SUMMARYMicroHuorimetric and flow cytometric analyses were used in nuclei of cortical cells of roots in the endomycorrhizal system Allium porrum L. + a Glomus sp., strain E., (from Rothamsted Station), in an attempt to explain nuclear hypertrophy. No variation of DNA content in comparison with the controls was observed in mycorrhizal roots. An increase of fluorescence (about 25 "o) vvas observed in mycorrhizal root nuclei stained with an undersaturating concentration of DAPI. This can be explained by a greater accessibility of DNA to Huorochromes owing to a lower degree of chromatin condensation, as confirmed by ultrastructural data.Our results suggest a possible explanation of nuclear hypertrophy and, further, show that variations of chromatin condensation can occur in mature plant cell nuclei, even though this, in differentiated plant cells, is generally considered to be independent of the cellular functions.

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