Nuclear and nucleolar protein during the cell cycle in differentiating Pisum sativum vascular tissue

Squash preparations of Pisum sativum fourth internode tissue were stained with a combined Feulgen and dinitrofluorobenzene (DNFB) procedure. Nuclei from differentiating xylem vessel elements, phloem sieve tube elements and phloem fibres were measured for their DNA and protein contents with a Zeiss scanning cytophotometer linked to an interactive computer system. Nuclei were examined from both slowly growing and more rapidly growing internodes. A computer program was constructed to calculate nuclear protein alone as well as the ratio of DNFB (protein) to Feulgen (DNA) staining in each 0.5 X 0.5 micron measuring point. Nuclei were assigned to each of ten interphase fractions based on DNA content, nuclear area and percent condensed chromatin. There was a slight increase of nuclear protein during G1, a gradual increase in S, and a continued, often sharper, rise as G2 proceeded. In all three cell types, there was, on the average, a higher protein content throughout interphase in nuclei from the more rapidly growing internodes than from the slower growing ones. A population of fibre nuclei designated G0, however, differed from phloem and xylem G0 nuclei in the pattern of protein change. The nucleolar protein/DNA ratios of xylem nuclei increased in G1, showed no significant change in S, but increased thereafter.