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.
- 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.