Increased DNA levels in centrally located endosperm nuclei are shown to be related to endosperm development in Zea mays. Mitotic activity sharply decreases in endosperm cells 10-12 days after pollination. At this time nuclear size and DNA content per nucleus (where C = haploid content) sharply increase until peak levels are reached at about 14-18 days after pollination. Mean DNA content per endosperm nucleus in strain A188 was shown by Feulgen cytophotometry to increase to about 90C by this peak stage, with the pattern being remarkably consistent over four consecutive growing seasons. Some individual nuclei achieved levels of >200C. Most other strains compared during one growing season averaged even higher peak levels of DNA per nucleus than did A188. Individual nuclei in those strains reached levels as high as 690C. A decrease in DNA level was observed in older endosperms with most strains. Endosperm mutant strains did not show a significant reduction in DNA. Opaque-2 mutants in several backgrounds achieved higher levels of DNA per nucleus. DNA levels from F1 endosperms did not indicate heterosis. Regardless of differences in DNA content, the pattern of DNA increasing as development proceeds followed by a DNA decrease was observed for most strains. Cytological studies reveal much variation in chromatin strandedness, a maximum of three nucleoli, a maximum of three nucleolar organizer regions, and "'30 diffuse chromatin masses in older endosperm tissue. A form of DNA amplification, perhaps polytenization, appears to be occurring during endosperm development.The developmental biology of maize (Zea mays L.) is important because of the prominence of maize in plant genetics research and its importance as a major agricultural food crop. Understanding the development of the endosperm of maize becomes especially important because this tissue makes up 85-90% of the mature kernel dry weight.Endosperm tissue obviously serves a critical role in overall kernel development.The initial ploidy level ofthe endosperm is 3X, because two polar nuclei ofthe central cell ofthe embryo sac fuse with one of the two sperm nuclei of the pollen grain. Within several hours, the initial triploid nucleus undergoes rapid and synchronous divisions that ensue for several days. Both nuclear divisions and cytokineses eventually cease in the more central regions of the tissue, whereas these activities persist for the longest time only in the peripheral endosperm regions. The outermost layer of cells, the aleurone, cytologically behaves like a meristem providing additional cells to the interior region of the kernel. The nuclei of the nonperipheral cells continue to increase in size during the early period of endosperm development. Aside from the morphological aspects of kernel development reported by Lampe (1), Randolph (2), and Kiesselbach (3), the genetic and molecular behaviors of maize endosperm during the process of development have not been fully described. Nuclei of the central endosperm regions have been characterized with regard to ...

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Multiparametric flow cytometry was used to analyze the development of the endosperm in Zea mays L. during the period from 8 to 20 days after pollination (dap). Nuclear size, DNA content per nucleus, and frequencies of nuclei with varying properties were measured in preparations that included all of the endosperm nuclei of single kernels of the inbred strain A188. Characteristics of nuclear populations from different kernels on the same ear showed minimal variation. The dynamic changes of non-mitotic cells involved in endosperm development consisted of alternating periods of DNA replication with nonreplication. Seven rounds of DNA replication had occurred in some nuclei in the later developmental stages with the rate averaging approximately one round per 24-hour period. Analysis of the DNA levels in the nuclei showed an exact doubling pattern indicating an endoreduplication process, that is, replication of the entire genome during each round. The loosely organized polytenization of the chromatin occurred to varying extents among the nuclei within an endosperm. A weak positive correlation existed between DNA content and size of nuclei suggesting that DNA increases and nuclear growth may not be highly coordinated in this tissue. Increased proportions of the larger nuclei occurred in the later stages of endosperm development. Considering the entire endosperm, the average DNA content per nucleus at the 15-dap peak level was approximately 12.8 C constituting a 2.7-fold overall increase from 8 dap.