Plant Water Relations

PLANT-WATER RELATIONS 

  • Water is indispensable for all physiological activities in plants and is vital for sustaining life in all organisms. 
  • It serves as the primary medium for the dissolution of various substances.
  •  The protoplasm within plant cells consists mainly of water, serving as a solvent for different molecules and suspending several particles. 
  • In a watermelon, over 92% of its composition is water, emphasizing its significance. 
  • Water distribution within a plant varies, with woody parts containing relatively less water compared to soft, fleshy parts. 
  • Seeds, though appearing dry, contain essential water for their vitality and respiration. 
  • Herbaceous plants typically have around 10 to 15 percent of their fresh weight as dry matter, highlighting the predominant water content. 
  • Woody parts exhibit lower water content, while soft and growing parts are rich in water. 
  • Terrestrial plants absorb a substantial amount of water daily, but a significant portion is lost to the air through leaf evaporation, known as transpiration. 
  • A mature corn plant, for instance, can absorb nearly three liters of water daily, while a mustard plant takes in water equivalent to its own weight in about 5 hours. 
  • Due to the high demand for water, it often becomes the limiting factor for plant growth and productivity. 
  • Water scarcity can constrain both agricultural and natural environments, impacting overall plant health and development. 

 

Water Potential

 

  • Water Potential (Ψw) is essential for comprehending water movement in plants. Comprises solute potential (Ψs) and pressure potential (Ψp). 
  • Water molecules, possessing kinetic energy, exhibit rapid and constant random motion in liquid and gaseous states. 
  • Water potential is directly proportional to the concentration of water. This means that as the concentration of water molecules increases, the water potential also increases. 
  • In simpler terms, where there is a higher concentration of water molecules, there is a greater potential for water movement. 
  • In pure water, there are no solutes or pressure factors affecting the movement of water molecules. Therefore, the concentration of water is at its maximum, resulting in the highest water potential. 
  • Movement of substances down a free energy gradient is diffusion. Water moves from a system with higher water potential to one with lower water potential. 
  •  Water Potential is denoted by the Greek symbol Psi (Ψ). 
  • Expressed in pressure units, such as pascals (Pa). 
  • Water potential of pure water at standard temperatures and no pressure is considered zero. 
  • Solute Potential (Ψs) is the magnitude of the reduction in water potential due to the dissolution of solute in a solution. 
  • Solute potential is always negative; increases in solute molecules lead to a more negative Ψs. 
  • Pressure Potential (Ψp)

- Increase in Water Potential: Applied pressure greater than atmospheric pressure elevates water potential. 

- Plant System Example: Pressure builds up in plant cells due to water entry, making the cell turgid and increasing pressure potential. 

  • Relationship Between Water, Solute, and Pressure Potential:

 Ψw = Ψs + Ψp 

  •  Water potential of a cell is influenced by both solute and pressure potential. 
  • Positive pressure potential in plant cells contributes to turgidity. 
  • Negative pressure potential or tension in the water column aids water transport up a stem. 

 

 

 

Osmosis

  • Plant cells are enclosed by a cell membrane and a permeable cell wall. 
  • The central vacuole, containing vacuolar sap, contributes to the solute potential. 
  • The cell membrane and the tonoplast (vacuole membrane) play crucial roles in determining molecular movement in and out of the cell. 
  • Osmosis specifically refers to the diffusion of water across a differentially or semi-permeable membrane. 
  • The cell wall, being permeable, does not impede the movement of water and substances in solution. 
  • Osmosis occurs spontaneously in response to both pressure and concentration gradients. 
  • Water moves from higher to lower chemical potential or concentration until equilibrium is achieved. 
  • At equilibrium, chambers on either side of the membrane should have the same water potential. 
  • The central vacuole and its contents contribute to the solute potential, influencing water movement. 
  • Potato osmometer experiment demonstrates water movement into a concentrated sugar solution in the potato tuber due to osmosis. 
  • Another experiment using an eggshell membrane and sucrose solution highlights osmotic pressure and equilibrium dynamics.
  • Osmotic Pressure: External pressure applied to prevent water diffusion through a semi-permeable membrane is osmotic pressure. 
  • This pressure is influenced by solute concentration; higher concentration requires greater pressure to prevent water influx. 
  •  Osmotic pressure and osmotic potential are numerically equivalent, but their signs are opposite. 
  • Osmotic pressure is positive, applied to prevent water diffusion, while osmotic potential is negative.

 

 

 

Plasmolysis

 

  • Plant cell behavior concerning water movement is contingent upon the characteristics of the surrounding solution. 
  • Isotonic Solution:

- An isotonic solution is one where the external solution perfectly balances the osmotic pressure of the cytoplasm. 

- No net movement of water; cells maintain their original state. 

  • Hypotonic Solution:

- The external solution is more dilute than the cytoplasm. 

- Cells in hypotonic solutions swell due to the influx of water. 

  • Hypertonic Solution:

- The external solution is more concentrated than the cytoplasm. 

- Cells in hypertonic solutions shrink as water moves out of the cell.

 

 

 

  • The behavior of cells in various solutions underscores the dynamic nature of water movement across cell membranes. 
  • Plasmolysis occurs when water exits the cell, causing the plant cell membrane to retract from the cell wall. 
  • Happens in a hypertonic solution with more solutes outside compared to the protoplasm. 
  • Water Movement: Initially lost from the cytoplasm and then from the vacuole, leading to protoplast shrinkage. 
  • The space between the cell wall and the shrunken protoplast in a plasmolysed cell is filled by the extracellular fluid. 

 

 

  • In an isotonic solution, there is no net water flow inside or outside the cell. Cells are in equilibrium, termed as flaccid. 
  • Reversal Conditions: Placing cells in a hypotonic solution (lower solute concentration than the cytoplasm) allows water to diffuse in. Cytoplasm builds up pressure against the cell wall, known as turgor pressure. 
  • Turgor pressure is the pressure exerted by protoplasts due to water entry against rigid cell walls. Prevents cell rupture; responsible for cell enlargement and extension growth. 
  • In a flaccid cell, the Ψp is reduced or near zero as there is no significant water entry or pressure against the cell wall. 
  • Other than plants, organisms possessing cell walls include fungi, bacteria, and some protists. 
  • Cell walls contribute to structural support and distinctive shapes in these organisms.

 

Imbibition

 

  • Imbibition is a unique form of diffusion where solids, particularly colloids, absorb water, leading to a significant increase in volume. 
  • Classical instances of imbibition include the absorption of water by seeds and dry wood.

 

 

 

  • Prehistoric man utilized the pressure generated by wood swelling through imbibition to split rocks and boulders. 
  • Importance in Germination:

- Imbibition pressure is crucial for seedlings to break through the soil and establish themselves in the open. 

- Without this pressure, seedlings might struggle to emerge successfully. 

  • Imbibition is a form of diffusion since water movement occurs along a concentration gradient. 
  • Seeds and similar materials, being nearly devoid of water, readily absorb water during imbibition. 
  • A water potential gradient between the absorbent (solid) and the imbibed liquid is fundamental for the imbibition process. 
  • For successful imbibition, there must be an affinity between the adsorbant (solid) and the imbibed liquid. 
  • This affinity ensures effective absorption and expansion of the solid material.