1. Researcher, Institute of Production Engineering, Federal Research Center of Agriculture, Bundesallee 50, 38116 Braunschweig, Germany.
2. Researcher, Soils and Irrigation Unit, Servicio de Investigación Agroalimentaria, DGA, P.O. Box 727, 50080 Zaragoza, Spain.
3. Researcher, Dept. of Genetics and Plant Production, Estación Experimental de Aula Dei, Superior Research Scientific Council (CSIC), P.O. Box 202, 50080 Zaragoza, Spain.