Plant Physiology

1Water potential and the direction of flow

Water potential Ψ_w is the free energy of water per unit volume, measured against pure water at atmospheric pressure, which is defined as zero. Every solute added lowers it, so cell sap is always negative and the whole subject is read in negative numbers: −0.6 MPa is higher than −0.9 MPa because it is closer to zero. Water then moves down the gradient, from higher to lower potential, passively — nothing pumps it.

Two terms make up the total: Ψ_w = Ψ_s + Ψ_p. Solute potential Ψ_s is zero for pure water and negative for every solution. Pressure potential Ψ_p is positive when a turgid protoplast presses on its wall, zero in a flaccid cell, and negative only where water is under tension — the xylem sap, and the evaporating cell walls that pull on it. Osmosis is no more than this gradient acting across the semi-permeable plasma membrane.

Figure. Both cells are turgid, so both pressure potentials are positive; only the sum decides. Water crosses to B and keeps crossing until the two water potentials are equal — not until the two solute concentrations are.

2Two routes across the root, and the strip that closes one

Water entering a root hair can cross the cortex two ways. In the apoplast it travels through the continuous network of cell walls and intercellular spaces, never crossing a membrane, so it moves fast and is not selected. In the symplast it enters the cytoplasm and passes from cell to cell through plasmodesmata, crossing the plasma membrane once, at the very start.

At the endodermis the choice is taken away. Suberin laid down in the walls that each endodermal cell shares with its neighbours forms the Casparian strip, which is waterproof, so the apoplastic stream has to enter the cytoplasm of an endodermal cell. Everything that reaches the xylem has therefore crossed at least one living membrane, and that is where the plant selects its ions. Past the endodermis water re-enters the apoplast, because the xylem lumen is itself apoplast.

Figure. The upper arrow is the wall route and it stops dead at the strip; the short diagonal is where it is forced inward. The lower arrow is the cytoplasmic route, which never had to stop. Both end in the xylem, but only one of them was inspected on the way.

Following one water molecule inward

1. Into the root hairThe soil solution is the less negative of the two, so water enters the root hair passively.
2. Across the cortexMostly by the apoplast, which offers least resistance; the symplast carries the rest in parallel.
3. Stopped at the endodermisThe suberised Casparian strip seals the wall route, so water and ions must cross a plasma membrane.
4. Into the xylemTransport proteins load selected ions in, water follows them, and the lumen returns it to the apoplast.

3Transpiration pull: the plant's only negative pressure

More than 95 % of the water a root absorbs is lost again as vapour, almost all of it through the stomata; under 5 % is kept for growth and photosynthesis. That apparent waste is the engine. Evaporation from the wet walls of the mesophyll draws the water surface back into the pores of the wall, and the curved menisci put the water behind them under tension.

Because water molecules hydrogen-bond to one another (cohesion) and to the walls of the xylem (adhesion), that tension is transmitted down an unbroken column from leaf to root. Xylem sap is therefore under negative pressure — Ψ_p below zero, which is never true of a living cell's protoplast — so the column is pulled up, not pushed. Root pressure is real, but it is small, appears on still nights as guttation, and cannot lift water to the top of a tall tree.

Figure. Read the ladder upward and every rung is more negative than the one below it, which is the only condition water needs in order to climb. The single arrow is not a pump: it marks the direction the whole column is dragged when the top rung is made more negative still by evaporation.

How a leaf lifts water

1. Evaporation at the wallWater leaves the mesophyll cell wall as vapour and escapes through the open stoma.
2. Tension appearsThe retreating menisci drop the pressure of the water behind them: Ψ_p turns negative.
3. Cohesion transmits itHydrogen bonds hold the column together, so the pull is felt all the way down the xylem.
4. The column risesWater is drawn out of the root xylem, and the root in turn draws more from the soil.

4Phloem: loading builds a pressure that pushes

Xylem pulls; phloem pushes. Sucrose is loaded into the sieve tubes at a source — a photosynthesising leaf, or a storage organ mobilising its reserves — and the loading is active, worked against a concentration gradient and paid for with ATP from the companion cells.

Loading drops Ψ_s in the sieve tube, so its Ψ_w falls and water enters osmotically from the neighbouring xylem. The tube swells and its Ψ_p rises. At the sink sugar is unloaded, Ψ_s rises, water leaves again, and the pressure there stays low. The hydrostatic difference between the two ends drives bulk flow of the entire solution — Münch's pressure-flow hypothesis. Direction follows whichever organ is source and which is sink, so it is not fixed: a potato tuber is a sink all summer and a source in spring.

Figure. One arrow in the xylem, two in the phloem, and that asymmetry is the whole comparison: the xylem can only go up, while the phloem goes wherever the sink is — down to a root in summer, up to a fruit or a growing tip at the same time.

Pressure flow, one step at a time

1. Loading at the sourceSucrose is pumped into the sieve tube with ATP supplied by the companion cell.
2. Water followsThe sieve tube's Ψ_s falls, water enters from the adjacent xylem, and turgor builds.
3. Bulk flowThe pressure difference between source and sink pushes the whole solution along the tube.
4. Unloading at the sinkSugar is removed, water leaves with it, and the low pressure there keeps the gradient alive.

5Essential elements, and where a deficiency shows first

Seventeen elements are essential. Carbon, hydrogen and oxygen come from air and water; the other fourteen are minerals drawn from the soil. Nine of the seventeen are macronutrients, present above roughly 10 mmol per kilogram of dry matter, and eight are micronutrients, present below that — needed in traces, but no less indispensable, because essentiality is not a statement about quantity.

An element counts as essential only if the plant cannot complete its life cycle without it, no other element will substitute for it, and it takes a direct part in metabolism. Which leaf shows the deficiency first is then a question about mobility rather than about the element's job: a mobile element (N, P, K, Mg) is withdrawn from old leaves and sent to the young ones, so the old leaves fail first, while an immobile one (Ca, S, Fe, B) strands the young tissue that never received any.

Figure. Essentiality is a yes/no rule; abundance is not drawn. The figure teaches the mobility contrast that decides whether deficiency appears first in old or young leaves.

Fixing nitrogen in a root nodule

1. InfectionRhizobium in the soil invades a root hair of a legume and grows inward as an infection thread.
2. The nodule formsCortical cells divide, the bacteria become bacteroids, and vascular strands link the nodule to the stele.
3. Oxygen is kept outNitrogenase is destroyed by O₂, so leghaemoglobin scavenges it — and turns the nodule pink.
4. Ammonia is exportedN₂ + 8e⁻ + 8H⁺ + 16 ATP → 2 NH₃ + H₂, and the ammonia leaves as glutamine or asparagine.

6Glycolysis, Krebs and the chain: where each ATP comes from

Glycolysis splits glucose into two pyruvate in the cytoplasm, and releases no carbon dioxide at all. Pyruvate then enters the mitochondrial matrix, where the link reaction decarboxylates it to acetyl-CoA — the first CO₂ of the whole process — and the Krebs cycle strips the remaining two carbons from each acetyl group. All six CO₂ per glucose come from those two stages.

Very little ATP has been made by that point: four molecules by substrate-level phosphorylation, two in glycolysis and two in the cycle. What the first three stages really produce is reduced coenzyme — ten NADH and two FADH₂ — and it is the electron transport chain on the inner mitochondrial membrane that turns those into ATP, handing the electrons finally to O₂ to make water. Remove the oxygen and the chain backs up, NAD⁺ stops being regenerated, and the cycle halts with it.

Figure. Read the bottom row and the shape of the chapter appears: the first three boxes make almost no ATP but hand twelve reduced coenzymes to the fourth, which makes 34 of the 38. The compartment line along the top is worth as many marks as the yields — glycolysis is the only stage outside the mitochondrion, and the only one that releases no CO₂.

7Respiratory quotient: reading the substrate off the gases

RQ is the ratio of CO₂ released to O₂ consumed over the same interval, and it says what is being respired. A carbohydrate is already half oxidised, so burning it needs exactly as much O₂ as it yields CO₂ and RQ = 1. A fat carries far more hydrogen per carbon, so a large part of the oxygen goes into making water rather than CO₂ and RQ drops to about 0.7. An organic acid starts out more oxidised than a sugar, needs very little added O₂, and pushes RQ above 1.

One value needs no equation at all. A fermenting seed consumes no oxygen, so the denominator is zero and the quotient is infinite. A reading above 1 therefore narrows the possibilities to two: an organic acid is being respired, or the tissue has run short of air.

Figure. RQ reads the substrate: carbohydrate near 1, fat near 0.7, protein near 0.8. Bar height is the ratio itself.

8Five regulators, and what each is famous for

Three promoters and two inhibitors, though ethylene manages to be both. Auxin, made at the shoot apex and carried downward, drives cell elongation and enforces apical dominance. Gibberellin elongates internodes — bolting in a rosette plant — and induces amylase in germinating barley. Cytokinin drives cell division, releases the lateral buds that auxin was suppressing, and delays leaf senescence, so the two are antagonists at almost every point.

Abscisic acid is the stress hormone: it closes the stomata within minutes of water shortage and imposes seed and bud dormancy, which makes it gibberellin's opposite number. Ethylene is a gas, and most of its behaviour follows from that — it diffuses from one ripening fruit to its neighbours, hastens climacteric ripening, promotes senescence and abscission, and breaks dormancy in potato buds.

Figure. This is a schematic of the shoot axis, not a picture of a plant: the only things the shapes are claiming are up, down, and apex against lateral bud. On the left, auxin flowing basipetally from the tip keeps the two buds small. On the right the tip is gone, the flow with it, and the same two buds grow — which is all that pruning a hedge is doing.

Why pruning makes a bush

1. The apex makes auxinThe shoot tip is the plant's main site of auxin synthesis.
2. Auxin moves downPolar transport carries it basipetally, from the tip toward the base of the stem.
3. Lateral buds stay shutAuxin arriving from above holds the buds below it dormant — that is apical dominance.
4. Remove the tipThe supply stops, cytokinin now has the upper hand in the buds, and they grow out.