Histological observations

Tubers were sliced and fixed immediately with 2.5% glutaraldehyde in 0.1 M phosphate buffer (pH 6.8) at room temperature for 6 h. After fixation, samples were dehydrated using an ethanol series, and embedded in Technovit 7100 (Kulzer & Co., Hanau, Germany) as described previously (Yeung & Chan 2015). Sections of 3 µm were cut with glass knives on a Reichert-Jung 2040 Autocut rotary microtome. These sections were collected on slides and stained with Periodic acid–Schiff’s reaction for total insoluble carbohydrates, and counterstained with either 0.05% (w/v) toluidine blue O (TBO) in benzoate buffer for histology or 1% (w/v) amido black 10B in 7% acetic acid solution for protein staining (Yeung 1984). Images were captured digitally using a CCD camera attached to the microscope.