[nafex] Root growth basics from MSU
edforest55 at hotmail.com
edforest55 at hotmail.com
Tue May 22 09:10:45 EDT 2001
MI State UniversRoots: The Hidden Half of Growth
MSU has an excellent site for fruit growers, below is an example of
an article on root growth that can be found there:
Southwest Michigan District Extension Horticultural & Marketing Agent
Winter has come and the trees, vines and bushes that bear our
Michigan fruit crops have lost their leaves and gone dormant for the
winter. Fruit growers would be wrong to think that not much is
happening in their orchards and vineyards. The above ground portions
of our plants are dormant and the buds by some undetermined mechanism
are keeping track of chilling hours. We may think that the whole
plant is dormant but this is hardly the case. In the ground, the
roots are active and growing. Unlike the above ground shoots, which
are dormant and will not grow until completing their chilling
requirement, the only thing holding back root growth is soil
temperature. The roots are capable of growth at any time of the year.
All the roots need are conditions conducive to growth and the lack of
competition from other plant organs such as the shoots and fruit.
All of us know that our plants have roots. But we just don't think
about them. It's hard to image things we can not see, touch or feel.
Most of us, when we think of the roots imagine a subterranean mirror
image of the trunk. We often wonder how far from the tree do the
roots go? How deep are they? We busily take care of the plant above
ground but we seldom think about the roots. With this essay, I hope
to give you a little insight into what is happening under your feet
all year long.
We know the roots provide support for the tree. We are reminded of
this when trees fall over. We also know that the root system absorbs
and supplies water and nutrients to the tree. We are reminded that
the tree needs roots when new plants leaf out and then just sit there
not growing for the rest of the growing season. We often wonder what
could we do to increase growth. Then the next year, the tree takes
off, growing like a rocket. This situation often occurs when trees
are planted late or the soil conditions are not favorable for root
growth. The young plant is unable to produce enough of a root system
to supply the needs of the plant. When the demands of the leaves
overtake the ability of the roots to supply water, growth stops.
In woody plants, there are two kinds of roots; woody roots and white
roots. These two kinds are very different. The woody roots are in
essence the underground trunk of the tree. These are the roots that
we all think of when we think to the root system of a perennial
plant. There are also new white roots that are the result of active
growth. These young roots are an important site of mineral
absorption. White roots do not hang around long. The live span of a
young root is measured in days and weeks. As the white portion of the
root matures, it turns brown as the outer layers turn brown and die.
Nine times out of ten the remaining small brown root also dies.
Occasionally a brown root lives. The root thickens and becomes part
of the woody root system. Root survival is probably due to whether
there is still good growth in the root tips supported by that root.
Root growth is similar to shoot growth but there are major
differences. At the tip of an actively growing root is the root cap.
The root cap is a loose assemblage of cells and a slimly substance
called mucilage that protects and lubricates the root tip as it
forces its way through the soil. Behind the root cap is the zone of
active cell division were new cells are created. Immediately behind
is the zone of cell elongation. This is where the new cells begin
growth. Plant cells grow by pumping themselves full of water. They do
this by actively moving salt ions, usually potassium, into the cell.
They also move sugars into the cell. The name of the game is to be
saltier and sweeter that the water around you. This causes water to
move into the cell. The cells do not actively take up water. The
water flows into the cells because there are more salts and sugars
inside the cells than outside. This is the way all plant cells grow
and the way roots absorb water from the soil. As the cells absorb
water they increase in length and force the root tip deeper into the
Behind the zone of elongation is the zone of maturation where the
different types of cells develop. The xylem, long cells that conduct
water out of the roots and up the tree, form in the middle of the
root. On the outside of the root is the cortex that absorbs nutrients
from the soil. In-between the xylem and the cortex are two very
important layers of cells. The outer layer is the endodermis. This
layer of cells forms a thick waxy layer between all its cells called
the casparian strip. This thick barrier forces water and nutrients
moving into the xylem to go through the endodermis cells. This
barrier prevents leakage out of the root. Inside the endoderimis is
the pericycle. This layer still has the capacity for new growth by
renewed cell division. This is where new roots originate and where
thickening will occur if the root survives and becomes woody. As the
root ages the cortex dies and the endodermis is exposed to the soil.
This is when the white root turns brown and soon after that the root
usually dies. If the root lives, new roots from the pericycle may
burst out through the old covering or the root may thicken and become
There is a pattern and timing to root growth. The roots grow when the
shoots are not growing. Competition with other parts of the plant$is
the main determining factor to root growth. In fall, after harvest,
sugars made in the leaves was transferred to the shoots, trunk and
roots for storage and use during the next growing season. A large
portion of these storage compounds are stored in the roots. The roots
use this abundance of energy for growth and root growth is active in
the late summer and fall. Root growth begins to taper off as the soil
cools in the fall, but the roots are always capable of growth. As the
soil becomes cold in the winter, root growth ceases. This is not
because the roots are dormant as the buds are in the top of the tree,
but because the soil is too cool to support growth. The crown of the
tree, where the root system joins the trunk of the tree is the
transition zone between the dormant top and the active roots. This
transition zone, which often contains a graft union, is very
susceptible to winter cold injury. Conditions that favor late root
growth can predispose your plants to cold injury in the winter,
particularly if the ground is not covered by snow.
Root growth increases in the Spring. This is not because the roots
have become active but because the soil is now warm enough to support
faster growth. Root growth increases as the soil warms. Rapid root
growth continues through bud break and only begins to slow as the
shoots begin their growth. During the period of rapid shoot growth in
the early spring new root growth virtually stops and all the trees'
reserves are used in above ground growth. When shoot growth ceases in
early summer, root growth begins again. The rate of root growth
increases through the summer reaching a peak in early fall and then
declines as the soil cools. So, two of the main restraints on root
growth are soil temperature and competition with shoots and fruit.
The other factor determining root growth is soil moisture. Roots grow
were the conditions for growth are favorable. The old adage that
roots grow to water is WRONG. Roots grow where the water is. Roots in
dry soil show very little growth. In the spring, when the soil is
moist and the surface is warm, roots near the surface show the most
growth. As the soil dries out, roots near the surface die and new
roots grow deeper in the soil. As the season progresses, the depth of
active root growth moves progressively deeper and deeper in the soil.
With each rain or irrigation new roots appear near the surface. These
new roots absorb water and nutrients and then die.
Lets go back to the analogy of the upside down tree to represent the
root system. We can imagine that instead of one trunk that there are
several and the trunk and branches of this root system are composed
of woody roots. Now imagine that the leaves on this tree are not
green and flat like an apple or a grape leaf but long and thin like
the needles of a pine and white not green. Keep in mind that most of
these leaves (white roots) will fall off, many a week or two after
they form, and few become permanent woody stems. Unlike the leaves on
a pine tree, white roots can change into a woody roots (stems) and
grow more leaves (roots).
I find that thinking of white roots as leaves helps me to put the
process of root growth in a visual framework that I can understand.
The white roots really are like leaves. They are temporary structures
that absorb water and nutrients. Leaves are temporary structures that
absorb carbon dioxide and mix it with water to make sugar providing
the energy for plant growth and fruiting. When their job is done,
both roots and leaves are discarded by the plant.
As the growth of our imaginary (upside down) root tree begins in the
Spring, most of the roots sprout out close to the ground. As the
season progresses, more roots sprout deeper and deeper in the soil.
Instead of hanging on to the trunk all season, most of the roots fall
off after two weeks. Rapidly the white rooted region of the tree
moves downward in the soil, and every time it rains there is a new
flush of growth near the surface of the soil from the older root
system. Towards the end of the year, fewer new roots grow deep in the
soil. Most of the new growth occurs close to the soil surface were
the ground is warm, moist and has the most nutrients. Over the years
a frame work builds, with many branches close to the soil surface and
several large trunks. Most of these large trunks lie close to the
surface. Few roots are found deep in the soil. Forty to sixty percent
of the rootsystem of perrenial plants is in the uppermost layer of
the soil. On the large woody roots there are branches were the roots
have found water and nutrients over the years.
If you now take this root system and place it in the soil with a
fruit tree above we can visualize the annual cycle of root and shoot
growth. In the early Spring, many new roots grow close to the soil
surface and remove water from the surface soil layers. As the leaves
begin to appear on the tree, roots appear lower and lower in the
soil. While the leaves above ground last until the end of the growing
season, there are ten or twenty overlapping generations of new roots
in the soil. You can almost imagine a time lapse movie of tree
growth. Above the ground we see leaves form and shoots grow begin.
Below ground we see a zone of white moving downward in the soil,
broadening and narrowing as it moves downward. As active shoot growth
begins, root growth slows and the white zone fades. All through the
soil we see brief flashes of white as roots grow and die. As shoot
growth ceases, we see increased activity in the soil as new roots
form close to the surface and older, deeper roots die.
Almost half the sugars manufactured in the leaves are used and lost
through root growth, which we normally do not think about because we
do not see it. As I sit here and write this on a cold winter day, I
can look outside where the trees are standing with no leaves on them.
I know that in the soil roots are growing, getting ready to supply
that explosive burst of green, leafy new growth that comes every
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