropism experiments in school science project 
books make that same error. The key to a tropism is that it requires a bending 
in response to a directional environmental factor such as light or gravity.

In recent years, some scientists have found that roots grown without soil in 
humid air can exhibit a weak hydrotropism. Mutant plants whose roots lack the 
normal gravitropism response (gravitropism is the preferred term for geotropism) 
show the strongest hydrotropism. However, even the strongest hydrotropism is 
very artificial because it occurs in laboratory chambers in humid air, not in 
soil where roots naturally grow. Unlike hydrotropism, plants grown in soil 
readily exhibit phototropism and gravitropism.


References

Hershey, D.R. 1992. Is hydrotropism all wet? Science Activities 29(2):20-24.

Jaffe, M.J., Takahashi, H. and Biro, R.L. 1985. A pea mutant for the study of 
hydrotropism in roots. Science 230:445-447.

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