ONTOGENETIC CHANGES IN DNA CONTENT IN ROOTS OF THE WATER FERN AZOLLA FILICULOIDES

The DNA content of the apical cell and various other cells in the roots of Azolla filiculoides was determined by two-wavelength cytospectrophotometry. DNA content decreased markedly with increasing age of the apical cell; there was a similar but less pronounced trend in the other cell populations studied. These findings suggest the possibility of DNA amplification in very young roots with rapidly dividing cells. ON THE BASIS of anatomical data, the nineteenth century botanists (among others, Nageli and Leitgeb, 1868) who studied the apical cell characteristic of many bryophytes and lower vascular plants concluded that it must be an initial, that is, that all other cells of a given root or shoot are ultimately derived from its apical cell. This view was questioned first by Buvat and Liard (1953) and subsequently by several other investigators; the resulting controversy has been reviewed by D'Amato (1975) and Giffiord (1983). In recent years, evidence supporting the classical hypothesis has been obtained from studies of cell cycle kinetics, ploidy levels, and anatomy (see, for example, Kuligowski-Andres, 1977; Hebant, HebantMauri and Barthonnet, 1978; Polito, 1979, 1980; Vallade and Bugnon, 1979; Giffiord and Polito, 1 98 1; Kurth, 198 1; Giffiord and Kurth, 1982, 1983). The classical hypothesis concerns actively growing axes. However, determinate growth is quite common among lower vascular plants. It occurs in some shoots, such as aerial shoots of Equisetum, but is especially frequent in roots, perhaps because of the limitations imposed on import of photosynthate from the shoots by the lack of secondary growth. By the time a root attains its maximum length, cell division has presumably ceased, including division of the apical cell. Indeed, Gunning, Hughes and Hardham (1978) reported that root apical cells of Azollafiliculoides andA. pinnata divide only about 55 times; almost all of these divisions occur before the roots exceed 1 mm in length. Subsequent growth occurs mainly through di' Received for publication 20 August 1984; revision accepted 3 June 1985. 2 Present address: Department of Land, Air, and Water Resources, University of California, Davis, CA 95616. The authors thank V. S. Polito, T. L. Rost, and L. D. Gottlieb for critical reading of the manuscript. This research was supported by NSF Grant PCM 79-10683 to EMG. visions occurring in the derivatives3 of the apical cell and through cell enlargement The roots have a maximum length of about 3 cm. Similarly, in determinate shoots of Equisetum, most nodes have been formed by the time the shoot is a few centimeters tall; apparently, further growth occurs through intercalary divisions and cell enlargement rather than through mitotic activity in the apex. Thus, one would not expect to find high ploidy levels in the apical cell, at least not in indeterminate shoots or during the early phases of growth of determinate axes, as it is difficult to envision a polyploid cell forming diploid derivatives. After the apical cell stops dividing, it might conceivably become polyploid. High ploidy levels have been reported for the apical cells of several species and cited in support of the concept of mitotic quiescence of the apical cell (D'Amato, 1975), but in retrospect it appears that there may have been some methodological problems (Polito, 1980; Kurth, 1981). It is also possible that at least some of these data were obtained for axes in the later stages of determinate growth; this seems especially likely when one considers how small the roots of Azolla and aerial vegetative shoots of Equisetum still are when division of the apical cell ceases. Gifford, Polito and Nitayangkura (1979) reported on developmental changes in the mitotic behavior of several cell populations of Azolla filiculoides roots but did not address the issue of ploidy levels in the root. We here report on the DNA content of the apical cell and some other cell populations in A. filiculoides roots of diffierent ages. MATERIALS AND METHODS-Azolla filiculoides Lam. was grown on nutrient solution 3 Each cell derived from one mitotic division of the apical cell, as well as the group of cells formed by subsequent divisions of such a derivative cell, is termed a merophyte.