Authors would like to thank Didier Casane, Marion Hulin, Michel Laurin and Juliana Sterli for valuable comments. This work was financed by regular funding from the Centre National de la Recherche Scientifique, the Université Paris-Sud, and AgroParisTech.
Received: 23 September 2011
Revised and accepted: 7 March 2012
Published online: 4 July 2012
Editor: J.W. Arntzen