BIOL1030 Chap.6 Water Transport and Drought
Water Transport and Drought
Water Transport and Drought covers Lecture 4 of Unit 6, on how water moves from soil to canopy and how plants survive when it runs short. Water always flows down a gradient of water potential, which adds a solute term to a pressure term.
Roots take it in through the apoplast and symplast until the Casparian strip forces it across a membrane, and transpiration from leaves creates tension that cohesion and adhesion transmit down an unbroken xylem column. The chapter explains turgor and wilting, cavitation and embolism, how guard cells use potassium to open and close stomata, and the trade-off between carbon gain and water loss.
It ends with CAM and the physiological, structural and life-history traits of drought-adapted plants such as Mulga.
What this chapter covers
- 01
Water potential and its solute and pressure components
- 02
Turgor pressure and wilting
- 03
Apoplast, symplast and the Casparian strip
- 04
Transpiration, cohesion, adhesion and cavitation
- 05
Guard cells, potassium and the stomatal trade-off
- 06
CAM and drought traits in Australian plants
Worked example · free
Rank the water-potential steps from soil to air
- 1Order from highest to lowest water potential: soil, root, stem xylem, leaf, air.
- 1Because flow runs downhill in water potential, each compartment passes water to the next, ending with evaporation into the air.
- 1Evaporation through stomata lowers leaf water potential and puts the xylem column under tension.
- 1Cohesion between water molecules and adhesion to xylem walls keep the column continuous, so the tension draws water up from the roots.
Key terms
- Water potential
- The tendency of water to move, the sum of solute and pressure potential; water flows from higher to lower values.
- Casparian strip
- A barrier in the endodermis that blocks apoplastic flow and forces water across a membrane into the vascular cylinder.
- Cohesion
- The attraction between water molecules through hydrogen bonds that keeps xylem water columns continuous.
- Cavitation
- The breaking of a xylem water column under excessive tension, leaving an air-filled embolism.
- Vapour pressure deficit
- The drying power of air relative to a leaf; a high deficit makes stomata close.
- Crassulacean acid metabolism
- A photosynthetic pathway that opens stomata at night and stores CO2 as a four-carbon acid for use by day.
Water Transport and Drought FAQ
What makes water move into a plant cell?
Differences in water potential. Solutes lower the solute term and a turgid cell raises the pressure term, and water always flows towards the lower total. A cell with plenty of solute but little pressure can therefore draw water in until its pressure rises and the two sides balance.
What does the Casparian strip do?
It seals the walls of the endodermis so water and dissolved minerals can no longer slip around cells in the apoplast. Everything has to pass through a plasma membrane to reach the xylem, giving the plant control over which substances enter.
Why do plants close their stomata in drought if it stops photosynthesis?
Closing saves water and keeps xylem tension from rising to the point where cavitation breaks the water column. The cost is lower carbon dioxide uptake, so growth slows. Plants respond both to dry air and to dry soil when making that trade.
How do Australian plants cope with very dry country?
Mulga dominates central regions with under 300 mm of rain a year by carrying small, thick phyllodes, and another acacia reaches the driest regions with spine-like ones. Pigface stores water in succulent tissue and spinifex rolls its leaves to cut water loss.
What is cavitation and why does it matter in drought?
When tension in a xylem water column becomes too great, the column can break and air enters the conduit, an event called cavitation that leaves an embolism. Embolised conduits stop carrying water; plants can sometimes recover, but repeated failure during severe drought can kill branches, which is one reason stomata close as soils dry.
Exam move
Practise the water potential method until it is automatic: add the two terms for each compartment, compare the totals and state the direction, then repeat with a flaccid cell and a turgid cell. Redraw the path from soil to air from memory and annotate where the Casparian strip, cohesion, adhesion and stomata act.
Finally, build a three-column table of drought responses, physiological, structural and life-history, with one named Australian or lecture example in each row, and explain one of them as a carbon-versus-water trade-off. Close by explaining in two sentences why wilting and cavitation are different failures, since questions often test that boundary.
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