Chapter 7: Transport in Plants

Introduction

Transport in plants refers to the movement of water, minerals, nutrients, and organic compounds throughout the plant body. Unlike animals, plants lack a circulatory system with pumping organs. Instead, they utilize various physical and biological mechanisms to transport substances across short distances (cellular level) and long distances (tissue and organ level).

Means of Transport in Plants

Diffusion

• Movement of molecules from a region of higher concentration to a region of lower concentration
• No energy expenditure (passive process)
• Rate depends on concentration gradient, temperature, and molecular size
• Examples: Entry of CO₂ and O₂ through stomata, movement of small molecules across cell membranes

Facilitated Diffusion

• Movement of specific molecules across membranes through special proteins
• No energy expenditure (passive process)
• Requires carrier proteins or channel proteins
• Exhibits specificity and saturation
• Examples: Glucose transport, ion transport

Active Transport

• Movement of substances against concentration gradient
• Requires energy expenditure (ATP)
• Involves specific carrier proteins (pumps)
• Exhibits specificity and saturation
• Examples: Mineral ion uptake by root cells, loading of sucrose into phloem

Osmosis

• Movement of water molecules across a semipermeable membrane
• From region of higher water potential to region of lower water potential
• Special case of diffusion
• Important for water absorption by roots

Osmotic Pressure and Osmotic Potential

• Osmotic pressure: Pressure required to prevent osmotic water movement
• Osmotic potential (Ψs): Negative of osmotic pressure; contribution of solutes to water potential

Plasmolysis and Deplasmolysis

• Plasmolysis: Shrinkage of protoplast due to water loss in hypertonic solution
• Deplasmolysis: Recovery of plasmolyzed cell when placed in hypotonic solution
• Incipient plasmolysis: Initial stage of plasmolysis when protoplast just begins to separate from cell wall

Imbibition

• Adsorption of water by solid colloids causing increase in volume
• Important in seed germination
• Generates considerable force
• Depends on water potential gradient and affinity between adsorbent and liquid

Water Potential

• Denoted by Ψw (psi)
• Measured in pressure units (MPa, bars, atmospheres)
• Sum of osmotic potential (Ψs) and pressure potential (Ψp)
• Ψw = Ψs + Ψp
• Water moves from higher to lower water potential

Factors Affecting Water Potential

• Solute concentration: Increases solute concentration decreases water potential
• Pressure: Increases pressure increases water potential
• Gravity: Affects water potential in tall plants
• Matric potential: Water binding to surfaces (significant in soil)

Long-Distance Transport Systems in Plants

Xylem Transport (Ascent of Sap)

• Transport of water and minerals from roots to aerial parts
• Unidirectional movement
• No energy expenditure at the transport level

Root Pressure Theory

• Osmotic pressure generated in root cells
• Causes upward movement of water and minerals
• Responsible for guttation and spring sap flow
• Limited to short plants or short distances
• Not significant in tall trees

Capillarity

• Rise of water in narrow tubes due to adhesion and cohesion
• Contributes to water movement but insufficient for tall trees
• Limited by radius of xylem vessels

Transpiration Pull and Cohesion-Tension Theory

• Most widely accepted mechanism for ascent of sap
• Proposed by Dixon and Joly
• Involves:
  • Transpiration: Loss of water vapor from aerial parts
  • Cohesion: Mutual attraction between water molecules
  • Adhesion: Attraction between water molecules and xylem walls
  • Surface tension: Property of water creating a continuous water column
• Creates tension (negative pressure) that pulls water upward
• Forms a continuous water column from roots to leaves

Phloem Transport (Translocation of Organic Solutes)

• Transport of organic substances (mainly sucrose) throughout the plant
• Bidirectional movement (source to sink)
• Energy-dependent process

Munch Pressure Flow Hypothesis

• Most widely accepted mechanism for phloem transport
• Involves:
  • Source: Region of sugar production (mature leaves)
  • Sink: Region of sugar utilization/storage (roots, fruits, young leaves)
  • Pressure flow: Movement due to hydrostatic pressure gradient
• Process:
  1. Active loading of sucrose into phloem at source
  2. Water enters phloem by osmosis, creating high pressure
  3. Mass flow of solution toward sink due to pressure gradient
  4. Unloading of sucrose at sink
  5. Water leaves phloem by osmosis, maintaining pressure gradient

Factors Affecting Phloem Transport

• Rate of photosynthesis
• Sink strength and demand
• Temperature
• Phloem loading and unloading mechanisms
• Plant growth regulators

Plant-Water Relations

Transpiration

• Loss of water vapor from plant surfaces, primarily through stomata
• Acts as driving force for water movement through plant
• Regulated by stomatal opening and closing

Types of Transpiration

• Stomatal transpiration: Through stomata (80-90% of total)
• Cuticular transpiration: Through cuticle (5-10%)
• Lenticular transpiration: Through lenticels (1-5%)

Factors Affecting Transpiration

• Environmental factors:
  • Light intensity
  • Temperature
  • Relative humidity
  • Wind velocity
  • Soil water availability
  • Atmospheric pressure
• Plant factors:
  • Number and distribution of stomata
  • Stomatal behavior
  • Leaf area and orientation
  • Leaf structure (cuticle thickness, etc.)

Significance of Transpiration

• Creates transpiration pull for water absorption and transport
• Cools leaf surface through evaporative cooling
• Maintains turgidity and shape of cells
• Helps in mineral salt transport
• Influences rate of photosynthesis

Stomatal Regulation

• Controlled by guard cells surrounding stomatal pore
• Opening mechanism:
  1. Active transport of K⁺ into guard cells
  2. Decrease in water potential
  3. Water enters guard cells by osmosis
  4. Guard cells become turgid and curve (due to radial arrangement of microfibrils)
  5. Stomatal pore opens
• Closing mechanism: Reverse of opening process
• Influenced by:
  • Light
  • CO₂ concentration
  • Water availability
  • Abscisic acid (ABA)
  • Temperature

Guttation

• Exudation of liquid water from hydathodes at leaf margins
• Occurs under high humidity conditions when transpiration is suppressed
• Driven by root pressure
• Common in herbaceous plants in early morning

Uptake and Transport of Mineral Nutrients

Soil-Plant-Water Relationship

• Soil serves as reservoir for water and minerals
• Water availability affected by:
  • Soil particle size
  • Soil structure
  • Organic matter content
  • Soil solution concentration

Mechanisms of Nutrient Uptake

• Passive absorption: Along concentration gradient
• Active absorption: Against concentration gradient, requires energy
• Ion uptake shows:
  • Selectivity
  • Accumulation against concentration gradient
  • Carrier-mediated transport

Translocation of Mineral Ions

• Primarily through xylem with transpiration stream
• Some ions also transported through phloem
• Redistribution occurs from older to younger parts

Practical Applications

• Understanding plant transport helps in:
  • Irrigation management
  • Fertilizer application
  • Drought resistance breeding
  • Understanding plant adaptations to different environments
  • Controlling plant diseases that affect vascular tissues