Not a straight forward question with an easy answer and not really relevant to this forum but I'll give it a try. Then I suggest you go over to the Seed Saving forum and/or the Vegetable Gardening forum for more info in the earlier discussions there.

First "toxins leached from sun-heated plastics" is by and large not considered a valid issue in gardening. In part because they exist all around us and the only way to avoid ingesting any sort of toxins is to give up eating and breathing. So all one can do is minimize exposure when possible.

But it would depend on exactly which plastics you are referring to, which grade they are, and made of which components. There are multiple types of plastic, all with very different components and the only ones of concern are those components that are water soluble. Water soluble contaminants can be absorbed by plants in limited amounts despite the natural filtering system of most plants.

Second, some plants are more effective filters than others so once one know the exact form of toxin at issue one can either a) grow the plants best known to filter them or b) grow the plants best known to absorb them and clean them from the soil.

Third, plants are far more prone to transfer infectious plant diseases to the seeds than they are to transfer toxins but even seeds known to be collected from infected plants can be treated effectively.

So the bottom line is exactly what "toxins" are we talking about that would be found in plastics you would use for raising food?

Neonicotinoids are nicotine-based insecticides, systemic insecticides that have been proved to be plant-absorbed and pollen-contaminating. Thus they are harmful to bees and other pollinators. But AFAIK there has been no research to support that they are hazardous to humans, especially since we don't normally consume pollen and the fruit produced by pollination shows no neonicotinoid presence.

Hope this helps.

Dave