Bonnardot, V., ARC-Institute for Soil, Climate and Water (ISCW) of the Agricultural Research Council, Private Bag X5026, 7599 Stellenbosch, South Africa. Current address: University Rennes 2, COSTEL LETG-UMR 6554 CNRS, 2 Place Recteur H. Le Moal, 35 043 Rennes Cedex, France, France