4.1 Types of Vascular Tissues and Translocation
Vascular plants possess a specialized transport system composed of vascular tissues (xylem and phloem) to transport water, inorganic minerals, and organic nutrients throughout the organism.
1. Vascular Tissues and Adaptations
- Xylem Tissue:
- Xylem Vessels: Long, continuous hollow tubes formed by dead cells joined end-to-end without end walls or cell contents (protoplasm). Lignified secondary walls provide structural support and prevent collapse under negative pressure.
- Tracheids: Elongated dead cells with tapered ends and pitted walls allowing lateral movement of water.
- Function: Transports water and dissolved mineral salts unidirectionally from roots to leaves.
- Phloem Tissue:
- Sieve Tubes: Formed by living elongated cells (sieve tube elements) arranged end-to-end with perforated sieve plates. They lack nuclei, vacuoles, and ribosomes at maturity to allow unimpeded flow of organic sap.
- Companion Cells: Adjacent living cells densely packed with mitochondria, rough endoplasmic reticulum, and ribosomes. Connected to sieve tube elements via plasmodesmata to supply ATP and metabolic support.
- Function: Transports organic substances (mainly sucrose and amino acids) bidirectionally from source to sink.
2. Concept of Translocation
- Definition: Translocation is the transport of organic solutes (primarily sucrose and amino acids) in the phloem from photosynthetic sources (e.g., leaves) to non-photosynthetic sinks (e.g., roots, fruits, growing shoots, and storage tubers).
- Mass Flow / Pressure-Flow Hypothesis:
- Loading at Source: Sucrose is actively loaded from photosynthetic mesophyll cells into companion cells and then into sieve tube elements using ATP energy.
- Water Potential Drop: High sucrose concentration in sieve tubes lowers water potential ($\psi$), causing water from adjacent xylem vessels to enter sieve tubes by osmosis.
- Hydrostatic Pressure Generation: Accumulation of water generates high hydrostatic pressure at the source end of the sieve tube.
- Bulk Mass Flow: Phloem sap flows down the hydrostatic pressure gradient toward the sink region (where pressure is lower due to sucrose unloading and water exiting back to xylem).
- Unloading at Sink: Sucrose is actively or passively unloaded from sieve tubes into sink cells for usage or starch storage.
4.2 Transport of Water and Inorganic Mineral Salts
1. Route of Water Entry into Roots
- Water enters root hair cells via osmosis down a water potential gradient.
- Pathways across the Root Cortex to the Stele:
- Apoplast Pathway: Water moves exclusively along non-living cell walls and intercellular spaces by capillary action without crossing plasma membranes.
- Symplast Pathway: Water moves from cell to cell through living cytoplasm connected by plasmodesmata.
- Casparian Strip: A continuous waxy ring of suberin in the endodermal cell walls blocking the apoplast route. Water is forced to enter the cytoplasm (symplast pathway), allowing selectively permeable endodermal cells to regulate mineral ion entry into the central vascular cylinder (xylem).
2. Forces Driving Water Movement in Xylem
- Transpirational Pull (Primary Force): Loss of water vapor via transpiration creates negative pressure (tension) in mesophyll cells, pulling a continuous water column upward through xylem vessels.
- Capillarity (Cohesion and Adhesion):
- Cohesion: Hydrogen bonding between water molecules keeps the water column unbroken and continuous.
- Adhesion: Attraction between water molecules and hydrophilic xylem walls prevents the column from pulling apart or sliding downward under gravity.
- Root Pressure: Accumulation of mineral ions active transport into xylem vessels lowers root water potential, drawing water in and creating positive pressure pushing xylem sap upward.
- Responsible for guttation: Exudation of liquid water drops through specialized pores called hydathodes at leaf margins, occurring during conditions of high humidity and low transpiration (e.g., cool nights).
4.3 Phytoremediation
Phytoremediation is an eco-friendly environmental biotechnology utilizing plants to absorb, accumulate, transform, or detoxify pollutants (e.g., heavy metals, hydrocarbons, radioactive elements) from soil and water.
1. Mechanism and Applications
- Hyperaccumulators absorb contaminants via root systems and translocate them to shoots and leaves without suffering cellular toxicity.
- Examples of Phytoremediator Plants:
- Eichhornia crassipes (Water Hyacinth): Absorbs heavy metals like cadmium, lead, and nickel from polluted wastewater.
- Helianthus annuus (Sunflower): Absorbs heavy metals, zinc, and radioactive isotopes (e.g., Cesium-137, Uranium) from contaminated soil.
- Pistia stratiotes (Water Lettuce): Cleans water contaminated with heavy metals and agricultural runoff.
- Pteris vittata (Chinese Brake Fern): Accumulates arsenic from soil.