NUCLEAR DEGENERATION IN EPIDERMAL-CELLS OF DRYING ONION BULB LEAF BASES

We tested the null hypothesis 'that activated nuclei and nucleoli in o uter-epidermal cells of newly exposed equatorial tissue of the turgid leaf bases of white onions (exposed to the ambient atmosphere by remov al of two dry and two turgid leaf bases) remained in that state as the tissue dried' by following nuclear macromolecules (total nucleic acid , DNA, RNA, total protein, histone, and non-histone protein; compared with T-0 = 100%) and nucleolar morphologies over a 5-day period. The n uclei became activated within 6 h and remained in that state for 2-3 d ays [increases in RNA, non-histone protein, and volume of major nucleo li occurred by T-12 (about 191; 177, and 289%, respectively) and appea rance of the minor nucleoli between T-12 and T-24 (activation of silen t rRNA cistrons)]. Combined nucleolar (major and minor) volumes decrea sed to 228% by T-24 and to 150% by T-48. Minor nucleoli were visible a t T-24 and T-48. DNA (DAPI) remained unchanged over that period of tim e. At the T-96 sampling, all nuclear indices had decreased to levels b elow those obtained at the lime of exposure to the ambient atmosphere; minor rRNA cistrons had became silent genes; nuclear volume was about 89%, of the original volume; and, nucleolar volume (major nucleoli) w as about 93%. The percentages for nuclear indices at T-120 were DNA, 8 5% of T-0; RNA, 35%; histone, 87%; non-histone protein, 47%; nuclear v olume, 81%; and nucleolar volume, 67%. Of interest is the lack of chan ge in major nucleolar morphologies between T-96 and T-120 although the y decreased in volume during that period. We infer that the karyoskele ton (nuclear matrix) had undergone irreversible degeneration after T-4 8 and that the cells had passed the point-of-no-return in the senescen ce pathway by T-120. We propose that this model for cell senescence an d death (drying of turgid leaf bases to form the dry, dead outer cover ing of the bulbs) simulates post-harvest storage conditions and will p rove helpful to those studying cellular senescence mechanisms and asso ciated host-pathogen interactions in plants.