Circadian rhythms of the L-ascorbic acid level in Euglena and spinach.

Plant defenses against photo-oxidative stress have been studied almost exclusively with respect to stress responses, and little is known about how non-enzymic antioxidants change under constant conditions without a time cue or an environmental stress. Here, we show that, in both the flagellated alga Euglena gracilis Z and the angiosperm Spinacia oleracea L., the potent antioxidant L-ascorbic acid (Asc) displays a circadian rhythm with a maximum at subjective midday, a physiological state reflecting that attained at noon under daily light/dark cycles.

Circadian rhythms of resistance to UV-C and UV-B radiation in Euglena as related to "escape from light" and "resistance to light".

Radiation-induced stress, either from visible or UV light, is strongest at midday. We found that, in the absence of stress or time cues, Euglena gracilis Z was the most resistant to UV-C and UV-B at subjective midday, whether judged from immediate or reproductive survival. The circadian UV-resistance rhythms were free-running in stationary cultures under 1-h light/1-h dark cycles or continuous darkness, indicating that cell-cycle dependent DNA susceptibility to UV was not involved.