A widely used standard is Pisum sativum, but the absolute values which have been assigned to it are divergent; this is in sharp contrast to the findings of Baranyi and Greilhuber (1995, 1996) that the genome size of P. sativum is stable worldwide. Pisum sativum has the advantage of being intermediate in genome size among angiosperms, poor in or devoid of inhibitors, well established for genome size stability, and neither rich in nor completely devoid of heterochromatin. It is easily available and germinates fast, and responds equally well to different isolation buffers (Loureiro et al. 2006a). Therefore, it has all the qualifications of a primary standard, against which secondary standard species can be calibrated. Its 1C-value is presently best taken as 4.38 pg or 4.284 Gbp, which is the mean value obtained by four laboratories using laser and lamp-based flow cytometers (Dole-zel et al. 1998). A very similar 1C-value of 4.42 pg has been measured with Feul-gen densitometry by comparison with Allium cepa (Greilhuber and Ebert 1994). Marie and Brown (1993) report an almost 5% lower value (i.e. 4.185 pg/1C) for P. sativum ''Express Long'', when calibrated with Petunia hybrida ''PxPc6'' (1.425 pg/1C, Solanaceae), which had been calibrated with female CRBCs (1.165 pg/ 1C). Dolezel et al. (1998) assumed 1C = 4.545 pg for Pisum sativum after calibration against human leucocytes with 1C = 3.5 pg, but the latter value seems to be the upper limit for the human (see above).
Thus, there is great interest in a unique standard which fulfils all demands -the ''plant gold standard'', against which all other plant standards can be calibrated. An Arabidopsis thaliana mutant with knocked-out flavonoid production is being reviewed as a potential standard (M. D. Bennett et al., personal communication), in which the 2C, 4C, 8C, and 16C peaks could be used, the first peak representing 0.321 pg DNA (314 Mbp), the final peak, 2.569 pg (2.512 Mbp).
However, reduced peak height at the higher C-levels may limit the use of this species as a standard. While such a ladder meets the most frequent 2C-values in angiosperms, higher C-values need other standards. It seems, that a set of standard species covering the whole range of DNA content in angiosperms cannot be circumvented. Unfortunately, consensus on a unified set of standard species with agreed C-values has not been achieved.
An overview of species used in the literature for standardization is presented in Chapter 18 (Table 18.2), and occasionally large variations of assumed C-values are recognized. A list of nine species and the values obtained by four laboratories are presented by Dolezel et al. (1998). These data also give the impression of some variation between teams, notwithstanding the application of best practice rules.
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