sure there is ,My UV is 12 or 13 on the index each and every day during the summer and I don't consider my plants any trippier than the indoor or imported stuff.
There must be something else to it imho ...
like yesum said , its the whole package ,
not just one thing , uv , heat , sun intensity , dryness ,
and growing plants that are accustomed to this over many generations i guess ,
all these things combined,
gets some pretty high uv where i live at 3000 ft in the tropics ,
up to 17 at specific times of the year ,
one of the highest rates of skin cancer in the world ,
might be from the amount of white people living here ,
whose skin is just not made for that sort of punishment ...
its not recommended going out into the sun for long without covering oneself ...