2.6. Flow cytometry
Spike tips of 1 cm length were chopped, using a razor blade, in nuclear extraction buffer obtained from the High Resolution Kit for Plant DNA (Partec, Münster, Germany) provided by the manufacturer of the flow cytometer. The extraction buffer was a low pH solution containing Triton X-100. The extract was filtered through a 50-mm nylon mesh and the medium with the isolated nuclei was collected. A buffer with the fluorescent DNA stain 4,6-diamidino- 2-phenylindole (DAPI), also part of the standard reagent set (Partec) was added, and the solution was vortexed. The DNA content of the isolated nuclei was analyzed using a flow cytometer (Ploidy Analyzer, Partec). DNA levels were obtained from a total of 5000 nuclei. On the basis of histograms obtained from cytometry, the ratio of nuclei with decreased DNA was calculated as described in Yamada et al. (2006). Although itis unclear whether DNA masses detected by flow cytometry (FCM) have functions as nuclei, DNA masses were regarded as nuclei in the present study.