Lecture 22 Transport 1

Introduction & Learning Objectives

• End of-course double-lecture block on plant transport; current lecture focuses on water.
• By the end students should be able to:
  • Describe how water is transported from soil to leaves.
  • Explain how plants regulate water use via anatomy, physics and stomatal behaviour.
  • Recognise how transport limitations constrain plant growth form (e.g. maximum height).

Vascular System Overview

• Vascular tissue is complex & multifunctional (nutrients, water, gases, hormones, heat & waste management, immunity).
• Two parallel conduits:
  • Xylem – mainly water (+ minerals, heat, some signals).
  • Phloem – mainly sugars (+ hormones, immune molecules).
• Xylem transport labelled “brilliant & brainless”:
  • Brainless: open system (water enters roots, exits leaves; no closed circuit).
  • Brilliant: requires no metabolic energy; powered by physics + special cell architecture.

Water Loss from Leaves & Transpiration

• Photosynthesis sites (spongy mesophyll) are water-saturated; gas exchange requires open stomata.
• Open stomata leak vast quantities of water (“incredibly wasteful”).
• One square centimetre of leaf can possess ≈ 6 000 vein endings that discharge water into mesophyll.
• Huge share of all plant water loss occurs through stomata.

Root Water Uptake

• Primary source = soil water.
• Water entry points:
  • Cell-wall pores in roots.
  • Root hairs (single-cell projections, no waxy cuticle ⇒ high permeability; dramatically enlarge absorptive surface).
• Goal: deliver water to the stele (central vascular cylinder).

Physical Principles Driving Water Movement

1. Capillary Action

• Emerges from water’s cohesion (water–water attraction) & adhesion (water–solid attraction).
• Demonstrated by water creeping up narrow glass tubes or wicking into paper towels.

2. Hydrostatic Pressure

• Positive (push) or negative (suction) pressure moves a continuous water column; illustrated with a U-tube experiment.

3. Diffusion / Osmotic Pressure

• Water crosses semi-permeable membranes toward regions of higher solute concentration to equalise concentrations.

Water Potential (\Psi)

• Total driving force for water flow:
  \Psi = \Psip + \Psi\pi
  where \Psip = hydrostatic (pressure) potential, \Psi\pi = diffusion/solute/osmotic potential.
• Convention: pure water \Psi = 0. Wet soils have high (close-to-zero) values; dry air has highly negative values.
• Water always moves from higher (less negative) to lower (more negative) \Psi.

Pathways Inside Roots

• Apoplastic route – through cell walls & intercellular spaces; no membranes crossed; very fast.
• Symplastic route – through cytoplasm via plasmodesmata; crosses plasma membrane at least once.
• Endodermis/Casparian strip forces water to switch to symplastic route before entering xylem; benefits:
  • Filters toxins & pathogens.
  • Prevents back-flow to drying soil.
  • Allows slight positive pressure build-up (root pressure).

Xylem Architecture & Efficiency

• Produced by meristems; cells stack, lose contents, leaving long hollow pipes (vessel elements in angiosperms, tracheids in gymnosperms).
• Transport is fully apoplastic – analogous to a fire-hose vs bucket brigade (continuous stream vs hand-to-hand buckets).

Tension–Cohesion Theory (Ascent of Sap)

1. Water vapour diffuses from moist leaf air spaces to drier external air via stomata.
2. Loss is replaced by evaporation of the water film on mesophyll cell walls.
3. Curvature of the retreating meniscus increases surface tension, creating negative pressure that pulls water from xylem into mesophyll.
4. Cohesion transmits this tension all the way down the continuous xylem column to the roots, while adhesion keeps water in contact with vessel walls (chimney-climber analogy).

• Example gradient (MPa):
  • Wet soil \approx -0.3 to -0.03
  • Trunk xylem \approx -0.8
  • Leaf bulk tissue \approx -1
  • Leaf air spaces \approx -7
  • Atmosphere (dry day) < -100

Stem Diameter Fluctuations

• Diurnal negative pressure can physically "squeeze" stems – measurable shrinkage during midday peak transpiration; relevant for forest carbon inventories.

Self-Regulating Nature of Transpiration

• Warmer, drier micro-sites ⇒ greater mesophyll dehydration ⇒ larger xylem tension ⇒ faster local water delivery.
• Sun-exposed side of crown can pull water faster than shaded side; dynamic throughout day.

Stomatal Closure: Risks & Trade-Offs

• Closing stomata ≈ “plant holding its breath”.
• Three key purposes:
  • Prevent exhaustion of water reserves.
  • Avoid collapse of xylem vessel walls under extreme negative pressure.
  • Prevent cavitation (snapping of the water column, introduction of air).
• Costs: sharply reduced photosynthetic CO₂ uptake → energy deficit.

Cavitation, Vessel Diameter & Annual Rings

• Large-diameter vessels move water most efficiently but are cavitation-prone.
• Spring: abundant water & mild demand → plants lay down large vessels (earlywood) to maximise efficiency.
• Summer: hotter, drier, steeper \Psi gradient → plants produce narrower vessels (latewood) to reduce cavitation risk; pattern produces visible growth rings.

Why Aren’t Trees 300 m Tall? – Competing Hypotheses

1. Mechanical failure – rejected; wood is over-engineered and could support > 200 m.
2. Hydraulic limitation – water column tension + gravity ultimately exceed cavitation threshold.
3. Carbon balance – increasing non-photosynthetic mass versus relatively constant leaf area limits whole-plant energy budget.

Case Study: Coast (Giant) Redwoods

• Tallest living trees: > 130 m; angiosperm record (Eucalyptus regnans) ≈ 90 m.
• Researchers climbed crown to measure xylem pressure & leaf physiology along height gradient.
• Cavitation threshold for redwood tracheids ≈ \Psi = -2\,\text{MPa}.
• Model + measurements show column tension reaches ≈ -2 MPa at 120–130 m during midday; risk zone.
• Trees cope by closing stomata → tension drops (safer) but photosynthesis stalls.
• Crown-top leaves are small, thick, low photosynthetic capacity; seedlings at ground level exhibit broad, high-rate leaves → confirms water-stress constraint.
• Conclusion: Hydraulics sets upper limit; carbon balance becomes secondary consequence.

Analogies & Real-World Relevance

• Fire-hose vs bucket brigade: continuous conduit far outperforms serial transfer.
• Fire-department ladder limits mirror hydraulic limits in skyscraper firefighting.
• Medical parallel: air embolism in human blood vessels ~ cavitation in xylem; both potentially fatal.
• Forest carbon accounting must standardise measurement time due to stem diameter fluctuation.

Numerical & Formula Recap

• Water potential definition: \Psi = \Psip + \Psi\pi.
• Pure water: \Psi = 0.
• Cavitation safety margin in redwoods: failure at -2\,\text{MPa}; midday tension approaches this at 120–130 m.
• Leaf vein density: ≈ 6 000 vein endings · cm^{-2}.

Ethical / Ecological Implications

• Understanding hydraulic limits aids conservation of giant trees vulnerable to hotter, drier climates.
• Insight into cavitation informs breeding of drought-resistant crops.

Looking Ahead

• Next lecture: phloem transport, sugar allocation & photosynthetic adaptations.