John,
There are three references you should probably take a look at if you
haven't already:
Dickson et al. 1992, Nuclear DNA content variation within the Rosaceae.
American Journal of Botany. 79:1081-1086. They used the protocol found in:
Arumuganathum and Earle. 1991. Estimation of nuclear DNA contents of
plants by flow cytometry. Plant Molecular Biology Reporter. 9:229-233.
This protocol uses PI (plus RNase). The third referece uses H0: Moyne et
al. 1993. Relationship between cell ploidy and regeneration capacity of
long term Rosa hybrida cultures. Plant Science. 93:159-168.
Good luck!
Wendy