Plants really are not effected till you hit 40C plus.
My tanks sit at about 90-94F, fish tend to drop before plants will.
A few plants like Lace plants seem to have issues with hot water.
I've not noticed any issues otherwise.
California has some spots in the Mojave and Death Valley(125F typically in July/Aug, sometimes even hotter) and the Sororan deserts that are the hottest in the world, there are springs oasis and they have aquatic plants there also. I'm sure some spots in Oz hit 45C or so and have springs/plants as well as the Okvango in Africa's Kalahari.
Regards,
Tom Barr