Unit-I: Water Relations and Phloem Transport
1. Basic Concepts: Diffusion, Osmosis, and Water Potential
Understanding plant physiology starts with the physical forces governing the movement of water.
- Diffusion: The passive movement of molecules from a region of higher concentration to a region of lower concentration.
- Osmosis: The movement of water molecules through a semi-permeable membrane from a region of higher water potential to lower water potential.
- Water Potential (Ψ): A measure of the free energy of water in a system. It determines the direction in which water will flow. Pure water has a potential of zero at standard pressure.
2. DPD and Components of Water Potential
Water potential is influenced by various internal and external factors.
- Diffusion Pressure Deficit (DPD): The reduction in the diffusion pressure of water in a solution compared to pure water. Modern physiology describes this as the negative of water potential.
- Osmotic System: Plants act as osmotic systems where the cell wall, cell membrane, and vacuole regulate water entry.
- Components: Water potential is primarily calculated as:
Ψw = Ψs + Ψp
Where Ψs is solute potential (always negative) and Ψp is pressure potential (usually positive in living cells).
3. Water Absorption and Movement Pathways
Roots are the primary organs for water acquisition from the soil.
- Water Absorption by Roots: Occurs mainly through root hairs via osmotic and non-osmotic mechanisms.
- Apoplastic Pathway: Movement of water through cell walls and intercellular spaces. It is relatively fast but blocked by the Casparian strip in the endodermis.
- Symplastic Pathway: Movement of water through the cytoplasm of cells, connected by plasmodesmata.
- Trans-membrane Transport: Water crosses cell membranes (vacuolar and plasma membranes) as it moves from cell to cell.
4. Transpiration and Stomatal Movement
Transpiration is the loss of water vapor from the aerial parts of the plant.
- Factors Affecting Transpiration: Light, temperature, humidity, and wind speed.
- Stomatal Anatomy: Stomata consist of a pore surrounded by specialized guard cells.
- Stomatal Movement: Regulated by changes in the turgidity of guard cells. Accumulation of solutes (like K+ ions) lowers water potential, causing water to enter and open the pore.
- Anti-transpirants: Substances (like ABA or certain waxes) applied to plants to reduce water loss without significantly affecting CO2 uptake.
5. Cohesion-Tension Theory and Aquaporins
How water moves to the top of tall trees against gravity is explained by several physical properties.
- Cohesion-Tension Theory: Water molecules stick together (cohesion) and to the xylem walls (adhesion). Transpiration at the leaves creates a "tension" or pull that draws the water column upward.
- Aquaporins: Membrane proteins that act as "water channels," facilitating the rapid movement of water molecules across cell membranes.
6. Phloem Translocation and Source-Sink Relationship
Phloem is responsible for the long-distance transport of organic nutrients (primarily sucrose).
- Translocation: The movement of photosynthates through sieve tube elements.
- Source: Parts of the plant that produce or export sugars (e.g., mature leaves).
- Sink: Parts that consume or store sugars (e.g., roots, fruits, growing buds).
- Source-Sink Relationship: The direction of transport is determined by the nutritional needs of various plant organs and can change during different growth stages.
7. Pressure Flow Model: Loading and Unloading
The Münch Pressure Flow Model is the most widely accepted mechanism for phloem transport.
- Phloem Loading: Sugars are actively transported into sieve tubes at the source. This lowers water potential, causing water to enter from the xylem and creating high turgor pressure.
- Phloem Unloading: At the sink, sugars are removed from the sieve tubes. Water follows by osmosis, reducing the turgor pressure.
- Pressure Flow: The difference in turgor pressure between the source and the sink drives the mass flow of the phloem sap.
Exam Tip: Always remember the formula Ψw = Ψs + Ψp. In numericals, Ψs (solute potential) is always a negative value because adding solutes always lowers the free energy of water.
Common Mistake: Confusing Diffusion and Mass Flow. Diffusion is slow and operates over short distances, while Mass Flow (in Xylem and Phloem) is fast and moves entire volumes of fluid over long distances due to pressure differences.
Frequently Asked Questions
- Q: Why is the apoplastic pathway faster than the symplastic pathway?
A: Because water moves through the cell walls (apoplast) which offer less resistance than the cytoplasm and plasmodesmata (symplast).
- Q: What is the role of ABA in stomatal movement?
A: Abscisic Acid (ABA) acts as a stress hormone that triggers the exit of K+ from guard cells, causing them to lose turgidity and close the stomata to conserve water.