Sunflower (Helianthus annuus) leaves contain compounds that reduce nuclearpropidium iodide fluorescence

Sunflower leaves have unidentified compounds that interfere with propidium iodide (PI) intercalation and/or fluorescence. Independently prepared pea l eaf nuclei show greater PI fluorescence than nuclei from pea leaves simulta neously processed (co-chopped) with sunflower leaves. Differences in fluore scence persist after mixing the PI-stained pea and the co-chopped pea/sunfl ower samples, i.e. PI staining protects the nuclei from the effects of the inhibitor. The current results are significant to practical flow cytometric determination of plant nuclear DNA content. They show: (1) simultaneous pr ocessing of nuclear samples from the target and the standard species is nec essary to obtain reliable DNA estimates; (2) a test for the presence of inh ibitors should be conducted; and (3) when inhibitors are present caution sh ould be taken in interpreting differences in estimated DNA content. The pre viously reported environmentally-induced variation in DNA content in sunflo wer populations is most simply explained by variation in the amount of envi ronmentally-induced inhibitor that interferes with intercalation and/or flu orescence of PI. Intraspecific variation of DNA content for Helianthus annu us needs to be re-evaluated using best practice techniques comparing physio logically uniform tissues that are free of inhibitors. The best estimate fo r 2C DNA content of H. annuus used in this study is 7.3 pg. (C) 2000 Annals of Botany Company.