Intraplant variation in nuclear DNA content was quantified in DAPI-stained cells of Laminaria saccharina and Alaria esculenta. In both species, meristoderm, cortical and medullary cells were examined from vegetative fronds. In addition, DNA was quantified during sporogenesis, in free-swimming zoospores and gametophytes of A. esculenta. Zoospores and gametophytes had 1C (= N) levels of nuclear DNA in all cells, but vegetative cells of sporophytes varied from 2C to 16C. In A. esculenta nuclear DNA was significantly correlated with cell length, total cell volume and nuclear volume, whereas in Laminaria saccharina these correlations were not apparent. During sporogenesis of Alaria esculenta, maximal nuclear DNA levels of 8C were present in post-meiotic nuclei. During four subsequent mitotic divisions (to form 64 zoospores per sporangium at the 1C level) only one cycle of DNA synthesis occurred. These results show that mitosis can be independent of DNA synthesis in Laminariales, and that endopolyploidy is a generalized feature of multicellular plants.
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