Weak luminescence from cotyledons of Cicer arietinum L. induced by sudden freezing and thawing: The role of superoxide, free radicals and singlet oxygen in the phenomenon

Cotyledons of etiolated seedlings of Cicer arietinum L., when subjected to sudden freezing and thawing, were found to emit luminescence of weak intensity. The presence of oxygen and iron compounds enhanced the intensity of the emission. In contrast, superoxide dismutase, catalase, mannitol, uric acid, vitamin C and t-butyl alcohol inhibited the emission. α-Tocopherol, which is a free radical scavenger, also inhibited the emission. The generation of singlet oxygen in the system was inferred from the suppression of emission by 2,5-dimethyl furan, which is a specific singlet oxygen scavenger. The buffer extracts prepared with D₂O, instead of H₂O, produced enhanced luminescence. These results suggest that singlet oxygen plays a role in the generation of the weak luminescence. Studies on the spectral distribution also revealed the existence of singlet oxygen as the principal emitter. An increase in UV absorption at 233-234 nm was observed with increasing time after the freeze-thaw treatment. This result is indicative of the accumulation of conjugated diene hydroperoxides.