Physiological and Canopy Temperature Responses to Drought of Four Penstemon Species

Available water for urban landscape irrigation is likely to become more lim-ited because of inadequate precipitation and the ever-increasing water demand of a growing population. Recent droughts in the western United States have also increased the demand for low-water-use landscapes in urban areas. Penstemon species (beard -tongues) are ornamental perennials commonly grown in low-water-use landscapes, but their drought tolerance has not been widely investigated. The objectives of this study were to determine the effects of water availability on the morphology, physiology, and canopy temperature of Penstemon barbatus (Cav.) Roth 'Novapenblu' (Rock Candy BlueVR penstemon), P. digitalis Nutt. ex Sims 'TNPENDB' (DakotaTM Burgundy beardtongue), P. xmexicali Mitch. 'P007S' (Pikes Peak PurpleVR penstemon), and P. strictus Benth. (Rocky Mountain penstemon). Twenty-four plants of each penstemon species were randomly assigned to blocks in an automated irrigation system, and the substrate volumetric water content was maintained at 0.15 or 0.35 m3.m23 for 50 days. The decreased substrate volu-metric water content resulted in a decreased aesthetic appearance of the four penstemon species because of the increased numbers of visibly wilted leaves and chlorosis. Plant growth index [(height + (width 1 + width 2)/2)/2], shoot number, shoot dry weight, leaf size, and to-tal leaf area also decreased as the substrate volumetric water content decreased, but the root-to-shoot ratio and leaf thickness increased. Photosynthesis decreased, stomatal resis-tance increased, and warmer canopy temperatures were observed when plants were dehy-drated. Additionally, as substrate volumetric water content decreased, the leaf reflectance of P. barbatus and P. strictus increased. Penstemon digitalis, which had the highest canopy-air temperature difference, was sensitive to drought stress, exhibiting a large proportion of visi-bly wilted leaves. Penstemon xmexicali, which had the lowest root-to-shoot ratio, had the lowest shoot water content of the species studied and more than 65% of leaves visibly wilted when experiencing drought stress. Penstemon barbatus and P. strictus, native to arid regions, exhibited lower canopy-air temperature differences and better aesthetic quality than the other two species. Under the conditions of this study, Penstemon barbatus and P. strictus ex-hibited better drought tolerance than P. digitalis and P. xmexicali.