| August
10, 2006: Close your eyes and think of your favorite
plant. What do you see? Flowers, leaves, stems, fruit? Very
few people visualize the root system at all. Likewise, most
agricultural research focuses on what is easy to see above
the ground—not on what we can’t see below.
Because of this and the tedious process of taking and measuring
root samples, we conveniently forgo extensive root analysis
and end up de-emphasizing the role that root growth and root
physiology play in producing healthy plants.
Although unseen and under-appreciated, the compelling fact
remains: Without roots there are no shoots.
At The Rodale Institute, we use the motto “Healthy
Soil, Healthy Plants, Healthy People” to focus our mission.
Healthy plants start with their connection to healthy soil
in the root zone or by a connection to the soil through beneficial
mycorrhizal fungi. (For more on mycorrhizae, see Fact
Sheet: Mycorrhizae.) The interface between the soil and
the plant is the zone immediately adjacent to the plants roots
or the mycorrhizae hyphae (long, microsopic strands—up
to 18-feet long—attached to the roots). This zone is
known as the rhizosphere.
Here, microorganisms are more numerous, and their activity
improves the structure of the soil. Microorganisms in this
zone interact with roots and mycorrhizal exudates, forming
a community which interacts biologically with parts of the
soil.
The survival of naturally growing plants depends on a delicate
balance between the root system and the shoot system. As the
top of the plant grows larger and larger, the leaf area and
water loss through transpiration also increase.
The increased water loss through leaves is compensated by
water absorption from an increasingly active and growing root
system. The enlarging shoot system also requires greater amounts
of minerals that are absorbed by the increasing root system.
Simply put, everything above is supported and nurtured by
everything below. The importance of roots—although not
fully appreciated by humankind—is appreciated by the
plant, which dedicates up to one third of all its food from
photosynthesis to root support.
Dr. Francis Zee, a horticulturalist and former colleague
at the USDA’s Agriculture Research Service, proclaimed
the balancing of root and shoot growth has the greatest ability
to promote a healthy plant. The root system comprises a significant
portion of the entire dry weight of any plant—about
one quarter to one third, depending on the functional role
or structure of the root.
Roots, aided by protuberances (root hairs) and associated
mycorrhizal fungi, generate a huge soil contact zone facilitating
the absorption of water and minerals, as well as for anchoring
and supporting the plant. Root hair cells and mycorrhizal
hyphae are relatively short-lived, but are present in great
numbers. They are rapidly produced as their tips continuously
thrust through the soil exploring for water and nutrient resources.
Work done at The Rodale Institute in collaboration with USDA-ARS
research scientists show that mycorrhizal fungi are an important
and overlooked part of plants root system. Together the root,
root hair and mycorrhizal fungi provide a botanical web not
only exploiting the soil resource but also sensing its condition.
Other research shows that this biological association is extremely
important in providing the catalyst (biochemical boost) for
building productive soil by increasing the soil organic matter.
(For more on these processes, check out Cedar
Meadow Farm Field Day.)
The mycorrhizal hyphal body and a group of specialized cells
in the root form the xylem tissue, which becomes the main
pathway for the transport of water and minerals. All mineral
elements are ultimately absorbed from the soil together with
water and are transported upward through the xylem. Organic
acids and chelating agents from the mycorrhizal fungus allow
absorption of materials that are barely absorbable by the
plant alone.
Since the ratio of the different elements in the xylem sap
is quite different from that in the soil, it is clear that
mineral uptake through root cells must be a selective process.
Mycorrhizal fungi have been shown to be extremely important
for mobilizing otherwise immobile nutrients in soil and for
extending the web that captures water for plants in dry environments.
While root hairs absorb nutrients within a 1mm zone around
them, with the aid of myccorrhizal fungi this zone can extend
to 10cm.
Several conditions must be satisfied if the green plant is
to acquire adequate quantities of minerals for its growth.
First, the minerals must be in a form suitable for absorption
by the cells of the root. Usually this means they must be
dissolved in the soil solution, but oftentimes they may be
released from soil particles through the solubilizing activity
of roots. This shows the importance of root growth and the
spatial distribution of roots in the soil.
Second, the soil must be well aerated so root cells can carry
out their oxidative process for mineral uptake.
Third, the transport system in the plant must function efficiently
in delivering minerals to the recipient cells.
There is competition for minerals by root absorption versus
leaching minerals through soil as water percolates past the
root region. Hence the deeper and more developed the rooting
zone, the more efficient and effective the plant is in absorbing
these soluble nutrients.
By contrast, shallow root zones lead to less spatial opportunity
for the plant to absorb nutrients. 
Next
time: Part 2
highlights long-unreleased research that documents greater
root function under organic systems.
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