1. New Zealand Institute for Crop & Food Research Limited, Private Bag 4704, Christchurch, New Zealand.
2. Corresponding author. E-mail: LiP@crop.cri.nz
3. Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, IN.
4. New Zealand Institute for Crop & Food Research Limited, Private Bag 11 600, Palmerston North, New Zealand.