Sugar transport, storage, and demand

Picture the trident maple, one of this course's instructional example trees, in early summer. Out of sight in its pot, fine white root tips are pushing through the substrate, building new cells, respiring day and night, and spending energy to take up mineral nutrients (Lesson 7). Yet roots grow in the dark and cannot make sugar from light and air. Nearly all the sugar they use was made in leaves they will never touch. How does it reach them?

Each spring, the maple also unfolds a whole set of new shoots and leaves before any of them can feed it. Where does that sugar come from? Following the sugar answers both questions.

Sources and sinks

Leaves make sugar by photosynthesis (Lesson 6), but every living cell uses it, so sugar has to move from the parts that have extra to the parts that need it.

A source is any part that makes or releases more sugar than it uses, so it has sugar to send out: a mature leaf in good light, or storage tissue turning its saved starch back into sugar.

A sink is any part that uses or stores more sugar than it makes, so it must take sugar in. Growing shoot tips, young leaves, the cambium that thickens the trunk and branches, roots, flowers, fruits, and seeds are all sinks.

These roles can change. A young leaf imports sugar while it is small, then exports for the rest of its working life, often starting well before it reaches full size. Storage tissue is a sink while it fills and a source while it empties. Fine roots, which cannot make sugar, are always sinks.

How the phloem moves sugar

Sugar travels from sources to sinks in the phloem, the living inner bark you met in Lesson 3. It moves dissolved in water, in most trees mainly as sucrose (table sugar), through long channels of living cells called sieve tubes. This movement is called translocation.

What moves it is pressure. The widely accepted explanation, called pressure flow, works in three steps:

1. Loading at the source. In a mature leaf, sugar is moved into the phloem of the veins until the solution there is very concentrated.
2. Water follows. Water tends to move across cell membranes toward wherever dissolved substances are more concentrated, a process called osmosis. So water from the neighboring xylem seeps into the loaded phloem, and the pressure inside rises, much as air pressure rises in a pumped-up tire.
3. Unloading at the sink. At a root tip, a growing shoot, or a storage cell, sugar is taken out of the phloem to be used or stored. Water leaves too, and the pressure there stays low.

The solution flows from high pressure to low, from source to sink, like water in a garden hose running from the tap to the open end. Water in the xylem moves differently (Lesson 8): it is pulled up through dead, hollow cells by evaporation at the leaves. Sugar is pushed through living cells by pressure built at the source.

Go deeper · optional, not assessedInside the phloem, and an insect that reveals the pressure

Each sieve tube is a file of living cells, called sieve-tube elements, joined end to end through end walls perforated like a sieve. As each of these cells matures, it loses its nucleus and most of its internal machinery, leaving a nearly open channel lined by a living membrane. A narrow companion cell beside it keeps it alive and, in many plants, spends energy from respiration to pump sugar in; in many trees, sugar may instead enter more passively, and how each species does it is still being studied. Conifers such as the nursery juniper use a similar arrangement of differently shaped cells.

Aphids show that the phloem is under pressure. An aphid slips its needle-like mouthparts into a single sieve tube, and the pressure pushes the sugary solution into it with little sucking needed. Aphids take in more sugar than they can use and excrete the excess as sticky honeydew, which is why aphids on the tender shoot tips of a bonsai often leave a glossy film on the leaves below.

Direction follows demand

Because the flow runs from where sugar is loaded to where it is unloaded, its direction depends on where the sinks are. Sugar from the same leaves can travel up to a growing shoot tip, down the trunk to the roots, or sideways into a fruit, and neighboring sieve tubes in one branch can carry sugar in opposite directions at once. Sinks draw most on the nearest sources, so upper leaves feed more of the shoot tips and lower leaves send more to the trunk and roots.

The flow changes through the year

In a temperate deciduous tree such as the trident maple, the sources and sinks change through the year, and the flow follows them. Before you explore the map, predict where spring growth gets its sugar.

Now check your prediction. The map opens on “Summer growth.” Switch Season to “Bud break,” then “Late season,” and watch the Source and Sink labels, the amber dashes in the trunk cutaway, and the starch grains. To follow a route stop by stop, press “Start tracing.”

In summer, sugar left the mature leaves in two directions at once: out to the shoot tips and down to the cambium and roots. At bud break, the trunk flow ran upward from storage to the opening buds, and the starch grains shrank. Late in the season, most of the leaves' sugar ran down to storage, and the grains grew. As each new leaf starts exporting in spring, the tree shifts from living on its savings to living on its income.

Other trees keep different rhythms. Evergreens such as the nursery juniper keep foliage that can make sugar whenever conditions allow, so their spring growth draws on older foliage as well as on stored starch. A tropical tree such as the ficus needs no winter rest and grows in flushes, bursts of new shoots and leaves, each one a sink.

Reserves: savings for spring and for setbacks

Sugar a tree does not need right away is mostly converted to starch and kept as reserves in living cells of the roots, trunk, branches, and bark. Reserves pay for spring growth before new leaves can, keep living cells respiring through winter, fund recovery when leaves or roots are lost, and supply defenses against pests and decay (Lesson 14). In a deciduous tree they are often lowest soon after the spring flush, when new growth has spent them and the new leaves have only begun to repay them.

Now sort the parts of a tree yourself. The sorter opens on “Midsummer, healthy tree”: choose Source, Sink, or “Neither now” for each numbered part, then check your answers and watch where the arrows go. Then predict what changes “After heavy leaf loss” before checking it.

After heavy leaf loss, storage became a source again, as at bud break. Losing most leaves in early summer, to caterpillars or hail, takes a tree's sources at the moment its savings are lowest. The tree often pushes out replacement leaves from buds meant for next year, drawing its reserves down further, while roots, cambium, and fruit compete for what little remains. Rebuilding takes healthy new leaves, good light, and time, sometimes more than one season.

You cannot see reserves in a photograph or in leaf color. You infer them from the tree's history: how much healthy foliage it has carried, in what light, for how long, and what has been taken from it. Growers' talk of a tree's “energy” is best read as this carbohydrate supply and its recent spending (Lesson 14).

Cutting the phloem but not the wood

Because water and sugar travel in different tissues, one can stop while the other keeps running. Removing a complete ring of bark from a branch, down to the wood, cuts the phloem all the way around but leaves the xylem whole. This is girdling (Lesson 3).

Now test your prediction. Switch on “Remove a ring of bark” and step through the three times, from “At first” to “Months later,” reading the status list as you go. Then switch on “Wrap in moist moss” and step through them again.

Water kept crossing the ring through the wood, and sugar still reached the growing tip, but sugar heading down stopped at the upper edge. That edge swelled into callus, a rounded roll of new tissue, while the lower edge changed little. Auxin, a hormone made in shoot tips and young leaves, collects there too, because it moves down mainly through the cambium and the living cells beside it, which the ring also removed. Among its many roles, it encourages roots to form; Lesson 12 shows how it shapes growth. Left bare, the wood dried; wrapped in moist moss, the upper edge grew roots.

That is the principle behind layering, making a new plant by growing roots on a stem while it is still attached to its parent. An air layer does it above ground: once the new roots are well developed, the branch is cut free below them and grown on its own. Trident maples generally root well this way; many trees root far less readily. Course 13, Propagation and Creating New Material, will teach the technique.

What this means for your bonsai

Leaves feed the tree, but mostly nearby. Together, the leaves supply every sink, including the roots in the pot. But a mature branch runs mainly on its own leaves, so stripping or shading its foliage cuts supply most to that branch and the trunk and roots below it, and a branch kept bare or deeply shaded for long can die back even on a healthy tree (Lesson 4). Removing all the leaves on purpose, called defoliation, spends reserves the same way; it suits only particular healthy, vigorous trees and goals (Course 14, Refinement and Long-Term Stewardship).

A long, leafy branch feeds what lies below it. Much of a vigorous low branch's sugar moves into the trunk beneath it and on to the roots. Growers use this when they let a sacrifice branch grow freely to thicken the trunk, then remove it (Lesson 12).

Flowers and fruit are strong sinks. A heavy crop competes with shoots, roots, and storage and can slow other growth; Course 14 will also cover managing it.

Anything that girdles bark cuts the supply. A wire, label tie, or string left around a thickening trunk or branch can girdle it, and so can gnawing rodents; deep wire bite (Lesson 3) is a partial version of the same injury. A complete girdle around the main trunk, below all its branches, cuts every leaf off from the roots: the leaves may stay green for months while the roots use up their reserves, but the tree usually dies.