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Ideal Root System: role of root ecologist-physiologist
Akira Yamauchi
School of Agricultural Sciences, Nagoya University,
Chikusa, Nagoya 464-01
Roots support a whole plant growth with various functions. Recent studies
have revealed the morphology and physiology of individual root, while much
less is known about root system of an individual plant or a plant community,
which is consisted of such individual roots.
During the course of the Green Revolution, a concept of "Plant Ideotype"
was proposed that deals with plant shoot. As a fruit of intensive and extensive
basic studies, the Green Revolution finally produced high yielding varieties
for wheat and rice based on the concept. One of the most significant findings
in those studies was that the architecture of leaves in shoot that effectively
captures solar energy is the key trait in addition to high photosynthetic
rate of individual leaf for achieving high dry matter production when the
crop is grown in community. These facts lead us to speculate that hierarchy
may exist in root function; the function of root system is not simply the
addition of function of individual root, but different levels of function
can be expressed when individual root works together in a root system as
an integrated organization.
Many researchers seem to assume that a big root system is good. If the
thing is that simple, then we do not need to study to pursue the ideotype
of root system. As scientists specializing shoot part of crop, root scientists
have difficulties in identifying a desirable root system architecture that
ensures the highest function. To find an ideal root system, at least two
conditions must be clearly specified. First, we need to pinpoint a precise
goal; what plant trait needs to be maximized or optimized with such ideal
root system? Examples of traits are water extraction in soil depths, lodging
resistance, P uptake in acidic soil and so on. Consequently, second, growth
conditions, especially those in soil, need to be explicitly defined.
Passioura's group developed wheat line that is adapted to the environment
where crop depends only on residual soil moisture. This line grows with
only a few seminal roots and thus has great hydraulic resistance, and thereby
can save soil moisture till anthesis, while other lines consume most of
the stored water during vegetative and suffer from water deficit during
anthesis, which results in substantial yield reduction. In favorable years,
the line plastically produces more nodal roots that effectively collect
available soil water so that its growth become comparable with other lines
with more extensive root system. Apparently, such strategy can not be generalized
for the identification of an ideal root system in drought-prone area. However,
it is an example of excellent approaches that identify the ideal root system
in specific environment of target area.
There is another important aspect for the ideal root system; dynamic trait
of root growth and function. The ideal root system can be differed along
with plant growth progression and environmental changes that frequently
occur during the growth. It is also important to understand that a plant
root system consists of roots of different natures in morphology, physiology
and genetic control, and thus responses in development and function to
environment. As such, the phenotypic plasticity is one of the key traits
that needs to be taken into serious consideration for the ideal type of
root system.
There have been very limited cases in which root traits were intentionally
and effectively incorporated in crop breeding program. Root ecologists
and physiologists first need to identify the above-mentioned root ideotype
in specific environment and provide breeders with such information. Second,
great efforts need to be exerted to develop easily accessible techniques
for screening such traits. In such a way, root scientists can contribute
to increased crop production in various environment agronomically and through
breeding.