Most organisms are comprised of at least 70% or more water. Some plants, like a head of lettuce, are made up of nearly 95% water. When organisms go dormant, they loose most of their water. For example, seeds and buds are typically less than 10% water, as are desiccated rotifers, nematodes and yeast cells. Earth is the water planet (that’s why astronomers get so excited about finding water in space). Water is the limiting resource for crop productivity in most agriculture.
• In general, water always moves down its water potential gradient from areas of higher water potential to areas of lower water potential.
• Water potential is typically measured as the amount of pressure needed to stop the movement of water.
• The unit used to express this pressure is the megapascal (MPa).
The three factors that most commonly determine water potential are:-
- Solute concentration
WHAT IS WATER POTENTIAL?
Water potential is the potential energy of water relative to pure free water (e.g. deionized water) in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects including surface tension. Water potential is measured in units of pressure and is commonly represented by the Greek letter (Psi). This concept has proved especially useful in understanding water movement within plants, animals, and soil.
Components of water potential
Much different potential affect the total water potential and sum of these potentials determines the overall water potential and the direction of water flow:
= 0+ p+ s+ v+ m
• 0 is the reference correction, is the solute potential,
• p is the pressure potential,
• s is the gravimetric component,
• v is the potential due to humidity, and
• m is the potential due to matrix effects (e.g., fluid cohesion and surface tension)
COMPONENT OF WATER POTENTIAL
1. Pressure potential
Pressure potential is based on mechanical pressure, and is an important component of the total water potential within plant cells. Pressure potential is increased as water enters a cell. As water passes through the cell wall and cell membrane, it increases the total amount of water present inside the cell, which exerts an outward pressure that is retained by the structural rigidity of the cell wall.
The pressure potential in a living plant cell is usually positive. In plasmolysed cells, pressure potential is almost zero. Negative pressure potentials occur when water is pulled through an open system such as a plant xylem vessel. Withstanding negative pressure potentials (frequently called tension) is an important adaptation of xylem vessel.
Pure water is usually defined as having a solute potential of zero, and in this case, solute potential can never be positive. The relationship of solute concentration (in molarity) to solute potential is given by the van ‘t Hoff equation:
m – The concentration in molarity of the solute,
i – The van ‘t Hoff factor, the ratio of amount of particles in solution to amount of formula units dissolved,
R – The ideal gas constant, and T is the absolute temperature.
3. Matrix potential
When water is in contact with solid particles (e.g., clay or sand particles within soil) adhesive intermolecular forces between the water and the solid can be large and important. The forces between the water molecules and the solid particles in combination with attraction among water molecules promote surface tension and the formation of menisci within the solid matrix. Force is then required to break these menisci. The magnitude of matrix potential depends on the distances between solid particles–the width of the menisci and the chemical composition of the solid matrix. In many cases, matrix potential can be quite large and comparable to the other components of water potential discussed above.
It is worth noting that matrix potentials are very important for plant water relations. Strong (very negative) matrix potentials bind water to soil particles within very dry soils. Plants then create even more negative matrix potentials within tiny pores in the cell walls of their leaves to extract water from the soil and allow physiological activity to continue through dry periods.
4. Gravity (Ψg):
Contributions due to gravity which is usually ignored unless referring to the tops of tall trees.
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