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Hydrotropism (hydro- "water"; tropism "involuntary orientation by an organism, that involves turning or curving as a positive or negative response to a stimulus") is a plant's growth response in which the direction of growth is determined by a stimulus or gradient in water concentration. A common example is a plant root growing in humid air bending toward a higher relative humidity level.
This is of biological significance as it helps to increase efficiency of the plant in its ecosystem.
The process of hydrotropism is started by the root cap sensing water and sending a signal to the elongating part of the root. Hydrotropism is difficult to observe in underground roots, since the roots are not readily observable, and root gravitropism is usually more influential than root hydrotropism. Water readily moves in soil and soil water content is constantly changing so any gradients in soil moisture are not stable.
Thus, root hydrotropism research has mainly been a laboratory phenomenon for roots grown in humid air rather than soil. Its ecological significance in soil-grown roots is unclear because so little hydrotropism research has examined soil-grown roots. Recent identification of a mutant plant that lacks a hydrotropic response may help to elucidate its role in nature. Hydrotropism may have importance for plants grown in space, where it may allow roots to orient themselves in a microgravity environment.
A class of plant hormones called auxins coordinates this root growth process. Auxins play a key role in bending the plants root towards the water because they cause one side of the root to grow faster than the other and thus the bending of the root.
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