Primary function for a chemical defense? Nicotine does not protect Datura stramonium L from UV damage

The chemical defenses of plants are thought to play many different functional roles, all of which in some way decrease the costs of deploying the defense. However, few of these other functions have been tested with metabolites that are clearly defensive. Nicotine is a potent chemical defense with a high molar extinction coefficient in the UV, is a potent quencher of free radical pecies produced by UV absorption in vitro, and is known to increase in concentration after exposure to UV radiation. Hence this chemical defense may also function to protect against UV-induced damage. We examined nicotine's potential photoprotective role in two experiments by culturing Datura stramonium L plants in hydroponic solutions containing nicotine at two concentrations, and by exposing nicotine-containing and nicotine-free plants to UV radiation, which allowed us to manipulate nicotine contents independently of UV exposure. Plants with high levels of nicotine were not more resistant to UV-induced damage but, instead, exhibited greater decreases in photosynthetic capacity than did nicotine-free controls. Proposed multifunctional roles for chemical defenses deserve closer scrutiny.