Chapter 7: Transport in plants.

Structure of xylem vessels

Lignified, hollow, dead, long tubes joined end-to-end with perforation plates and pits for lateral movement

Role of lignin in xylem vessels

Strengthens walls, prevents collapse under tension, waterproofs vessel

Function of bordered pits

Allow lateral water movement and bypass of embolisms

Lignin patterns in xylem

Annular, spiral, reticulate patterns give flexibility and strength

Xylem functions

Long-distance transport of water and mineral ions; mechanical support

Phloem sieve tube elements structure

Living cells with reduced contents, sieve plates, connected end-to-end

Companion cell role

Provide ATP and metabolic support for loading/unloading

Plasmodesmata role

Cytoplasmic channels enabling symplastic movement and phloem loading

Transpiration

Loss of water vapour from mesophyll by evaporation and diffusion through stomata

Leaf water pathway

Xylem → mesophyll → cell walls/air spaces → stomata → atmosphere

Cohesion–tension mechanism

Evaporation creates tension pulling cohesive water column up xylem

Transpiration stream

Continuous upward water/mass flow from roots to leaves

Cavitation and embolism

Air bubble formation breaking column; pits allow bypass

Root hair cell adaptations

Large surface area, thin cell wall, many mitochondria, steep gradient

Apoplast pathway

Water moves through cell walls/intercellular spaces

Symplast pathway

Water moves through cytoplasm via plasmodesmata

Vacuolar pathway

Water moves through vacuoles and cytoplasm

Casparian strip role

Suberin band forcing water into symplast for selective uptake

Active mineral uptake

Proton pumps and co-transport lower water potential for uptake

Root pressure

Ion accumulation lowers water potential generating small upward push

Factors increasing transpiration

Higher light, temperature, wind; lower humidity

Humidity effect

Low humidity steepens gradient increasing transpiration

Wind effect

Removes boundary layer increasing diffusion

Temperature effect

Increases evaporation/diffusion

Light effect

Stimulates stomatal opening increasing loss

Stomatal control

K⁺ uptake lowers water potential causing guard cell turgor increase

Cuticle function

Reduces water loss from leaf surface

Mesophyll structure

Thin walls, large SA, many air spaces promoting evaporation

Potometer use

Rate of water uptake approximates transpiration under controlled conditions

Potometer limitations

Measures uptake not direct loss

Importance of transpiration

Provides water for turgor, photosynthesis, and mineral transport

Translocation

Movement of assimilates (sucrose, amino acids) through phloem

Source

Region producing/exporting sucrose (e.g., leaf)

Sink

Region using/storing sucrose (e.g., root, fruit)

Phloem loading

Sucrose actively loaded into sieve tubes lowering water potential

Phloem pressure generation

Water enters sieve tubes increasing hydrostatic pressure

Mass flow along phloem

Pressure gradient drives sap from source to sink

Phloem unloading

Sucrose removed at sink raising water potential

Evidence for mass flow

Aphid stylet pressure, radioactive tracers, ATP requirement

Aphid stylet experiment

Sap exudes due to positive pressure in phloem

Ringing (girdling) experiment

Sugars accumulate above ring showing downward translocation

Radioactive tracer evidence

¹⁴C sucrose tracing shows source→sink movement

Xerophyte stomatal adaptations

Sunken stomata, stomatal crypts, hairs, fewer stomata

Xerophyte leaf adaptations

Thick cuticle, rolled leaves, reduced leaf surface area

Xerophyte water storage

Succulent tissues storing water

Hydrophyte adaptations

Large air spaces, stomata on upper surface, reduced xylem

Roles of mineral ions

Nitrate for amino acids, Mg for chlorophyll, K for stomata, Ca for cell walls

Mineral uptake mechanisms

Active transport via proton pumps, ion channels, co-transporters

Wilting

Loss of turgor when transpiration exceeds uptake

Water potential gradient in plant

Soil > roots > stem > leaves > air

Stomatal density variation

Species/environment influence transpiration potential

High soil salt effect

Lowers soil water potential reducing absorption

Mycorrhizae

Fungal symbiosis increasing phosphate and water uptake

Phloem-mobile vs xylem-mobile nutrients

Sugars/hormones in phloem; ions in xylem

Signalling in phloem sap

Hormones/RNAs transported for whole-plant coordination

Aquaporins

Proteins increasing membrane water permeability

Temperature effect on phloem

Alters viscosity and loading/unloading rates

Flooding adaptations

Aerenchyma, adventitious roots, reduced stomatal opening

Inhibitor experiments

Blocking proton pumps reduces phloem loading

Ion uptake: active vs passive

Active needs ATP; passive follows electrochemical gradients

Hydraulic conductivity

Influenced by vessel diameter, pit resistance, embolism risk

Vessel diameter trade-off

Wide vessels increase flow but risk cavitation

Cavitation repair

New xylem growth, root pressure refilling

Sieve plate function

Allow sap flow while giving structural support

Phloem sap velocity factors

Pressure gradient, tube diameter, sieve resistance

Companion cell specialisation

Transfer cells with wall ingrowths for high loading

Polymer trapping

Converts sucrose to larger sugars to maintain gradient (some species)

Phloem unloading types

Passive diffusion, active transport, or metabolic conversion

Phloem retrieval

Reabsorption of solutes that leak from sieve tubes

Xylem–phloem interaction

Water returned to xylem after unloading

Vascular bundle arrangement

Monocot scattered; dicot in ring with cambium

Vascular cambium role

Produces secondary xylem and phloem

Annual ring formation

Earlywood/latewood alternation due to seasonal variation

Cork cambium

Produces periderm replacing epidermis in woody stems

Girdling effects

Interrupts phloem causing root starvation

Potometer setup precautions

Airtight seal, cut under water, avoid air bubbles

Transpiration experiment controls

Keep light/temp/wind/humidity constant

Tracer dyes in xylem

Show xylem flow paths experimentally

Species variation in water use

Differences in stomatal control, vessel anatomy, root depth

Irrigation management relevance

Understanding transpiration helps reduce water loss

Crop breeding targets

Improved root systems, xylem safety, stomatal responsiveness

Integrated transport system

Xylem and phloem coordinate water, ion, and assimilate movement across whole plant