What is Hydrotropism?
tropism is the directed growth towards (positive tropism) or away (negative tropism) from a stimulus. The following tropisms are the also important for plants:
Hydrotropism is a plants growth response to water concentrations. The response can be positive (towards the water) or negative (away from the water). Roots, for instance, are positively hydrotropic. That means that they grow towards moist soils to avoid draught stress. Once a root cap has sensed water it bends and then the root grows towards it.
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.
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.
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.