Allelopathy--Wheat
I have perused many articles that contain published results of research into these perceived effects. A summary of the results from this extensive literature search follow.
● Both wheat residue and wheat seedling allelopathy are currently being researched for their potential utilization in weed management.
● Results reported from greenhouse experiments where wheat straw or wheat straw leachate was used have shown a perceived adverse allelopathic effect of wheat straw on early growth of soybeans.
● Results from field studies using the same variables as those used in greenhouse studies are not correlated with those of the greenhouse studies; i.e., the same results have not been obtained from field studies.
● In field studies where leaving wheat straw on the soil surface or incorporating the straw into the soil resulted in stunted early growth of soybeans, adding supplemental N at soybean planting overrode the effect. In treatments where wheat straw was removed or burned, the effect did not occur. Thus, N immobilization rather than allelopathy probably played the major role in the measured effects.
● Wheat plants produce and release toxic substances that inhibit several weed species, and many of these compounds have been identified. This has led to the perception that wheat allelopathy has the potential for the management of weeds.
● It is believed that the constant exposure of weed plants to continuously released toxic allelochemicals would create a stress environment for the weeds, thereby resulting in a reduced impact of the target weed in a cropping system.
● Wheat accessions/genotypes/varieties differ in their allelopathic effect toward other plant species.
● Research on wheat allelopathy toward weeds has progressed to the identification of wheat allelochemicals and further to the identification of genetic markers associated with this trait. Presently, there is no significant agronomic achievement from this knowledge.
● Wheat varieties have not been developed that have enhanced biosynthesis of allelochemicals for weed control, although this field of research and development is of interest to a number of researchers.
In summary, there is no evidence that the phytotoxicity of wheat straw on soybeans is a field problem. Thus, perceived allelopathy effects that were measured in the greenhouse but not in field environments are likely tempered by the soil environment. Also, N immobilization by the wheat straw probably results in lack of sufficient available N needed for early-season soybean growth before nodulation and subsequent N fixation occurs.
Even though past research indicates that wheat varieties release allelochemicals that have an adverse effect on some weed species, this has not led to a weed management system that incorporates this concept.
A 1982 Weed Science article states that further screening of different weed species against wheat allelopathic toxins in field situations is needed before any possibility of utilizing wheat allelopathy in weed control. In 2012, this is still true.
From Molecular approaches in improving wheat allelopathy published in 2005, “Wheat allelopathy has great potential in integrated weed management systems. Concerted research efforts have been made toward the development of wheat varieties with high allelopathic activity”. However, in 2012, wheat varieties that may have been released with this trait are not advertised in the US.
Articles used to develop the above summary are:
Techniques for identifying tolerance of soybean to phytotoxic substances in wheat straw, by Herrin, Collins, and Caviness (Crop Science, Vol. 26, 1986).
Soybean-Wheat Doublecropping: Implications from straw management and supplemental nitrogen by Hairston, Sanford, Pope, and Horneck (Agronomy Journal, Vol. 79, 1987).
Molecular approaches in improving wheat allelopathy, by Wu.
Allelopathy in wheat, by Wu, Pratley, Lemerle, and Haig (Ann. Appl. Biol., Vol. 139, 2001).
Allelochemicals in wheat: cultivar difference in the exudation of phenolic acids, by Wu, Haig, Pratley, Lemerle, and An (J. Agric. Food Chem., Vol. 49, 2001).
Evaluation of seedling allelopathy in 453 wheat accessions against annual ryegrss by the equal-compartment-agar method, by Wu, Pratley, Lemerle, and Haig (Aust. J. Agric. Res., Vol. 51, 2000)
Allelopathic potential of wheat straw on selected weed species, by Steinsiek, Oliver, and Collins (Weed Sci., Vol. 30, 1982)
Composed by Larry G. Heatherly, Nov. 2012, larryheatherly@bellsouth.net