RESUMEN
Nonspecific light loss by the cell-wall-plus-cytoplasm (CWC) can cause a 50% increase in Feulgen absorption units in peanut root-tip nuclei as determined by scanning at 450 nm, whereas this phenomenon is not evident with chicken erythrocytes. A two wavelength scanning method of subtracting nonspecific 450 nm absorption from 550 nm Feulgen absorption values eliminated the nonspecific light loss in CWC, However, the two wavelength scanning method is time consuming and somewhat impractical with a regular scanning microdensitometer such as Vickers M85. Elimination of the problem of nonspecific light loss is suggested by careful determination of background setting with the spot position close to the nucleus in CWC. The accuracy of the CWC background setting method was further tested by comparison with subtraction method. The use of plant nucleis as an internal standard in plant DNA measurements was also evaluated. Significant variation among the replicate slides due to the variation in pine nuclear DNA amounts was observed and plant nuclei generally are not reliable internal standards. Mature chicken erythrocytes are recommended as an internal standard because the cell type and metabolic state is known.