1.1 Water potential #
1.1.1 Components, units, and a transpiring leaf #
Water moves from higher water potential to lower. Ψ is the chemical potential of water per unit volume, so the unit is pressure, the megapascal (1 MPa = 10 bar). Pure free water at atmospheric pressure is the reference and has Ψ = 0. The working equation is
Ψ = Ψs + Ψp + Ψg
with a matric term Ψm added where surfaces bind water, as in dry soil and dry seed. Solute potential Ψs is zero in pure water and negative in any solution. Pressure potential Ψp is positive under turgor and negative under tension. Gravitational potential Ψg rises by about 0.01 MPa per metre. It is negligible in a cell and costly in a tree: 30 m costs about 0.3 MPa before friction. An older form, Ψ = P − π with π positive, is the same equation if Ψs = −π. Mixing the two conventions double-counts solute.
An illustration of the arithmetic, not a universal measurement: Ψs = −1.2 MPa and Ψp = +0.7 MPa gives Ψ = −0.5 MPa. A neighbour at −0.8 MPa draws water from that cell. A xylem vessel might have Ψs = −0.1 MPa and a tension Ψp = −1.0 MPa, so Ψ = −1.1 MPa. The negative pressure is real pressure below atmospheric, carried by liquid water.
For water vapour, Ψ = (RT / Vw) ln(relative humidity). At about 20°C the prefactor is about 135 MPa, so 50% relative humidity is near −95 MPa and 90% is still near −14 MPa. Only air extremely close to saturation approaches the Ψ of a leaf. The large drop is across the stomatal pore. Evaporation at curved menisci in the mesophyll walls is why a transpiring leaf has a negative Ψ, and why that value falls further as the soil dries. A moist soil may sit near −0.05 to −0.1 MPa. The permanent wilting point, about −1.5 MPa, is a convention for many crops, not a constant of every species.
1.1.2 Osmosis, turgor, and plasmolysis #
Osmosis is net water movement across a membrane toward lower Ψ. Solutes that cannot cross lower Ψs and pull water. Plant membranes pass water readily and sucrose poorly, so a sucrose gradient is fully expressed. Water entering a cell presses the protoplast against the wall. That positive Ψp is turgor. Herbaceous leaves stand up because of it, and they wilt when it falls to zero. Wilting is loss of pressure, not the sudden disappearance of water.
Plasmolysis is what happens in a bathing solution more negative than the cell’s Ψ at zero turgor. Water leaves, Ψp falls to zero, and the protoplast shrinks away from a wall that the solution still supports. At incipient plasmolysis, Ψ = Ψs, which is how tissue Ψs is estimated. A milder solution only reduces turgor: the cell equilibrates at a smaller volume, sap a little more concentrated, Ψp lower but still positive. A leaf drying in air does not plasmolyse. Nothing holds the wall out, so the wall buckles with the protoplast. That collapse is cytorrhysis. Reserve plasmolysis for the bath.
1.2 The path from soil to air #
1.2.1 Aquaporins and the continuum #
Aquaporins are channels of the major intrinsic protein family. Plasma-membrane PIPs and tonoplast TIPs are the groups to learn. They raise hydraulic conductivity. They do not set direction, which remains ΔΨ, and flow through them is passive. Many PIPs close when the cytosol acidifies, one reason root conductivity falls in a flood. Phosphorylation opens some; calcium participates in closing others; drought can also remove them by changing expression. A root can therefore change resistance in minutes without new anatomy.
The soil–plant–atmosphere continuum is one path. Evaporation lowers mesophyll Ψ, tension is transmitted in the xylem, and water is drawn from the soil. Flow equals driving force divided by resistance, with soil, root, xylem, stomata, and the boundary layer in series. In the liquid the force is ΔΨ. Into the air it is a difference in vapour concentration across the pore. The steepest drop is usually at the stomata, until dry soil or an embolism makes another resistance larger. Across the root, water moves in the apoplast, the symplast, and across membranes. The endodermis blocks the apoplast. Chapter 2 explains the Casparian strip.
1.2.2 Cohesion–tension, the evidence, and cavitation #
The cohesion–tension theory, stated by Dixon and Joly in the 1890s, says sap is pulled, not pushed. Hydrogen bonds give water its cohesion, so tension at the leaf is transmitted down a continuous column. Adhesion to hydrophilic walls stops the column pulling away. Mature xylem conduits are dead pipes, not pumps. The column is metastable: below the vapour pressure of water, vapour is the favoured state, and liquid persists only because nucleating a bubble has an energy barrier. Pure water in a clean capillary bears extreme tension. Xylem sap fails earlier because pits and dissolved gas provide nuclei.
The evidence is direct. In a Scholander pressure chamber, a cut shoot is pressurised until xylem sap returns to the cut surface. That balancing pressure estimates the tension before the cut. It deepens as transpiration rises or the soil dries, and it relaxes when stomata close. Stems shrink slightly by day as tension pulls the walls in, and recover at night. Severing a conduit lets air in and stops flow in that file. A pressure probe can nucleate its own bubble and then report only a mild vacuum. Methods that do not puncture the conduit agree with the pressure chamber.
Cavitation forms a vapour cavity, and the conduit becomes an embolism until it is refilled or replaced. The usual route is air seeding through the largest pore of a pit membrane. The difference required is about 2τ/r, near 1.5 MPa for a pore of radius 0.1 µm. Wider vessels carry more water, flow rising with the fourth power of radius, and they embolise more easily. Narrow conifer tracheids, sealed by torus–margo pits, are safer and slower. New xylem removes embolisms for certain, and root pressure can refill herbs when tension collapses overnight. Refilling under sustained tension is not settled, and these notes do not treat it as established.
1.2.3 Root pressure and guttation #
Root pressure is real and secondary. Living root cells pump ions into the xylem, Ψs falls, water follows, and if the shoot is not transpiring a positive pressure builds. It is usually well below 0.2 MPa. That is enough for spring bleeding of grapevine or birch, and enough to refill some conduits in herbs overnight. It is not enough for a tall crown. Thirty metres already costs about 0.3 MPa against gravity, and friction costs more. Root pressure is also greatest when transpiration is least. It has the wrong size and the wrong time of day to be the midday motor.
Guttation is liquid water forced through hydathodes, unregulated pores over vein endings at leaf margins or tips. Grasses, tomato, and strawberry show it on humid nights when the soil is wet. The drops contain solutes. Dew is different: it condenses from the air onto a cold surface and is not exuded.
1.3 Stomata #
The study of Stomata provides fundamental principles governing cellular architecture, molecular interactions, and physiological regulation.
Opening And Closing Of Stoma
1.3.1 Blue light, potassium, and malate #
A pair of guard cells bounds the pore. Dicot guard cells are kidney-shaped. Grass guard cells, including those of cereals, are dumbbell-shaped, with subsidiary cells that lose turgor as the guard cells gain it. Radial cellulose microfibrils make a guard cell bow rather than balloon, so rising turgor opens the pore. Opening is therefore an accumulation of solute.
Blue light, perceived by phototropins, phosphorylates the plasma-membrane H+-ATPase through the kinase BLUS1. A 14-3-3 protein holds the phosphorylated pump on. Protons leave, the membrane hyperpolarises, often beyond −150 mV, and K+ channels such as KAT1 admit K+. Malate, made from starch via PEP carboxylase, and chloride from outside balance the charge. In Vicia faba malate is a major counter-ion. Vacuolar K+ can reach several hundred millimolar, Ψs falls, water enters, and the pore opens within minutes. Red light opens stomata mainly because photosynthesis lowers intercellular CO2.
1.3.2 ABA closes the pore #
Abscisic acid closes stomata in the light if the water deficit is severe. The fast path is ion efflux, not new transcription. ABA binds PYR/PYL/RCAR receptors, the complex inhibits PP2C phosphatases such as ABI1, and the kinase OST1 (SnRK2.6) is released. OST1 phosphorylates SLAC1 and QUAC1, so chloride and malate leave. The membrane depolarises, GORK lets K+ out, and the H+-ATPase is switched off. Ψs rises, water leaves, and the pore shuts. Calcium-dependent kinases can also phosphorylate SLAC1. Calcium reinforces closure. It is not the only route.
The ABA at the guard cell is made in the leaf, especially in the vascular tissue, when leaf turgor falls. A drying root can also raise ABA in the xylem, but the leaf does not wait for that delivery: a hydraulic signal is enough to start leaf synthesis. Alkaline xylem sap, common in drought, favours ABA reaching the epidermis rather than being taken up by mesophyll on the way. A shut pore still loses a little water through the cuticle. Chapter 7 places closure among the slower drought responses.
1.4 Key terms, summary, and self-check #
Key terms. Water potential; Ψs; Ψp; Ψg; turgor; plasmolysis; aquaporin; cohesion–tension; cavitation; embolism; air seeding; root pressure; guttation; phototropin; malate; ABA; SLAC1.
Summary. Water flows down Ψ. A transpiring leaf is negative because wall menisci are under tension and the air is a far steeper sink. Turgor is positive pressure against the wall; plasmolysis is the protoplast leaving that wall in a bath, not the buckling of a wilted leaf. Aquaporins set conductivity, not direction. Cohesion transmits leaf tension, as the pressure chamber and daytime stem shrinkage show, until air seeding embolises a conduit. Root pressure and guttation are real at night and far too small for a tall crown at midday. Blue light drives K+ and malate into guard cells; ABA empties them through SLAC1.
Self-check.
1. A cell has Ψs = −1.2 MPa and Ψp = +0.7 MPa. What is Ψ, and which way does water move if the apoplast is at −0.8 MPa?
2. Why is air at 90% relative humidity still a steep sink for a leaf near −1 MPa?
3. What does the pressure chamber report, and why can root pressure not replace transpiration pull in a 30 m tree?
4. What is air seeding, and why is a wider vessel both useful and risky?
5. Which solutes raise guard-cell turgor in blue light, and which channel does ABA open to let anions out?
