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Nuclear conditions in the meristems of resting seeds
of Triticum durum Silvana AVANZI. A. BRUNORI and F. D'AMATO Institute of Botany of the University, Pisa, Laboratory for the Applications of Nuclear Energy in Agriculture, C.N.E.N., Rome, and Institute of Genetics of the University, Pisa, Italy DNA labelling with 3H-thymidine and DNA cytophotometry of Feulgen stained material have been used to study the sequential development of meristems in the embryo of Triticum durum : caryopses have been collected at different intervals during the last three weeks of development up to maturity (field grown plants) (AVANZI et al. 1969). It has been found that the embryonic shoot apex completes its development at ca. 63% water content in the seed ; other meristems-apices of primary root and the seminal roots of the first and second pair, primordia of leaves 1, 2 and 3-complete their development at 47-48% seed water content, or lower. At seed maturity, the shoot apex, the leaf primordia 1, 2 and 3 and most apices of the second pair roots consist exclusively of cells with a 2C nuclear DNA content, corresponding to the G1 phase of the nuclear cycle in meristem cells. On the contrary, the meristems of the primary root and of the first pair roots contain different proportions of cells with 4C nuclei (G2 Phase of the nuclear cycle) in addition to 2C cells (Table 1). An analysis of the labelling index (percentage of labelled interphases after feeding with 3H-thymidine) and mitotic index (cells in mitosis in percent of cells scored) in the meristems of the embryos has shown that, at late stages of embryo development, DNA synthesis is stopped earlier than mitosis ; but the interval between the inhibition of DNA synthesis and the inhibition of mitosis is different in different meristems. Consequently, in some meristems - leaf primordia, apices of most seminal roots of the second pair-mitotic activity lasts long enough to deplete completely the meristems of 4C (G2) cells ; in the remaining root meristems, this "depletion phase" is less efficient, the degree of this efficiency decreasing progressively in the succession: roots of second pair->roots of first pair->primary root (this meristem contains the highest proportion of 4C relative to 2C cells). If mature seeds are germinated in water containing 3H-thymidine and the first mitotic cycle is observed in primary root apices, it is found that the nuclei first entering mitosis are unlabelled (nuclei in G2 in the resting seed) ; to these labelled mitoses follow (nuclei in G1 in the resting seed) (AVANZI et al. 1963). As to the implications of these findings in studies on chromosome breakage and on developmental processes reference is made to our original papers. Literature Cited AVANZI, S., A. BRUNORI and F. D'AMATO 1969. Sequential development of meristems in the embryo of Triticum durum. A. DNA autoradiographic and cytophotometric analysis. Developmental Biology 20 : 368-377. AVANZI, S., A. BRUNORI, F. D'AMATO, V. NUTI RONCHI and G. T. SCARASCIA-MUGNOZZA 1963. Occurrence of 2C (G1) and 4C (G2) nuclei in the radicle meristems of dry seeds in Triticum durum. Its implications in studies on chromosome breakage and developmental processes. Caryologia 16 : 553-558. (Received Feb. 24, 1970) |