SUMMARY: Nutritional shifts up of synchronized or random populations of (strain ML30 and derivatives of B/r and K12) to a richer medium were followed (nutritional pulse) or not (shift-up) by shifts down to the original poor medium. In most cases, the first postshift division was clearly postponed by a shift-up or a nutritional pulse. This delay of the first division was almost constant, whatever the cell age at the time of the transition, and, for the nutritional pulse only, whatever the time spent by cells in the rich medium (between 2 and 25% of the generation time characteristic of poor medium). Following a shift-up, the new steady state of division rhythm and of mean cell mass was reached at about the second postshift division, whereas after a nutritional pulse, it took three generations to return to the steady state prevailing in poor medium. When both the poor and the rich medium were varied, the extent of the postponement of cell division after a nutritional pulse increased when the amplitude of the stimulus (i.e. the difference in richness) was increased. This was not the case with a simple shift-up, where it seemed that the postponement was compensated in part by the accelerating effects of the rich medium.


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