Changes in DNA and in nuclear and nucleolar dry mass and area in senescing parenchyma cells of corn cob and stalk tissues.

Corn (Zea mays L.) cob and stalk parenchyma tissues were used for a study of senescence using quantitative interference microscopy and quantitative Feulgen cytophotometry. In both parenchyma tissues, nuclear dry mass (NDM), nucleolar dry mass (NoDM), nuclear area (NA), and nucleolar area (NoA) increased during cell elongation and then decreased prior to cell death. These changes were more distinct in corn cob parenchyma tissue than in stalk parenchyma tissue. DNA content (determined for cob parenchyma, center section, only) decreased over the entire study period, slowly at first, them more rapidly. Because NoDM and NoA trends in cob parenchyma tissue were similar to those of NDM and NA but with earlier decreases, we conclude that nucleolar degeneration is an early event in corn parenchyma cell senescence. Loss of DNA also occurs earlier than loss of NDM in cob parenchyma tissue. It is further suggested that the increases in NDM as DNA content decreased represents nuclear protein and RNA content increases prior to senescence. However, no indication of nucleoli degeneration was found in stalk parenchyma cells even though NDM and NA decreased prior to cell death.