1 Plant Science Laboratories, School of Biological Sciences, University of Reading, Whiteknights, PO Box 221, Reading RG6 6AS, UK.
2 British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK; Gateway Antarctica, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. Electronic address: pcon@bas.ac.uk.