Biology · Plant Structure, Transport & Nutrition

Water uptake, xylem transport and transpiration

Two cells can contain different amounts of dissolved material without water moving toward the more concentrated one. Pressure can change the result. To understand both cell hydration and water ascent, we need a quantity that includes these contributions rather than a rule based only on water amount.

Cells, Membranes & Organisation introduces selective membranes, osmosis and connections between cells. Energy, Enzymes & Metabolism introduces respiration, carbon fixation and ATP coupling. Their necessary ideas are restated here. Plant Diversity & Evolution provides the wider evolutionary context; on a first reading, remember that vascular tissues belong to particular plant lineages. These lessons use selected vascular land plants rather than a universal plant body. Chemistry's concentration and energy chapters and Physics's fluid dynamics chapter offer optional return paths.

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By the end, you can

Predict a local water-movement direction from total water potential, then connect it to xylem flow and a whole-plant water balance.

Water potential combines several influences

Water potential, written Ψ, expresses the energetic tendency of water to move relative to a specified reference. It is commonly expressed in megapascals, MPa, an energy-per-volume unit equivalent to pressure. Pure water at the reference pressure and temperature is assigned zero. A higher potential means a greater tendency for net passive water movement toward a connected region of lower potential.

For neighbouring solutions at equal height in our local exercise, Ψ = Ψs + Ψp. Ψs is the solute contribution; dissolved solute makes it more negative. Ψp is the pressure contribution; positive pressure raises the total. Gravity and interactions with a solid matrix are omitted from this small comparison. They matter in tall plants and in dry soils or tissues.

Consider A with Ψs = −0.8 and Ψp = +0.5 MPa. Its total is −0.3 MPa. B has Ψs = −0.5 and Ψp = +0.1 MPa, giving −0.4 MPa. Across a membrane permeable to water and impermeable to the model solute, net water initially moves from A to B. A has the more negative solute contribution, but its pressure raises its total above B's.

These totals do not specify the quantity transferred. Permeability, area, duration and changes in volume and pressure matter. As water moves, the contributions can change. Equal total potentials can occur with different solute and pressure contributions, so equal potential does not mean equal concentration or equal water content.

Pressure can reverse a concentration-only prediction

The comparison is local and initially assigned. Water movement changes concentrations and pressures as the compartments respond.

Read the diagram in words

A: −0.8 + 0.5 = −0.3 MPa. B: −0.5 + 0.1 = −0.4 MPa. C: −0.9 + 0.5 = −0.4 MPa. A initially supplies net water to B across the stated membrane. B and C have equal totals despite different contributions.

Pure water at reference atmospheric pressure and the same temperature

MPa
Description	Drawing x coordinate	Ψs: solute contribution	Ψp: pressure contribution	Ψ: total
A	0	-0.8	0.5	-0.3
B	4	-0.5	0.1	-0.4
C	8	-0.9	0.5	-0.4

Pressure can reverse a concentration-only prediction
Process	From	To	Difference in total water potential (MPa)
Initial net movement A → B	A	B	0.1
Equal totals; no net passive movement from this difference	B	C	0

Pressure can reverse a concentration-only prediction
Description	Compartments separated	Drawing x coordinate
Selective membrane	A ↔ B	2
Selective membrane	B ↔ C	6

Potential contribution or total in MPa
Minimum	Maximum	Tick values
-1.2	0.8	-1.2, -0.8, -0.4, 0, 0.4, 0.8

Ψs: solute contribution

Ψp: pressure contribution

Ψ: total

Ψ = Ψs + Ψp

Higher total → lower total

Direction does not determine quantity transferred

Constructed local membrane comparisons at equal height and temperature. Water permeates; model solute does not. Gravity and matrix contributions omitted. Displayed values describe initial conditions, not a dynamic solution.

Evaporation can pull a continuous liquid pathway

Within a transpiring leaf, water evaporates from moist internal surfaces. Vapour then moves toward the external air, often through stomata. Evaporation changes the liquid boundary conditions and can put the connected xylem water under tension. Cohesion within the liquid and the conduit walls support a continuous pathway through which water is pulled.

This cohesion-tension explanation does not require a heart-like organ pumping upward. In much transpiring xylem, pressure is below the atmospheric reference. The conduit still contains liquid: negative gauge pressure is not an empty tube. Cavitation and gas-filled interruptions can reduce its capacity to conduct.

Roots supply water when the external and internal conditions permit uptake. Root pressure can contribute under some conditions, but it is not a universal explanation for water reaching a tall canopy. Resistance along the route and the gravitational contribution must also be overcome. A potential gradient supplies a direction; tissue conductance helps determine the resulting flow.

Stomatal opening, air humidity, temperature and boundary-layer conditions influence water loss. Closing stomata can reduce transpiration but may also restrict carbon dioxide entry. These effects connect transport to metabolism without making the whole stream an actively pumped fluid.

A flow and a water stock answer different questions

In the constructed interval, the plant begins with 30 water-account units and takes up 120. It loses 110 by transpiration and incorporates or retains 10 in a separately counted new-tissue pool. Its original mobile-water pool ends at 30: 30 + 120 − 110 − 10 = 30.

The unchanged pool does not mean that no water moved. Uptake and losses were large while the ending stock stayed constant. Conversely, a plant can lose stored water when loss exceeds uptake. A single ending mass cannot reconstruct all the underlying fluxes.

The accompanying route diagram shows progressively lower assigned potentials from soil to root, stem and leaf, then to air. It is a schematic of a transpiring state, not an equilibrium portrait, an estimate of tree height or a linear rule for computing the interval's transfers.

Continuous transport can leave the ending stock unchanged

The route illustrates a transpiring state. The ledger conserves water across its stated boundaries without computing fluxes from potential values.

Read the diagram in words

The assigned potential order is soil −0.1, root −0.3, stem −0.5, leaf −0.8 and air −100 MPa. The interval ledger separately starts at 30, adds 120, loses 110 to air and transfers 10 to new tissue, ending at 30. With uptake of only 90 and the other entries unchanged, it ends at zero.

MPa
Description	Drawing x coordinate	Drawing y coordinate	Assigned total potential
Soil water	0	8	-0.1
Root route	3	7	-0.3
Xylem through stem	3	4	-0.5
Leaf liquid route	3	1	-0.8
External air	7	1	-100

Continuous transport can leave the ending stock unchanged
Process	From	To
Uptake where conditions permit	Soil water	Root route
Continuous liquid pathway	Root route	Xylem through stem
Liquid delivery	Xylem through stem	Leaf liquid route
Evaporation followed by vapour loss	Leaf liquid route	External air

Water-account units per assigned interval
Water-account term	Balanced assigned interval	Assigned deficit interval
Initial mobile-water stock	30	30
Water uptake	120	90
Transpiration	110	110
Transfer to the separate new-tissue pool	10	10
Ending mobile-water stock	30	0

Water-account units per assigned interval
Water-account term	Amount in the stated account
Increase in the new-tissue water stock	10

Assigned total potential

Mobile-water stock

Input from outside

Loss to air

Transfer to separately counted new-tissue pool

Ending mobile stock

Arrow direction is schematic; arrow size is not measured flow

Constructed transpiring state and separate water-account interval. Potential values are not used to calculate the ledger fluxes. No height, conductance, survival threshold or real rate assigned.

Worked example

Equilibrium, then a separate deficit

A third local compartment C has Ψs = −0.9 and Ψp = +0.5 MPa, giving −0.4 MPa. C and B therefore have equal totals despite different contributions. With the stated selective membrane and unchanged conditions, their total-potential difference gives no net passive water movement; individual water molecules can still cross both ways.

For a separate assigned interval, begin with 30 mobile-water units, uptake 90, transpiration 110 and a transfer of 10 into new tissue. The ending mobile stock is 30 + 90 − 110 − 10 = 0. Those numbers reach the model's available-stock limit; extending the same losses into another interval would require new supply or changed fluxes.

This arithmetic does not predict when a real plant wilts or whether it survives. Water distribution, tissue elasticity, thresholds and responses are absent. It teaches the distinction between a balanced interval, a depleted stock and an impossible negative inventory.

Check your understanding

D has Ψs = −0.7 and Ψp = +0.4 MPa; E has −0.4 and +0.05 MPa. Which way does water initially move across the stated water-permeable membrane?
Show reasoning
D totals −0.3 MPa and E totals −0.35 MPa. Net passive water movement is initially from D to E, the lower total. The concentration contribution alone would miss the pressure effect.
Does water in a xylem conduit at negative gauge pressure have a negative amount of water?
Show reasoning
No. Gauge pressure is measured relative to a reference pressure; a negative value describes tension relative to that reference. The conduit can contain liquid water. Pressure and water amount are different quantities.
A mobile stock begins at 20, receives 70, loses 65 to the air and transfers 5 to new tissue. What is its ending stock, and does an unchanged stock imply zero transport?
Show reasoning
It ends at 20 + 70 − 65 − 5 = 20. The stock is unchanged while 70 units entered and 70 left the mobile pool through two different destinations. Endpoint stock and interval transport are distinct.

What to carry forward

Use total potential to predict direction, conductance to explain flow capacity, and a material balance to track changing water stocks.

References

Resources supporting this lesson’s explanations and examples.