|MadSci Network: Botany|
Nick, to my knowledge you cannot just vary the amount of starch in the roots. However, there are varieties of a plant known as Arabidopsis that naturally don't have a whole lot of starch in their root cells. I don't think those seeds are commercially available. You may want to contact the AIMS (Arabidopsis Information Management System)/ABRC (Arabidopsis Biological Resource Center). The web address is http://aims.cps.msu.edu/ aims/. I hope this helps!
Try the links in the MadSci Library for more information on Botany.