Loss of DNA and euchromatin in senescing leaf cells of Allium

The leaves of the chive, Allium schoenoprasum, have an average life-span of 53 days, then they fade from top to bottom, in the same sequence as cells originated. Starting during the adult phase, the amount of DNA per nucleus decreases significantly. Nuclei of senescent cells exhibit about 15% less DNA than nuclei of juvenile cells. Electron-microscopic investigations have shown that the diffuse chromatin is lost from the nuclei, followed by shrinkage of the space left. Quantitation was achieved by measurement of the percentage of condensed chromatin per nucleus. Senescence starts in different tissues, and in individual cells of a tissue, asynchronously. Chloroplasts undergo structural changes after initiation of aging in the nuclei. The short life-span, in spite of a relatively high DNA content (11.8 pg), is suggestive of a programmed senescence in Allium leaves.