Consequences of stoichiometric error on nuclear DNA content evaluation in Coffea liberica var. dewevrei using DAPI and propidium iodide

The genome size of coffee trees (Coffea sp.) was assessed using flow cytometry. Nuclear DNA was stained with two dyes [4',6-diamino-2-phenylindole dihydrochloride hydrate (DAPI) and propidium iodide (PI)]. Fluorescence in coffee tree nuclei (C-PI or C-DAPI) was compared with that of the standard, petunia (P-PI or P-DAPI). If there is no stoichiometric error, then the ratio between fluorescence of the target nuclei and that of the standard nuclei (R-PI or R-DAPI) is expected to be proportional to the genome size. Between-tree differences in target : standard fluorescence ratios were noted in Coffea liberica var. dewevrei using propidium iodide and DAPI. For both dyes, between-tree differences were due to a lack of proportionality when comparing locations of the coffee peak and the petunia peak. Intraspecific genome size variations clearly cannot explain variations in the target : standard fluorescence ratio. The origin of the lack of proportionality between target and standard fluorescences differed for the two dyes. With propidium iodide, there was a regression line convergence point, and no between-tree differences were noted in this respect, whereas there was no such convergence with DAPI. An accurate estimate of genome size can thus be obtained with PI. Implications with respect to accessibility and binding mode are discussed.