10.4 Translocation and Storage

10.4 Translocation and Storage

Water in the xylem of an angiosperm moves from roots toward leaves under tension. Sugar does not ride that stream. Sugar moves in the phloem by the pressure-flow hypothesis. The direction depends on which organ is supplying sugar and which organ is using it. The same plant can send sugar down to a root and up to a young leaf at the same time, in different sieve tubes.

Sources, sinks, and living tubes

A source is an organ that releases sugar into the phloem. A mature leaf is the usual source, because photosynthesis makes more sugar than that leaf needs. A sink is an organ that takes sugar out. Growing root tips, developing fruits, flowers, and young leaves are common sinks. A young leaf is a sink until it exports more sugar than it imports. After that it is a source. The source and the sink of the moment set the direction. Phloem flow is not locked from root to leaf the way the transpiration stream is.

The conducting cells are sieve-tube elements, also called sieve-tube members. They are alive at maturity, which separates them from tracheids and vessel elements, but they lose their nuclei as they mature. Their end walls are sieve plates, porous cross-walls that let sap pass. A companion cell beside each sieve-tube element keeps its nucleus and dense cytoplasm. The two cells come from one division and stay linked by plasmodesmata. The companion cell helps load and unload sugar. This cell pair is angiosperm phloem.

Loading, pressure, and bulk flow

At a source, sugar, usually sucrose, is actively loaded into the sieve-tube elements. Companion cells do much of the work. A common method spends energy to pump protons out of the cell. Sucrose then enters together with protons through a cotransporter. Species differ in the membrane route, but the working model is active loading at the source.

Loading raises the sugar concentration inside the sieve tube. Water potential falls, so water follows by osmosis from the neighboring xylem. The incoming water raises turgor pressure at the source end. At the sink, sugar is unloaded into cells that are growing or storing. Some unloading spends energy, and some follows a concentration gradient, depending on the organ. As sugar leaves the sieve tube, water leaves too, and pressure falls. Phloem sap then moves in bulk from the high-pressure source toward the low-pressure sink.

Nothing like cohesion-tension is pulling that sap up a dead pipe. Pressure flow is a push created by osmosis at the two ends. The loading step uses metabolic energy. The long-distance flow itself is bulk movement down the pressure gradient. Reverse the source and the sink, and you reverse the flow. A filling fruit can draw sugar down from the leaves. A sprouting storage organ can send sugar up toward new shoots.

Phloem sap is a watery solution rich in sugar, mostly sucrose, plus amino acids and some minerals. It is far more concentrated in sugar than xylem sap. Name the tissue before you name the mechanism. Phloem carries sugar by pressure flow. Xylem carries water and minerals by cohesion-tension. The pressure-flow mechanism is not the cohesion-tension pull.

Storage forms

Plants do not leave the whole sugar supply dissolved in the phloem. They store it. The usual long-term carbohydrate is starch, built into grains inside amyloplasts. Starch is common in storage roots such as cassava, in tubers, and in seeds such as wheat and corn, where the endosperm or the cotyledons hold it. Some plants store sucrose in vacuoles. A sugar beet stores sucrose in the root. Sugarcane stores sucrose in the stem. Many seeds store oils, which hold more energy per gram than starch. Sunflower seeds are a familiar example. The chemical form changes with the species. The rule does not: a storage organ is a sink while it fills, and it can become a source when the plant withdraws the reserve.

A tuber that changes jobs

A potato tuber is a swollen underground stem, not a root and not a fruit. The eyes are nodes, and each eye contains a bud. While the tuber is filling, mature leaves are the sources. They load sucrose into the phloem. Sap moves toward the tuber, which is the sink. Cells in the tuber unload the sugar and convert much of it to starch, and the tuber swells. In that season the tuber is a sink.

When the tuber sprouts, enzymes break starch down to sugars that can be loaded into the phloem. The tuber is now a source, and the new shoots are sinks until their leaves export sugar. Source and sink follow sugar flow. A growing root tip can be a sink while xylem water in that same root moves toward the leaves.

While a tuber is filling, xylem water moves upward and phloem sugar moves down to the tuber. After sprouting, phloem sugar can move from the tuber into the new shoot. The xylem remains the water path, and pressure flow remains the sugar path. The source of the moment sets the phloem direction.