1. Department of Mathematics and Statistics, University of Otago, Dunedin 9054, New Zealand
2. Invermay Agricultural Centre, AgResearch, Mosgiel 9053, New Zealand
3. Department of Biochemistry, University of Otago, Dunedin 9054, New Zealand
4. Palmerston North Research Centre, New Zealand Institute for Plant & Food Research Limited (Plant & Food Research), Palmerston North 4442, New Zealand