Investigation into allelopathic effects of weeds on the germination and early growth of spring wheat has been investigated in recent years, but one of the first investigations that caught the eye of Jim Bauder, soil and water quality specialist at Montana State University/Bozeman still is quite interesting. It happens that this research into the allelopathic effect was done by two high school students.

Bauder wrote about it as his agronomy note number 255 quoting Jeremy Brimhall and Marshall Overcast’s research, the two students at Northern Toole County Sunburst High School in Montana. He also credited the student’s teacher Lawrence Fauque. The rest of this write-up comes from Bauder’s number 255 report.

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?