1. 2Section of Plant Biology and
2. 2,5Department of Land, Air and Water Resources, University of California, Davis, California 95616–8627 USA;
3. 3S.E.P.F.‐I.N.R.A. 86 600 Lusignan France; and
4. 4Department of Plant and Microbial Biology, University of California, Berkeley, California 94720–3102 USA