SEASONAL-VARIATION IN THE NUCLEAR GENOME SIZE OF RAY CELLS IN THE VASCULAR CAMBIUM OF FRAXINUS-AMERICANA

The nuclear DNA content in ray cells from the 1-year-old vascular camb ium of white ash (Fraxinus americana L.) trees was determined at inter vals during the annual cycle of cambial activity and dormancy by using Feulgen microspectrophotometry. By 10 September, these cells had ente red dormancy in Gi with a normal DNA distribution and a minimal averag e DNA content of 2.65 pg. The average amount of DNA increased to 3.51 pg by 30 November, remained at this elevated value until at least 30 M arch, when the cambium was still dormant, then declined to the minimum level on 1 May and 10 June, when the cells were mitotically active. T he springtime decline appeared to occur both before and during cell di vision. Between 1 May and 10 June, the prophase (4C) and telophase (2C ) DNA contents decreased significantly. The amount of nuclear DNA meas ured by microspectrophotometry was verified by using flow cytometry an d image analysis. The results support the view that there is an annual oscillation in the nuclear genome size of shoot meristematic cells in tree species native to the northern temperate zone.