Cambial Variants (Anomalous Secondary Growth) Fig. 8.7.1 Machaerium (In Spanish)
4.1 Plant respiration #
4.1.1 The shared path, and the alternative oxidase #
Glycolysis, the citric acid cycle, and oxidative phosphorylation are Biochemistry’s. The plant substrate is sucrose or starch. Sucrose synthase yields UDP-glucose and fructose and costs less ATP than invertase, which matters when oxygen is short. A leaf in the light still runs its mitochondria: photorespiratory glycine is decarboxylated there, and a partial citric acid cycle supplies 2-oxoglutarate for GOGAT.
Coupled respiration through complexes I to IV is the default. Alternative oxidase takes electrons from ubiquinol and reduces O2 to water without pumping protons, so the energy leaves as heat. Cyanide blocks complex IV and does not block the oxidase. The oxidase is sensitive to SHAM. It runs when a high ATP/ADP ratio, or a reduced ubiquinone pool, restricts the cytochrome path, so carbon skeletons keep turning and fewer electrons leak to superoxide. Reduction of the protein, and pyruvate, activate it. Non-pumping NAD(P)H dehydrogenases do a related job. In an ordinary leaf the point is flexibility, not heat.
4.1.2 Heat in skunk cabbage and the aroids #
Thermogenesis is the specialised use. The spadix of eastern skunk cabbage, Symplocarpus foetidus, respires through the alternative oxidase fast enough to hold tissue near 20ยฐC in air near or below freezing. The heat volatilises the scent that attracts pollinators.
In arum lilies and Philodendron the sterile appendix runs many degrees above the air for a few hours on the night of pollen release, by the same uncoupled oxidation of carbohydrate. These are pollination devices. A wheat leaf does not run the alternative oxidase in order to stay warm.
4.2 Sucrose and the phloem #
4.2.1 Where sucrose is made, and what the sap contains #
Sucrose is made in the cytosol of source mesophyll. Triose phosphate leaves the chloroplast only in exchange for phosphate. Sucrose-phosphate synthase makes sucrose-6-phosphate, and a phosphatase frees sucrose. The synthase is active when hexose phosphates are high, is inhibited by phosphate, and is quiet in the dark through phosphorylation. If export lags, phosphate returns to the chloroplast too slowly, triose phosphate stays inside, and starch accumulates. At night starch is broken largely to maltose, maltose leaves, and sucrose export continues. The leaf feeds the plant after sunset as well as before.
Sieve-tube elements are alive but enucleate. The companion cell keeps them supplied through plasmodesmata. P-protein and callose seal a damaged plate, so a cut stem does not bleed phloem the way a spring birch bleeds xylem. Aphid stylet exudate is the clean sample, because the tube is under positive pressure. The sap is dominated by sucrose, commonly a few tenths molar and sometimes near 1 M, with amino acids, K+, and signals such as the FT protein of Chapter 6. Sucrose is the cargo because it is non-reducing, a disaccharide moves two hexoses at once, and invertase is kept out of the tube. Some families also move raffinose sugars or sugar alcohols. Glucose is not the long-distance sugar.
| Xylem | Phloem | |
|---|---|---|
| Force | Tension from transpiration; rarely root pressure | Turgor difference, source higher than sink |
| Cargo | Water and mineral ions | Sucrose, amino acids, signals |
| Cells | Dead conduits | Living sieve tubes and companion cells |
| If cut while transpiring | The vessel sucks air | The sieve tube can exude, then seals |
4.2.2 Apoplastic and symplastic loading #
Apoplastic loading is the pattern in Arabidopsis and in most crops. SWEET facilitators let sucrose out into the wall beside the sieve elementโcompanion cell complex. A sucroseโH+ symporter, SUC2/SUT, takes it up into the companion cell, driven by the H+-ATPase, and concentrates it above the mesophyll level. Few plasmodesmata cross that boundary. Blocking the pump or the symporter blocks export.
Symplastic loading keeps plasmodesmata open into the companion cells. In many such plants the concentration step is polymer trapping. Sucrose diffuses in through narrow pores and is converted to raffinose and stachyose. Those larger sugars cannot diffuse back, but they can move on into the sieve tube. Cucurbits are the familiar case. A third pattern, passive symplastic loading, is likely in some trees that simply hold mesophyll sucrose above the sieve-tube concentration. The two mechanisms to draw are the symport and the polymer trap.
4.2.3 Pressure flow, and a sink that changes job #
Mรผnch’s hypothesis of 1930 is the long-distance motor. Loading at the source lowers sieve-tube ฮจs, water enters from the xylem, and turgor rises, to the order of 1 MPa. At the sink, sucrose is removed, water leaves, and turgor falls. Bulk flow through the sieve pores follows the pressure difference. Water returns in the xylem, so Chapters 1 and 4 are one hydraulic system. A given sieve tube flows one way at a time. Different tubes can feed different sinks, so the plant as a whole is not limited to one direction.
The evidence is the positive pressure of stylet exudate, the sugar gradient from source to sink, and the effect of a phloem girdle, which stops export while intact xylem still delivers water. Diffusion is not an alternative. A small sugar takes of the order of ten years to diffuse a metre in water. Phloem delivery over that distance takes hours, often at speeds of the order of a metre an hour. Unloading and the choice among sinks are local biochemistry. The hypothesis does not decide them. Sucrose that leaks on the way is retrieved, or the pressure would fail.
A source exports. A mature leaf is the usual one. A sink imports: apices, young leaves, fruits, seeds, and storage organs. A dicot leaf is a sink until about half grown, then a source. A developing fruit is a strong sink. The seed has no plasmodesmata to the mother, so sugar crosses an apoplast and invertases keep the gradient steep. If the fruits are removed, sugar backs up and photosynthesis slows. Sugar beet stores sucrose. A potato tuber converts it to starch, so unloading does not stall. Sink strength is size times the rate of use, and it changes from flowering to seed fill.
4.3 Key terms, summary, and self-check #
Key terms. Alternative oxidase; sucrose-phosphate synthase; sieve tube; apoplastic loading; polymer trap; Mรผnch pressure flow; source; sink.
Summary. Plant respiration is the standard path plus non-phosphorylating dehydrogenases and a cyanide-resistant alternative oxidase that releases heat. Skunk cabbage and thermogenic aroids use that heat in flowering. Sucrose is made in the cytosol and loaded either by an H+ symport from the apoplast or, after symplastic entry, by conversion to larger oligosaccharides. Pressure flow carries it from a turgid source to a less turgid sink. A half-grown leaf changes from sink to source, and a fruit or a storage root is a sink whose strength is the rate at which it uses the sugar.
Self-check.
1. Why does skunk-cabbage respiration continue in cyanide, and why does that path make heat rather than ATP?
2. Why is sucrose, rather than glucose, the sugar in sieve-tube exudate?
3. What does the sucroseโH+ symporter concentrate, and what does raffinose synthesis achieve in a polymer-trap loader?
4. In the Mรผnch hypothesis, where is turgor higher, and why is diffusion not a serious alternative over a metre?
5. Name an organ that is a sink and later a source, and say why converting sucrose to starch helps a tuber stay a sink.
