1. Departamento de Nutrición Vegetal, Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Cientı́ficas, Apdo 202, 50080 Zaragoza, Spain (M.A.M., P.R.E., I.I.-O., M.C.R., J.F.M., M.B.)
2. Biology Department, University of San Diego, San Diego, California 92110 (L.M.B.)
3. Biology Department, Reed College, Portland, Oregon 97202 (D.A.D.)
4. Institute of Grassland and Environmental Research, Plas Gogerddan, Aberystwyth SY23 3EB, United Kingdom (F.R.M., A.J.G.)