Photodynamic Effects of Rose Bengal on Senescent Flax Cotyledons

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Abstract
Senescent cotyledons, excised intact from 6-d-old Flax (Linum usitatissium) seedlings, were treated with Rose Bengal, a photo-sensitizing agent. In continuous light, chlorophyll breakdown was increased from 44% in untreated cotyledons to 73% in Rose Bengal-treated cotyledons. Depletion of carotenoid and linolenic acid was enhanced by 25% and 39% respectively over the same period. After 48 h the decline in uncoupled photosynthetic electron transport was increased from 58% to 82% in isolated chloroplasts. Over the same period CO2 exchange was greatly inhibited in treated cotyledons. Rose Bengal had little effect on dark incubated cotyledons, suggesting that singlet oxygen generated by photo-sensitized reactions was responsible for the enhanced deterioration of structure and function of senescent cotyledons and the prime site of action was the photosynthesis apparatus. These photodynamic effects are discussed in relation to the possible involvement of toxic oxygen species in the senescence of leaf tissue.