Herbicides can stimulate plant growth

Summary Low dose stimulations by toxicants have long been observed. Great controversies exist concerning the interpretation of these observations, spanning from believing that they are a general stress response occurring for all chemicals, to simply being an experimental artefact resulting from poorly growing control plants or from biomass allocation between plant parts. This study investigates the growth response and biomass allocation pattern of barley exposed to 10–15 doses of eight different herbicides. The results show that the globally most widely used herbicide, glyphosate, together with the sulfonylurea, metsulfuron‐methyl, can induce a real stimulation in biomass growth of approximately 25% when applied at doses corresponding to 5–10% field rate. The other six herbicides tested did not induce consistent hormesis, thereby undermining the theory of hormesis being a general stress response. Biomass allocations between plant parts did take place, but were not the cause of the hormetic growth stimulations. The results demonstrate that plant physiological responses to low herbicide doses cannot be extrapolated from our knowledge of effects of higher, commercially used, doses. Other physiological mechanisms seem to be triggered in the low dose‐range, and the investigation of these mechanisms poses new challenges for agronomists, environmentalists and plant physiologists.