Hydrotropism• By

• Slide_Maker4u• (Abhishek Sharma)

Introduction• Hydrotropism (hydro- "water";

tropism "involuntary orientation by an organism, that involves turning or curving as a positive or negative response to a stimulus")[1] 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.[2] Water readily moves in soil and soil water content is constantly changing so any gradients in soil moisture are not stable.

This qs, 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.[3] Hydrotropism may have importance for plants grown in space, where it may allow roots to orient themselves in a microgravity environment.[4]

Mechanism

• 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.

The greater growth of roots in moist soil zones than in dry soil zones is not usually a result of hydrotropism.[5] 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 couple millimeters.

Misconceptions

Why is being Hydrotropic so Important for Plants?

This ability to bend and grow the root towards a moisture gradient is essential because plants need water to grow. Water, together with soluble mineral nutrients, is taken up by the root hairs. Then, in vascular plants (also called higher plants or tracheophytes), water and minerals are transported to all parts of a plant through a lignified transport system called xylem. The second transport system in vascular plants is called phloem. The phloem also carries water, not with soluble minerals, but mainly with soluble organic nutrients instead. These organic nutrients are delivered by photosynthesis and are called photosynthate.

Actually, this plant growth

response is not easy to study. Studies are conducted in labs and not in the natural environment. Yet, more and more is learned about the complex nature of this plant growth process. Popular plants to study this effect are: pea plant (Pisum sativum), corn plant (Zea mays) and thale cress (Arabidopsis thaliana).

A study showed that a gene in the roots is essential for the ability to detect water. Plant mutants that lack this gene, showed no response to moisture.

Studies

Actually, this plant growth response is not easy to study. Studies are conducted in labs and not in the natural environment. Yet, more and more is learned about the complex nature of this plant growth process. Popular plants to study this effect are: pea plant (Pisum sativum), corn plant (Zea mays) and thale cress (Arabidopsis thaliana).

A study showed that a gene in the roots is essential for the ability to detect water. Plant mutants that lack this gene, showed no response to moisture.

Scientists are also interested in the interaction between hydro- and gravitropism. The findings are important to understand how plants grow in space and therefore zero gravity. Here on earth, generally spoken, root growth direction is the result of positive hydrotropism and negative gravitropism, yet other environmental stimuli (tropisms) may interfere and interact as well.

These processes simply need to be more studied. Every scientific study will increase our understanding of these complex mechanisms.