1. 1Plants, Photosynthesis and Soil, School of Biosciences, University of Sheffield, Sheffield S10 2TN, U.K.
2. 2Electron Bio-imaging Centre, Diamond Light Source, Didcot OX11 0DE, U.K.
3. 3School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.