1. Department of Plant Nutrition, Aula Dei Experimental Station, Spanish Council for Scientific Research (CSIC), Apdo. 202, E-50080 Zaragoza, Spain, and Groupe Photosynthèse et Télédétéction, LURE/CNRS, Bât 203, Centre Universitaire Paris-Sud, B.P. 34, 91898 Orsay Cedex, France