Abstract
Cucumber (Cucumis sativus L. cv Poinsette) plants were sprayed with 20 mM solution of 5-aminolevulinic acid (ALA), the precursor of tetrapyrroles, and then incubated in darkness for 14 h. Upon transfer to sunlight (800 W m-2), the plants died after 6 h of exposure due to photodynamic damage. The photosystem II (PSII) and photosystem I (PSI) photochemical reactions were impaired. Intact chloroplasts, isolated from the control and ALA-treated plants in the dark were exposed to weak light (250 µmoles m-2 s-1). Within 30 min, PSII activity was reduced by 50%, and the variable fluorescence was significantly reduced. Thylakoid membranes prepared in darkness from control and 20 mM ALA-treated plants were illuminated (250 µmoles m-2 s-1) in the presence of scavengers of active oxygen species. The singlet oxygen scavengers histidine and sodium azide protected the thylakoid membrane linked function of PSII from photodynamic damage. However, the hydroxyl radical scavenger formate and the superoxide radical scavengers superoxide dismutase and 1,2-dihydroxybenzene-3,5-disulfonic acid failed to protect the PSII reaction. Non-phototransformable protochlorophyllide was the most abundant pigment in the thylakoid membranes isolated from ALA-treated plants and acted as a type II photosensitizer.
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