CELL-CYCLE IN RELATION TO INDUCED XYLEM DIFFERENTIATION - TRITIATED-THYMIDINE INCORPORATION IN CULTURED TUBER EXPLANTS OF HELIANTHUS-TUBEROSUS L

The occurrence and timing of periods of DNA synthesis in cells destined to differentiate after 48 h in cultured tuber explants of H. tuberosus (Jerusalem artichoke) were estimated by tritiated thymidine autoradiography. All tracheary elements were derived from cells that had undergone at least 1 round of [3H]TdR incorporation. The last period of [3H]TdR incorporation ended 7-10 h before the appearance of the secondary wall. Three periods of [3H]TdR incorporation were observed, after 12, 20 and 30 h; tracheary element precursor cells entered each round of incorporation 2-4 h earlier than the remainder of the cell population. Apparently, tuber parenchyma cells go through 3 rounds of DNA synthesis, each of which is followed by mitosis, before differentiating at 48 h.