NUCLEAR (DNA, RNA, HISTONE AND NONHISTONE PROTEIN) AND NUCLEOLAR CHANGES DURING GROWTH AND SENESCENCE OF MAY APPLE LEAVES

Quantitative interference microscopy was used to determine changes in nuclear and nucleolar indices (dry mass and cross-sectional area) in u pper and lower epidermal cells and adjacent leaf-margin hair cells of the May apple (Podophyllum peltatum L.) leaves over a 42-day period (a fter leaves emerged above the ground litter). These indices decreased in a highly correlated manner. A ploidy variation may exist between ep idermal cells and leaf-margin hair cells. Using the leaf-margin hair c ells model, six nuclear macromolecule indices (total nucleic acid, DNA , RNA, total nuclear protein, histone and non-histone protein), nuclea r volume, nucleolar volume and perinucleolar volume (measured using qu antitative epifluorescence-phase contrast microscopy) all declined wit h age (42-day study) in a highly correlated manner. The degeneration o f the nucleus and nucleolus in the three leaf locations studied follow ed the patterns observed for programmed cellular senescence and death (necrosis) in epidermal cells of onion leaf bases (stored tissue; leaf bases did not contain chlorophyll) and human epithelial cells (buccal ; cervical). We conclude that the epidermal cells and leaf-margin hair cells from green leaves of the May Apple are ideal for the study of p rogrammed cell senescence and death in plants, especially for the part itioning of this process into the study of: the point-of-no-return (so lubilization of the karyoskeleton and loss of non-histone proteins and RNA associated with the karyoskeleton from the nucleus); nuclear pycn osis (loss of nuclear dry mass and volume and loss of nuclear internal support structure); chromatin condensation, margination along the inn er nuclear envelope; and DNA-histone degeneration; degeneration of the nucleolus and loss of the perinucleolar zone of exclusion. The charac terization of chlorenchyma cells during the 42-day period should now b e undertaken (leaf senescence as indicated by the beginning of yellowi ng about 35 days after emergence) to determine whether these cells wit h functional chloroplasts undergo nuclear changes like those lacking f unctional chloroplasts. (C) 1996 Elsevier Science Ireland Ltd.