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
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.
time: Part 2 highlights
long-unreleased research that documents greater root function
under organic systems.