DNA flow cytometry describes the use of flow cytometry for estimation of DNA quantity in cell nuclei. The method involves preparation of aqueous suspensions of intact nuclei whose DNA is stained using a DNA fluorochrome. The nuclei are classified according to their relative fluorescence intensity or DNA content. Because the sample preparation and analysis is convenient and rapid, DNA flow cytometry has become a popular method for ploidy screening, detection of mixoploidy and aneuploidy, cell cycle analysis, assessment of the degree of polysomaty, determination of reproductive pathway, and estimation of absolute DNA amount or genome size. While the former applications are relatively straightforward, estimation of absolute DNA amount requires special attention to possible errors in sample preparation and analysis. The article reviews current procedures for estimation of absolute DNA amounts in plants using flow cytometry, with special emphasis on preparation of nuclei suspensions, stoichiometric DNA staining and the use of DNA reference standards. In addition, methodological pitfalls encountered in estimation of intraspecific variation in genome size are discussed as well as problems linked to the use of DNA flow cytometry for fieldwork. Reliable estimation of absolute DNA amounts in plants using flow cytometry is not a trivial task. Although several well-proven protocols are available and some factors controlling the precision and reproducibility have been identified, several problems persist: (1) the need for fresh tissues complicates the transfer of samples from field to the laboratory and/or their storage; (2) the role of cytosolic compounds interfering with quantitative DNA staining is not well understood; and (3) the use of a set of internationally agreed DNA reference standards still remains an unrealized goal.
