The Importance of Root-Cap Mucilage for Plant And Soil

Abstract

One of the significant differences between root cap and root body is their growth direction. The oldest cells of the root cap lie on the periphery and are not protected against the environment. Therefore they can take up water from the outside very easily. As a consequence, their elongation is much greater than that of the inner cells. Thus, the tissue is formed like an envelopment over the root body, similar to a cap. The tissue produces more or less great amount of mucilage which covers the root body after the proximal end of the root cap tissue. The tissue and the mucilage hinder the water entrance in the cells of the root body from the outside. The production of mucilage is closely connected with the development of the cap tissue. The youngest cells of the root cap receive the carbohydrates in the form of transported sugar. No starch granules, or only small ones, are found in these cells. In the older cells the sugar is transformed to starch. These cells are filled with large starch granules which are the source for the production of mucilage. The mucilage accumulates between plasmalemma and cell wall and in the intercellular. At the same time the middle lamella of the cells becomes mucilaginous and the cells loose from each other and go apart. When the water supply is high, a great amount of cells lies isolated in the mucilage. The cells continue the production of mucilage. Many Golgi complexes with active dictyosomes and areas with active endoplasmatic reticulum can be seen. The development of the tissue of the root cap explains the function of the starch granules as one step of metabolism, but not as a particular acceptor of gravity (statoliths). This also demonstrates the fact that roots of taxa such as those of ASTERACEAE, which have inulin as a storage material instead of starch, react to gravity in the same way. In general, we can distinguish in the root cap meristematic cells, storage cells, and mucilage secreting cells. The mucilage protects also the root body against bacteria and toxic substances.In the soil the mucilage improves the structure of the root channels and the stability of soil. According to its swelling and shrinking it compensates the changing of the temperature and water supply resulting from the climatic conditions. As nutrient for the micro-organisms it improves their activities in decomposing dead plant material. In addition, the mucilage supports the bending growth of roots by the strengthening of the asymmetric cover of the root body by the root cap tissue. In this way it also supports the penetration of the soil by the roots in different directions. The asymmetry of mucilage and of the root cap tissue is caused by gravity and by differences in environment conditions on the sides of the roots. These differences include temperature, water, nutrients and salt supply, as well as one-sided contact of the roots with solid material.