Evaluation location: Arizona, United States
The leaf samples used in the studies were obtained from plants in an experimental elm plantation located in the city of Holbrook, AZ. The plantation was established during 1995-1998, and each accession was represented by at least five single-tree replicates at the time of sampling. Elm seedlings 2-4-yr-old and 30-45 cm in height at the time of planting were used in establishing the plantation using a completely randomized design with 10 single-tree replicates per taxa. Seedlings were planted on a 21 rows x 14 columns grid at 3.6- by 3.6-m spacing. The soil around each plant was mulched using a 1-sq m piece of black shade cloth covered with pine wood chips. Plants were watered regularly with a drip irrigation system that supplied water to the base of individual plants. Detailed description of the experimental plantation can be found in Bosu et al. (2007). Undamaged single leaves were removed from each of the plants in the field and immediately cut with dissection scissors into ~1-sq cm pieces that encompassed at least the leaf midrib. The cut leaf samples were then placed in labeled glass vials containing 4% glutaraldehyde fixative and 0.01M sodium cacodaylate buffer solution, pH 7.2, before processing for scanning electron microscopy observation (St-Laurent et al. 2000). The samples were dehydrated in a series of ethanol concentrations of 25, 50, 70, 95, and 100%. Samples were then critical point dried using carbon dioxide, mounted on aluminum stubs, and sputtered with gold under vacuum. Trichome type and density were assessed with a scanning electron microscope (15 kV; LEO 435 VP, Leica, Cambridge, United Kingdom) on the abaxial leaf surface.