I'm planning to do an experiment where I extract burnt toast and test how that affects the growth of mung beans. I know burnt food contains PAHs, but what are some other chemicals present and how would they affect plant growth?
First of all, one should have look a look on the ingredients of your toast. It mainly consist of starch, proteins (2% maybe), fat (a few %), water and some inorganic compounds such as NaCl. Well Salt normally won´t interact with anything below 1400°C so no need to look at this. Water just evaporates, i think we must not discuss that here. Fat: that's more interesting because fat is known to degenerate into different compounds when heated.
But the definitely most interesting reactions will be present when one heats carbon hydrates together with protein: so called Maillard Reaction(s) those kind of reactions are still issues of ongoing research although this type of reactions have been discovered long ago. As a good starter on the various compound which can occur during Maillard Reaction you might read Mottram or Monnier. Especially acrylamide could be of interest when looking a toxicity. I don't have a certain paper in mind but i know that there has been research about toxicity of acrylamide in fish and other animal. So it's likely that there was some research about plants.
When you look at starch and protein separately you will encounter heat degeneration as well as caramelisation.