I've been curious as to which photosynthetic cycle St. John's wort uses to synthesise glucose and more complex carbohydrates. I know it probably won't be the crassulacean acid metabolism (CAM) cycle seeing how that applies mostly to plants that require a limited amount of water as opposed to St. John's wort which tends to prefer moisture. I suspect the answer to my question will be C3 seeing how they dominate in the plant kingdom and I know that St. John's wort requires relatively cool climates to grow successfully -- a trait characteristic of C3 plants seeing how high temperatures tend to cause excessive and wasteful photorespiration.
It's C3, according to the Illinois plant information network. (See CO2 fixation on this page.) This is the best I could find.