The photoprolaytic processes of 7-hydroxy-coumarin (Umb) were investigated by-steady-state and time-resolved-fluorescence techniques. We found that-the Umb compound. is a photoacid with pK(a)* approximate to 0.4 and a rate constant of the excited-state proton transfer (ESPT), to water of 2 X 10(10) s(-1). Umb is also a photobase and accepts an excess proton in solution and also directly from weak acids like acetic acid. When. Umb is adsorbed on cellulose it also functions as a photoacid and a photobase. Hydroxycoumarins are known to accumulate next to fungal-, bacterial-, and viral-infected regions in the leaves and stems of plants in general and also in trees. We propose that these compounds when irradiated by sunlight UV, combat the fungi or bacteria by excited-state proton-transfer reactions. These photoprotolytic reactions provide a universal resistance mechanism to infections in plants.