Important and rapid variations of Fru-2,6-P2 were observed during light-on and light-off transitions in both spinach and mustard: Fru-2,6-P2 underwent tremendous variations showing an 80% decrease within 13 minutes in both plants. In mustard cotyledons, during both day and night periods, Fru-2,6-P2 showed a decrease during the first hour, followed by a regular 240% increase. In spinach, the same behavior of Fru-2,6-P 2 was observed, except that once the concentration of the sugar had doubled, it remained constant. In contrast to mustard, spinach Fru-2,6-P2 presented a much higher level during the night than during the light period. In 3 day old germinating oat, red light illumination for 5 min was accompanied by a 48 h halt of elongation and by a 180% increase of Fru-2,6-P 2 20 min after irradiation, followed by the total disappearance of this sugar 80 minutes after light treatment. The present work shows the highly dynamic capacity of the regulatory sugar Fru-2,6-P2 in plants with respect to short, mid and long term light treatments. This indicates an important cellular messenger role of Fru-2,6-P 2 in the regulation of carbohydrate metabolism in response to light changes.