Autoradiographic studies of the synthesis of nuclear DNA in various tissues during leaf development of Xanthium pennsylvanicum

Abstract

The synthesis of nuclear DNA and the incorporation of H 3-thymidine into leaf nuclei of various tissues has been studied during the entire course of leaf development of Xanthium pennsylvanicum. Leaf plastochron index (L.P.I.) developed by Erickson and Michelini (1957) was used for designation of developmental stages. The autoradiographic slides yielded data on the percentage of labeled nuclei and the average number of grains per nucleus in the upper and lower epidermis and palisade layer and in the mesophyll cells. The percentage of labeled nuclei indicated a relative number of cells engaged in DNA synthesis of the above tissues, whereas the average number of grains per nucleus was indicative of the amount of the radioisotopic precursor incorporated into nuclear DNA of a single cell. The amount of DNA synthesis decreased significantly with the age of the leaf in the course of development. At L.P.I. 1.72, fewer cells were engaged in DNA synthesis and also a smaller amount of the radioactive precursor was incorporated in a single nucleus than at L.P.I. −0.56. After L.P.I. 1.72, the amount of synthesis decreased rapidly. However, the amount of incorporation of the radioisotope on a single cell level increased sharply starting with L.P.I. 1.72. In leaves older than L.P.I. 2.78, few cells were engaged in DNA synthesis, but they incorporated more of the H 3-thymidine per nucleus than at any younger plastochronic age. The amount of DNA synthesis in epidermal and mesophyll layers decreased significantly between L.P.I.'s −0.56 and 1.72 and dropped very rapidly thereafter. In the palisade layer, however, DNA synthesis followed a different pattern. Between L.P.I.'s −0.56 and 0.94, it was constant, but smaller than in the other tissues. At the plastochronic range of L.P.I. 0.94 and 2.2, an almost linear increase was noticeable. During this period of development a highly significant difference in the amount of DNA synthesis was demonstrated between the palisade cells and cells of any other tissue. The nucleic acid synthesis in the palisade layer continued longer and at a significantly higher rate than in the epidermal and mesophyll cells of the lamina for at least one plastochron.