You could select a plant in the following paper in which genome size and ploidy level are given:

Marie, D. and S. C. Brown. 1993. A cytometric exercise in plant DNA histograms, with 2C values for 70 species. Bio. Cell 78: 41-51.

Alternatively, you can choose a plant from the Plant DNA C-values Database:

http://data.kew.org/cvalues/

You must use the plants have the basic number for this crop 

Thank you all for your kind suggestions. The analysis is done.