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In plants experiencing environmental stress, the formation of reactive oxygen is often presumed. In this study, singlet oxygen was detected in broad bean (Vicia faba) leaves that were photoinhibited in vivo. Detection was based on the reaction of singlet oxygen with DanePy (dansyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole) yielding a nitroxide radical (DanePyO) which is EPR active and also features lower fluorescence compared to DanePy. The two (fluorescent and spin) sensor fuctions of DanePy are commensurate, which makes detecting singlet oxygen possible with a spectrofluorimeter in samples hard to measure with EPR spectroscopy [Kálai, T., Hideg, É., Vass, I., and Hideg, K. (1998) Free Radical Biol. Med. 24, 649−652]. We found that in leaves saturated with DanePy, the fluorescence of this double sensor was decreased when the leaves were photoinhibited by 1500 μmol m^-2 s^-1 photosynthetically active …