Modification of the radiosensitivity of barley seed by post-treatment with caffeine. Kinetics of decay of caffeine-reactive oxygen-sensitive sites.

In dry barley seeds (3-1 per cent moisture content), the maximal level of oxic damage (class III damage) is reached within the first 40 min of post-hydration in oxygenated water at 5 +/- 1 degrees C. The decay of the gamma-ray-induced oxygen-sensitive sites requires, however, about 120 min. The mechanisms leading to partial protection against the class III damage are initiated when caffeine is present during the first 30 min of oxygenated post-hydration. If added after 30 min of oxygenated hydration, caffeine has no protective action. For the potentiation of an oxygen-independent component of damage, caffeine has to be present during the first 240 min of oxygen-free hydration. These observations involving a physiologically inert system raise questions regarding physico-chemical vis-a-vis biochemical mechanisms of caffeine effect on irradiated systems and these have been briefly discussed.