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Summary.-Water potential, hydration sites (MRI), DNA content of nuclei and mitotic index of the meristems of adventitious roots have been established before and after immersion of Sesbania rostrata stem. Before immersion, meristems of adventitious root primordia show a low water content, a null mitotic index and an unimodal distribution of DNA content, all these features are characteristic of a nuclei blockage in Go., phase of the cell cycle. After immersion, root growth is correlated with an enhancement of hydration, water potential and resume of mitotic divisions. The quiescent status of the adventitious root primordia of S. rostrata stems is the first example of a reversible growth arrest due to a nuclei blockage in G 0 -1 of the root meristematic cell cycle.