The movement of water is fundamental to the survival of terrestrial plants, facilitating nutrient transport and maintaining structural rigidity. Water travels from the soil, through the roots and stems, and eventually evaporates from the leaves in a continuous process called transpiration. This flow is governed by differences in potential energy, which scientists quantify using the concept of water potential. This potential measures the tendency of water to move, calculated as the difference in energy between the water in a plant system and pure water under standard conditions.
The Context: Water Potential and Its Components
Water potential is the measure of the free energy of water, which predicts the direction of water movement. Water always moves spontaneously toward a region of lower (more negative) potential. Pure water at atmospheric pressure is assigned a water potential of zero, meaning all other solutions and plant tissues will have a water potential that is zero or negative. This potential energy is expressed in units of pressure, most commonly megapascals (MPa).
The primary factors influencing the overall water potential are the Solute potential and the Pressure potential. These two components are combined in the simplified water potential equation: Water potential = Solute potential + Pressure potential. Solute potential accounts for the effect of dissolved substances, which reduce the water’s free energy, making it a negative value in all plant cells. Pressure potential is the physical pressure exerted on the water, which can be positive, negative, or zero.
In living plant cells, Pressure potential is often a positive value known as turgor pressure, which is the internal force of the water pushing against the rigid cell wall. Conversely, in non-living xylem vessels, water is frequently under tension, or negative pressure, as it is pulled upward by transpiration. To calculate the Pressure potential, one must first accurately determine the other variables in the equation. This calculation involves finding the Solute potential.
Calculating Solute Potential: The Necessary Variable
Solute potential is a direct measure of the osmotic effects of dissolved solutes on the water’s energy status. Since the addition of any solute lowers the water potential, the resulting Solute potential value is always negative. Solute potential is calculated using the van’t Hoff equation: Solute potential = -iCRT. This formula determines the contribution of solutes to the overall water potential of a solution.
The variable ‘\(i\)‘ represents the ionization constant, which accounts for the number of particles a solute dissociates into when dissolved in water. For a non-ionizing substance like sugar, ‘\(i\)‘ is 1, but for a salt like sodium chloride, ‘\(i\)‘ is 2. The term ‘\(C\)‘ is the molar concentration of the solute, measured in moles per liter (M).
The variable ‘\(R\)‘ is the pressure constant, also known as the ideal gas constant. This constant is \(0.00831\) when the pressure potential is expressed in megapascals. Finally, ‘\(T\)‘ is the temperature of the solution, which must be expressed in the absolute scale of Kelvin. The negative sign preceding the equation ensures the resulting Solute potential value correctly reflects the reduction in water potential caused by the dissolved particles.
Methods for Determining Pressure Potential
Once the Water potential and the Solute potential are known, the Pressure potential can be found algebraically by rearranging the main equation. The calculation becomes Pressure potential = Water potential – Solute potential. This formula allows researchers to determine the internal turgor pressure of a cell by measuring the overall water potential of the tissue and then subtracting the calculated Solute potential. For instance, if the total Water potential of a leaf is measured as \(-0.8\) MPa and the Solute potential is calculated as \(-1.2\) MPa, the Pressure potential is a positive \(0.4\) MPa.
The total water potential of a plant tissue is most commonly measured directly in the field using a specialized device called a pressure bomb, or Scholander pressure chamber. This instrument works by placing a detached leaf or stem segment inside a sealed chamber, with the cut end protruding slightly outside. Compressed gas, typically nitrogen, is slowly introduced into the chamber, increasing the external pressure on the tissue.
The pressure is increased until the xylem sap is forced back out of the cut end of the stem or petiole, becoming visible as a small droplet. The pressure gauge reading at the exact moment this fluid appears is called the balancing pressure. This balancing pressure is equal in magnitude but opposite in sign to the tension, or negative pressure, that originally existed in the xylem. This reading represents the total Water potential of the sample.