The nuclear DNA content of over 100 important plant species was measured using flow cytometry and propidium iodide staining. Arabidopsis has a 2C nuclear DNA content of 0.30 pg (145 Mbp/1C), which is twice the commonly cited value. Rice has a 2C value about three times that of Arabidopsis, while tomato has a 2C value of about 2.0 pg, larger than commonly cited. Several horticultural crops, including fruit trees like apricot, cherry, mango, orange, papaya, and peach, have genome sizes only two or three times that of Arabidopsis. The small genome sizes of rice and horticultural plants should facilitate molecular studies of these crops. The nuclear DNA contents of over 100 important plant species were estimated by flow cytometry and are presented in Table I. This table includes major crop plants, some widely used in molecular studies, as well as Arabidopsis thaliana. For some species, one or more nuclear DNA values have been published, while for many others, no such values have been reported. The information may be helpful to plant biologists interested in genome analysis or in the relationship of nuclear DNA content to plant physiology and ecology. For flow cytometric analysis, suspensions of intact nuclei were prepared by chopping young leaf tissues in MgSO4 buffer, stained with propidium iodide, and treated with DNAase-free RNAase. The DNA content of 2C nuclei was calculated using nuclei from chicken red blood cells (CRBC) as internal standards. The absolute DNA content of CRBC nuclei was estimated as 2.33 pg/2C. Arabidopsis has been reported to have the smallest genome among flowering plants. In our survey, Arabidopsis had the lowest value, with 2C nuclei containing 0.30 ± 0.012 pg DNA. This is about twice the widely cited genome size. Arabidopsis showed multiploidy in many of its tissues, with nuclei from leaves producing four or five distinct peaks of fluorescence, representing the 2C, 4C, 8C, 16C, and 32C complements of the genome. The fourth (16C) peak coincides with or falls very close to the peak for nuclei of CRBC. These results suggest that Arabidopsis exhibits developmental regulation of multiploidy.The nuclear DNA content of over 100 important plant species was measured using flow cytometry and propidium iodide staining. Arabidopsis has a 2C nuclear DNA content of 0.30 pg (145 Mbp/1C), which is twice the commonly cited value. Rice has a 2C value about three times that of Arabidopsis, while tomato has a 2C value of about 2.0 pg, larger than commonly cited. Several horticultural crops, including fruit trees like apricot, cherry, mango, orange, papaya, and peach, have genome sizes only two or three times that of Arabidopsis. The small genome sizes of rice and horticultural plants should facilitate molecular studies of these crops. The nuclear DNA contents of over 100 important plant species were estimated by flow cytometry and are presented in Table I. This table includes major crop plants, some widely used in molecular studies, as well as Arabidopsis thaliana. For some species, one or more nuclear DNA values have been published, while for many others, no such values have been reported. The information may be helpful to plant biologists interested in genome analysis or in the relationship of nuclear DNA content to plant physiology and ecology. For flow cytometric analysis, suspensions of intact nuclei were prepared by chopping young leaf tissues in MgSO4 buffer, stained with propidium iodide, and treated with DNAase-free RNAase. The DNA content of 2C nuclei was calculated using nuclei from chicken red blood cells (CRBC) as internal standards. The absolute DNA content of CRBC nuclei was estimated as 2.33 pg/2C. Arabidopsis has been reported to have the smallest genome among flowering plants. In our survey, Arabidopsis had the lowest value, with 2C nuclei containing 0.30 ± 0.012 pg DNA. This is about twice the widely cited genome size. Arabidopsis showed multiploidy in many of its tissues, with nuclei from leaves producing four or five distinct peaks of fluorescence, representing the 2C, 4C, 8C, 16C, and 32C complements of the genome. The fourth (16C) peak coincides with or falls very close to the peak for nuclei of CRBC. These results suggest that Arabidopsis exhibits developmental regulation of multiploidy.