All plants respond dramatically to changes in phytohormone levels.

The use of plant hormone products usually corresponds to a crops value. High value crop growers quickly recognize they cannot afford to not use these products because of the exceptional crop responses they can produce when applied in a timely fashion.

We often observe instances where cultural management practices produce a profound negative effect on a crop, because the impact on phytohormone levels is not appreciated.

As an example, abscisic acid (ABA) is the phytohormone which leads to the development of good fruit coloration. When fruit does not contain enough trace minerals, ABA levels remain low, and apples (or any other fruit) remain green instead of coloring well. Some apple growers use foliar applications of ABA to enhance fruit coloring. In parallel they may also use water deprivation to improve fruit storability. However, water deprivation results in elevated levels of ABA.

The combination one-two punch of water deprivation and a foliar of ABA often causes early fruit drop. After all, abscisic acid gets its name because it triggers fruit abscission. This combination of cultural management practices having misunderstood consequences costs apple producers a lot of yield and profits. It’s like shooting off both your feet at once.

For those growers producing crops other than apples, no need to feel relief to early. The odds are good that similar foot shooting practices have become commonplace for your crops as well.

With this background context in mind, I find it intriguing to learn that low level glyphosate residues in the soil profile result in changed phytohormone profiles in crops.

It seems a logical expectation this would occur, given glyphosate’s disruption of the shikimate pathway, but before this paper, no one looked at how soil residues would affect following crops.

Here are a couple of highlights from a great article:

– Glyphosate disrupts the shikimate pathway which is the basis for several plant metabolites. The central role of phytohormones in regulating plant growth and responses to abiotic and biotic environment has been ignored in studies examining the effects of glyphosate residues on plant performance and trophic interactions.

– Plant hormonal responses to GBH residues were highly species-specific.

– Potato responded to GBH soil treatment with an increase in stress-related phytohormones abscisic acid (ABA), indole-3-acetic acid (IAA), and jasmonic acid (JA) but a decrease in cytokinin (CK) ribosides and cytokinin-O-glycosides. (An expression of reduced root growth. JK)

– Our results demonstrate that ubiquitous herbicide residues have multifaceted consequences by modulating the hormonal equilibrium of plants, which can have cascading effects on trophic interactions.

– Accordingly, the consequences to non-target plants can range from growth stimulating to changes with their biotic environment. Herbicide residues are ubiquitous and it is necessary to unravel their consequences for ecological interactions and their involvement in shaping evolutionary processes (Riedo et al., 2021). In conclusion, to elucidate the full picture of effects of GBH residues, it requires thorough understanding of the “soil legacy” including the study of soil microbiota and how it is affected by persistent herbicide use.

While this research is relevant for all soils and crops that have had glyphosate applied historically that has not yet degraded, it is of particular relevance to tree crops where lots of glyphosate has been applied in the tree row, or any soil with a history of generous glyphosate applications.

It is worth reading the paper, and noting how phosphate applications effect glyphosate solubility and crop performance.

Have you observed any changes in plant expression where soils had higher levels of glyphosate residue?

1. Fuchs, B. et al. A Glyphosate-Based Herbicide in Soil Differentially Affects Hormonal Homeostasis and Performance of Non-target Crop Plants. Front. Plant Sci. 12, 787958 (2021).