Does DNA Endoreduplication Occur During Differentiation of Secondary Xylem and Phloem in Abies balsamea?

The amount of nuclear DNA was determined microspectrophotometrically in cells of the vascular cambium and differentiating secondary xylem and phloem in Abies balsamea (L.) Mill. Feulgen-stained radial sections were prepared from 1-yr-old shoots of mature trees sampled during (May 21) and after (June 5) the period of high mitotic activity of the cambium. Differentiating xylem and phloem cells had an unreplicated DNA level, indicating that differentiation proceeded from the G1 phase of the cell cycle. The DNA content did not change during differentiation, indicating a lack of endoreduplication, whereas the nuclear size increased during cell expansion and decreased during cell maturation until the nuclei broke down. The genome size in fusiform cambial cells and differentiating xylem and phloem elements decreased between the two sampling dates. This decrease agrees with our previous observation that the genome size in fusiform cambial cells decreases during the growing season and increases during dormancy, probably as a result of loss of amplified sequences followed by DNA amplification.