Abstract:The role of acid phosphatase (APase) played in the differentiation and dedifferentiation of the secondary xylem in Eucommia ulmoides Oliv. in respect of enzyme localization was studied. APase was initially prominent along the nuclear membrane and in the dictyosomes of the young xylem parenchyma cells, and present in the whole nucleus in the highly differentiated cells. At a later stage of differentiation of the xylem parenchyma cells , APase was concentrated in the debris of other organelles and along the cell wall in the mature cells. APase aggregated in the nucleus, plasma membrane and pits of the immature vessel element, from which part of the plasma membrane became indistinct or disappeared and the nucleus eventually degenerated. APase mainly aggregated along the primary cell wall of the nearly mature vessel element, and in the secondary cell wall in the mature vessel element. On the 1st and 2nd day after girdling, the enzyme was sparsely scattered in the cytoplasm during the dedifferentiation of the xylem parenchyma cells. On the 4th and 7th day after girdling, APase was present in the cytoplasm of the dilated xylem parenchyma cells in the surface layer and also that of the adjacent dedifferentiating parenchyma cell. On the 14th and 21st day after girdling, few APase was found in the cytoplasm and none in the cell wall of the inner immature dedifferentiating vessel element. The localization of APase in the secondary xylem formed from the new cambium was as similar as that in the normal vascular tissue, and that in the mature and nearly mature vessel element that could not dedifferentiate after girdling was the same as that before girdling. Comparison of the distribution of Apase activity between the differentiation and dedifferentiation stages of the secondary xylem indicates that different isoforms of APase may respectively play functions in the protoplast degeneration and the secondary wall formation during programmed cell death of the secondary xylem. The intensity of APase activity may be an important decisive characteristic in cell dedifferentiation.