1. Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, U.K.
2. School of Life Sciences, Oxford Brookes University, Headington, Oxford OX3 0BP, U.K.
3. Max-Planck-Institut für Molekulare Pflanzenphysiologie, Am Mühlenberg 1, 14476 Potsdam-Golm, Federal Republic of Germany