Variability of hormetic dose responses of the antiauxin PCIB on Lactuca sativa in a plant bioassay

Some herbicidal agents that damage plants at high doses stimulate their growth at low doses. This phenomenon of hormesis constitutes an alternative possible use of herbicidal agents that is, however, compromised by the apparent variability of the phenomenon. However, studies demonstrating and quantifying this apparent variability are lacking. The auxin‐inhibitor PCIB 2‐(p‐chlorophenoxy)‐2‐methylpropionic acid] was therefore investigated to determine whether interassay variability of stimulatory effects exceeds those of inhibitory effects and which hormetic quantity is most variable. Reparameterisation of the dose–response model used to estimate the hormetic dose range allowed including this feature as an explicit parameter. In bioassays with lettuce and root length as response variable, the variability of PCIB effects was evaluated in 33 complete dose–response assays. In a meta‐analysis, the occurrence of PCIB hormesis proved highly reproducible. However, the variability of effects was dose‐dependent and increased with decreasing dose. The response before the maximum stimulatory response and the absolute magnitude of hormesis proved most variable. Comparing the frequency distribution of effective doses demonstrated there was a risk of a previously hormetic dose causing a loss of hormesis or inhibitory effects in a subsequent experiment. Therefore, selecting a hormetic dose that will induce hormesis under any circumstances will be a major challenge.