5-AMINOLEVULINIC ACID-INDUCED PHOTODYNAMIC DAMAGE OF THE PHOTOSYNTHETIC ELECTRON-TRANSPORT CHAIN OF CUCUMBER (CUCUMIS-SATIVUS L) COTYLEDONS

Cucumber (Cucumis sativus L. cv Poinsette) plants were sprayed with 20 millimolar 5-aminolevulinic acid (ALA) and then incubated in dark for 14 hours. Upon transfer to sunlight (congruent-to 800 watts per square meter) the plants died after 5 hours of exposure due to photosensitization reaction of metalloporphyrins. Due to the photodynamic damage, photosystem II (PSII), photosystem I (PSI), and whole chain reactions were impaired. PSII activity was more susceptible to photodynamic damage than PSI. The variable fluorescence was significantly reduced in ALA-treated plants within 1 hour of exposure to sunlight. At low temperature (77-degrees-K), the PSI fluorescence peak height (F734) was drastically reduced and blue shifted by 6 nanometers. The photodynamic damage was irreversible; rather, it continued upon dark incubation of ALA-treated cucumber plants exposed to sunlight for 15 minutes. In the latter experiment, continued production of malondialdehyde during dark treatment suggests the degradation of unsaturated membrane lipids.