1. Department of Chemistry, University of Florence, Via Gino Capponi 7, 50121 Florence, Italy, Department of Soil Sciences and Plant Nutrition, University of Florence, P.le delle Cascine 18, 50144 Florence, Italy, BIP-CNRS, 31, Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France, and Biologisches Institut II, University of Freiburg, Schänzlestrasse 1, 79104 Freiburg, Germany