Measurement of fresh and dry densities of suspended plant cells and estimation of their water content

A new method that considers the osmotic pressure of a culture broth for measurement of fresh cell density was developed. The water content of suspended plant cells was calculated using the value of the density obtained by the method. In the method, the optical density (O.D.) of cells and cell aggregates in a sedimentation equilibrium state in various solutions of known density and equal osmotic pressure to that of the culture broth was measured. The fresh cell density and density distribution of heterogeneous cell aggregates could be measured quantitatively. It was found that the fresh cell density of Catharanthus roseus (a dicotyledonous plant) had a relatively narrow distribution (1.010-1.028 g/cm³) with light cell aggregates; however, Oryza sativa (a monocotyledonous plant) had a wide distribution (1.030-1.064 g/cm³) with relatively heavy cell aggregates. In both cell lines, the distribution of fresh cell density at the logarithmic growth phase was more localized in the heavier density area compared with that at the stationary phase, and the small cell aggregates group had a heavier density distribution than the large cell aggregates group. The dry cell density was measured by a pycnometer method designed for plant cells using tridecane as immersion liquid. The water contents % (v/v)] of C. roseus (92.2% at 7 d and 95.0% at 12 d) and O. sativa (82.6% at 7 d and 89.8% at 12 d) were calculated by using the values of both the fresh cell density and the dry cell density. The maximum cell concentration in suspension culture was calculated from the value of the solid content of cells on the assumption that the volume of water existing in the spaces among cells is half the total volume of the culture vessel. The result showed that the maximum cell concentration in a suspension culture of C. roseus was about half that of the O. sativa culture.