The above-mentioned tips will only work when sufficient water and nutrients
are supplied by the roots. According to botanical research findings, the size
and quantity of leaves and the height of a tree in nature are determined by
water and nutrients absorbed in soil and the total energy obtained from
photosynthesis.
You need to prune the roots to ensure they supply enough water and
nutrients. The roots absorb water and nutrients in soil by root hair at the
root tips. Root hair has a rather short life span, and new root hair constantly
replaces old withering one. The most important factor in increasing water and
nutrient absorption is not to have thick and long roots but to increase their
total amount. Indeed, more roots can absorb more water and nutrients.
For naturally growing trees, large and thick roots such as the taproots are
necessary for supporting the shoots. But a bonsai tree does not need thick
roots including the taproot. On the contrary, the taproot will inhibit the
growth of finer roots. So during root pruning, you should focus on pruning
the taproot and the thick roots.
Like the shoots, new lateral roots will grow out after root pruning, which
increases the total number of roots. More roots will provide more water and
nutrients to the tree, which is advantageous to the development of lateral
buds after shoot pruning. On the other hand, without root pruning there will
be few roots, and so the shoots will be poor. Therefore, root pruning is
indispensable to achieving the effect of shoot pruning.

Special Column

Plant hormone

Pruning makes the lateral buds grow, so the shoots and roots can grow
to adjust their mutual balance. All these activities are due to the effects of
plant hormones such as auxins and cytokinins. Plant hormores are
chemicals that regulate the growth of the plant. There are several
categories of them, but until now many aspects of these hormones have
not been understood.
Auxins and cytokins are not just two specific hormones, but they refer
to two families of chemicals. They have various effects, and their main
effect is to promote cell divisions. The most typical one is in forming the
scab-looking knobby calluses which cover the wounds after the tree is cut
or wounded in order to keep out germs and prevent tree juices from
flowing out.
Mutual effects of auxins and cytokinins in the growth of the shoots and roots
Auxins are primarily synthesized in the apical buds in the shoots. They
reach the roots through phloem in the stem and trunk. When the auxins
concentration in the roots has reached a certain threshold, many lateral
roots will grow out. Thus, a plant with active apical buds will form many
branching roots. On the other hand, auxins can also inhibit the roots from
growing larger. According to the results, as the auxins concentration
increases, the plant grow out many tiny roots.
Cytokinins are mainly synthesized in the apical meristem in the root tips.
They are transported from the roots to the shoots with water and
nutrients through xylem in the stem and trunk. Cytokinins will accumulate
in the apical buds. This boosts the vigor of cell divisions there and thereby
increases the vitality of the shoots and leaves. Therefore, it is correct to
say more roots or more apical meristems poduce more cytokinins, and so
shoots will grow more vigorously. Moreover, vigorously growing shoots will
increase the amount of auxins synthesized in the apical buds and sent to
the roots, which further boosts cytokinins synthesis there.
These findings are scientific explanations of the fact that the trees with
ample roots have abundant shoots.
They also explain why plants stop growing in winter. As the soil
temperature drops in winter, the roots become less active and so
cytokinins synthesis decreases. Consequently, cell divisions stop in the
apical buds.

Special Column

The reason why the lateral buds grow after being
pruned――Apical dominance

Usually the apical buds alone are stimulated into growing, whereas the
growth of all lateral buds below them is suppressed. This is called apical
dominance. Shoots do not grow in lateral direction but grow outwards.
Pruning is to cut off apical buds so as to remove the apical dominance.
We consider the following hypotheses to explain the effects of auxins,
cytokinins, and apical dominance.
Dormant lateral buds will grow immediately after being applied
cytokinins. Also, if we spray cytokinins to the entire tree, germination of
the lateral buds will be evidently be boosted. Thus we can conclude that the
apical dominance occurs because the cytokinins concentration in the apical
buds is too high.
In other words, the cytokinins concentration is the highest in the roots
where they are synthesized, their concentration gradually lowers as they
are transported upwards through xylem. In the topmost apical buds where
cytokinins accumulate, their concentration is especially high. Auxins
produced in the apical buds promote cells' absorption of cytokinins, so
cytokinins accumulate especially in the apical buds with active auxins
production. In contrast, cytokinins do not accumulate in the lateral buds,
so the growth of the lateral buds is suppressed.
The optimal auxins concentrations for promoting the growth of the apical
buds and lateral buds differ. We have seen high concentration of auxins
promotes the growth of the apical buds but suppress that of the lateral
buds. We may consider a second hypothesis that lower auxins
concentration is needed to promote the growth of the lateral buds. Indeed,
if we prune the apical buds and place agar gel containing auxins on the
wounds immediately after the lateral buds grow out, we will observe the
growth of the lateral buds are suppressed. This confirms our hypothesis.
In addition, the growth of the lateral buds after pruning the apical buds
is the results of both the decrease in auxins and the increase in cytokinins.
Besides the above mentioned plant hormones, other new chemicals have
been discovered and identified more recently.

All plants develop from a single shoot and grow new shoots, on which new
shoots grow out. As the shoot ends develop and branch for several times,
the plant grows bigger. Meanwhile, the stems grow thicker to support the
increased weight of the plant, and to transport more water and nutrients.
Similarly, the roots keep growing.
For the shoots and roots to grow thicker, cell reproduction must occur in
the horizontal direction. This is called the secondary growth. It occurs in the
cell division tissue known as the vascular cambium in the stems. It should be
noted that some plants do not have secondary growths. But almost all plants
used for bonsai have secondary growth. As the cells develop in vascular
cambium, their cell walls thicken. After the cell die, their cell walls remain and
harden, and become firm supportive tissue. Grass has  short secondary
growth, whereas trees have extended secondary growths. In this way trees
make most of their stems, which are known as wood. Secondary growth
occurs both in shoots and roots.

Water, nitrogen, and inorganic minerals in soil are absorbed by root hair and
then transferred upwards through the vessels in xylem to the main root,
trunk, branches, and eventually leaves and buds at the shoot ends. Water is
broken down into oxygen and hydrogen by the energy in sunlight in the
leaves, and photosynthesis produces carbohydrates such as sugar and
starch by combining the hydrogen with the carbon dioxide absorbed from the
stoma in the leaves. About 20% of water is not used in this process, instead
it evaporates from the stoma in the leaves.

A plant is composed of cells and fibers. Cells support various vital life
activities of the plant by cell divisions. Fibers build and shape the cell walls
surrounding cells. Indispensable to all these life activities are carbohydrates.
There are two types of cell activities. One concerns the growth of the
plant, i.e the enlargement of its size, and the other its reproduction. For the
plant to grow larger, the ends of the stems and branches will bud and the
tree will grow taller. The branches will grow longer and the leaves will grow
denser. As the roots penetrate deeper into the soil, the trunk and branches
will grow thicker. The tree's wounds are also healing as it repairs itself. To
produce offspring, the tree will flower and eventually bear fruit.
Fibers make up the cell walls that surround the cells and make them rigid.
In their development, cells first divide to produce more cells. Then the cell
walls thicken to fix the plant tissues. The support provided by the cell walls
enable the vessels transporting water and nutrients to maintain their shapes.
The leaves and branches can keep their shapes for the same reason. Indeed,
a plant can support itself on the ground because of the cell walls. Most
tissues in the trunk and branches are composed of thick layers of cell walls
left by dead cells.
Carbohydrates are the building materials for cells and fibers.
Carbohydrates combine with each other to form fibers. They react with
nitrogen absorbed from roots into amino acids and proteins, which construct
the shapes of plant cells. All above-mentioned processes require
carbohydrates that are produced in the leaves and stored in the trunk and
roots.

For trees growing in nature, the stem size is determined by the amount of
water and nutrients transported through the vessels in xylem and that of
carbohydrates transported through sieve tubes in phloem. In addition, the
stem size should have enough wood to support the tree's weight. For a
bonsai plant, you may neglect the weight issue, so the other two factors are
more important.
Here are some findings from the botanical research.

1. When a single stem branches into two, the cross section area of the
   stem before branching is more or less the same as the sum of the
   cross section areas of the two branches.
   
2. The cross section area of a stem is proportional to the total amount of
   leaves on the stem up to its end.
   

Simply put, the more a shoot branches out upwards, the thicker the shoot
grows. So if you want to thicken a shoot, you should increase the number of
branching at that shoot and increase the amount of leaves on it.
For example, to thicken trunk while making “kokejun” (taper), you can just let
more branches grow out of the trunk and let the leaves grow denser.
Meanwhile, you should gradually reduce the amount of branches and leaves
on the shoots as you move up the trunk, and cut off the topmost branches
and leaves. As shoots grow via branching, this method can be applied to any
part of a tree.

Carbohydrates produced in
the leaves move to the
branches, stems, and roots
through sieve tubes in
phloem. Eventually, they are
transported to the buds and
root tips where cell division is
active, or to the reserve
tissue in the roots and the
stems to be stored. The
opposite can also happen as
carbohydrates move from the
storage tissues in the stems
and roots to the leaves and
buds. Water, carbohydrates,
and other nutrients are
transported through xylem
and phloem via changes in
osmotic pressure and the
capillary phenomenon.

BONSAI STYLE SECRET OF SHAPING

6. Leaf Pruning

Physiology of Shaping

End of shoot