Trade-offs of different types of dose-responses following temporal gradients of Pb stress in <i>Tillandsia usneoides</i>

The dose-response relationship between toxicants and organisms is the most fundamental principle in toxicological risk assessment. However, multiphasic nonlinear responses are poorly understood, and the underlying mechanisms remain elusive. In this study, we subjected the indicator plant Tillandsia usneoides to 5 or 10 time gradients of 1 mM Pb and assessed the response patterns of five damage markers and eight resistance markers in the leaves. The results showed that in the assay with five time gradients, 10 out of 13 biomarkers showed biphasic responses and two biomarkers showed multiphasic responses. In the assay with 10 time gradients, six biomarkers showed biphasic responses and five biomarkers showed multiphasic responses. The results confirmed that increasing the number of dose gradients facilitates the detection of multiphasic dose responses. Additionally, the occurrence of nonlinear responses was found to be a common phenomenon. Further analysis of the frequency of trade-offs showed that the occurrence frequency of trade-off 2 (toxicants do not affect one marker but stimulate other markers) was always higher than that of trade-off 1 (toxicants worsen one marker while stimulating other markers) for both damage and resistance markers, and that trade-off 2 was more likely to appear in resistance markers. This suggests that as the duration of stress increases, the body produces more resistance substances, which coordinate with one another to maintain internal balance. This study suggests that the trade-off theory can be effectively used to elucidate the complex relationship between a toxicant and T. usneoides.