Steady-state and time-resolved fluorescence techniques were employed to study the photophysics and photochemistry of trans-resveratrol. trans-Resveratrol is found in large quantities in fungi-infected grapevine-leaf tissue and plays a direct role in the resistance to plant disease. We found that trans-resveratrol in liquid solution undergoes a trans-cis isomerization process in the excited state at a rate that depends partially on the solvent viscosity, as was found in previous studies on trans-stilbene. The hydroxyl groups of the phenol moieties in resveratrol are weak photoacids. In water and methanol solutions containing weak bases such as acetate, a proton is transferred to the base within the lifetime of the excited state. When resveratrol is adsorbed on cellulose (also a component of the plant's cell wall), the cis-trans process is slow and the lifetime of the excited state increases from several tens of picoseconds in ethanol to about 1.5 ns. Excited-state proton transfer occurs when resveratrol is adsorbed on cellulose and acetate ions are in close proximity to the phenol moieties. We propose that proton transfer from excited resveratrol to the fungus acid-sensing chemoreceptor is one of the plant's resistance mechanisms to fungal infection.