Ultraweak light emission, free radicals, chilling and light sensitivity

Ultraweak light emission (UWLE) was measured from leaves of various chilling-sensitive (Lycopersicon esculentum, Cucumis sativus and Phaseolus vulgaris) and -tolerant (Pisum sativum and Vicia faba) plants after exposure to low (4-7 degrees C) temperature in the light. UWLE increased upon chilling treatment combined with illumination with 200 mu mol m(-2) s(-1) PAR in all plants, by about 30% in tolerant and by more than 100% in sensitive plants. It increased more when applied together with 400 mu mol m(-2) s(-1) PAR: by 90-100% and by 250-280% in chilling-tolerant and -sensitive plants, respectively. Free radical production was detected by spin-trapping EPR spectroscopy in thylakoid membranes isolated from the chilling-treated Lycopersicon esculentum and Vicia faba leaves. After 12 h chilling at 200 pmol m(-2) s(-1) PAR, free radical production was approximately 3 times greater in the former than in the latter species. The same ratio was approximately 6 if chilling was carried out at 400 mu mol m(-2) s(-1) PAR, indicating the role of photooxidative stress in chilling injury. Our results also confirm that the stress-induced increase in UWLE is an indicator of free radical production and offers the possibility bf using UWLE for monitoring the effect of chilling on temperature-sensitive plants in an early stage.