Herbicides can stimulate plant growth

Summary Low dose stimulations by toxicants have long beenobserved. Great controversies exist concerning theinterpretation of these observations, spanning frombelieving that they are a general stress response occur-ring for all chemicals, to simply being an experimentalartefact resulting from poorly growing control plants orfrom biomass allocation between plant parts. This studyinvestigates the growth response and biomass allocationpattern of barley exposed to 10–15 doses of eightdifferent herbicides. The results show that the globallymost widely used herbicide, glyphosate, together withthe sulfonylurea, metsulfuron-methyl, can induce a realstimulation in biomass growth of approximately 25%when applied at doses corresponding to 5–10% fieldrate. The other six herbicides tested did not induceconsistent hormesis, thereby undermining the theory ofhormesis being a general stress response. Biomassallocations between plant parts did take place, but werenot the cause of the hormetic growth stimulations. Theresults demonstrate that plant physiological responses tolow herbicide doses cannot be extrapolated from ourknowledge of effects of higher, commercially used,doses. Other physiological mechanisms seem to betriggered in the low dose-range, and the investigationof these mechanisms poses new challenges for agrono-mists, environmentalists and plant physiologists.Keywords: hormesis, plant traits, trade-off betweentraits, glyphosate, metsulfuron-methyl, growth stimu-lation, herbicides, biphasic dose–response curves.C