Absorption and transport of water in plants . They are essential functions for the growth and development of plants .
Water can be absorbed through virtually any plant surface, but in land plants almost all of the water is absorbed through the roots and only a small portion through the aerial organs. The water is absorbed fundamentally by the radical hairs and other areas of the root , and then its transport to the aerial part must be carried out by the tissues of the xylem.
The xylem is the most important tissue in the transport of water, it is formed by several different types of living and non-living cells , among which the tracheal elements can be pointed out, through which practically all the transport of water is carried out , Living fibers and parenchymal cells are also found in the xylem.
The tracheal elements (tracheas and tracheids) constitute the water conductive system and are the cells most directly related to the transport of water in the plant. Both are more or less elongated, have lignified secondary walls and are dead during their functional phase. There is no cytoplasm inside that prevents the passage of the liquid, which facilitates the efficient transport of waterin relatively large quantities. The apical walls are perforated, typical characteristics of both tracheal and tracheal cells. In the more evolved vessels, the end walls may be completely missing, so there is no obstacle to prevent the passage of water through the cell .
If we take a large number of tracheal cells and splice them at their ends, we will obtain a long tube-shaped structure. This is exactly the arrangement that they find in the cells that form the vessels, joined together by their terminal walls, and form what is called a vessel or xylem duct. The vessels of the xylem tissue form a network of ducts that extend through all regions of the plant, and provide all living cells with an easily accessible water supply, being of primary importance to the plant, not only to maintain its turgidity. , but also for the transport of other substances (for example mineral elements) that can be carried from one cell to another by the movement of water.
The set of vessels is the main route by which water is transported in angiosperms. However, there are no vessels in conifers, and in this group it is the tracheids that constitute the main route of water circulation. These tracheids are long spindle cells, provided with sharp angled, perforated end walls. These end walls of the tracheids overlap, and thus constitute a continuous path for the movement of water. Of course, the movement of water in a group of tracheids is much less direct and encounters much greater resistance than in a trachea system.
Although the tracheas and tracheids are oriented in the plant in a vertical direction and the movement of water takes place predominantly in this direction, there is also some lateral movement. The side walls of the tracheas and tracheids are punctured by numerous pits, pores through which water can pass.
Absorption of water by the roots
Practically the plants carry out all the absorption of water through the root system and mainly through the root region where the absorbent hairs are found. The water that penetrates through the absorbent hairs and other cells of the epidermis of the root , does so as a result of a diffusion pressure gradient. Regularly, the diffusion pressure deficit of the root cells is greater than that of the soil dissolution , whereby water penetrates into the roots coming from the soil. As the solute concentration of the cells increases or the turgor pressure decreases, the cell diffusion pressure deficit will increase and as a result the absorption of water will increase.
We can affirm that most of the absorption of water takes place by means of an osmotic mechanism (passive absorption). However, some researchers estimate that there may be some active, non-osmotic absorption, from which an expenditure of metabolic energy is required ; so much so that theories that explain water absorption are often called passive absorption theories and active absorption theories.