The "hero" of the story is a biologist/botanist that lives in the 30th century. All is fine...high-tech city, lots of flying cars, robots flying around. She works at a big lab, with a room next door testing some time travel to future teleporter. She works in a greenhouse inside the lab.
One day, BOOM! there is a explosion in the lab next door and she and her little greenhouse are teleported three millennia into an post total-disaster future in which technology is no longer a thing (not how it used to be, they have electricity, but no cars/planes/computers/etc).
The story moves forward from there. There is some "she is the chosen one" silliness. To prove that she is the chosen one, she is obligated to fight at an arena (but she gets away with an excuse to visit her greenhouse one last time before the fight, after one of her friends tells her that the creature that she's going to fight is like an gigantic lizard) with an terrible reptilian beast about 7 times her size.
She manages to knock out the beast with little effort, just by standing there, almost shoving some leaves of a plant up the beast's snout. Then the fight is over, and the breeder of those gigantic voracious lizards asks her what the hell did she do to bring it down with such ease? He's been breeding them for years and never seen anything like it. She says she used a plant and shows him the leaves. The breeder asks why and how they work.
And then I HAVE NO IDEA.
It's really important to plot the explanation of why, and no matter how much research I do, I don't seem to be able to craft an plausible explanation of why these plants affect these lizards, how she knew it would work (it was a shot in the dark, but that's not the explanation she gives to him, though that's clear in the novel).
I researched some plants that are toxic for lizards, but even so I can't think of an plausible explanation for it to cause an knockout in seconds on a beast that big. Any suggestions on how I should craft this particular plant and its properties and why they work?