Water Relation: Water is the most important factor for all the vital functions of plant. Plant cannot survive without it. It is important solvent and consist most of the parts of protoplasm .Usually 75% quantity of water is found in cytoplasm. In leaf or petals its amount is 90% of the total weight. In hydrophytes, water quantity exceed up to 98% while in xerophytes its amount 60% amount or lesser than this. Water also plays important role in the process of photosynthesis. It is a good solvent the mineral cannot be absorb unless they are not soluble in water. Most of the water enters the plant through the roots and then passes into different parts through cells and tissues. The movement of water from one cell to another cell depends upon the water relation of all plant cell. In mature plant cell, there is a large central vacuole. Vacuolar membrane or tonoplast and plasma membrane are selectively permeable whereas cell wall is freely permeable. Selectively permeable membrane allows the passage of some substances but not other some of the terms to understand water relation are diffusion, osmosis, osmotic pressure, osmotic potential, wall pressure, turgor pressure, water potential, etc.
If a solution and pure water is separated by semi-permeable membrane, the solvent molecule diffused into the solution. This diffusion of solvent molecules into the solution through semi- permeable membrane is known as osmosis. Sometimes it is also known as osmosis diffusion. In this case, there are two solution of different concentration separated by semi-permeable membrane,the diffusion of solvent will takes place from a less concentration solution into the more concentration solution till both the solution attain equal concentration.
Diffusion means to spread or to flow out or to extend in all direction or to disperse. I f a small bottle filled with some gas or vapour is opened at a certain place in a room very soon its molecule become evenly distributed throughout the available space in that room. Similarly if a solute is placed in its solvent it is dissolved and its particle move so that they are equally distributed throughout the container. This movement of particles or molecules from a region of higher concentration in order to equalize the concentration of two region is called diffusion. The rate of diffusion or gases is faster than liquid and solid. The solid substances are also diffuse in liquid substances but with a slow speed. The pressure exerted to tendency of the particles of a substance to diffuse is known as diffusion pressure which is directly proportional to the number or concentration of the diffusing particles. Therefore diffusion pressure takes place always from a region of higher diffusion pressure to a lower diffusion pressure i.e along the diffusion pressure gradient. Diffusion is influence by the density of diffusing particles, temperature, distance, concentration gradient.
Ascent os SAP
Plants absorb water from the soil. Water after being absorbed by the roots is distributed to all parts of the plants and the excess of it is loss through transpiration. In order to reach the top most part of the plant , water has to move upward through the stem. This upward movement of sap from the root to the tip of the plant is called the transport of water and minerals or the ascent of sap.It can be studied under two headings. Pathway of ascent of sap and Mechanism of ascent of sap.
The mesophyll cells of the leaf withdraw water from the xylem tissue by osmotic diffusion and becomes turgid. From the surface of the turgid cells, water gets evaporated andcollects in the intercellular spaces. Thus, the air present in the intercellular spaces becomes saturated while the atmospheric air is usually unsaturated except just after the heavy rain due to which the water vapour diffuses outwardly from intercellular spaces to the environment through stomata, cuticle and lenticleDepending upon the plant parts involved in the transpiration, the transpiration is classified into three types; Stomatal Transpiration , Cuticular Transpiration and Lenticular Transpiration.