Water potential(Ψ w)
The movement of substances in plants depends on water potential which is represented by the symbol ψ. Water potential(Ψ w) is a fundamental concept that helps us to understand water movement. Solute potential (Ψ s) and pressure potential(Ψ p) are the two main components that determine water potential.
In both liquid and gaseous substances, the molecules are not fixed to their position and are free to move. Hence in both liquid and gaseous substances, the moment of molecules is rapid and constant. It is observed that the greater the concentration of water in a system, the greater the kinetic energy or water potential. Therefore pure water will have the highest water potential. If two systems containing water are brought to contact the random moment of molecules will result in a net moment of water molecules from the system with higher energy to the system with lower energy. Thus the water will start moving from the system containing water with higher water potential toward the system which contains lower water potential. The process of movement of substances over a concentration gradient is called diffusion. Water potential is denoted by the Greek symbol ψw and is expressed in the units such as Pascal. By convention, the water potential of pure water at standard temperatures which is not under any pressure is taken to be zero.
Whenever a solute is dissolved in pure water, the solution has fever-free water molecules, and the concentration of water decreases which in turn reduces water potential. Therefore all the solutions have lower water potential than pure water. The water potential gets reduced due to the dissolution of solute particles and the magnitude of this solute is called solute potential which is denoted by ψs and is always negative. As the concentration of solute molecules increases the solute potential decreases or becomes more negative. For a solution at atmospheric pressure, water potential is equal to solute potential.
If a pressure greater than atmospheric pressure is applied to pure water our solution its water potential increases and is called pressure potential. Due to diffusion, the movement of molecules takes place between cells in plants. If we consider a cell which having higher water potential, since the concentration of molecules surrounding that cell is greater, due to diffusion of the molecule from surrounding cells to the cell placed at the center. This makes the cell turgid by building pressure against the cell wall. This is called pressure potential. Pressure potential is denoted by Ψ p. Pressure potential is usually positive.
Water potential is affected by both solute and pressure potential. The relationship between water potential, solute potential, and pressure potential is that the water potential is equal to the sum of solute potential and pressure potential.
ψw = ψs + ψp
Water Potential
Plants do not have a circulatory system to transfer molecules from one place to the other. Water and minerals collected from the soil by the roots have to reach all parts of the plant, including the tip of the growing stem. The food synthesized by leaves through photosynthesis has to reach all parts of plants including the route tips embedded deep inside the soil. Throughout the path, the movement of molecules has to take place from cell to cell through the cell membrane. In most plants, the substances which are needed to be carried from one place to another are water, mineral nutrients, organic nutrients, and plant growth, regulators. Transport of molecules over a short distance may take place by diffusion but transport over a long distance through the xylem and phloem vascular system is called translocation.