In principle the toxicity effects are additive, As and Cd are mainly detoxified by phytochelatins, Pb can also be complexed to PCs, but forms also strong complexes with carboxylates and some inorganic anions. In which for did you add Cd? Could it be that the anion (e.g. SO42-) forms an extracellular complex with Pb and decreases the toxic effect in this way?

This depends on plant species, physiological condition , age of plants and concentration of metal used at particular time. yes they are additive, but can compete for uptake, their transport and localisation in system accordingly responsive

I agree with Enrico Martinoia. It is possible that the addition of Cd salts results in a depletion of soluble Pb2+ from the nutrient solution. Plants treated with a mixture of toxic elements show normally an additive stress response, not antagonistic. You should test the composition of your nutrient solution with an appropriate chemical equilibrium software (try visual MINTEQ 3.0 (http://www2.lwr.kth.se/English/OurSoftware/vminteq/). You may also need to test other physiological stress parameters (oxidative stress, generation of reactive oxygen species, photosynthetic activity, etc.) to complete your observations, and assess the behaviour you observed.

Hellow. Several years ago we did experiments with young spring barley seedlings, grown in nutrient solution. We got that plants, which were adapted to low concentrations of CdSO4, were more tolerant not only to Cd, but also to Pb impact. The adapted plants had significantly better growth of dry biomass and lower TBARs content as compared with non-adapted plants. So it shows, that in some cases Cd may decreas Pb impact. However, Cd and Pb was in mixed in nutrient solution, plants were treated with Cd first, and then were moved to nutrient solution with Pb.

Hi again!! This is not surprising, because plant metabolism was "primed" and tolerance mechanisms were exerted in your Cd-pretreated experiment. But it is different if plants are treated at the same time with two or more pollutants. It is crucial to establish the timing of application and doses (not too high) to promote tolerance.

Hello. I agree - If you want to emulate the natural conditions of contaminated field, or if you want just to study the effect of two of more toxic substances, it is better to add all of them concurrently to avoid the hardening (priming) effect of this used as a first one.

Concerning the phytochelatins (PC, hint by Enrico), their contribution depends on whether HM stress is acute or prolonged. PC serve more as a metal storage than the part of detoxification, and they are synthesised to bind more ions mostly on the first stage of stress. When HM acts longer, organic acids synthesis is the main detoxification pathway. But whether the stress is "acute" or "prolonged", depends on factors listed by S N Mishra :).