Investigations on the induction of plant hormesis by herbicidal compounds – mechanisms & practical implications

Stimulatory effects of low doses of herbicidal compounds on plants have been known for quite some time and in particular the appearance of hormesis with the currently most widely used herbicidal ingredient – glyphosate – re-entered an interest of research in this area. Nevertheless, the majority of reports on this hormesis phenomenon stem from studies which have been designed for other purposes. Therefore, our knowledge on hormetic effects of herbicides on plants is still very limited.

The phenomenon of herbicide-induced hormesis in plants is important for agricultural practice in two ways. On the one hand, there is the possibility to specifically use hormesis to increase crop yield and, on the other hand, hormetic effects can occur as side effects of regular herbicide applications when weeds or crops are inadvertently exposed to low doses (e.g., drift deposition, application errors, herbicide resistance, efficiency gaps). With respect to a targeted use of hormesis mainly the fact is problematic that hormesis is the result of a complex interplay between low-dose application and numerous influencing factors and, therefore, occurs variable. The demonstration of this variability and the quantification of its magnitude depending on various factors (e.g., mixtures with other toxins, environmental factors, response parameter) therefore represent one of the central tasks of the project. A further focus is the investigation of a possible contribution of hormesis to the population dynamics of herbicide resistant weeds. Due to the low sensitivity of resistant weeds it is possible that the approved herbicide application rate represents a hormetic dose that promotes resistant weeds in their growth and thus indirectly promotes the development of resistant weeds via an increased competitive ability, reproduction rate, and/or metabolic rate. The projected illustration of this phenomenon and its importance as a function of the resistance mechanism and resistance factor represents an important contribution to the understanding of the population dynamics of herbicide-resistant weeds. In addition, the project includes studies on the occurrence and interaction of hormetic effects with additional low-dose phenomena, in particular the phenomenon of selective toxicity occurring at ultra-low doses and the associated displacement of the size distribution within a total population.

The main goal of the proposed research project is therefore to develop a fundamental under­standing of herbicide hormesis that allows for a better assessment of its importance for plant production. However, the results are similarly important for agronomists as for environmental scientists and plant physiologists.