Department of Mathematics and Physics, Center of Molecular Plant Physiology, Royal Veterinary and Agricultural University, Thorvaldsensvej, Fredriksberg, Denmark. ekstrom@dina.kvl.dk|Department of Mathematics and Physics, Center of Molecular Plant Physiology, Royal Veterinary and Agricultural University, Thorvaldsensvej, Fredriksberg, Denmark. ekstrom@dina.kvl.dk|Department of Mathematics and Physics, Center of Molecular Plant Physiology, Royal Veterinary and Agricultural University, Thorvaldsensvej, Fredriksberg, Denmark. ekstrom@dina.kvl.dk|Department of Mathematics and Physics, Center of Molecular Plant Physiology, Royal Veterinary and Agricultural University, Thorvaldsensvej, Fredriksberg, Denmark. ekstrom@dina.kvl.dk