Allelopathy according to the world of Jeremy Brimhall and
Marshall Overcast
Jim Bauder, Soil and Water Quality Specialist, MSU/Bozeman
Being the Soil and Water Quality Specialist with Montana
Extension Service has its benefits! One of those benefits is the opportunity to
work with promising young scientists - high school students taking part in
science projects. I probably get involved in 20 a year. And, it is amazing what
you can learn. For instance, this note - more about allelopathy - comes from
excerpts from a report of an investigation carried out by two students from
Northern Toole County Sunburst High School, Montana. Way back in the winter
some time Jeremy and Marshall sent me an email and said they had read about
allelopathy in our Agronomy Notes and wanted to see if allelopathy really
existed in Northern Toole County. At first I was a little skeptical, but after
the third email and two phone calls, I knew these guys were onto something -
like Springers on the scent of a pheasant, our border collie on the cats
crossing the imaginary boundary of your back yard.
Anyway, not too long ago a report came across my desk - from
Jeremy and Marshall. One day they spent nearly an hour on the phone with me and
Matt Rinella, a graduate student in weed science - picking our brains. So, now
I want to share what these two shining scientists found out. I’ll give all
credit for this summary to Jeremy and Marshall and the excellent mentoring from
Mr. Lawrence Fauque, their teacher.
The title (abbreviated) of this more than modest investigation
was “the allelopathic effects of selected weed exudates on the germination and
early growth of spring wheat”. They hypothesized that certain plants, namely
kochia, wild oat, and wild buckwheat, produce either root exudates
(allelochemicals) or some other mechanism associated with water soluble
chemicals that inhibit or adversely affect the germination and growth of spring
wheat. They then set out a series of very rigorous and very controlled
experiments to find out if their hypotheses were true.
Bottom line - from the abstract: Wild oat and wild buckwheat
showed no effect on emergence, rate of growth, average vegetative dry weight or
final height and had no effect on germination of spring wheat. In contrast,
kochia had a significant negative effect on the emergence and average
vegetative dry weight of spring wheat grown in pots.
A little background about this whole allelopathy thing - from
the experts. “Allelopathy refers to the
exchange of any chemicals known as secondary metabolites of plant, fungal, or
microbial origin that influence the growth and development of other plants or
microbes. Plants produce many organic compounds that they do not use
directly. These compounds are known as
secondary metabolites, and can appear in all parts of a plant, including
leaves, flowers, fruit, stems, roots, rhizomes, seeds, and pollen. Plants eventually release their secondary
metabolites into the soil during germination, growth, or decomposition. The purposes of secondary metabolites are
various, and scientists know very little about them. Secondary metabolites such
as alkaloids, phenolics, flavonoids, terpenoids, and glucosinolates which do
not play a role in primary metabolic processes essential for a plant’s
survival, and are produced as offshoots of primary metabolic pathways.” And,
according to Jeremy and Marshall, allelopathy is significant for weed-crop
ecology in three respects: (1) as a factor affecting changes in weed
composition, (2) as a source of weed interference with crop growth and yield,
and (3) as a possible tool in reducing crop losses from weeds. If the
allelochemicals produced by certain seeds that affect crops can be identified, then
scientists may be able to neutralize their effects or lessen their
severity. If that is possible, then
weeds that are a nuisance in agriculture could be controlled in a more natural
manner.
Allelopathic potential has been suggested for only about 90
species of weeds and only one allelopathic effect has been proven to exist. So,
the work of Brimhall and Overcast seems pretty significant at this point.
Little did I know that plants can demonstrate two different types of
allelopathy: (1) true allelopathy involves one plant releasing exudates that
immediately affect another plant; (2) functional allelopathy occurs when
exudates released by one plant affect another as the result of transformation
by microflora. I also didn’t know that true allelochemicals can be released by
a plant in many ways. Volatilization
occurs when plants release allelochemicals in gaseous form through their
leaves. Other plants are affected when
the absorb these chemicals from the air.
Leaching occurs when plants store allelochemicals in their leaves. When their leaves fall to the ground and
decompose, they release these chemicals into the soil. Exudation, which is probably the most common
method, occurs when plants release allelochemicals directly into the soil
through their roots. Functional allelopathy usually arises from decomposition
of the exudates of plants or parts of the plants themselves by microflora. The products of the decomposition affect
other plants.
Anyway, back to the studies of Jeremy and Marshall: kochia (Kochia
scoparia), wild oat (Avena fatua), and wild buckwheat (Polygonum
convolvulus) were tested on spring wheat (Triticum aestivum). In one
of their studies, the three weed plants were grown in sterilized and
unsterilized soil. These same pots were then used to grow spring wheat. Wild
buckwheat (Polygonum convolvulus) and wild oat (Avena fatua)
plants grown in both sterilized and unsterilized soil demonstrated no
significant allelopathic affect on the emergence, rate of growth, final height,
or average vegetative dry weight of spring wheat plants. These young scientists concluded that root
exudates of wild buckwheat and wild oat plants have no significant effect on
the early growth of spring wheat. Significance was found, however, in some of
the experiments involving kochia (Kochia scoparia) plants. Kochia plants
grown in unsterilized soil affected the total emergence of spring wheat such
that it was significantly less than the respective control group. Kochia grown in sterilized soil also
affected the total emergence of spring wheat negatively, but to a lesser
extent. The average vegetative dry
weight of spring wheat plants was significantly reduced by root exudates of
kochia grown in sterilized soil but not in unsterilized soil.
In their second study, the same three weed plants were grown
in pots and then the soil in which the weeds had been grown was leached. The
leachate (drainage water) was used as a water stock for germination studies of
spring wheat in petri dishes. Leachate containing water soluble root exudates,
if at all present, of the three weed species demonstrated no significant effects on the total
germination of spring wheat seeds in petri dishes. This occurred for all three weed species grown in both sterile
and unsterile soil.
Bottom line here: The young scientist team of Jeremy Brimhall
and Marshall Overcast did a great job of investigating allelopathy of wild oat,
wild buckwheat, and kochia with respect to spring wheat. Their observations:
something happens when kochia precedes spring wheat that reduces the
performance of spring wheat. Allelopathy? According to our understanding of the
process - it looks like something worth following up on. I’m looking forward to
more great discoveries from these two scientists.
*This note is part of the Agronomy Notes series, a weekly news release and fact sheet series prepared by Dr. Jim Bauder, Extension Soil and Water Quality Specialist at Montana State University. Past Agronomy Notes can be viewed at http://agnotes.org/. For additional information on this subject, for subscription (no charge) to this Agronomy Notes series, or for questions about Agronomy Notes, Jim Bauder can be contacted by e-mail at jbauder@montana.edu, or by calling (406) 994-5685 at MSU.
Categories: Plant Nutrition, Weed
Management, Wheat Fertility
Date: 2000