Hydrotropism

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Hydrotropism is a directional growth response in which the direction is determined by a gradient in water concentration but a most common example is that of plant roots growing in humid air bending toward a higher relative humidity level. Hydrotropism is very difficult to observe for roots grown in soil for several reasons, such as:

Roots in soil are not readily observed without disturbing the soil.
Gravitropism of roots are usually stronger than root hydrotropism (Takahashi et al., 2003)
Water is not a strongly directional factor unlike gravity for gravitropism, light for phototropism and touch for thigmotropism. Water readily moves in soil and soil water content is constantly changing so any gradients in soil moisture are not stable.
The relative humidity in soil pores is almost 99% even when the soil water content is low enough that the plant is permanently wilted unless more water is added to the soil. Relative humidity gradients in soil will be below those required for hydrotropism for roots in humid air.

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 (Eapen et al., 2003). Hydrotropism may have importance for plants grown in space, where it may allow roots to orient themselves in a microgravity environment. (Takahashi et al., 1992).

[edit] Misconceptions

The greater growth of roots in moist soil zones than in dry soil zones is not usually a result of hydrotropism (Hershey, 1992). Hydrotropism requires a root to bend from a drier to a wetter soil zone. Roots require water to grow so roots that happen to be in moist soil will grow and branch much more than those in dry soil.
Roots cannot sense water inside intact pipes via hydrotropism and break the pipes to obtain the water.
Roots cannot sense water several feet away via hydrotropism and grow toward it. At best hydrotropism probably operates over distances of a few millimeters