The authors wish to acknowledge the assistance of B. PAGES (INRA - UE Pech Rouge) for technical assistance in the laboratory, and also Drs. B. LACOMBE and H. SENTENAC (INRA Montpellier), and M. GITNARD (INRA Avignon) for their technical and scientific advice. Thanks also to Dr. K. HUNTER (ARC Infiruitech, Stellenbosch SA) for his helpful suggestions oil the experimental design, and Dr. M. KELLY (University of Montpellier 1) for helpful discussions and for the English translation of this article. This work was partially financed by the Embassy of France in Peru and the Peruvian government (CONCYTEC), the French ministry of Foreign affairs (Eiffel Doctoral grants programme), and the General Counsel of the Wraith, France.